Heat Rate Improvement Reference Manual Training Guidelines
Heat Rate Improvement Reference Manual Training Guidelines TM-114073 Training Manual, December 1999
EPRI Project Manager P. Ruestman
EPRI 3412 Hillview Avenue, Palo Alto, California California 94304 • PO Box 10412, 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 •
[email protected] • www.epri.com
DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT EPRI Duke/Fluor Daniel
ORDERING INFORMATION Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, P.O. Box 23205, Pleasant Hill, CA 94523, (925) 934-4212. Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. POWERING PROGRESS is a service mark of the Electric Power Research Institute, Inc. Copyright © 1999 Electric Power Research Institute, Inc. Inc. All rights reserved.
CITATIONS This document was prepared by Duke/Fluor Daniel DFo1A 2300 Yorkmont Road Charlotte, North Carolina 28217-4522 Principal Investigator R. Snyder
EPRI 3412 Hillview Avenue Palo Alto, CA 94304 Principal Investigator or Author J. Tsou
This document describes research sponsored b y EPRI. The publication is a corporate document doc ument that should be cited in the literature in the following manner: Heat Rate Improvement Reference Manual, Training Guideline, EPRI, Palo Alto, CA, 1999. TM-114073.
i
ACKNOWLEDGEMENT EPRI wishes to acknowledge members of the Heat Rate Interest Group for providing guidance in development of this manual and training training guide. EPRI also wishes to to acknowledge the following people for reviewing and providing comments to the draft manual and training guideline. Duane Hill, Dairyland Power Cooperative Wes Hull, Central and South West Services Sam Korellis, Illinois Power Company"
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ABSTRACT Performance optimization of fossil power plants has always been a high priority within the electric power industry. industry. However, it has become of paramount importance in meeting the the challenges mandated by operating within a competitive environment. Recently, many power producers producers have have downsize downsized d and curren currently tly lack experien experienced ced staff staff require required d to maintain maintain optimal performance. Thus, a resource was needed to capture the lost experience to aid in the retraining of less experienced personnel. The objective of this project was to produce a manual to be used by power producers as a training tool and reference source for the development of heat rate and performance engineers. This document provides required information to to understand thermodynamic properties properties and precepts precepts,, guidance guidance on how to to use them and and methods methods of determin determination ation to assess their impact on system performance. This training guide, a compliment to the reference manual, used EPRI CS-4554 Heat Rate Improvement Guidelines as a basis for development of the program. Specifically, this manual includes: •
A description of the properties of water, water, its phases, and the determination determination of each. A discussion of the Steam Tables and Mollier diagram and how each is used to find the properties properties of water/s water/steam. team. A brief brief discuss discussion ion of the Ideal Ideal Gas Gas Law. Law.
•
A definition and application of the concepts of the first law of thermodynamics and required energy conversion calculations calculations to power plant components. The relationship is used to develop an understanding of how plant parameters are affected by the operation of the components.
•
A review of the principles and applications of fluid flow. Discussion includes pumps and pump operation for forced fluid flow.
•
A discussion of the concept of thermal efficiency and the methods employed to maximize efficiency.
•
An explanation of the various modes of heat transfer and the equations used with each mode. It gives an introduction introduction to nucleate boiling boiling and the factors affecting DNB. A discussion of natural circulation and a brief discussion on heat exchangers are also covered.
•
An explanation and review of power plant systems, which include the water/steam cycle, boiler fuel, air and flue gas systems, as well as, balance of plant systems.
•
An introduction to the ‘Heat Rate Improvement Reference Manual’, the purpose, organization and use of the manual.
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REVIEW OF THERMODYNAMIC THERMODYNAMIC PROPERTIES (1)
1. OVERVIEW: This Lesson Plan describes the properties of water, its phases, and the determination of each. It also includes a discussion discussion of the Steam Tables and Mollier diagram and how each is used to find the properties properties of water/steam. A brief discussion of the Ideal Gas Law is also included.
2
TERMINAL OBJECTIVE: At the end of this class the student should have a working knowledge of Thermodynamic princ principles iples that can be used by those involved in the Heat Rate Improvement Program. This will be accomplished by meeting the requirements of the following enabling objectives. ENABLING OBJECTIVES:
1. Define each of the following terms: 1.1 Temperature 1.2 Pressure 1.3
Density and Specific Volume
1.4
Enthalpy
1.5
Entropy
1.6
Specific Heat Capacity
1.7
BTU
2. Convert a known temperature from one scale to another. 3. Convert a known known pressure from one one scale to another. another. 4. State and and define the different different phases phases of water. 5. Explain each of the following 5.1
Saturation Temperature
5.2
Latent heat
5.3
Quality
5.4
Sensible Heat
6. Given a set of conditions and using the steam tables, determine the thermodynamic properties and phases of water. 7. Given a set of conditions using the Mollier diagram determine determine the properties and phases of water.
3
8. Using the ideal gas law, solve solve problems relating to pressure, temperature temperature and volume of an ideal gas.
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LESSON OUTLINE
1. INTRODUCTION 2. PRESENTATION 2.1
Fluid Properties
2.2 Temperature 2.3 Pressure 2.4
Specific Volume
2.5
Enthalpy
2.6
Entropy
3. DETERMINING THE PROPERTIES OF WATER 3.1
Steam Tables
3.2
Phase of Water
3.3
Mollier Diagram
4. GAS RELATIONSHIP 4.1
Ideal Gases
5
1. INTRODUCTION 1.1
Overview This lesson covers covers the properties of water/steam water/steam and the phases of water. It also covers the steam tables and Mollier diagram and their use in the power plant.
1.2 Objectives
2. PRESENTATION 2.1
Fluid Properties The thermodynamic properties of a fluid are measurable or quantifiable characteristics characteristics of the fluid and include the following: Temperature
Internal Energy
Pressure
Enthalpy
Specific Volume
Entropy
2.2 Temperature A. Definition: A measure measure of the average average molecular kinetic energy: (Thermal Driving Head) B. Temperature Scales 1. Absolute °R and °K:
• •
R = °F + 460(459.69 °) K = °C + 273
2. Relative °F = (1.8 x C) + 32 oF is used most often, but oR is used when absolute temperatures are required. 2.3 Pressure A. Definition: Force per unit area (P=F/A) B. Scales 1. PSIA = (Absolute Pressure) a) Pressure above a perfect vacuum b) Atmospheric Atmospheri c pressure + gauge pressure 2. PSIV = Pressure measured below a reference reference (atmospheric (atmospheric pressure) 6
a) PSIG = Gage pressure = pressure pressure measured measured from atmospheric b) (PSIG = PSIA - ATMOS) 3.
Inches of Hg Pressure (1 PSIA~- 2" Mercury Hg) Hg)
4. Inches of Hg Vacuum = PSIV x 2 Specific Volume (V) and Density ( ρ)
2.4
A. Definition of Specific Volume: Volume per unit mass. v = Volume = ft3 Mass lbm B. Definition of Density ( ρ): The inverse inverse of Specific Volume Volume 1 = lbm v ft3 C. Specific Volume Volume and Density are affected by temperature temperature and pressure. Pressure: as pressure
v
ρ
Temperature: as temperature
↑
↓
↑
↓
↓
↑
↑
↓
↑
↓
↓
↑
1. Example:Using the Steam Tables Tables find the density density of a saturated saturated liquid liquid at 200oF. v = 0.016637 ft 3 /lbm
ρ = 1/v = 1/0.016637 = 60.1 lbm/ft 3 2. Now raise its temperature to 300 oF. v = 0.01745 - ft 3 /lbm
ρ = 1/v = 1/0.01745 = 57.3 lbm/ft 3 2.5
Internal Energy A. Definition: Thermal Thermal energy energy stored within a substance itself. itself. This is due to the position and movement of the molecules or atoms which make up a substance in relation to each other. B. Enthalpy (h) 1. Definition a) Sum of the the internal energy and pv (pressure x specific specific volume) (flow) energy. 7
b) Energy content of one pound mass of a fluid at a given given temperature temperature and pressure. Units of heat energy are in BTU’s which stands for British Thermal Units. C. h = specific enthalpy; h = u + pv, where where u = Specific Internal Internal Energy in BTU/lbm 1. h = BTU/lbm To convert pv to BTU/lbm divided by Joules Constant = 778 ft- 1bf/BTU pv = Flow energy due to pressure and volume. pv = BTU J lbm 2. Example: Find the internal specific energy of s saturated aturated steam at 1000 psia. h = u + pv = h - pv J
J
u = 1192.9 BTU - (1000 lb) (144 in 2) (0.44596 ft3 /lbm) lbm in2 ft2 778 ft - 1bf BTU u = 1110.4 BTU lbm 3.
Notice that nearly all the enthalpy was was internal energy. Generally a change in internal energy results in a change in temperature, but not always. PROOF: For a saturated saturated liquid at 1000 1000 psia, find the internal internal energy. energy. U = H - pv = 542.6 BTU - (1000)(144)0.02159 (1000)(144) 0.02159 j lbm 778ft_-_1bf BTU U = 538.6 BTU lbm At 1000 psia, the change in internal energy is from 538.6 to 1110.4 BTU/lbm, but the temperature temperature remained constant. The only change change was a change from a saturated liquid to a saturated vapor.
D. Total Enthalpy (H) 1.
Total energy of a given mass (H = U + pv)
2. To find total enthalpy, enthalpy, simply simply multiply specific enthalpy times the amount of mass present. H = (h)(m) 8
2.6
Entropy (S) A. Definition: A measure measure of the unavailable unavailable energy in a fluid @ a given given temperature and pressure. Units: OR:
BTU on an absolute scale lbm-oR BTU lbm - oF on a relative scale
We are more interested in Entropy changes changes (Delta S = S final - Sinitial) than specific values of Entropy. 1. Example:Find Delta S when when energy energy is added added to a saturated saturated liquid liquid @ 100 oF and changes the saturated liquid to a saturated vapor @ 300 oF Delta S = S final - Sinitial Sf@ 100 oF = 0.1295 BTU/lbm oF
Delta S = Sg - Sf
Sg@ 300oF = 1.6351 BTU/lbm oF Delta S = 1.6351 - 0.1295 Delta S = 1.5056 BTU/lbmoF 2. The point is, as heat is added to this example, example, more of the energy energy of of the liquid-vapor is unavailable. Also the reverse is true for removing energy which results in a decrease in entropy. 3. The change change in entropy (Delta S) is used to account for the energy energy that has been made unavailable for work. 4. For example: Use the main condenser. a) When steam is condensed, the temperature temperature remains constant, but the entropy decreases. b) If condenser condenser pressure pressure is is 1 psia, saturated saturated steam steam exhausts exhausts from from the turbine into the condenser condenser and condenses with with no subcooling. Find the heat rejected .(qrej)
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qrej = T (Delta S) at 1 psia S f = 0.1325 BTU/lbm oF Sg = 1.9781 BTU/lbm oF T = oF + 460 = 101.74 + 460 = 561.74oR
1) qrej = (561.74)(0.1326 - 1.9781) 2) qrej = 1036.12 BTU's Rejected
5. Notice the qrej is equal to h fg which is the Latent Heat of Condensation. The BTU's are the BTU's or heat given to the Condenser Circulating Circulating Water (CCW) System. 6. Qualitatively: Qualitatively: We can say that S of condensate condensate decreased when heat was removed and S of CCW increased as heat was added. 7. Summary: The energy energy that is is available available in the the condensate condensate to do work (useful) per lbm oF has increased. increased. This is mainly due to the inadequacies inadequacies of the working fluid and the process. process. Also, the value of h fg indicates that amount of energy that has become unavailable in our work process is S . B. Discussion of T-S Diagram 1000 PSIA
2200 PSIA 1000
2 PSIA
900 CRITICAL POINT
800
) F 700 ( E R U 600 T A R 500 E P M 400 E T
SUBC. LIQUID Sf
SUPER HEATED VAPOR Sg
300
WET VAPOR Sfg
200 100 0 0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
ENTROPY ( BTU/lbm U/lbm - R 43
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1. We assume that at 32oF, S = 0 (Even though S = 0 at O oR). 2. Saturation Line: Every single point on the line, to the left of the critical critical point is where a saturated liquid exists. 3. Critical Point: At these conditions of temperature (705.47 oF) and pressure (3208.2 psia), the following f ollowing is true. a) There is no difference in in specific volume between a saturated saturated liquid liquid and a saturated vapor. b) There is no difference in enthalpy between a saturated liquid and a saturated vapor. c) There is is no difference in entropy between a saturated saturated liquid liquid and a saturated vapor. OR vf = vg vfg = 0
hf = Hg hfg = 0
Sf = sg sfg = 0
At the critical point, the liquid/vapor liquid/vapor acts like a perfect gas. 4. Subcooled Region: Region left of critical critical point and left of saturated saturated liquid line. 5. Wet Vapor Region: Area under saturation line. 6. Constant Pressure Lines: From the point where it touches the saturated liquid line, it is horizontal until it touches the saturated vapor line. 7. Enthalpy Lines: Range from 100-1800 (bottom-top) and extend horizontally horizontally across the entire diagram. 8. In order to locate a point on the diagram for any condition condition except except at saturated conditions, you must know two properties of the liquid, wet vapor, or superheated steam. 9. If you are a saturated liquid at 212 oF. a) Pressure is found by noting where where constant pressure line touches sat. liquid curve for 212 oF. b) Enthalpy is the horizontal line crossing crossing through through the sat. liquid liquid line for 212oF. c) Entropy is the vertical line crossing crossing through through the sat. liquid liquid line for 212oF. d) Ex. a saturated liquid at 212 oF. P = 15 psia h = 1150 BTU/lbm Ss = 1.75 BTU/lbmoR
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10. Go back to condenser condenser example example a) 1 psia, saturated liquid = 100 oF by T-s diagram. b) The saturated steam changed to saturated liquid. Note that enthalpy and entropy decreased. Sinitial = 1.98
Sfinal = 0.12
c) Heat rejected from condenser condenser was heat added to Condenser Condenser Circulating water.
3. DETERMINING THE PROPERTIES OF WATER 3.1
Steam Tables A. The Steam Table consist of 3 separate tables 1. Table 1. Saturated Steam: Temperature Table a) Consists of columns for: 1) Temperature 2) Pressure - corresponds corresponds to temperature temperature for saturation saturation conditions. 3) Specify Volume 4) Enthalpy 5) Entropy b) The v, h, and s columns each have have values values for saturated saturated liquid liquid (v f) saturated vapor (vg), and the change (v fg) from liquid to vapor. 2. Table 2. Saturated steam: Pressure Table a) This table table is set up the the same as table except the temperature temperature and pressure columns are reversed. 3. Table 3. Superheated steam a) This table is set up differently. It consists of: 1) Abs pressure column with sat. temperature temperature in parentheses. parentheses. 2) Across the top is temperature - degrees Fahrenheit. This represents the actual temp of the steam. 3) Sh column represents the degrees super heat. 4) It then then has columns for v, h, and s. 4. The last part of the steam tables is a conversion conversion factors factors chart used for converting from one parameter to another.
