Steam Turbines

Engineering Encyclopedia Saudi Aramco DeskTop Standards

STEAM TURBINES

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : General Engineering File Reference: AGE-102.06

For additional information on this subject, contact PEDD Coordinator on 874-6556

Engineering Encyclopedia

Rotating Equipment Steam Turbines

Section

Page

INTRODUCTION............................................................................................................. 3 COMMON TYPES OF STEAM TURBINES IN USE AT SAUDI ARAMCO ..................... 4 Principle of Operation ................................................................................................ 4 Curtis Stage.......................................................................................................... 5 Other Types of Stages.......................................................................................... 6 Turbine Classification................................................................................................. 7 Classifications of Mechanical Drive Turbines ....................................................... 7 Turbine and Cycle Efficiency...................................................................................... 8 Types of Steam Turbines......................................................................................... 10 Backpressure or Non-Condensing Turbines....................................................... 10 Condensing Steam Turbines .............................................................................. 14 Extraction Turbines............................................................................................. 16 MECHANICAL COMPONENTS AND THEIR FUNCTIONS .......................................... 18 Trip and Throttle Valve............................................................................................. 18 Governor Valve ........................................................................................................ 18 Steam Chest ............................................................................................................ 19 Hand Valve .............................................................................................................. 19 Nozzles .................................................................................................................... 19 Blades...................................................................................................................... 19 CALCULATIONS........................................................................................................... 20 Example Calculation - Theoretical Steam Rate, Actual Steam Rate and Outlet Temperature.................................................................................................. 20 Example Calculation - Efficiency of an Operating Turbine ....................................... 25 Turbines With Saturated Exhaust Steam............................................................ 29 Efficiencies of Steam Turbines for Use in Calculations ...................................... 29 Use of Hand Valves to Maximize Efficiency........................................................ 30 THEORETICAL STEAM RATE TABLES....................................................................... 31 Performance Curves ................................................................................................ 31 COMMON OPERATING PROBLEMS........................................................................... 33 WORK AID 1: CALCULATION OF THEORETICAL AND ACTUAL STEAM RATES AND OUTLET TEMPERATURE....................................................................... 34 WORK AID 2: CALCULATION OF TURBINE EFFICIENCY AND HORSEPOWER FROM STEAM CONDITIONS............................................................ 36 WORK AID 3: CALCULATION OF EFFICIENCY FROM INLET STEAM CONDITION AND BRAKE HORSEPOWER ................................................................. 38 WORK AID 4: ESTIMATING EFFICIENCY FACTORS FOR SINGLE-STAGE NON-CONDENSING TURBINES.................................................................................. 40

Saudi Aramco DeskTop Standards

i



WORK AID 5: COMMON OPERATING PROBLEMS................................................... 41 GLOSSARY .................................................................................................................. 42 REFERENCES.............................................................................................................. 44

LIST OF FIGURES

Figure 1. Steam Turbine - Principle of Operation ------------------------------------------------ 4 Figure 2. Nozzles and Blades ------------------------------------------------------------------------- 5 Figure 2a. Steam Turbine Blade Types------------------------------------------------------------- 6 Figure 3. Turbine Isentropic Efficiency-------------------------------------------------------------- 8 Figure 4. Cycle Efficiency ------------------------------------------------------------------------------ 9 Figure 5. Backpressure (Non-Condensing) Turbine------------------------------------------- 10 Figure 5a. Typical Multistage Backpressure Steam Turbine -------------------------------- 12 Figure 5b. Typical Single Stage Backpressure Steam Turbine ----------------------------- 13 Figure 6. Condensing Turbines --------------------------------------------------------------------- 14 Figure 6a. Typical Condensing Steam Turbine ------------------------------------------------- 15 Figure 7. Extraction/Condensing Turbines------------------------------------------------------- 16 Figure 7a. Typical Extraction/Condensing Steam Turbine ----------------------------------- 17 Figure 8. Steam Path Through Turbine----------------------------------------------------------- 19 Figure 9. Use of Mollier Diagram for Steam Turbine Calculations ------------------------- 22 Figure 10. Efficiency of an Operating Turbine -------------------------------------------------- 27 Figure 11. Typical Turbine Performance Curve ------------------------------------------------ 31 Figure 12. Typical Performance Curve - Extraction Turbine -------------------------------- 32 Figure 13. Typical Efficiencies, Single Stage Turbines, Non-Condensing "Normal Efficiency" Type ------------------------------------------------------------ 40