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3.2
Specific Heat A. Definition: Specific heat capacity (c) 1. Heat required to cause cause 1 lbm of any substance to change change by 1 oF. 2. Sensible heat - heat added added that raises the temperature temperature of water. c = BTU lbm oF
3.3
Phases of Water There are 5 exact phases of water that we consider in the power industry. A. Subcooled Liquid (Compressed Liquid) 1. Liquid below the boiling point. 2. Enthalpy (h) of a subcooled liquid liquid is determined by one of the following following methods. a) Definition: Heat required to cause 1 lbm lbm o off any substance to change by 1oF. Heat added that raises the temperature of water is "sensible heat". c = BTU lbm oF b) Subtract 32oF from the temperature and use the units of enthalpy (BTU/lbm). NOTE: NOTE: Be Belo low~ w~- 300 300oF, this is a fairly accurate method. But > 300 o F, the accuracy drops due to changes in the Specific Heat Capacity of the fluid, i.e., it takes more and more heat to cause the temperature to change by 1 oF as its temperature increases. c = Specific Heat Capacity (Assume 1.4 BTU/lbm oF for Reactor Coolant) c) The most desirable (most accurate) accurate) method method to find h is to look up temperature of liquid in the steam tables and assume h sc = hf STM Table hf
T-32 Method
Actual
hf
Assume at 400psi (Example on two methods) hsc @ 100oF =>
68
68
13
69.5
hsc @ 200oF =>
168
168
169
hsc @ 300oF =>
269.7
268
270.3
hsc @ 400oF =>
375.1
368
375.3
Conclusions 1) Note the increasing error. 2) Use steam table method B. Saturated Liquid 1. Water at the boiling point 2. The properties of H f, sf, vf, Tsat, Psat are found in the Saturated Steam Tables. 3. Example: find hf @ 100 psia
100 oF 300 psia 300oF 100 psia
C. Wet Vapor 1. A combination combination of saturated liquid and and saturated saturated steam steam at the boiling point. 2. Enthalpy is determined by: hwv = hf + x(hfg)
Where hwv = Enthalpy Wet Vapor hf = Enthalpy Liquid x = Quality of Vapor hfg = Latent Latent Heat of Vaporization
(hfg = The latent heat of vaporization vaporization or condensation) Numerically, h fg is the amount of heat which must be added to 1 lbm of a saturated liquid to change it to 1 lbm of steam or the amount of heat which must be removed to change 1 lbm of saturated steam to 1 lbm of saturated liquid.
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3.
x = Quality = amount of vapor in a wet vapor. x = 100%-m (m = % moisture in a wet vapor)
4. Example: Find h fg @ 1 psia 100 psia 300 oF 1000 psia 5. Example: Find enthalpy of wet wet vapor @ 500 psia if m = 15%. a) hwv = hf + (xhfg) D. Saturated Steam 1. Steam at the boiling point (no moisture, 100% vapor). 2. The properties of h g, sg, vg, Tsat, Psat are found on the saturated steam tables. 3. Example: Find:
hg
vg
sg for the following
460oF 180oF 400 psia 1000 psia E. Superheated Steam 1. Steam above the boiling point 2. The properties properties of h, s, v, v, P, and and T are are found in the superheated steam tables. 3. Superheat term refers to # of of deg degrees rees above T sat. 4. Example: Find T sh @ 400, 450, 500 oF when @ 100 psia. 3.4
Mollier Diagram
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MOLLIER
A I S P 5 4
280
1172 BT Us 240
E N I L . T A S
A I S P 5 1
MOISTURE
A. A pictorial pictorial representation representation of steam tables (does not not contain specific volume) volume) called a H-S diagram. 1. Find h of steam at saturation saturation with with x = 100% and 900 psia. a) From Mollier on saturated steam line h = b) From steam tables h= 2. Find the the temperature temperature and h of a system system at 500 psia psia and 20 o superheat. a) T =
h=
3. Find the the enthalpy enthalpy and and temperature temperature of a system system at 300 psia psia for the following. a) Saturated: b) m = 4% : Note that since we are under saturation line; c) m = 12%: temperature is constant.
4. GAS RELATIONSHIP 4.1
Ideal Gases
16
A. Ideal gases gases are described as gases comprised of molecules that do not interact with one another. another. The pressure exerted exerted by an ideal gas is the force exerted on the surface by the collision collision of these molecules. molecules. The forces exerted by the molecules increase with the absolute KE as measured by temperature. Similarly, the pressure pressure exerted by the molecules molecules increases as the density of the molecules increases. increases. So we can say say that, P α PT Since ρ = m/v, we can substitute substitu te and rearrange as follows: P α m/v T, or P v = T for 1 lbm. If we compare initial and final conditions on a particular system of ideal gas we can write the equation. (P1 V1)/T1 = P2V2 /T2 This is the ideal gas law. B. The Ideal Gas Law is actually a combination, combinat ion, of 2 laws; Charles Law and Boyles law. 1. Charles Law states: The volume of a given mass of gas, maintained at constant pressure, varies linearly with temperature or, V1
T1 =
V2
T2
2. Boyles Law states: For a fixed fixed mass of gas at a constant temperature, the volume of gas is inversely proportional to its pressure or, P1V1 = P2V2 C. All pressures pressures and temperatures must be in absolute absolute scales. D. Examples: If 20 cu ft f t of Nitrogen at 15 psia is heated from 73 oF to 150oF, what will be the pressure if the volume remains constant? P1V1
P2V2 =
T1
T2
15 1b/in2 (20 ft3)
P(20 ft3) =
(73 + 460) oR
(150 + 460) oR
17 psia = P2 17
REVIEW OF THERMODYNAMIC THERMODYNAMIC PROPERTIES (2)
1
1. OVERVIEW This Lesson Plan defines and applies the concept of the first law of Thermodynamics and Energy conversions conversions to power power plant components. This relationship relationship is used to develop an understanding of how plant parameters are affected by the operation of the components.
2
TERMINAL OBJECTIVE: At the end of this lesson the students will have a working knowledge knowledge of the First Law of Thermodynamics and Energy Conversions. ENABLING OBJECTIVES:
1. List and define the six energy forms considered considered in the study of Thermodynamics. 2. Identify the properties which which indicate a change in each of the six energy forms. 3. State the First Law of Thermodynamics 4. State the Continuity Continuity Equation and apply it in determining determining mass flow rate, volume flow rate and velocity changes in power plant components. 5. Describe the energy conversions conversions which occur in a moving fluid and relate them to changes in observable parameters and fluid properties for: 5.1
Constant diameter pipe
5.2
Nozzle and venturi
5.3
Throttling device
5.4
Pump
5.5
Turbine
6. Plot the throttling process process on a Mollier Diagram and and determine fluid properties upstream and downstream, given appropriate information. 7. Define pump efficiency. 8. Determine turbine work work and power given given appropriate appropriate information. information. 9. Define turbine efficiency.
3
LESSON OUTLINE 1. INTRODUCTION 1.1
Purpose of this lesson
1.2
Overview
2. THE ENERGY FORMS ASSOCIATED WITH THERMODYNAMICS THERMODYNAMICS 2.1 Energy 2.2
Forms of Energy
3. THE FIRST LAW OF THERMODYNAMICS APPLIED 3.1
First law of Thermodynamics Thermodynamics
3.2
The Steady Flow Energy Equation
3.3
Power
4. APPLICATIONS OF THE GENERAL EQUATION 4.1
Constant Diameter Pipe
4.2
Nozzle and Venturi
4.3
Throttling Throttlin g Device
4.4
Pump
4.5
Turbine
5. ATTACHMENT 1
4
1. INTRODUCTION 1.1
Purpose of this lesson A. Develop concepts of energy conversion within power plant components. components. B.
Apply the concepts concepts in order to enable the student student to predict the effects on system parameters.
C. Predict the effects of changing changing operational operational conditions on plant parameters. D. Develop the building blocks which will be used in the power plant cycles lesson. 1.2
Overview of the lesson - Review objectives.
2. ENERGY AND THE ENERGY ENERGY FORMS ASSOCIATED WITH THERMODYNAMICS 2.1 Energy A. Energy is defined as the capacity for producing producing an effect 1. The effect produced is frequently mechanical work thus the common definition is "the capacity for doing work." 2. Energy in some forms is intangible but the effects produced produced can can usually usually be evaluated B. General Methods of Classifying Energy 1.
Stored energy and energy in transition transitio n a) Energy in Transition 1) Momentary energy form; in an intermediate intermediate state state betw between een tw two o or more stored forms 2) Begins and ends as stored energy 3) Heat and and work work are are the forms of energy in transition transition b) Stored Energy 1) Energy associated with or contained in systems or bodies such as the working fluid 2) The stored energy forms we will be dealing with are: (a)
Kinetic
(b)
Potential
(c)
Flow or Pv
(d)
Internal
5
2. Mechanical and Thermal Energy a) Mechanical Energy 1) Energy which stems from the position position or motion of of relatively relatively large bodies 2) Forms of Mechanical Energy (a)
Kinetic
(b)
Flow or Pv
(c)
Potential
(d)
Mechanical Work
3) Transfer of mechanical mechanical energy energy is is by physical contact between large masses either directly or through a machine such as a moving shaft or piston 4) Unit of mechanical mechanical energy is the foot pound pound force (ft-lbf) b) Thermal Energy 1) Energy associated with the configuration configuration and motion of of molecules 2) Characterized Characterized by its ability to be transferred from one body to another by temperature difference alone 3) The mechanism mechanism of thermal energy transfer is through through the collision of molecules or electromagnetic waves waves 4) Forms of thermal energy (a)
Internal energy
(b)
Heat
5) Unit of of thermal energy is the British Thermal Unit (BTU) (BTU) c) Relating mechanical and thermal energy 1) Joules equivalent, the mechanical mechanical equivalent equivalent of heat is the mathematical relationship between units used to express mechanical energy energy and thermal energy. energy. Symbol J 2) 778 ft lbf = 1 BTU or J = 778 ft lbf / BTU
6
C. Kinetic Energy Energy - Stored mechanical mechanical energy energy due to the mass and velocity that the mass has or the work to bring mass up to observed velocity
→ 1. KE = MV2
gc = 32 lbm ft
2gc
lbf sec2
2. The parameter parameter that is an indication of the KE KE of a fluid is the velocity velocity
→ KE α V2 D. Potential Potentia l Energy (PE) - Stored mechanical energy associated with the elevation relative to a reference elevation or work done in bringing a mass from Ref to Height Z. 1. PE = mgz / gc or PE per unit mass = gz / g c 2. Height is an indication of PE E. Flow work or displacement displacement energy(Wf) - stored mechanical mechanical 1. The Energy Energy ne necessary cessary to maintain a continuous continuous steady flow of a stream of fluid 2. Flow work is not not present present unless there is flow 3. Wf is the product of the force acting on any cross-section cross-section of the stream and the distance through which the force must act to cause any selected mass to pass that cross section 4. Thus W f = Pv P = Pressure v = Specific volume of the fluid 5. Derivation Derivation - optional Wk = Force (F) x L F = P(press) A(area) Volume (V) = A(area) L(length) V also equals Mass(M) v (specific volume) Therefore AL = Mv PAL = PMv therefore Wk = PMv and the work per unit mass is Wf = pv
7
F. Internal Energy Energy (u) - Stored thermal thermal energy energy associated associated with with the molecules and atoms of a substance 1. Majority is contained contained as vibrational KE of the molecules and and atoms of a substance a) Temperature is a measure of th this is energy b) Also energy associated with the spin spin of molecules c) And the spacing between molecules 2. The energy energy distribution between these various various forms of internal energy is what causes the heat capacity of a substance to vary Example: At low temp Cp of H20 ≅ 1 BTU/ lbm °F In this condition, almost all of a BTU added goes into the vibrational KE and therefore will show up as a temperature change. At high temperatures and pressures C p H20 ≅ 1.4 BTU / lb- °F In this case, a portion of the BTU added is going into increasing the spin and spacing of the molecules => show up as an increase in C p because since all the BTU is not directly related to the increase in vibrational KE.(thus temperature change) Example Problem: 1) Find the the h, Pv, u for a saturated liquid at 500 °F 2) hstm table = 487.9 BTU/lbm 3) Pv = PSAT x v =
↑ from stm tables 2 (680.86 lbf/in2)(144 in2 /ft )(.02043 ft3 /lbm) ____________________________ __________________________________ ______
=
778 ft-lbf/BTU Pv = 2.57 BTU/lbm 4) U = h-Pv = 487.9 487.9 BTU/lbm BTU/lbm - 2.57 BTU/lbm BTU/lbm U = 485.33 BTU/lbm 3.
During a phase change all the energy goes to increasing distance between molecules (internal potential energy), therefore there will be no increase in temperature of the substance.
8
G. Work - Force Force applied in moving moving something something through through a distance distance 1. W = F x D 2. Units - ft-lbf 3. Work as such cannot be be stored, but work work done on a system system will will show up as an increase of one or more of the stored energy forms. 4. Work done on system system is evidenced by a mechanical device passing through the boundary and is moved by some external source 5. Work is done by a system ifif the mechanical mechanical device device is passing through the boundary and the force which causes movement of this device is developed from within the system. (Mechanical output such as turbine) 6. Heat - Transient Thermal Energy - Energy Energy transferred from one region to another due to a temperature difference. a) Methods of heat transfer 1) Conduction - Heat Heat transfer through a medium medium 2) Radiation - Heat Heat transfer without a medium medium via electromagnetic electromagnetic radiation 3) Convection - Heat Heat transfer transfer by by th the e combined combined action action of conduction, storage, and mixing of a fluid between regions of high and low temperature.
3. THE FIRST LAW OF THERMODYNAMICS APPLIED 3.1
First law of Thermodynamics Thermodynamics A. Energy can neither be created nor destroyed destroyed but only transformed transformed B. When applied to a system - Net Net energy entering the the system will will equal the net energy leaving the system plus the energy accumulated within the system or Energy in = Energy out + Energy Accumulated
3.2
The Steady Flow Energy Equation A. Statement of the first law considering considering all energy energy forms assuming steady flow Energy accumulated = 0 in steady flow. B. Energy in = Energy out KE1 + PE PE1 + Pv1 + u1 + qin + Won = KE2 + PE2 + Pv2 + u2 + qout + (Wby) Divide the mechanical terms by J to get all in BTU's 9
3.3
Power A. Power is defined defined as the rate rate of doing work B. The General Energy Equation becomes a power equation when multiplied by M the mass flow rate in lbm/hr C. Some equivalents 1hp = 2545 BTU/hr = 42.42 BTU/min = 33000 ft lbf/min = 550 ft lbf/sec 1kw = 3.4 x 10 3 BTU/hr = 1.34 hp D. This will be applied to the turbine, turbine, pump, and heat exchanger exchanger later
4. APPLICATION OF GENERAL ENERGY EQUATIONS 4.1
Energy balance on fluid flowing through a pipe A. Assumptions: -Water is incompressible -Water temperature is ambient -Area1 = Area2 -No change in height -Steady flow conditions B. KE1 + PE PE1 + u1 +Pv1 + Qin +Won = KE2 + PE PE2 + u2 + Pv Pv2 + Qout +Wby C. Since no change in area => KE 1 = KE KE2 D. No change in height => PE 1 = PE2 E. No heat in or out and no work done on or by the system => Qin & Qout = 0 W by & W on = 0 F. Therefore, we are left with: U1 + Pv Pv1 = U2 + Pv2 U1 - U2 = v(P2 - P1)
∆U = v∆P
10
G. Recall that: h = u + Pv J 1. (u1 + Pv)-(u2 + Pv) = ∆h J J2 2. u2 > u1 due to friction 3. Pv2 < Pv1 due to drop in fluid energy due to friction H. The conversion conversion of flow energy (Pv) (Pv) to internal energy energy (u) due to friction friction is called "Head Loss". The effects of head loss will be discussed further in the fluid f luid flow lesson. 4.2
Energy Balance on a Nozzle/Venturi A. Assumptions: 1. Mass flow in = mass flow out 2. Fluid is water = > incompressible 3. No friction 4. Area1 > Area2 5. Area1 = Area3 B. Converging section - nozzle 1. PE1 + KE1 + Pv Pv1 +U1 + Qin + Won = PE2 + KE2 + Pv Pv2 + U2 + Qout + Wby a) ∆PE = 0 - no change in height b) ∆U = 0 - no friction c) Qin & Qout = 0 d) No work done by the system => W by = 0 e) No work done on the system => W on = 0 2. This leaves: KE 1 + Pv1 = KE KE2 + Pv2 Pv1 - Pv Pv2 = KE2 - KE KE1
∆Pv = ∆KE 3. The continuity continuity equation can be applied to determine determine w which hich of the energy energy terms increased. a) Since we know that in a steady flow condition o
the M
o
M 11
into a
=
out of
system
, we can say that
the system
→
→
ρin Ain Vin = ρout Aout Vout →
→
ρAV1 = ρAV2 - Since the fluid is is water and relatively incompressible then ρ1 = ρ2. →
→
Since A2 < A1 then V2 > V1 4. Parameter property changes
• • • • •
Velocity increases Pressure decreases Enthalpy decreases Temperature Increases - not noticeable Entropy Increases - slightly
5. Application a) Steam turbine nozzles to convert pressure to velocity b) Air ejector nozzle to create low pressure area c) Flow measurement
→ 1) ∆P α ∆V2 2) Recall ∆Pv = ∆KE
→ and KE = 1/2mv2 gc o
also V α velocity o
V = Volume Flow Rate (GPM) 3) then o
∆P α V2 To make an equality use Venturi constant K giving o
V = K (∆P)1/2 12
K varies based on friction and geometry and will be affected by fouling of the Venturi 4) For mass flow rate o
o
M = V x density or o
M = k (∆P)1/2 (ρ) d) Diverging section 2 to 3 1) The Reverse Process Occurs 2) Since A3 = A1 (a)
Velocity decreases from point 2 to point 3
(b)
Pressure will increase
(c)
Neglecting friction Pressure will return to the value at point 1 and velocity will drop to the initial value
3) Effect of friction will will result result in some pressure pressure drop across across the the device (a)
This effect is minimized by the smooth transition
(b)
The orifice is not smooth and significant pressure drop will occur.
4) Application (a)
Inc. NPSH on pump suction
(b)
Steam Jet
(c)
Pump Volute
(d)
Accommodate expansion
13
4.3
Energy Balance on a Pump NOTE: NOTE: Sketch pump on board with suction as Point 1 and Discharge Point 2. A. Assumptions: 1. No heat transferred in or out 2. No friction o
o
3. Steady flow conditions => M in = Mout 4. The fluid is water => ρin = ρout 5. Suction diameter = Discharge diameter B. Evaluation using General Energy Equation 1. Pv1 + U1 + KE KE1 + PE1 + Qin + Won = Pv2 + U2 + KE KE2 + PE2 + Qout + Wby a) ∆KE = 0 - at point 2 as compared to point 1 Since A1 = A2 b) ∆pe = 0 - no noticeable difference in height c) ∆u = 0 - no friction assumption d) Qin & Qout = 0 - no heat in or out
14
e) No work by the system => W by = 0 f)
Wpump = Pv Pv2 - Pv1
Wpump = v (∆P) 2. ∆P is called Pump Head and is expressed as: a) PSID b) Feet of water (2.31 (2.31 ft of water = 1 psi) or43 psi = 1 ft. c) Sometimes ft-lbf C. Pump power o
o
1. Wpump = M x Wpump o
Power = M x v∆P 2. If all the the power supplied by the prime mover mover was imported as flow work work the pump would be 100% efficient 3. Pump Efficiency o
ηpump = output = M V∆P (power into head) input
Brake HP (input by prime mover)
a) Pump efficiency ≅ 85 - 99% Due to friction and other losses in the pump D. Parameter & property changes across pump 1. Pessure Increases 2. Velocity constant (assume same disch & suct diameter) 3. Enthralpy increases 4. Entropy increases due to friction friction and other losses 5. Temperature increases not noticeable 6. U↑ 4.4
Energy Balance on a Turbine NOTE: Sketch Turbine Turbine on board board with inlet as point point 1 and exhaust exhaust as point 2 A. Assumptions: 1. No friction => constant entropy o
o
2. Qin and Qout = 0 o
o
15
3. Min = Mout => no extraction flows 4. Inlet pressure is 1000 psia 5. Condenser pressure is 1 psia NOTE: In the turbine steam is expanded through the stages so specific volume increases. B. Energy Balance 1. KE1 + PE1 + Pv Pv1 + U1 +Qin + Won = KE2 + PE2 + Pv Pv2 + Q2 +U2 + Wby a) Qin and Qout = 0 No heat transfer b) No work done on => W on = 0 c) ∆KE ≅ 0 Since the steam expands through the turbine and v ↑, the area of the exhaust is made larger to accommodate the expansion. expansion. The result is that there is little difference in velocity between inlet and outlet. d) PE - overall height being looked at is 10 to 15 feet. Any change in PE in this case would be very small compared to the other changes. 2. U1 + Pv1 = U2 + Pv2 + Wby rearranged rearranged to: Wby = (U1 + Pv1) - (U2 + Pv Pv2) and since h = U + Pv then: J Wby = Wturb = ∆h C. Turbine Power o
Turbine power = M stm ∆h o
Turbine power = M stm (hstm - hexh) D. Property and Parameter Changes 1. Enthalpy decreases 2. Velocity unchanged
16
3. Pressure decreases 4. Temperature decreases 5. Specific volume increases 6. Entropy increases for real turbine; constant for ideal ideal E. Plot turbine turbine on Mollier Mollier Diagram Diagram Real and Ideal NOTE: For Real turb. must know h or % M to get work, Ideal assume const. entropy F. Turbine efficiency 1. In Ideal Ideal Turbine there will be a larger ∆h for a given pressure drop 2. The ∆h for the real turbine working between the same two pressures will be smaller. Due to losses in the turbine Entropy Entropy increases increases indicating some of the steam energy has become unavailable for conversion to work. 3. Turbine efficiency a) ηturb = Wturb real Wturb Ideal
= ∆h real
∆h ideal
b) Typically the turbine is designed designed to be most efficient at full load 85% - 92% G. Example: Given: Inlet conditions; conditions; 900 900 psia o
Msteam = 5 x 105 lbm/hr Outlet conditions; 1 psia, 28% moisture
17
Find: Real Turbine power in HP and KW Ideal turbine power in HP Turbine efficiency 0
P turb Real = M stm (hstm - hexh) 5 x 105 lbm/hr (1195 BTU - 815 BTU) = 74.8 x 10 3 hp 2.54 x 103 BTU/hr-hp or = 55.9 x 103 KW or = 3.4 x 103 BTU/hr-KW P turb Ideal = 5 x 10 5 lbm/hr (1195BTU-781 BTU)= 81.5 x 10 3 hp 2.54 x 103
ηturb = 74.8/81.5 = 91.7%
18
ATTACHMENT 1
1. What happens to each one of the following parameters as the diameter of a pipe gets smaller (increase, decrease or stay the same)? 1.1
Enthalpy
1.2
Entropy
1.3
KE
1.4
PE
1.5
Work
2. A pump takes suction on the hotwell at 28" vac. vac. & 107o F and discharges at a pressure of 350 psia. Pump flow rate is 1 x 107 lbm/hr. What is the pump work? (In HP) Assume suction is a Saturated liquid. 3. Given the following: Psm: 1000 psia o
M - 14 x 10 6 lbm/Hr Pcond - 28" Hg Vac Turb. Exit Quality = 83% What is ideal Turb. work, real turb work and turbine efficiency? eff iciency?
4. EXTRA: A pump has a ∆h = 3 btu/lbm. btu/lbm. If the pump head is 500 psi, how much of the ∆h is due to ∆u? (Inlet temp. = 370°F)
19
5. Given the following conditions:
• • •
NC press = 2235 psig PRT press = 0 psig NC-32 (PORV) Leaking by.
Find the following for the downstream downstream fluid: 5.1
Phase
5.2
Temp.
5.3
Quality
5.4
Entropy
5.5
Enthalpy
6. A 'C' HTR DRN Pump takes suction on the 'C' HTR DRN tank at 5 x 106 lbm/Hr. SUCT pressure is 1 psig and discharge pressure is 650 psia. The suction temp. is 210 o F. What is pump horsepower?
20
ANSWERS 1. 1.1
↓
1.2
↑
1.3
↑
1.4
→
1.5
→
2. 28" = 1 psia 107°F v = .016155 ft 3 /lbm o m = 10 x 106 lbm/Hr Wk = 10 x 10 6 lbm/Hr (.016155 ft3/lbm) (349 lb/in 2) (144 in2 /ft2) 9
= 8.11 x 10 ft-lbf/hr (1 hr/3600 sec.)(.0018182 ft - lbf sec) HP •••••••••••
= 4100 HP •
•
•••••••••••
o
o
3. WkI = M (∆h)
WkR = M ∆h
= 14 x 106 lbm/Hr (1192.9 - 775)
= 14 x 10 6 lbm/Hr(1192.9 - 930)
= 5.85 x 109 BTU/Hr
= 3.67 x 109 BTU/Hr
η = WkR /WkI = 3.67 x 109 5.85 x 109
= 62.9%
4. ∆h = ∆u + ∆Pv 3 BTU/lbm = ∆u + (500 lbf/in2) (144 in2 /ft2)(.01824 ft3 /lbm) 778 Ft-lbf BTU 21
3 BTU/lbm = ∆u + 1.688 BTU/lbm 1.31 = ∆u
5. h = ~1118 BTU’s phase = Wet Vapor Temp = 212°F X = 97% S = 1.71
6. Wk = 5 x 106 lbm/Hr (634 lb/in2)( 144 in2 /ft2 ) (.0167 ft3 /lbm) = 7.6 x 109ft-lb/Hr (1 Hr/3600 sec.) (.0018182 ft - lbf sec) HP = 3850 Horsepower
22
REVIEW OF THERMODYNAMIC THERMODYNAMIC PROPERTIES (3)
1
OVERVIEW This lesson is a review review of the principles and applications applications of fluid flow. Discussion will will include pumps and pump operation for forced fluid flow.
1. REFERENCES: 1.1
Introduction to Thermodynamics; Thermodynamics; Kurt C. Rolle
1.2
Pump Handbook; Karassik, Krutzsch, Frasser, Messina
2
TERMINAL OBJECTIVE At the end of this lesson the student will have a working knowledge knowledge of the principles and applications of fluid flow. He will also understand understand pumps and pump operation for forced fluid flow. ENABLING OBJECTIVES
1. Define the following: 1.1
Head Loss
1.2 Friction
2. Explain how a change in each of the following will affect friction friction Head Loss. 2.1
Friction Factor
2.2
Pipe Length
2.3
Pipe Diameter
2.4
Fluid Velocity
3. Describe the two types types of flow that can occur in a system: 3.1
Laminar
3.2 Turbulent
4. Explain the purpose of a pump. 5. Explain the theory theory of operation for a centrifugal pump. 5.1
Define pump head.
5.2
Explain how flowrate, head, and power vary with pump speed.
6. Define NPSH available and NPSH required. 7. Define Cavitation. 7.1
List the conditions and parameters that affect cavitation.
7.2
Explain how cavitation is detected.
7.3
Explain how cavitation is prevented or stopped.
3
8. Define pump runout. 8.1
Explain the problems associated with runout.
8.2
Explain plant design features that limit runout.
9. Define pump shutoff head. 10. Explain the theory of operation of a positive displacement pump. 10.1 Explain how to vary vary the capacity of of a positive displacement pump. 10.2 Explain how a Pd pump pump is affected by cavitation and runout. runout.
11. Define Define "water Hammer"
4
LESSON OUTLINE
1. INTRODUCTION 1.1
Overview
1.2 Objectives
2. PRESENTATION 2.1
Fluid Flow
2.2 Headloss
3. PUMPS AND PUMP OPERATIONS 3.1
Centrifugal Pumps
3.2
Series and Parallel pump operations.
3.3
Closed System pump Operation
3.4
Open System Pump Operation
3.5
Positive Displacement Pumps
4. PLANT PROBLEMS 4.1
Water Hammer
5
1. INTRODUCTION 1.1
This lesson lesson will will discuss: discuss: Fluid flow, factors affecting fluid flow, flow, pumps and pu pump mp operation.
1.2
Cover appropriate objectives.
2. PRESENTATION 2.1
Fluid Flow A. Definition: Fluid flow is the movement of a fluid from one point point to another in a system. B. Development of Fluid Flow. 1. No flow can be developed within a system until a driving force is established. 2. This difference difference in pressure (the driving driving force) can be created by by two two methods: a) By a pump b) By a difference difference in density of a fluid. 3. Recall that the energy balance performed on the straight pipe showed showed that energy conversions took place between points one and two. 4. Remember that P1V1 > P2V2 and U2 > U1V, where V is the specific volume. The change in pressure pressure between the two points points (Delta P), or differential pressure, resulted due to the flow f low of fluid within the pipe. 5.
The pressure loss in the pipe was a conversion of flow energy (P v) to internal energy (U) caused by the friction between the fluid and the pipe wall. This conversion is called Headloss.
6. Friction is the resistance to movement. 2.2 Headloss A. Definition: Headloss is the conversion of flow energy to internal internal energy energy due to friction. B. Headloss (HL) is dependent on any factor in a piping system which will change or vary the amount of friction in that system.
6
C. The factors that affect headloss are: 1. Pipe length: as the length of pipe pipe increases, increases, headloss will increase. 2. Friction factor: as the the roughness roughness of the pipe increases or the the v viscosity iscosity of the fluid increases; the friction, headloss and pressure drop will increase. 3. Fluid velocity: as the fluid velocity (squared) (squared) increases, the friction, headloss and pressure drop will increase. 4. Pipe diameter: as the pipe diameter increases, the friction, headloss and pressure drop will decrease. D. An equation equation is used to illustrate the terms affecting headloss and and is known known as Darcey's equation: where: F is the friction 1. hL = FLV2 friction factor D 2 gc L is the length of pipe V2 is the fluid velocity squared D is the pipe diameter 2. When a system is designed, built and then then operated, operated, the h L equation becomes: hL = KV KV2 2gc Because F, L, D are constant, the constant k is substituted and H L is proportional to the V 2. The velocity of the fluid will have have a direct effect on the type of flow in the pipe. E. Operators have some control over system headloss in the following ways. 1. Valve position position changes changes will will change change the friction felt by by the fluid and the velocity of the fluid. 2. Changing the system system lineup lineup by adding more components components or removing components also changes the friction factor. 3. Varying the speed of a single pump or the configuration configuration of several several pumps within the system will vary the velocity term.