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INTRODUCTION Steam turbines are used by Saudi Aramco to drive electric generators, gas compressors and certain critical pumps. Steam turbines are prime movers. They are used extensively in gas plant and refinery applications because the processes that are used in these plants are exothermic and generate high-pressure steam (600 psig) as a by-product. Steam turbines convert the pressure and heat energy of steam to mechanical energy (work).


3



COMMON TYPES OF STEAM TURBINES IN USE AT SAUDI ARAMCO Principle of Operation The two major components of a steam turbine are nozzles and blades. The blades are sometimes called buckets. Nozzles are stationary; blades rotate. Steam contains energy in the form of pressure and temperature. Nozzles convert this energy into velocity energy. In a nozzle, the pressure drops and the velocity increases (Figure 1).

Figure 1. Steam Turbine - Principle of Operation

The high-velocity jets from the nozzles strike the blades and cause them to move. In the moving blades, velocity energy is converted to mechanical work, or power. Blades are located in rows on rotating wheels. Nozzles are arranged on stationary wheels, between the rotating wheels.


4



A stage contains one row of nozzles, followed by one row of blades (Figure 2). Turbines may be single-stage or multistage.

Figure 2. Nozzles and Blades Curtis Stage A Curtis stage is a special kind of stage that takes a relatively high pressure drop. It is used for single-stage turbines and as the first stage in most older design multistage turbines. Present day multistage turbine design uses a Rateau stage since material and blade attachment methods allow higher blade operating stresses. However, present day single-stage turbines still utilize a single Curtis stage. Although a Curtis stage is approximately 10% less efficient than a single Rateau stage, it is cost effective to retain the Curtis stage in this application because the base efficiency of a single-stage turbine is in the 35-40% range. A Curtis stage has one row of nozzles, followed by three rows of buckets. The sequence is as follows: 1.

Nozzles

2.

Rotating buckets that develop power

3.

Fixed buckets that turn the direction of the steam

4.

A second row of rotating buckets, that develop more power.

All of the pressure drop takes place in the nozzles. Only velocity changes in the three rows of buckets.


5



Other Types of Stages In a multistage turbine, each stage after the first one has one row of nozzles (stationary) and one row of blades (rotating). These stages may be the "Rateau" type, also known as impulse type, or the "reaction" type. Refer to Figure 2A for the definition of impulse and reaction blading.

Figure 2a. Steam Turbine Blade Types Present day multistage turbines are of a hybrid design in that they utilize impulse blading for the initial stages and higher efficiency reaction blading in the final stages, where the volume flow is the greatest.


6



Turbine Classification Turbines are divided into two classes, power generation and mechanical drive. Power Generationturbines drive electric generators at constant speed because the frequency of the generated power must be constant. Because the turbine runs at constant speed, features can be designed to give a very high efficiency. Tolerances between the moving and stationary parts are very close. Complex staging can be used. Mechanical Drive turbines are used for driving machinery such as compressors and pumps, where variable speed is usually required. Some efficiency is sacrificed in order to increase mechanical strength. Tolerances are larger, and fewer stages are used. Classifications of Mechanical Drive Turbines Mechanical drive turbines may be General Purpose or Special Purpose. General Purpose Turbines are used for low power applications. They are covered by API Standard 611 and 32SAMSS-009 and are mass produced without regard to specific customer requirements. They are limited to steam supply conditions of less than 600 psig and 750ºF. They also operate at speeds less than 6000 rpm. General purpose turbines are usually single-stage turbines that may exhaust to a low pressure steam circuit, a condensing system or to the atmosphere. Since they may be less reliable than other turbines, their applications are limited to non-critical equipment. They are used as drivers for equipment that has a spare, such as pumps and fans. Such equipment is "spared;" that is, it always has a backup. Steam turbine driven pumps in Saudi Aramco gas plants and refineries utilize general purpose turbines.