7
F. There are are two two categories categories that that flow will fall into. One type type results results in low headloss and another type causes high amounts of headloss. 1. Laminar flow - this is fluid flowing in layers layers and occurs in in low flow systems (v < 10 ft/sec.) a) In this type of flow, layers layers of fluid near the pipe wall covers the roughness of the pipe wall. As far f ar as headloss is concerned, this will reduce friction, headloss and the pressure drop. b) One problem problem with laminar flow is that the layers layers of fluid act like insulation and reduce the heat transferred into or out of the fluid. 2. Turbulent flow - this is fluid flowing flowing with with random motion in the system. system. (v >12 ft/sec) a) This type type of flow creates creates more more friction between the molecules molecules themselves and between the water molecules and pipe wall. Because of increased friction, the headloss and pressure drop increase. b) One advantage advantage to turbulent flow is that the random mixing action of the fluid will enhance the heat transfer process. c) Friction is related to the type type of flow in a pipe through through Reynolds Reynolds number (R#). R# is related to velocity: as v ↑ --> R#↑ R# = Vav D v
R# is unitless Vav = average velocity
v = viscosity
D = pipe diameter For R# < 2000 the flow is Laminar. From 2000-3000 the flow is in Transition. For R# > 3000 the flow is turbulent.
3. PUMPS AND PUMP OPERATIONS 3.1
Centrifugal Pumps A. Principle of Operation - As Fluid Fluid enters the suction of the pump, it undergoes undergoes a pressure drop. The impeller then increases increases the velocity of the fluid as the fluid moves along the impeller impeller vanes. The fluid also sees an increase in area which will convert some of the velocity to pressure (A ↑ → v↓ → p↑ ). The fluid then passes into the volute where the rest of the velocity increase from the impeller is converted to pressure by another area increase. B. Characteristics 1. For a given speed the pump head will decrease decrease as volume flow rate increases. (Where pump head = discharge discharge press - suction pressure) pressure)
8
2. Centrifugal Pump Laws •
a) The change in V α the change in speed. ( ∆V α ∆N) •
•
V1 / V2 = N1 / N2 where:
N = speed or RPM •
V = ft3 /time b) The change in pump head α the change in pump speed, squared. ( ∆Hd α ∆N2) 2
2
Delta P1 / Delta P2 = (N1) / (N2) where
Delta P = pump head (psid or ft of water) Pump head = Disch press - Suction Press.
c) The change in pump power is α the change in pump speed, cubed. ( ∆P α ∆N3) 3
3
P1 / P2 = (N1) / (N2)
where: P = power d) Example: Given; variable speed pump at 2000 RPM Delta P = 250 psid, V = 3000 gpm, P= 5000 HP Find V, Delta P, and P at a speed of 4000 RPM 1) V α N; V1 /V2 = N1 /N2 V2 = (N2 /N1) V1 = (4000/2000) 3000 gpm = 6000 gpm 2) Delta P α N2 2
2
Delta P1 / Delta P2 = (N1) / (N2) 2
2
Delta P2 = [(N2) / (N1) ] Delta P1 2
2
Delta P2 = [4000 / 2000 ] 250 psid Delta P2 = 1000psid 3) ∆PWR α N3
3
3
P1 / P2 = (N1) / (N2) 3
3
P2 = [(N2) / (N1) ] P1 9
3
3
P2 = [4000 / 2000 ] 5000 HP P2 = 40000 HP 3. Series and Parallel Pump Operation a) Series Configuration Configurati on Consider a condensate condensate and feed system. system. Imagine a single single pump that takes a suction on the hotwell and pumps it to the boiler. The pump must create enough Delta P to overcome the headloss associated with the components in the system plus the difference in pressure from the condenser condenser to the boiler. This job would require require a pump with many stag stages. es. The option is to design a system system with more pumps in series series.. Each pump will create a Delta P to overcome headlosses and provide suction pressure for the next pump in the system. 1) Each pump pump has its own own characteristic characteristic curve, each produces produces some head for a given volume flow rate. 2) When placed in series, series, the first pump increases the pressure pressure in the system. The discharge discharge of pump #1 is now the suction line to #2 pump which will also create its own Delta P. Since the pumps add their pressures independently, the total or combined head curve is merely the addition of pumps 1 and 2. 3) In series series pump operation, the capacity capacity for flow is no greater than the capacity of one pump. 4) In an operating system, the characteristic characteristic curve for that system has not changed but the operating point (the intersection of the pump and system curves) has. The increased pump pump head creates higher volume flow rate. Higher volumetric flow rate (V) in turn creates more headloss and shifts the operating point up the system characteristic characteristic curve. 5) The above above explanation explanation is the same as describing the operation operation of one multistage pump where each stage is an individual pump. b) Parallel Configuration In order to increase the flow capacity of any system, additional pumps are added in parallel. This is done instead instead of using one variable speed pump pump to reduce power consumption. ( ∆P α ∆N3) 1) Each pump pump has its own own characteristic characteristic curve, each produces produces some head for a given volume flow rate.
10
2) When placed in parallel, parallel, each pump d draws raws suction from the same point and discharges discharges to another another common point. Since each pump sees the headloss of the system the Delta P of each pump is not additive as it is in series configurations. configurations. 3) The total capacity of both pumps will will be the addition addition of both pumps flow rate. 4) Adding the second second pump in parallel parallel causes causes the fluid to accelerate thus thus the headloss increases. increases. We see an increase in volume flow rate and a small increase in head of the pumps at the new operating point. 5) Note the the increased increased pump head for the second pump addition. addition. Generally the increase in pump head is limited by the maximum head of the largest of the two pumps. 6) This design is desirable for high volume flow requirement where the Delta P is less of a concern. C. Problems 1. Cavitation a) Definition - cavitation cavitation is the formation of vapor voids in the low pressure area of the pump followed by their collapse in the high pressure region. b) Cavitation is caused caused by insufficient pressure pressure available at the suction of the pump. 2.
Net Positive Suction Head a) Net positive positive suction suction head available is the absolute pressure pressure at at the suction of the pump minus the vapor pressure of the fluid at the same point. b) Required NPSH - required amount of pressure above the vapor pressure necessary to prevent cavitation for some given volume flow rate (V). This pressure will will be equal to the pressure drop drop in the pump from the inlet flange flange to the eye of the impeller. impeller. It is determined at the factory and will be plotted on the pump curves. c) Example: Instructor make up example for NPSH-Available NPSH-Available d) Results of cavitation 1) Pitting and subsequent erosion of impeller 2) Flow oscillations oscillatio ns 3) Pump vibration 4) Overheating
11
e) Indications 1) Vibration and noise 2) Motor current fluctuations 3) Fluctuations in discharge discharge pressure and/or flow rates. f)
Prevention Methods 1) Head tank to create a static pressure at pump suction. 2) Booster pump to increase pressure to suction suction of next pump. 3) Large suction pipe to convert KE to pressure increase. 4) Subcooled liquid to raise NPSHA (lower vapor pressure). 5) Pressurize entire system. 6) Close down on discharge valve - this will will decrease fluid velocity velocity and reduce the pressure drop in the pump and thus reduce the required NPSH.
3. Other centrifugal pump problems a) Pump Runout Runout - the maximum maximum flow rate at the low lowest est anticipated anticipated system head for a given system system design and pump selection. Pumps which operate in an oversized system can reach their maximum flow rates because of not enough system headloss. Correctly sized sized pumps can reach runout due to ruptures in the system which which drastically reduces reduces or eliminates headloss. headloss. Pumps running in parallel may runout if one pump trips. 1) Results (a)
Pump efficiency decreases
(b)
Eventual flow loss (possible cavitation problems)
(c)
Overheat motor and/or pump
2) Prevention methods (a)
System design - choose correct pump for system.
(b)
Throttle discharge to prevent high flow rates.
(c)
Create some minimum static head the pump must always discharge against.
b) Pump Deadhead Deadhead - pump is running running w with ith little or no flow. 1) Results (a)
Overheat pump and/or motor.
(b)
Efficiency Efficien cy is very low
12
(c)
So much heat put into water (W p) that temperature will increase; therefore, a greater pressure is required to prevent cavitation.
2) Prevention Methods (a) 3.2
Supply a flow path which will produce the manufactured suggested minimum safe flow rate. (i.e. recirc line) line)
Positive Displacement Pumps A. Principle of Operations - the pump is usually a reciprocating reciprocating piston device which draws the fluid into a chamber on one stroke and then compresses the trapped fluid before releasing it into the system. B. Types of Positive Displacement Pumps 1. Reciprocating 2. Rotary 3. Screw C. Characteristics 1. For a given given speed, the volumetric volumetric flow rate is constant. constant. Ideally, the volume flow rate will not vary (for that speed) until the mechanical leakage increases when the pump operates at extremely high discharge pressures. 2. The head head (discharge (discharge pressure minus suction suction pressure) pressure) is dependent dependent on the system the pump discharges into. D. Problems 1. Cavitation a) A positive positive displacement displacement pump will will cavitate cavitate and and the results, indications, and prevention are basically the same as for a centrifugal pump. 2. Runout a) A PD Pump will not suffer runout.
4. PLANT PROBLEMS 4.1
Water Hammer A. Defined as a force wave wave of fluid striking an an obstruction in a pipe (e.g. closed closed valve, abrupt turn) B. Causes 1.
Starting a pump with the discharge valve open.
13
2. Initiating flow in an unvented system. 3.
Rapidly closing a valve with flow in the line.
C. Effects 1. Causes rattling in pipes. 2. Loud Noise. 3. Damage to piping, hangers, components. D. Prevention 1.
Always initiate flow in an unvented, unfilled system slowly.
2. Use good sense and judgement when starting pumps and and opening/closing valves.
14
REVIEW OF THERMODYNAMIC THERMODYNAMIC PROPERTIES (4)
1
OVERVIEW: This lesson plan discusses the concept of Thermal Efficiency and the methods employed to maximize efficiency..
1. REFERENCES: 1.1
Elements of Applied Thermodynamics: Thermodynamics: Johnson, Brocket,
1.2
Brock, Keating: Naval Institute Press.
2
TERMINAL OBJECTIVE At the end of this lesson the student will understand the concept of Thermal Efficiency and the methods employed to maximize efficiency.. ENABLING OBJECTIVES
1. Define Thermal Efficiency and discuss its relationship to the heat and work in a Thermodynamic Cycle. 2. Discuss the Rankine Cycle Cycle in terms terms of the processes involved, and the component corresponding to each process. 3. Discuss the effects of the following following on Thermal Efficiency: Efficiency: 3.1
Feedwater Heater Operation
3.2
Condenser Pressure
3.3
Condensate Depression
3.4
Turbine Load
3
LESSON OUTLINE 1. INTRODUCTION TO POWER PLANT CYCLES 1.1
Purpose of the lesson
2. PRESENTATION 2.1
Power Cycles
2.2
Thermal Efficiency and the Second Law
2.3
Rankine Cycle A. Elements of the Cycle B. Cycle Efficiency Efficie ncy
2.4
Design Improvements In Efficiency A. Feedwater heating
2.5
Operational Effects on Efficiency A. Condenser Pressure B. Condensate Subcooling C. Throttling D. Operating Power Level E. Other Considerations Considerations F. Indications of Changing Efficiency
4
1. INTRODUCTION TO POWER PLANT CYCLES 1.1
Purpose of this lesson A. Define Power Cycles B. Tie individual individual components discussed previously previously into the plant C. Develop the concept of Thermal Efficiency, discuss the the conditions conditions which which effect it. 1. Design 2. Operational
2. PRESENTATION 2.1
Power Cycles A. Definition 1. A Thermodynamic Thermodynamic Cycle is a recurring series of thermodynamic thermodynamic processes used for transforming energy into a useful effect. 2. For a power cycle the the energy energy is in the form of heat and the useful effect is mechanical work. B. Elements of the Thermodynamics Thermodynamics Cycle 1.
A working substance a) Acts as the medium medium for transport of energy energy through through the the cycle. cycle. b) Steam/water is the working substance in power cycles.
2. An engine a) The device where thermal energy of the working working substance is converted to mechanical work. b) The steam turbine is the engine engine for power power cycles cycles we we will will be discussing. 3.
A source or high temperature temperat ure energy reservoir a) Supplies energy as heat to the working substance b) The boiler boiler furnace furnace and the fuel supplied provides the heat source.
4.
A sink or low temperature temperatu re energy reservoir a) Absorbs energy as heat from the working substance either directly or through an intermediate heat transfer device known as a receiver. b) The lake, river, ocean, ocean, cooling cooling towers, towers, etc will be the sink. c) The Condenser is a receiver. 5
5.
A pump - moves the working substance from the low pressure region of the cycle to the high pressure region.
C. More elements or components components may be introduced into the cycle cycle in order order to improve performance, as we will see later. 2.2
Thermal efficiency and the second law of thermodynamics A. For a work producing cycle the thermal thermal efficiency ( η) equals the output divided by the input. OUTPUT INPUT
1.
η=
2.
For a cycle where heat is converted to work.
η=
Net work out Heat Input
B. Applying the first law of thermo to the basic cycle. 1.
Energy Balance a) Energy into cycle = Energy out of the cycle. b) Qadded = Work out + Q rejected rearranging; rearranging; Work out = Qadded - Qrejected
2. Thermal efficiency can be expressed as:
η = work out Q added substituting for Work out:
η = Qadded - Qrejected Qadded 3. The first law does does not restrict how how the energy conversion takes place place nor to what extent. C. The Second law of Thermodynamics 1. Early efforts to increase the the cyclic cyclic work work produced produced from a given heat input by reducing the heat rejected suggested the possibility of reducing this waste. This led to the Second Law. 2. No engine, engine, actual or ideal, when operating in a cycle can can convert convert all the heat supplied it into mechanical work. D. The Carnot Cycle
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CARNOT CY CL E CARNO
A
T H E R U T A R E P T L M E T
B
C
D
S1
S2 ENTROPY 15
1. Used here to illustrate the basic basic relationships relationships which effect cycle cycle efficiency. a) Most efficient cycle conceivable though not practical. b) Serves as a standard standard of comparison for all heat cycles cycles in use today. today. 2. Composed of four reversible thus ideal processes. a) Constant temperature heat addition. 1) Heat is is added to the working fluid at the source source temperature temperature from a to b. 2) The heat added is the area under the a-b Process Process line on the TS Plot. (T ∆S1-2) b) Isentropic expansion from b to c gives the work output c) Constant temperature heat rejection from c to d. 1) Heat is is transferred transferred from the working working fluid at the sink temperature 2) The heat rejected in in the area area under the c to d process line on the T-S Plot. (T sink∆ S2-1) d) Isentropic compression from d to a. 3. The work work produced produced by by the cycle is the difference between between the heat heat added and the heat heat rejected. Area a b c d on the the T-S plot. 7
4. Cycle efficiency a) Efficiency Relationship
! added − Q ! rejected Q work 1) η = ! = ! added Qadded Q ! 2) Recall that Q added = Tsource ∆S1-2 ! Q rejected = Tsink ∆S2-1 3) Substituting and dividing out ∆S Tsource − Tsink Tsource
η =
Tsink Tsource
η =
1−
b) Means of increasing Carnot Efficiency I NCREASE NCREASE CARNOT CYCL E EFFICI EFFICI ENCY ENCY ?