7



Special Purpose Turbines are used for large power loads. The specifications are covered by API Standard 612 and 32SAMSS-010. They are manufactured to specific customer orders and requirements. These services are usually not spared; therefore, the turbine must be highly reliable. Because these turbines are large machines, efficiency is important, and multistage designs are used. The most common applications are gas compressors and large pumps. Steam turbines that drive centrifugal and axial compressors in Saudi Aramco gas plants and refineries are special purpose turbines.

Turbine and Cycle Efficiency Turbine isentropic efficiency is the efficiency of the turbine. It is the actual work produced by the turbine divided by the ideal or isentropic work that is expected for the given steam conditions (Figure 3). Turbine Isentropic Efficiency =

Actual Work Ideal (Isentropic) Work

For Given Steam Conditions, P1, T1, and P2

Figure 3. Turbine Isentropic Efficiency This efficiency is commonly called "thermodynamic," "engine," or "turbine" efficiency.


8



Cycle Efficiency involves more than the steam turbine. A steam cycle includes a steam generator, the turbine and a means of disposing of the exhaust steam (Figure 4). Exhaust steam that will be used by another process is useful heat output. If exhaust steam is condensed, the heat of condensation is lost. Cycle efficiency is defined as work output plus any useful heat output divided by the fuel fired in the steam generator.

Cycle Efficiency =

Work + Useful Heat Fuel Fired

Figure 4. Cycle Efficiency

This efficiency is commonly called thermal efficiency.


9



Types of Steam Turbines Steam turbines are used in several ways depending on the needs for process steam in the plant. The three major types are backpressure, condensing, and extraction. Backpressure or Non-Condensing Turbines Backpressure or non-condensing turbines are used when there is a need for shaft work and a requirement for steam for process heating (Figure 5). High-pressure steam is fed to the turbine. The turbine produces work. The steam leaving the turbine is at medium to low pressure, between 225 and 15 psig. This remaining steam is then distributed to those parts of the plant that need the steam energy to produce heat or as inlet steam to steam turbines having a lower inlet steam pressure design.

Figure 5. Backpressure (Non-Condensing) Turbine


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The backpressure arrangement has a high cycle efficiency. No energy is lost. All of the energy in the incoming steam is used to make work or for process heat. This assumes that there is a use for the medium to low pressure steam and that it will not be vented to the atmosphere. Turbines for backpressure service are simple and relatively low cost. They also have relatively high turbine efficiencies, typically 60% to 80%. On the other hand, a large rate of steam is required for each horsepower produced, because only part of the pressure energy in each pound of steam is used.


11



Figure 5A shows a typical multistage backpressure steam turbine.

Figure 5a. Typical Multistage Backpressure Steam Turbine


12



Figure 5B shows a typical single-stage backpressure steam turbine.

Figure 5b. Typical Single Stage Backpressure Steam Turbine


13



Condensing Steam Turbines Condensing turbines are employed where there is no suitable use for the exhaust steam (Figure 6). Exhaust steam is condensed by means of cooling water or air-cooled fin condensers. The condensate is recovered and pumped back to the steam generating system.

Figure 6. Condensing Turbines

The outlet pressure from a condensing turbine is very low, usually between 4 and 6 inches Hg absolute, or 2 to 3 psia. Because of the low exhaust pressure, maximum pressure energy is extracted from each pound of steam. The theoretical steam rate (lb/hp-hr) is lower than for a backpressure machine. Because of the large pressure drop, droplets of water form at the exhaust end of the turbine.


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Figure 6A shows a typical condensing steam turbine.