T H E R U T A R E P T L M E T S1
S2
INCREASE TEMP FOR HEAT ADDITION. INCR. WORK w/o INCREASING Qrej. • APPLICATION: HIGHER Tave SUPER HEAT MSRs FW HEATING LIMITS ON MATERIALS • DECREASE TEMP FOR HEAT REJECTION. LIMITED BY SINK TEMP
ENTROPY 22
1) Increase the temperature at which heat is added (a)
Increases the work out without increasing Q rejected.
(b)
This concept has application in the real world
• •
Raising Temperature Using superheat
8
• (c)
Feedwater heating
Constant temperature heat addition is not possible with real working fluids resulting in a sacrifice of work. Example is preheat to sat. temp. of feedwater.
2) Reduce the temperature temperature at which which the heat heat is rejected
2.3
(a)
Lowers Qrejected for a given Qadded with a corresponding improvement in workout.
(b)
This principle is limited by the available heat sink such as lake temperature.
The Rankine Cycle A. Elements of the Cycle 1. Represents the simplest steam cycle. Serves as the starting point for further refinements. 2. Working fluid - Water in the vapor and liquid phases.
E R U T A R E P M E T
a1
b
a c
d
ENTROPY
RANKINE C YC LE
24
3. Heat addition in the Steam Generator. a) Subcooled liquid from the pump discharge discharge enters the boiler boiler at boiler pressure. (Point a) b) A portion of the heat heat added to the feedwater goes to raising the temperature to saturation (point a 1)
9
c) The majority majority of of the heat added added goes goes to vaporizing vaporizing the liquid (constant temperature) from Point a 1 to b. d) The heat added added per per lbm. in the boiler is: q = hsteam − hfeed e) The average average temperature temperature at which the heat is added during the preheat portion is lower than during vaporization which results in a lower efficiency than the carnot cycle. 4. Expansion in the Turbine a) Ideal turbine b to c 1) Constant entropy expansion to condenser pressure 2) Enthalpy is converted to work work per lbm = hsteam − hfeed 3) Wet vapor vapor is exhausted to the condenser. b) Real turbine b to c. 1) Losses due to throttling, moisture, friction etc. result result in less of the energy being converted to work 2) Entropy increases from inlet to outlet 3) Exhaust enthalpy for the real turbine turbine at the same pressure is higher than the ideal turbine indicating: (a)
Less work per lbm
(b)
More heat rejected per lbm
(c)
Lower cycle efficiency effici ency
5. Heat Rejection in the Condenser a) Wet vapor at condenser condenser pressure enters the condenser. condenser. b) The heat is rejected rejected to the cooling water, condensing condensing the steam, c to d. c) The heat rejected per lbm. is: q = hexh − hcondensate 6. Pump-d to a a) Constant entropy compression compressio n b) Raises the pressure pressure from from condenser condenser pressure pressure to boiler pressure. 7. Superheat/reheat Superheat/reheat brings the energy of the fluid back to a high level for use in the turbines.
10
Superheat E R U T A R E P M E T
ENTROPY
REHEATC YC YCL LE 31
2.4
Cycle Efficiency A. Feedwater Heating 1. Principle of Feedwater heating a) A portion portion of the steam flow is is extracted extracted from the turbine turbine after expanding through several stages. b) The extracted Steam Condenses in the the feedwater feedwater heater heater c) The heat heat of condensation is transferred transferred into into the feedwater returning to the Steam Generator. d) The drains drains from the feed heaters are returned returned to the feedwater feedwater system by the drain pumps. 2. Effect of Feedwater heating is improvement improvement of the cycle efficiency a) Raises the average average temperature of heat addition in the steam generator. 1) Feed heating raises the temperature temperature from a to a 1 2) Heat from the reactor reactor is added from a1 to b. 3) A larger larger portion portion of the heat is added at a higher temperature, improving efficiency.
11
b) Less heat rejected 1) Some of the energy energy in the steam is recirculated into the feedwater rather than being rejected in the condenser. 2) This results results in less total steam flow through the turbine turbine but the the effect improves efficiency. 2.5
Operational Effects On Efficiency A. Condenser Pressure
OPERATIONAL EFFECTS: CONDENSER PRESSURE ! LOWER ABSOLUTE PRESSURE ! LOWER AVERAGE TEMPERATURE FOR
HEAT REJECTION ! LARGER DELTA h PER lbm STEAM E R U T A R E P M E T
ENTROPY
35
1.
Lower condenser pressure (higher vacuum) in general yields improved efficiency. a) Lower pressure means lower lower temperature at which which the heat is rejected. (The condenser is at saturation conditions) b) A larger larger portion portion of the steam enthalpy is converted converted to work. c) Less heat rejected.
2. Condenser Pressure is affected by: a) Condenser Cooling Water flow rate - lower lower flow rate yields yields higher higher condenser temperature and pressure b) Condenser Cooling Water inlet temperature - higher higher temperature yields higher condenser temperature and pressure.
12
c) Condenser heat load load - higher steam flow rate into the condenser condenser for a given Condenser Cooling Water inlet temperature and flow f low rate causes higher Condenser Cooling Water average temperature and condenser to Condenser Cooling Water ∆T. Condenser temperature and pressure will be higher. B. Condensate subcooling (condensate depression)
OPERATIONAL EFFECTS: CONDENSATE SUBCOOLING "EXTRA HEAT ADDITION REQ’d "EFFICIENCY REDUCED "REQ’d FOR PUMP NPSH ! CNS DESIGN MINIMIZES E R U T A R E P M E T
ENTROPY
36
1. Condensate depression occurs when the heat rejected exceeds heat which is required to condense the exhaust steam from the turbine resulting in subcooling of the condensate. 2. The extra extra heat rejected must be replaced by the the heat source. 3. Efficiency is reduced. 4. Some subcooling subcooling may be desirable in order order to provide NPSH for the Hotwell pumps. C. Throttling 1. Throttling the steam flow prior to admission to the turbine reduces efficiency. a) Throttling is constant Enthalpy. b) Turbine inlet pressure is lower c) Less of the enthalpy enthalpy is converted to work work in the turbine turbine in exhausting to the same condenser pressure 13
d) The throttling throttli ng loss eventually shows up as increased heat rejected. 2. Operational Considerations Considerations a) Sequential operation of governor valves b) Operation with with governor valves fully open at full power if possible. D. Superheat and Reheat e
E R U T A R E P M E T
a1
b
a c
d
f
ENTROPY
RANKINE C YC LE
25
1. Superheat and reheat reheat adds adds temperature temperature and energy to the cycle. 2. It also has the effect of increasing the qu quality ality of the fluid. E. Operating Power Level 1. In general, general, the plant is more efficient at full load load then at lower lower power power levels. a) Turbine is designed designed to be most efficient efficient at full load. load. b) Less throttling losses. 2. Increasing power level thus total total steam flow can can cause reduced efficiency if corrective action is not taken. F. Other methods of maintaining maximum efficiency. 1. Minimize auxiliaries 2. Fix steam leaks 3.
Fix air leaks into condenser
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4. Operation of SJAE condenser.
15
REVIEW OF THERMODYNAMIC THERMODYNAMIC PROPERTIES (5)
1
1. OVERVIEW This lesson covers the various modes of heat transfer and the equations used with each mode. It gives an introduction introduction to nucleate boiling and the factors factors affecting DNB. A discussion of natural circulation and a brief discussion on heat exchangers is also covered.
2
OBJECTIVES
1. Define 'Heat Transfer'. 1.1
State the three ways ways heat heat is transferred in a power plant.
2. Define 'Conduction' heat transfer. 2.1
Explain the variables variables that effect the rate rate of conduction heat transfer.
2.2
List the formulas used for conduction heat transfer.
2.3
Give an example of where conduction heat transfer occurs in in the power power plant.
2.4
Given a set of parameters, parameters, be able to work conduction problems.
3. Define 'Convection' heat transfer. 3.1
Explain the v variables ariables that effect the rate of convection convection heat heat transfer. transfer.
3.2
List the formulas used for convection heat transfer.
3.3
Give an example example of convection heat transfer transfer in the power power plant. plant.
3.4
Given a set of parameters, parameters, be able to work convection problems.
4. Define 'Radiation' heat transfer. 4.1
Explain the variables variables that effect the rate rate of radiation heat transfer. transfer.
4.2
Give an example example of radiation radiation heat heat transfer transfer in power plant.
5. Explain why a counter flow heat exchanger is the most efficient type of heat exchanger. 5.1
Using the appropriate appropriate heat transfer transfer formulas and g given iven information, information, be able to work heat exchanger heat transfer problems.
3
OUTLINE
1. INTRODUCTION 2. PRESENTATION 2.1
Heat
2.2 Conduction 2.3 Convection 2.4
Natural Circulation
2.5
Radiation Heat Transfer
3. HEAT EXCHANGERS 3.1
Heat Transfer in Heat Exchangers Exchangers
3.2
Types of Heat Exchangers
3.3 Applications
4. SUMMARY
4
1. INTRODUCTION This lesson will present the topic of heat transfer including the 3 modes of heat transfer, natural circulation, and heat exchangers. exchangers.
2. PRESENTATION 2.1
Heat- Energy transferred between two substances due to a temperature temperature difference. A. Rate of of Heat Transfer (Q) - Energy/unit Energy/unit time (BTU/hr) B. Heat is transferred by three methods 1. Conduction 2.
Convection
3. Radiation 2.2
Conduction - transfer of heat thru thru a material material due to a Delta Delta T across the the material. It involves no motion of the material itself, but is a result of collisions between the molecules of the material. A. Thermal Conductivity Conductivity (k) - the rate of heat transfer transfer between between opposite faces of a unit cube of material with a Delta T = 1 F. (BTU/hr - F- ft.) "k" varies with the type of material and temperature temperature range. The larger the value value of k, the better a material will conduct heat °
°
B. As the feedwater feedwater enters the lower lower boiler drum drum it is delivered up the inside inside of the boiler wall tubes. In these tubes the water conducts the heat trough the boiler tube walls from the heat input from the boiler furnace. The water, as it turns to steam, goes to the steam drum.. C. Conduction through a material material can be calculated calculated by: by: Q = K A Delta T Delta X Where K = thermal conductivity A = Area Delta T = Temperature difference Delta X = Material thickness D. If there are several materials materials together, together, such as as through a heat exchanger exchanger tube, the rate of heat transfer is a result of all the k's, A's, Delta T's, and Delta X's for the materials.
5
E. To simplify working with conduction conduction problems, problems, the terms k and Delta X have been combined to give the term "overall heat transfer coefficient" (U = BTU/hr- oF - ft2) and results in the formula: Q = UA Delta T (U also includes another variable to be discussed in convection heat transfer.) Used anytime heat is transferred across a material and the 'U' and 'A' terms are known. In the power plant it is used in problems problems relating to the boiler tubes, condenser tubes, Hx tubes, etc. 2.3
Convection - transfer of heat energy by the the combined combined action of conduction, conduction, energy storage, and mixing motion of a fluid between regions of high and low temperatures (i.e. the energy transfer between a surface and a flowing substance). If the fluid is being pumped, we have have forced convection. If flow is due to a change in the fluid density, we have natural convection. A. The rate of heat heat transfer (Q) (Q) due to convection will be dependent on on several properties of the fluid, such as; temperature, velocity, specific heat, viscosity, viscosity, etc. These variables variables have been combined into one factor called called the Nusselt Number (Nu). Also affecting Q are the thermal conductivity conductivity and the length length of fluid being observed. observed. To further simplify this, another another factor has been used to combine all of these variables; Convection Convection Heat Transfer Coefficient. hc = Nu K L This term is also included in the U term of Q = UA Delta T. Looking back at this formula for a moment, we see that we can increase the Q by doing three things: 1. Increase the Delta T (not desirable) 2. Increase the A (set by design) 3.
Increase U (done by increasing hc) Looking further, "U" can be increased by two methods: (1) decrease laminar layer or (2) break up the laminar layer
B. Reheat and superheat superheat sections sections of the boiler use use convection convection heat transfer. The superheat section take the saturated steam from the boiler steam drum and raises its temperature to the desired level. In this process any small droplets of water carried out of the drum are also evaporated. The reheat section takes steam exiting the HP turbine. This steam is piped back to the boiler to the reheat section so that the energy of the steam and the quality of the steam can be increased back to a superheat condition for use in the IP and LP turbines.
6
C. The formulas used for convective convective heat transfer are: are: 1. Q = MC Delta T and, 2. Q = M Delta h D. Uses 1.
Q = M C Delta T a) Used for heat transfer in medium with no p no phase hase changes and no boundary is crossed.
2. Q = M Delta h is a phase change, but no a) Used for heat transfer where there is a boundary is crossed. Example: 1) Feedwater to steam in the boiler 2) Steam to condensate in the condenser 2.4
Natural Circulation A. Mechanism 1. Natural Circulation occurs due to density difference between between fluids or two points in in the same fluid system. system. As a fluid is heated heated up its density decreases. Fluids of higher temperature, lower density have a natural tendency to rise to a higher elevation. Conversely, fluids with lower temperature, higher density have a tendency to fall to a lower elevation. a) Example ---------------Levels ------------- --Levels equal------------equal-------- ----70 F
180 F
Tank 'A' water has a higher density than tank 'B' because of the lower temp. Static pressure felt on either either side of the valve will be due to the difference in density between the tanks since there is no height difference. Flow will occur from tank 'A' to Tank 'B' until levels change sufficiently to cause the Delta P = 0.
7
b) In the above example example we we could place a heater in tank 'B' and and a heat heat exchanger exchanger in tank 'A' to remove heat and we would still only get flow until the levels changed changed to make Delta P = 0. To have continuous continuous flow between the tanks, a complete path from tank 'B' to tank 'A' and a completely filled loop is needed. With the heat source (heater), heat sink (heat exchanger), and return flow path, we can establish a small natural circulation flow.
COOLING FLOW (SINK)
A
B
HEATER (SOURCE)
2. The amount of flow we can can get from the above above system system can be aided further by elevating the heat sink (tank 'A') above the source (tank 'B'). The difference in height will cause a greater Delta P, increasing flow.
A
HEIGHT B
2.5
Radiation Heat Transfer Transfer - the emission emission of heat energy energy in the form of electromagnetic radiation radiation from a body by virtue of its temperature. Unlike conduction and convection, radiation heat transfer is independent of any medium and depends entirely on the absolute temperature of the radiating body. The rate of heat transfer is still dependent on the Delta T between two bodies. When conduction and convection cease due to loss of transfer medium radiation transfer will be the only means of heat removal.
2.6
Types of Heat Exchangers A. Counter Flow heat exchangers - In a counter flow heat exchanger the two fluids flow in opposite directions. directions. Because of this, the average average Delta T is at its maximum all along along the tubes. This gives gives the maximum heat transfer transfer of all the heat exchangers. exchangers.
8
B. Parallel flow - In parallel parallel flow exchangers exchangers the two fluids fluids flow in in the same direction. This results in a large large Delta T at the inlet, with a small delta T at the outlet. The heat transfer rate for a parallel heat heat exchanger is less less than for a counter flow f low heat exchanger of the same size. C. Cross-Flow 1. In a cross flow heat exchanger, exchanger, one fluid fluid flows flows across across the the tubes. tubes. These heat exchangers are of two types. a) Single pass- fluid makes one pass at right angles b) Multi-pass - fluid makes several several passes passes back and forth across the the tubes to set up an approximation of counter flow. 2.
The Main Condenser is a cross flow heat exchanger.
9
3. SUMMARY 3.1
Three methods of Heat Transfer A. Conduction B. Convection C. Radiation
3.2
Cover Objectives
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HEAT RATE IMPROVEMENT REFERENCE MANUAL: INTRODUCTION AND USE
1
1. OVERVIEW 1.1
This lesson will provide provide the student with with an introduction to the ‘Heat Rate Improvement Reference Manual’, the purpose, organization organization and use of the manual.
2. REFERENCES 2.1
Heat Rate Improvement Reference Manual, Duke/Fluor Daniel
2
LESSON OUTLINE
1. Purpose 2. The Heat Heat Rate Rate Improvement Improvement Reference Manual 2.1
Heat Rate Primer
2.2
Heat Rate Logic Trees
3. Fossil Steam Station Components 3.1
Thermal Kits
3.2 Boilers 3.3
Turbine
3.4
Plant Auxiliaries
3.5 Condenser 3.6
Cooling Towers
3.7
Feedwater Heaters
4. Elements of a Thermal Performance Monitoring Program 4.1
Goals
4.2
Initial Steps for Establishing Establishing a Heat Heat Rate Rate Monitoring Monitoring Program
4.3
Performance Tutorial
4.4
Performance Loss Monitoring and Trending of Key Parameters
4.5
Unit Performance Survey
5. Instrumentation and Testing Requirements for Heat Rate Monitoring Monitoring 5.1
Instruments and Performance
5.2
Testing Program
6. Cycle Isolation 7. Heat Rate Improvement Program 8. Appendix A and B 3
TERMINAL OBJECTIVE At the end of this class the students will have a working knowledge knowledge of the Heat Rate Improvement Reference Manual. ENABLING OBJECTIVES
1. At the end of this class the student will be able to: 1.1
State the the purpose purpose of of the Heat Rate Improvement Reference Manual.
1.2
Define HEAT RATE.
1.3
Discuss the following in detail: A. Design Heat Rate B. Best Achievable Heat Rate C. Actual Heat Rate D. Factors Affecting Plant Efficiency E. Controllable Controllable Losses F. Accounted for Losses G. Unaccounted for Losses
2. Read and interpret interpret the logic trees, how to modify them for a particular plant/facility and how to use them to develop decision criteria. 3. Discuss Thermal Kits. 4. Perform the following: 4.1
Loss calculations for various various plant components including turbines, boilers, condensers, cooling towers and feedwater heaters.
4.2
Set up thermal monitoring performance programs including: A. Deciding upon which plant parameters to monitor monitor B. Determining deviations from expected values C. Use of logic trees trees to identify possible possible causes causes of the the deviations deviations D. Plan an appropriate course of action to resolve performance deviations.
5. Discuss short vs long long term performance activities as follows: 5.1
Cycle Isolation Techniques 4
5.2
Improved Operation Practices
5.3
Preventive Preventive Maintenance Programs
5.4
Upgrades of Plant Instrumentation
5.5
Repair/Replacement Repair/Replacement of Components
5.6 Cost-Benefit.
5
1. PURPOSE - To define the performance performance standards standards necessary necessary to successfully manage a heat rate improvement plan. The information contained in the manual can enable management , staff and technical individuals to make their company more competitive and successful in the future production of electricity. 1.1
The value value of this is to measure how well the unit is doing its job in producing electricity. Decisions Decisions should not necessarily be made only to improve thermodynamic efficiency but rather to improve a company’s overall performance.
1.2
A thermal thermal performance performance program is actually the development development of performance performance parameters which characterize a unit’s operation.
2. The Heat Rate Improvement Improvement Reference Reference Manual - (NOTE: all references references to figure numbers indicate figures located in the manual) 2.1
Heat Rate Primer A. This chapter provides the user with definitions of heat rate 1. Heat rate rate - the amount of heat input into a system divided by the amount of power generated by a system. 2. As-designed heat rate rate - a tool that provides a definable benchmark for comparison and trending purposes. It is simply a curve generated from turbine heat balance curves, unit expected auxiliary consumption consumption and design boiler efficiency. 3. Best achievable heat rate rate - the same as the net heat rate obtained from unit acceptance test when the equipment was new and the unit was operated at optimum. This heat rate value is realistic and attainable for it has been achieved before. 4. Operating heat rate rate - calculated from the heat energy consumed by a unit or station for a specified time period regardless of the operating status of the unit or station. 5. Incremental heat rate rate - units within a utility system and within a power pool are dispatched (loaded upon the grid) based on their incremental heat rate and resulting cost curve. It is also used in production simulation for maintenance planning and projecting fuel procurement needs and for pricing of power for sale or resale. B. A summary summary of heat rate rate measurement measurement methods is also provided 1. Actual heat rate 2. Input/Output method 3. Output/Loss Method
6
C. Efficiency Factor Factor - a quick quick reference of unit performance performance in relation relation to what what it was designed to be. 2.2
Heat Rate Logic Trees Trees - a systematic approach to aid station engineering in identifying the root cause(s) of declining unit performance A. Heat Rate Losses Tree Tree - used to identify areas in the plant where heat rate degradation may be occurring without conducting expensive tests. 1. Structured to provide provide a process process by which decisions can be determined that narrow down the cause of the problem based on the available information. 2. A statement of the problem starts the tree. B. Major Cycle Component Tree Tree - identifies major areas in the plant cycle that have the potential for contributing to the overall problem. 1. Components such as boiler and turbine 2. Systems such as condensate/feedwater, condensate/feedwater, cooling water,auxiliary water,auxiliary systems and fuel handling. C. Logic Model Symbols. 1. “OR” gate - output occurs occurs if one one or more of the the input events occur.
‘OR’ Gate
2. “AND” gate - output occurs occurs ifif all of the input events events occur. occur.
7
‘AND’ Gate
3. “TRANSFER “TRANSFER IN” - indicates indicates that the logic model is developed further at the occurrence of the corresponding transfer out. Transfers can be used to simplify logic model construction by eliminating the need to develop duplicate branches.
“TRANSFER IN”
8
4. “TRANSFER “TRANSFER OUT” OUT” - denotes a transfer of a portion of the logic logic model to corresponding corresponding transfer in. Transfer out symbol can correspond with multiple transfer in symbols.
“TRANSFEROUT”
D. Logic Tree Application The engineer must first obtain data from various sources at the station including routine monitoring of selected plant performance parameters, special tests, outage reports, initial design documents and interviews with plant personnel. personnel. He must then convert the data into decision criteria and associate these with the appropriate areas of the plant. (use figures 2-4 through 2-22 of the Heat Rate Improvement Reference Manual to explain how the logic trees are used).
3. Fossil Steam Station Components 3.1
Thermal Kits - a collection of manufacturer turbine generator data in the form of secondary cycle diagrams, curves, equations and constants. This data is supplied to the purchaser in order to best describe the expected performance characteristics of the turbine generator. (A sample thermal kit is supplied, but specific plant data and thermal kits should be used if possible) A. Purpose - the thermal kits are used primarily for the following functions: 1. Standards for monitoring 2. Data for cycle model verifications and studies 3. Unit net capability calculations
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4. Turbine testing calculations 5. Corrected unit cycle heat rate or output computation B. Details - the following covers the definition, definition, explanation explanation and applications of the various items in thermal kits. 1. Heat Balance Balance - a diagram diagram of the unit’s secondary secondary cycle describing describing the expected conditions at a specific unit power level or at a valve point. The cycle conditions described in a heat balance include: a) enthalpies b) absolute pressures c) fluid temperatures d) main steam quality e) simplified normal cycle flow paths and flow rates f)
feedwater and condensate booster pumps enthalpy rises
g) gross generatoin h) turbine heat rate i)
cycle expansion end points
j)
fixed fixed mechani mechanical cal and and electr electrical ical turbine turbine gener generator ator losses losses
k) electrical generator conditions l)
calculation assumptions, units, steam tables used.
2. Turbine Heat Rate Rate Cu Curve rve - the heat rates and loads from the heat heat balances are used to form the turbine heat rate curve. This expected heat rate curve has been drawn through the locus of valve points. 3. Expansion Lines/Mollier Diagrams - the the steam steam conditions conditions during expansion through through the turbine are illustrated by a plot on a Mollier diagram. The thermal kit expansion lines lines may be used as a basis for comparison for test results and cycle model verification. If a test data expansion line shifts to the right, there has been a decrease in eficiency of that specific section. 4.
Extraction Pressure vs. Flow - the extraction zone pressure is determined by the turbine flow to the downstream stages. Either a graph or equation is supplied that defines the expected Extraction Extraction pressure as a function of turbine downstream flow for each extraction and for the first stage shell pressure.
5. Enthalpies and End Points - expected stage, extraction and expansion expansion line end point enthalpies are plotted against flow by some venders. These reflect the design at the turbine and its accessories.
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6. Leakoffs - this data may be used for cycle models models when when no test data is available. These may also be used as the baseline for comparison to monitoring/trend monitoring/trend results. a) Control, Stop or throttle valve stems and packings b) Turbine and gland steam seals 7.
Exhaust Loss Curve Curve - unrecoverable losses occur in the exhaust hood of each LP turbine. The total exhaust loss is plotted as a function of exhaust volumetric flow. The components are: a) Leaving loss - the wasted kinetic energy of the steam leaving the last stage. b) Hood loss - results from the pressure drop of of the steam passing passing through the exhaust hood. c) Turnup loss loss - occurs due to flow instabilities instabilities and recirculation recirculation found at very low exhaust flows. d) Shock wave pressure drop loss - due to the shock eaves formed at the turbine exhaust when the pressure drop across the last stage is greater than that required for sonic flow.
8. Choked (Limited) Condenser Pressure - each turbine generator unit has a specific LP turbine exhaust pressure below which unit performance starts to deteriorate, assuming steady flow and heat cycle conditions. Choke limited pressure is nearly a linear function of turbine exhaust end steam flow. Operation Operation at a condenser pressure pressure lower that the choke limited pressure results in loss of net output and a higher heat rate. 9. Turbine Section Efficiency and Effectiveness Effectiveness - vender vender supplied supplied curves curves as a function of flow. These reflect expected performance. Turbine test results are compared to these curves to determine which sections or components have performance levels that deviate from the norm. 10. Electrical Generator Generator Losses - the total generator loss is a function of power factor, load, hydrogen purity and hydrogen hydrogen pressure. This loss information is used for calculating the turbine used energy endpoint in order to calculate gross generation from cycle parameters. 3.2
Boilers Boilers A. Dry Gas Loss - high excess air and lower heat absorption in the boiler system can cause exit gas temperatures higher than expected resulting in o Dry Gas Loss. A 40 F rise in exit gas temperatures can raise the Heat Rate by one percent. High exit gas temperatures can be caused by: 1. Plugging or fouling of preheaters, preheaters, hot or cold side where where moisture moisture has formed due to reaching the dewpoint.
11
a) Ensure proper soot blowing for preheater. b) Periodic high pressure washing may be necessary if the pressure drop across the preheater starts to limit fan capablity. 2. Corroded or eroded eroded Preheater. Preheater. During the process process of combustion, sulfer in the fuel is converted to SO 2 and, dpending on the excess air available, part of the SO 2 is converted to SO 3. The SO3 reacts with any water vapor to form sulfuric acid (H 2SO4). a) Ensure proper operation of steam coils or preheater preheater bypass bypass damper damper to keep the preheater above the cold end minimum metal temperature (Figure 3-2). b) If steam coils are used, used, perform perform periodic periodic inspections inspections for leaks which would increase water vapor to the preheater. c) Open or or close plant windows windows and doors doors to circulate outside air through the plant and around the boiler to the Forced Draft fan suction to minimize steam coil usage. 3. Inadequate boiler soot blowing blowing can cause cause slag to build on heat absorbing surfaces and proper heat transfer cannot occur. 4. Air Preheater Preheater average air in-gas in-gas out out temperature temperature too high above above dewpoint. 5. Incorrect number of pulverizer mills in service service at a given given load - causes an increase in tempering air. This decreases the percentage of total air flow which goes through the preheater, raising exit gas tempertures. 6. Excess pulverizer pulverizer mill tempering air - causes low mill temperature. temperature. 7. Fuel/air control system - maintain O 2 as low as possible without adversely affecting combustion 8. Improper O2 monitoring system a) Calibrate or repair monitors b) Ensure location and q quantity uantity of monitors gives a representative representative indication. 9. Air inleakage in boiler, preheater or ducts. a) Run O2 rise test on boiler to locate air inleakage and make repairs. b) O2 readings should be taken at several locations simultaneously simultaneously to isolate cause of air inleakage. B. Dry Gas Loss Calculations (use Heat Heat Rate Improvement Reference Manual’ Manual’ section 3 along with actual plant data to perform calculations)
12
C. Unburned Carbon Carbon Loss - A flyash sample should be collected and analyzed analyzed for unburned carbon. The percent unburned carbon is also called Loss On Ignition (LOI). This can usually be traced to the following. 1. Improper excess air in the furnace 2. Poor mixing of the fuel and and air in the furnace 3. Pulverized Pulverized coal fineness is incorrect 4. High surface moisture moisture in the coal can lead lead to agglomeration agglomeration and have the same effect as coarse coal during the combustion process. D. Carbon Loss Calculations - (use Heat Rate Improvement Reference Manual’ section 3 along with actual plant data to perform calculations) E. Moisture Loss Loss - Boiler efficiency calculations calculations use use the Higher Higher Heating Heating Value (HHV) as the amount of heat generated in complete combustion of the fuel. Since these losses are related to the percent moisture and hydrogen in the fuel, improvements can only be made m ade by minimizing the exit gas temperatures discussed earlier. F. Moisture Loss Calculations - (use Heat Rate Improvement Reference Manual’ section 3 along with actual plant data to perform calculations) G. Radiation and Unaccounted for Losses (RUMA) 1. Radiation losses account for heat losses to the air through conduction, radiation and convection. The heat emanates from the boiler, ductwork and pulverizers. pulverizers. If the unit is equipped with hot side precipitators, they can be a source of significant gas temperature drop. 2. Unaccounted for losses losses include include difficult to measure measure losses that are are included in a heat balance to arrive at a guaranteed efficiency. These include heat lost in the ash leaving the furnace through the bottom ash hoppers and economizer hoppers and any apparent losses due to instrumentation errors. H. RUMA Loss Calculations - (use Heat Rate Improvement Reference Manual’ section 3 along with actual plant data to perform calculations)
13
3.3
Turbine Steam is admitted through control valves to the turbine where the thermal energy is converted to kinetic energy and then to mechanical energy by expansion through the turbine sections. For maximum efficiency the turbine stages contain a combination of impulse bucket and reaction blade designs. The method used to control the steam flow to the turbine at various loads affects the plant performance. Partial arc admission can be used where the control valves are throttled successively which adjusts the active nozzle area and the throttle pressure remains constant through the load range. In Full arc admission the control valves remain fully open and the load is changed by varying the boiler pressure or the boiler pressure can remain constant and all the control valves are operated together until the desired load is reached. Usually the best operation is a combination of fixed and variable pressure operation where the control valves are throttled to a valve point and reduced pressure operation is used in a particular load range. There are a number of factors dealing with the turbine which can affect the unit heat rate. A. Main Steam Steam Temperature - A throttle temperature temperature change change can affect affect the turbine load and heat rate. Curves supplied with the unit thermal kit are used to estimate the effects of temperature deviations deviations on the unit heat rate. B. Main Steam Steam Pressure - as with main steam temperature temperature a change in in pressure can affect the unit load in several ways. The curve for calculating heat rate improvements due to increased throttle pressure should be included in the unit thermal kit. 1. A 5% increase in initial pressure will result result in a 5% increase increase in in steam flow which in turn will cause a 5% unit load increase. 2. The increase increase in flow will will cause cause an increase increase in steam velocity velocity leaving leaving the the last stage, increasing the total exhaust loss. An increase in exhaust loss results in poorer low pressure turbine efficiency. 3.
The throttle available energy increases as the pressure increases.
C. Design Features Features - Design considerations considerations affecting turbine turbine efficiency include component design to minimize pressure drops, stationary and rotating blade design to obtain optimum steam velocity, and section design to minimize friction and leakage losses. D. Maintenance Items Items - during maintenance outages outages all packing seals seals should be inspected for wear and turbine blades and nozzles should be inspected for corrosion or erosion. 1. Solid particle erosion (SPE) of turbine turbine buckets buckets and blades, nozzles and control valves has been a problem of concern for many years. This damage can be minimized by water chemistry, thermal cycling, materil changes and elimination of air inleakage. Damage to turbine seals, nozzles or blades can normally be detected from performing enthalpy drop tests on the turbine quarterly. quarterly. 14
2. Enthalpy drop test test - (use ‘Heat ‘Heat Rate Rate Improvement Improvement R Reference eference Manual’ Manual’ section 3 for discussion of this test, instrumentation required, Unit operating setup, calculation and corrections) 3.4
Plant Auxiliaries Auxiliaries - Overall unit heat rate is calculated by dividing total Btu input by total net generation. Since gross generation is not used, the electrical auxiliaries used to operate the plant can affect aff ect the heat rate significantly. A. Each unit should should have a curve of expected auxiliaries for a given given load and it should be updated whenever equipment is added or removed. B. An effect on on the heat heat rate due to high plant plant electrical auxiliaries can be caused by the following: 1. Operating plant equipment equipment when when it is not needed for the plant status. status. 2. Operate equipment such as service water pumps and air compressors compressors only as needed. 3.
Maintain equipment whose power usage increases with deteriorating deteriorat ing performance such such as pulverizers pulverizers and pumps.
4. Maintain boiler ducts and and expansion expansion joints to prevent air inleakage inleakage to conserve FD and ID fan power. 5. Investigate possible installation of variable variable speed speed drives drives for fans instead instead of using dampers for air flow control. 6. Outdoor lighting should be controlled controlled by automatic sensors. 7. Maintain heating and air air conditioning conditioning controls for proper proper operation. operation. 8. Turn off personal computers when not in use, especially especially overnight. overnight. 3.5
Condenser A. The condenser condenser receives receives exhaust exhaust steam from the low pressure pressure turbine turbine and condenses it to liquid for reuse. B. The water water cooled cooled surface condenser is the most common type of condenser used in modern power plants. Efficiency increases as condenser absolute pressure decreases. With condenser pressure as low as possible the amount of heat rejected is lower and the amount of work of the turbine increases. This is accomplished by optimizing the heat transfer rate between the condensing steam and the cooling water, effectively allowing no leakage of air into the condensing space and minimizing any cycle leakage to the condenser which would add heat load. C. Things that affect the the heat heat transfer are:
15
1. Tube fouling fouling or restriction - detected detected by monitoring absolute back pressure and the terminal temperature terminal difference between the turbine exhaust and cooling water outlet temperature. Possible corrective or preventative measures: a) Backwashing arrangements arrangement s may be provided b) Sponge balls or or brushes brushes may be automatically automatically circulated through the condenser. c) Periodic condenser tube cleaning d) Chemical cleaning e) Install condenser pressure differential transmitters to monitor monitor tube tube restrictions 2.
Cooling water flow rate inadequate a) Place additional additiona l pumps in service b) Monitor flow rate c) Determine if cooling water blowdown blowdown is installed on CC CCW W inlet. It should be installed on the outlet. d) Test circulating circulating cooling cooling water water pumps for proper flow and rebuild as necessary e) Maintain clean racks or screens and waterboxes.
3. Cooling water temperature too high a) Place additional additiona l cooling tower cells in service b) Perform cooling tower maintenance 4.
Condenser backpressure backpressu re too high with proper cooling water a) Perform helium leak test for condenser inleakage b) Inspect steam jet air ejectors for proper operation c) Check incoming drain lines, feedwater heater high level dumps, minimum flow valves and steam traps for f or leakage or improper operation d) Isolate pressure sensing lines at at condenser condenser to check for instrument instrument line leaks.
3.6
Cooling Towers Condenser cooling water systems are either once through or closed systems. A once through system is where water is pumped from a river or lake through the condenser and the warmer water is returned to the source. A closed loop system rejects the heat to the atmosphere through the use of either a cooling tower or a body of water such as a cooling lake. Most newer plants use cooling towers because of the environmental environmental restraints. 16
A. Cooling tower performance is affected by the ambient wet bulb temperature, deterioration of the fill material, m aterial, fill silt buildup, icing on tower structure, low water loading and high water loading. B. Wet Bulb temperature temperature - Using Using the range and the tower outlet outlet temperature, temperature, the corresponding calculated wet bulb temperature can be found from the vender’s design curves using either the design or measured value for circulating water flow. 1. If the actual actual wet bulb temperature temperature is higher than the calculated the tower is performing at or better that expected. 2. If the actual wet wet bulb temperature is consistently consistently lower that the calculated then further testing or inspections are necessary to determine caused of the deficiency C. Deterioration of fill material - routine inspections inspections are are necessary necessary to survey survey damage or deteriorated fill material or to remove any debris D. Fill silt or algae buildup buildup - Purpose Purpose of the fill material is to increase the contact area between the air and water and to increase the water residence time. To maintain its maximum effectiveness buildup must be prevented by blowdown blowdown and proper water treatment. E. Low water loading can cause poor water water distribution and high water water loading can cause excessive air pressure losses. 1. Inspect distribution nozzles 2.
Clogged distribution distribut ion nozzles
3. Fan blade deterioration 4. Motor problems 3.7
Feedwater Heaters Heaters - Provide three purposes in the power plant. A. Provide efficiency efficiency gains gains in the steam cycle by increasing increasing the initial water water temperature to the boiler, reducing the amount of heat input required by the boiler’ B. Provide efficiency by by reducing reducing the heat rejected rejected in the condenser. condenser. C. Minimize thermal effects in the boiler. D. Items that can affect performance: 1. Improper heater level can cause cause flashing flashing in the drain drain cooler section and tube damage a) Check operation of automatic automatic controls controls and level instrumentation. b) Check for possible possible tube leaks in feedwater feedwater heater. c) Vent valves may not be set up properly. 17
2. Improper extraction line pressure pressure drops. Possible problem with extraction line check valve. 3. Tube fouling fouling due due to corrosion affects the heat transfer transfer in the heate heaterr and also increases the problem of deposition of oxides on heat transfer surfaces. a) Reduce the level of dissolved gases such as oxygen and carbon dioxide in feedwater and adjust pH of the feedwater. b) Clean tube bundles 4. Continuous vent orifice plugging 5. Channel pass partition/gasket partition/gasket leak E. Feedwater Heater Calculations Calculations (use Heat Rate Improvement Reference Manual’ section 3 along with actual plant data to perform calculations)
4. Elements of a Thermal Performance Monitoring Program Program (the students should refer to section 4 of the manu manual al throughout this section of study) 4.1
Goals A. The goal goal of a performance monitoring monitoring program program is to improve improve unit efficiency. These are needed because of increased fuel cost, increasing age of the units and their equipment, increased cost of capitol improvements and increased competition in the utility industry. B. Improvements should initially concentrate on activities activities that can be accomplished with little capitol investment in a relatively short time. 1.
Cycle isolation
2. evaluating selected parameters to improve operations control 3. Identify preventive preventive maintenance maintenance that can easily easily be conducted conducted on select equipment 4.2
Initial steps for establishing establishing a heat heat rate monitoring program: A. Evaluate cycle isolation for leaks or improperly positioned valves (Use Section 6 of the manual for details on cycle isolation) B. Determine which performance parameters parameters are are being monitored at the unit unit with existing instrumentation instrumentation (use Table 4-1 for f or a list of parameters that can be monitored at a typical fossil station). C. Obtain readings readings of the selected selected parameters from Table 4-1 and compare the readings with expected values. If no historical readings are available use Table 2-2 as a starting point. D. Determine the the magnitude of the parameters parameters deviation deviation from expected. An aid in this is to review Table 2-1 for utility experienced deviations. E. Determine the heat rate deviation deviation as shown shown in Table 4-4.
18
F. List the parameters deviating from expected expected in descending order of effect on heat rate. G. Select a parameter parameter to be investigated investigated from the list. Discuss Discuss the parameters parameters and their deviation with unit operators and other plant personnel to ensure its validity. H. Review logic trees or parameter diagnostic tables to identify identify components components effected and potential caused for the deviation. Modify the logic trees from one of the demonstration reports to suit the unit being evaluated. I.
Refer to performance parameter accounting manual such as found in Appendix B of the manual, or the Performance Tutorial in section 4. Prepare a list of possible corrections to the deviation.
J.
Review with operations and maintenance to determine determine the appropriate appropriate action for correction.
4.3
Performance Tutorial - This section of the manual is designed designed to be of assistance in identifying losses while operating the plant . Use this section as a guide to addressing various plant problems that have an effect on plant efficiency.
4.4
Performance Loss Monitoring Monitoring and Trending Trending of Key Parameters - This This section of the manual describes how the units performance can be surveyed for losses and trended to follow the effects of operation . A. Loss Monitoring Monitoring - required for determining determining how well a unit is being maintained and operated. A successful program consists of: 1. Gathering accurate operating data from adequate sources to provide provide a complete status of unit operating parameters parameters.. 2. The data data must then be incorporated into the proper calculations for determining actual performance losses for cost/benefit analysis. 3. If a source of degradation degradation is identified identified the plant staff staff can then pursue pursue determining the root cause for the degradation. 4. From the root root causes causes plant staff can optimize their preventive preventive maintenance program, their instrumentation requirements, operating practices and their performance parameter monitoring needs. B. Classification - It is is helpful to classify classify performance performance losses so responsibilities responsibilities for loss reduction can be more effectively delegated 1. Controllable Controllable losses - are performance losses that can be minimized minimized by by the plant operating personnel, such as: a) throttle pressure b) throttle temperature
19
c) hot reheat temperature d) condenser pressure e) make-up flow f)
feedwater heater terminal temperature difference
g) feedwater heater drain cooler approach temperature h) main steam desuperheater spray flow i)
reheat desuperheater spray flow
j)
auxiliary auxiliary electrica electricall loads loads
k) dry gas loss l)
carbon loss
m) coal weighing error 2. Accounted-for losses - those remaining performance losses for which which an effect on heat rate can be determined. These are usually correctable by maintenance a) reheater pressure drop b) extraction line pressure drop c) hydrogen loss d) moisture in fuel loss e) RUMA loss f)
turbine efficiency
g) miscellaneous h) light-off fuel 3. Unaccounted-for Unaccounted-for losses losses - performance performance losses for which which an effect effect on heat rate cannot easily be established. These may or may not be known to exist. a) heat loss to the condenser b) soot blowing c) steam coils usage d) plant auxiliary steam heating e) condensate/feedwater condensate/feedwater recirculation f)
improper valve alignment
g) excessive excessive turbine shaft seal leakages h) LP turbine efficiency i)
others
20
C. Quantification - to achieve total benefit from monitoring performance losses, losses, the effect on unit efficiency must be correctly quantified (use the example example in section 4) 4.5
Unit Performance Performance Survey Survey - not since the initial acceptance acceptance tests tests on many many fossil units has an overall level of performance been established, Since that time unit operating modes have changed, modifications have been made, coal quality has changed and equipment has aged. With this in mind it is the primary goal of the Unit Performance Survey program to determine the present level of overall performance and to improve it by identifying and minimizing losses. The main objectives are: (Use section 4 to identify particular items in each of the following to aide in presenting this section) A. To stress the importance importance of performance performance and to provide provide a means means for ‘on-the‘on-the job’ performanc performance e trainin training g for station station and and results results personne personnel. l. B. To establish establish a present level of performance performance for each fossil unit. C. To identify cycle and and equipment equipment problems and obtain information for use in problem resolution. D. To improve the present level of performance performance of each fossil unit by identifying and minimizing losses.
5. Instrumentation and Testing Requirements for Heat Rate Monitoring Monitoring 5.1
Instruments and Performance Performance - A problem problem w with ith the objective of efficient unit operation is one which provides the operators with the tools they require to maintain all important parameters at an optimal value. The operators should also be motivated to utilize this information to strive strive for efficient unit operation. The Thermal Performance program is jeopardized severely when instrument error exists. This is an unaccounted-for loss undetectable until a calibration is performed. A. ASME Performance test codes - if a plant component performance is suspect, based on on-line testing with normal instrumentation an ASME Code test can be performed to verify, identify and quantify the problems. B. Instrumentation - Once it has been decided decided which which performance parameters should be monitored, and the effect of each of these parameters on unit heat rate has been determined, the type if instrumentation to be used and the frequency of monitoring must be decided
5.2
Testing Program Program - (use section section 5 of of the manual for a list list of periodic periodic and special testing that may be performed)
6. Cycle Isolation
21
6.1
Each steam plant has a normal path for the flow of liquid or steam. These can be different at various loads. These paths are usually well represented on the heat balance diagrams provided by the vendors. What is not shown are the numerous paths which are available for liquid or steam to escape from the normal flow paths.
6.2
When flow is diverted diverted from the normal steam steam path is either either lost from the cycle completely or returned to a section of the cycle where energy is removed from the fluid without providing any useful work.
6.3
The only way to appropriately appropriat ely deal with with cycle isolation problems is to perform periodic cycle walkdowns. This will allow a utility to identify the particular losses that are occurring in a unit and schedule maintenance activities to correct these problems as required.
6.4
Cycle isolation method A. Prepare a detailed detailed cycle configuration checklist. This should should contain all the lines not used during normal operation and they should be isolated. This should also contain all lines which have steam traps to ensure steam trap operability. B. The walkdown walkdown consists consists of determining determining if there there is any leakage through through the isolation valve by checking either the downstream pipe wall temperature or by listening for flow C. Lines which which terminate at at the base slab drain or are vented vented to the atmosphere can be checked visually. D. Attention to the deaerator deaerator and feedwater vents can can be important E. A list of valves valves will will be produced produced from the walkdown. walkdown. This list may need to be reduced. The reduced list should be used on a frequency bases to check cycle condition. A number of these valves may need repair.
7. Heat Rate Improvement Program 7.1
The financial financial success success of electric utilities in in an increasingly competitive environment depends largely on improving improving plant performance to maintain or lower the cost of producing electricity while meeting ever more stringent environmental regulations. regulations. A heat rate improvement program can include cycle modifications, component modifications, improved maintenance practices or increased use of microprocessor based controls and instrumentation.
7.2
Cycle Modifications - examples examples of of cycle modifications include retrofitting a unit for variable pressure operation, installing variable speed drives on plant equipment and modification or addition of equipment for improved heat recovery.
22
A. Variable Pressure Operation - EPRI report GS-6772 presents a discussion of variable pressure operation for efficiency improvements, a portion of this report is highlighted in the manual. B. Variable Speed Speed Drives Drives - an option option increasingly increasingly being being considered considered in both new plant designs and life extension projects. Increasing fuel costs and improvements in technology and reliable have caused a decisive shift toward the use of this equipment. 1. There are three three basic ways a motor’s speed can can be changed: a) change the number number of poles poles on th the e motor motor b) change the slip of the motor c) change the frequency frequency of the energy supply to the motor 2. Two speed speed motors - a good good example example of changing changing the poses the make a speed change. These machines do not have the flexibility to meet more than two plant conditions. 3. Wound rotor motor drives drives are are a form of variable speed drive drive that has been found in generating stations throughout the years. 4. The most efficient and most commonly commonly used type of of variable variable speed speed equipment is the adjustable frequency synchronous synchronous motor. The benefits are reduced electrical auxiliary usage at reduced loads. C. Heat Recovery Recovery Modifications Modifications - a power plant plant Rankine Rankine thermal cycle cycle suffers tremendous energy losses, therefore it is important to recover even small heat losses. 1. Circulating water heat heat recovery recovery - more than half of the fuel burned is lost to the condenser cooling system. These are considered unavoidable, but it is possible to recover recover some of this waste energy energy for a secondary use such as air preheating. 2.
Air preheating evaluation results are shown in table 7-1, Coil design operating results in table 7-2
3. Heat pipe pipe air heater technology technology - are high performance performance heat heat transfer transfer devices that are simple, inexpensive and reliable over a long service life. There are no moving parts and positive seal connections reduce leakage to effectively zero while high heat transfer rates permit a lowweight, low-volume, low-cost package. (use section 7 for a detailed discussion) D. Component Modifications Modifications - there there are many areas in the the plant where where small investments can improve unit heat rate and provide cost savings. 1. Boiler duct expansion joint leaks
23
2. Boiler oxygen measurement is extremely important to proper proper boiler operation. The proper number of analyzers and the proper location is important 3. Windbox damper operation is critical critical for proper fuel and air mixing. mixing. 4. Accurate feedwater heater level controls should be installed installed and maintained. E. Maintenance Practices - if proper proper unit maintenance is not carefully planned and executed then unit thermal performance will suffer greatly. A good performance program should help drive the maintenance plan. 7.3
Cost Benefit Benefit Analysis Analysis -EPRI report TR-101249 describes the efforts of Southern Southern California Edison Company to improve heat rate at Ormond Beach Generating Station Unit 2. A brief of this report is in section 7 of the manual.
8. Appendix A and B 8.1
Appendix A - Procedure for calculating calculating expected unit net heat rate. rate. A. Purpose - to provide a standard for comparison with actual unit net heat heat rate. B. Use section section 8 to guide discussion of net heat heat rate calculations.
8.2
Appendix B - Performance Parameter Accounting Manual.
24
POWER PLANT SYSTEMS
1
1. OVERVIEW: This lesson will provide provide the student with a review review of power plant plant systems which will include the following: 1.1
The Water/Steam cycle
1.2
Boiler fuel, air and flue gas systems
1.3
Balance of plant systems
2. References: 2.1
Electric Generation Steam Stations, Skrotzki, Bernhardt
2
TERMINAL OBJECTIVE At the end of this lesson the student will understand power plant systems which will include the following: The Water/Steam cycle, Boiler fuel, Air and flue gas systems and Balance of plant systems ENABLING OBJECTIVES
1. Describe the Water/Steam cycle. 2. List the boiler fuels most most commonly commonly used. 3. Describe the air air and flue gas gas systems systems 4. Describe other major plant systems used to produce electrical electrical power.
3
LESSON OUTLINE 1. PLANT CYCLES 4.1 1.1 STEAM/WATER CYCLE 4.2 1.2 BASIC CYCLE
5. BOILER FUEL, AIR AND FLUE GAS SYSTEMS 6. BALANCE OF PLANT SYSTEMS
4
1.
PLANT CYCLES (Water and Steam) 1.1
The steam-water steam-water cycle cycle is the heart of the steam-electric steam-electric power power plant. This main cycle uses two primary types of equipment. A. Shell and and tube type heat heat exchangers exchangers (boilers, (boilers, superheaters, superheaters, economizers, economizers, condensers, heaters) 1. Bring two fluids close close to each other, other, one hot and the other cold, separated by a thin metal wall. 2. The Hot fluid transmits transmits its heat heat to the cold cold fluid. 3. Typical problems: a) Boiler tubes must contain water and steam at v very ery high pressures while the outer side of the tube can be at very high temperatures temperatures (3500 degrees) b) Condensers must handle handle tremendous tremendous volumes of exhaust exhaust steam steam and transmit heat between steam and cooling water whose temperatures differ by only 20 degrees. Occasionally air leaks by the sealing mechanisms into the condenser. This air can plate out on the surfaces of the condenser tubes increasing the heat transfer medium in which heat must be transferred. The exhaust pressure will increase and reduce the efficiency of the cycle. B. Rotating shaft equipment (pumps, fans, turbines) 1. Pumps are are used through out the steam-electric plant. Areas Areas where where they they are used are: a) Condensate/Feedwater Condensate/Feedwater pumps b) Chemical addition pumps c) Fuel oil pumps (oil fired plants) d) Boiler feed pumps e) Circulating water pumps 2. Fans are used to provide provide air to the boiler as: a) Forced draft fans b) Induced draft fans c) Pulverizer Pulverizer fans, primary air fans(Coal fired plants) 3. Turbines are used used to provide the rotating rotating motive motive force force for the generator generator and for the moving of fluids (turbine driven feedwater pumps)
5
1.2
The basic water-steam water-steam cycle is the same for all all steam-electric steam-electric power power plants. plants.
THE C YC LE:
BOILERS
PUM P
T UR BI NE
CONDENSER
9
A. Condenser hotwell 1.
The condenser hotwell provides a large quantity of boiler quality water to be circulated through the system to the boiler.
2. This hotwell condensate pumps take pumps take a suction on the hotwell and provide enough discharge discharge pressure to send this water through a number of condensate heaters a) The hotwell condensate condensat e pumps are usually relatively low pressure pumps and operates with a low suction head (hotwell is under a vacuum) b) These pumps pumps provide provide suction suction pressure pressure to a higher higher head pump. B. Condensate heaters 1. Condensate heaters provide preheating of the condensate prior to providing this water to the boiler.
6
2.
By preheating preheatin g the water, less energy has to be added to the water in the boiler to provide the quality of steam required by the turbine/generator thereby making the cycle more efficient.
3. Heating steam for the condensate condensate heaters heaters is provided to the heaters from extraction (bleed) steam from steam from different stages of the turbine. The steam provided to the heaters is compatible (temperature) to provide a gradual increase in the feedwater temperature. 4. The number number of stages of feedwater feedwater heating heating is dependent on the heating requirements requirements to make the cycle as efficient as is possible. Too few or too many heaters can make a marked difference in cycle efficiency. The number of heaters is a function of plant design. 5. Taking heaters out of service service will will effect cycle efficiency. C. Condensate/Feedwater Condensate/Feedwater is delivered to the boiler boiler at the required required pressure pressure by the boiler the boiler feedwater pump. pump . This pump can be a constant speed motor driven centifugal or variable speed turbine driven centrifugal pump. It can also be a positive displacement pump. 1. If a constant speed motor motor driven driven feedwater feedwater pump pump is used, varying varying the amount of feedwater to the boiler must be performed by boiler feedwater control valves. 2. If a variable speed turbine turbine driven driven pump is used, a combination of feedwater pump speed and feedwater control valve position is used to control boiler water level. 3. This water water delivered delivered to the boiler will have have energy energy applied applied such that the water will be turned into steam. WATER/STEAM WATER/ST EAM CYCLE Steam
r e s i R d e t a e H
r e m o c n w o D d e t a e h n U
7
D. Boiler 1. Types of boilers used: a) Natural draft (circulation) (circulation) - air is allowed to enter the furnace area of the boiler to feed the combustion process and the natural flow through the boiler to the stack is allowed to occur. b) Forced draft - air air is supplied to the furnace furnace area with forced forced draft fans and forced though the boiler sections to the stack. This type boiler produces better heat transfer to all areas of the boiler. 2. Heat left in the combustion gases leaving leaving the boiler is the largest single loss in a steam generating unit. Economizers are Economizers are used to recover some of that heat. The economizer is used to preheat feedwater going to the lower boiler drum. The hotter the water entering the boiler, the less energy is required to produce the steam required by the unit. 3. Water that is supplied is is heated as it flows flows up the boiler inside boiler boiler tubes 4. Heat produced produced in the boiler through the ig ignition nition and and burning burning of fuels must be transferred through the boiler tube wall to the water flowing inside. Anything that will reduce the heat transfer of this heat to the water will reduce cycle efficiency. 5. Steam is produced when the water is heated to the vaporization vaporization phase. 6. Superheaters take Superheaters take the saturated steam leaving the steam drum and raises it’s temperature to the desired level. 7. Steam leaves leaves the boiler boiler through through main steam lines lines to the high high pressure pressure turbine 8. Reheaters provide Reheaters provide for greater efficiency of the IP and LP turbines by increasing the energy energy of the steam prior to it reaching the IP and LP turbines. Reheaters also increase the cycle efficiency by using the combustion gases leaving the furnace. E. Turbine 1. The turbine turbine is the prime mover of of the generator generator to produce the required electrical power output. 2. There are numerous numerous arrangements of the high pressure and low low pressure turbines that are used to produce the desired electrical output, a) single casing, single flow b) single casing, double flow c) tandem compound, single or single/double flow d) tandem compound, triple exhaust
8
e) cross compound, double flow f)
cross compound, quadruple exhaust
g) triple cross compound 3. There is is usually a single high pressure pressure turbine, turbine, possibly possibly an intermediate pressure turbine and one to three low pressure turbines. 4. The high high pressure pressure turbine receives the high high pressure pressure steam steam from the boiler and produces the highest amount of torque on the turbine shaft. A large amount of the energy is removed from the steam by this turbine. 5. The steam is then piped to the reheater section of the boiler where additional temperature and energy is added and then it is directed to the IP and LP turbine(s). The blades on the IP and LP turbines are much larger than those on the high pressure turbine. These larger blades allow the turbine to use more of the steams remaining energy. energy.
THE C YC LE:
BOILER
PUM P
T URBI NE
CONDENSER
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F. Condenser
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1. The steam steam from the low pressure turbine is exhausted exhausted to the condenser where it is condensed back to a subcooled liquid to begin the water/steam cycle over again. 2. Circulating cooling water is supplied supplied to the condenser condenser through through condenser tubes to condense the steam. The circulating water can be supplied from a lake, river, ocean, cooling towers or any other large supply of cooling water.
2. BOILER FUEL, AIR and FLUE GAS SYSTEMS 2.1
Boilers use various various fuels to provide provide the necessary heat required required to produce steam. These fuels are classified as solid, liquid and gas. A. Solid fuels are coal (various types of coal coal are used), used), wood, coke and even trash. 1. Coal is ranked ranked by percent moisture, fixed carbon, carbon, oxygen oxygen and sulfur sulfur content. All of these are determining characteristics characteristics which define how much heat (BTUs) and ash content can be expected to be produced when burned. a) The higher higher the BTU content content the less fuel is required required to produce the the steam requirements of the plant. b) The lower lower the ash and sulfur sulfur content content the fewer emissions released from the plant and the less flyash that has to be removed. c) Flyash plates out out on the boiler boiler tubes, tubes, especially especially in in the superheat sections of the boiler. This reduces the heat transfer of the hot gases to the water/steam and, therefore, reduces cycle efficiency. d) Sulfur dioxide dioxide is a byproduct byproduct of the firing process process of coal. coal. The higher the sulfur content the more Sulfer dioxide is produced. This , when combined with water, produces sulfuric acid. 2. Coke is a byproduct byproduct of oil oil refineries refineries that may be burned in pulverizedpulverizedcoal-fired boilers. 3. Wood is used usually in the form of waste from lumbering lumbering or manufacturing processes. Special furnaces are required for this fuel. B. Liquid fuels are oil and coal tars 1. Fuel oils oils such as diesel diesel fuels to number 6 crude oils oils are used. Btu content, water, specific gravity, gravity, sulfur and flash point are the test that oils must undergo prior to use in boilers. C. Gas used is natural gas
2.2
Air and Flue Gas Systems
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A. Inorder for the fuel supplied supplied to the boiler boiler to burn burn and burn burn properly air must be added to the fuel mixture. This is usually provided by forced draft fans/primary air fans. fans . 1. Forced draft fans are controlled either from the control room. 2. Induced-draft fans are fans are used to “suck” the gases out of the boiler and deposit them to the stack at a slightly higher than atmospheric atmospheric pressure. B. Flue gas temperature leaving leaving the the economizer economizer is still quite quite high. Some of this energy can be removed and transferred to the inlet air through the use of air-heaters. air-heaters. Flue gas outlet temperature must be watched closely to ensure inlet air temperature doesn’t drop too low. If temperature gets too low water droplets can form in the air. This mixed with the sulfur products produces sulfuric acid. C. It has already already been stated stated that when when burning burning fuels byproducts byproducts of the the fuel burning process are produced and must be removed from the boiler and the stack emissions. 1. In the burning of coal slag is produced which falls to the bottom of the boiler. Systems are provided to crush this slag into particles that are sized such that they can be sluiced to a holding tank where trucks can remove them from the plant or sluiced to an area outside of the plant. 2. Flyash is also also produced produced when when coal is burned. burned. High High efficiency efficiency precipitators have been installed in the flue gas flow path to catch and funnel flyash to areas where it can be removed from the plant.
3. BALANCE OF PLANT SYSTEMS 3.1
Coal handing handing equipment equipment or fuel oil handing equipment as well as coal or fuel oil oil storage facilities must be provided. There needs to be enough fuel stored to operate the plant for an extended period of time. A. Coal must must be moved moved to conveyor conveyor belts, then crushed and and moved to other conveyor belts to move it to the top of the plant where it is stored again for daily use in the plant. This plant storage or ‘hoppers’ must be refilled daily. The coal is further crushed as necessary to be used in the furnaces in the boilers. B. Fuel oil must be stored stored in large large tanks and must be kept at a temperature temperature such that the oil will flow easily through the piping but not to hot to where is may ignite prematurely.
3.2
The circulating circulating water water system system will have some sort of intake screening system to prevent large objects from entering the condenser waterboxes and possibly blocking condenser tubes. This will cause a reduction in cycle efficiency.
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3.3
Demineralized Demineralized water water must be be either produced in in the plant plant or delivered to the plant by some means. The boilers require clean demineralized water water for use in the boilers. This reduces the amount of deposits and boiler downtime.
3.4
Various oil oil systems must be provided provided to lubricate lubricate turbine and pump bearings to prevent component failures. This will also require oil purification systems.
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