Figure 6a. Typical Condensing Steam Turbine Condensing turbines cost more than backpressure turbines, because they have more stages and larger diameters at the exhaust end of the turbine and the cost must include the condenser. Blades are longer and have complex shapes. The turbine efficiency is lower because high velocities exist in the final stages and because wet steam has a higher viscosity than dry steam. It is also necessary to protect against erosion of the blades in a condensing turbine. The condensing turbine also has a relatively low cycle efficiency, because only a portion of the steam energy is converted to work. A large part is lost in the condenser. In fact, more heat is transferred to cooling water or air than is converted to work. However, condensing turbines are necessary if power is required and there is no use for the exhaust steam.


15



Extraction Turbines Frequently there is a use for part of the steam from a turbine but not all of it. In this case, an extraction/condensing turbine can be used (Figure 7).

Figure 7. Extraction/Condensing Turbines High-pressure steam is fed to the inlet of the turbine. After one or more stages, medium or low-pressure steam is extracted from the turbine. The remainder of the steam proceeds through the low-pressure stages of the turbine and exits at normal condensing pressure, 2 to 3 psia. This portion of the steam is condensed and returned to the steam generator. The extraction/condensing turbine is used to balance steam requirements of a plant with power requirements. Maximum power is extracted from the steam that is used in processes. The low efficiency condensing part of the cycle uses only the surplus steam.


16



This type of turbine is more complex and therefore more expensive than either of the other two types. Figure 7a shows a typical extraction/condensing steam turbine.

Figure 7a. Typical Extraction/Condensing Steam Turbine


17



MECHANICAL COMPONENTS AND THEIR FUNCTIONS The steam path through a turbine is outlined in Figure 8.

Trip and Throttle Valve The trip and throttle valve is a manual (start up) valve and a safety device that shuts off the supply of steam in case of a malfunction or an Emergency Shutdown Signal (ESD). The usual malfunctions are: •

Overspeed of the machine

•

Loss of oil pressure

•

High vibration

•

An abnormal process condition

The inlet steam takes a minimum pressure drop when the trip valve is open. The trip valve is sometimes combined with the governor valve.

Governor Valve The governor valve is the main control for the rate of steam flow into the turbine. It acts with the governor to maintain the speed of the turbine. The governor valve may be a single valve, or for more complex machines, it may be multiple valves. It may be operated mechanically or hydraulically.


18



Steam Chest The steam chest is a chamber between the governor valve and the nozzles; in the steam chest, the steam pressure and temperature are at their highest values in the turbine. Steam Supply

Turbine Exhaust Trip and Throttle Valve

Governor Valve

Steam Chest

Nozzles and Blades

Hand Valve

Nozzles and Blades

(Single Valve Turbines Only)

Figure 8. Steam Path Through Turbine

Hand Valve Hand valves are usually provided on turbines that have a single governor valve. When the turbine is not operating at full load, the efficiency will be improved if some of the nozzles are closed off. Hand valves are used for this purpose. Note: If a turbine that is supplied with hand valves is used in an auto start or critical service (lube/seal oil pump), all hand valves should be open to ensure that maximum power will be available if required.

Nozzles Nozzles convert the pressure energy of the steam to kinetic or velocity energy.

Blades Blades convert steam velocity to mechanical energy.


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CALCULATIONS Theoretical Steam Rate is the amount of steam required to produce the horsepower if the turbine were an ideal machine. The units are pounds of steam per horsepower hour (lb/hp-hr). Actual Steam Rate (also called water rate) is the pounds of steam per horsepower hour that is required in a real (non-ideal) turbine. Condition of Exhaust Steam - The temperature and pressure of the inlet steam are known. The exhaust pressure is usually known, however the enthalpy of the exhaust steam must be calculated. The enthalpy determines the temperature of the exhaust if the exhaust steam is superheated. If the exhaust is saturated, the enthalpy determines the percent moisture.

Efficiency of an Operating Turbine - To monitor the performance of an operating turbine, the efficiency is calculated. The inlet steam temperature and pressure and the outlet steam temperature and pressure are known.

Example Calculation - Theoretical Steam Rate, Actual Steam Rate and Outlet Temperature The method used for predicting turbine conditions uses the Mollier Chart for steam. Work Aid 1 is a calculation form for this type of problem. The following example illustrates the calculation. Given:


Inlet steam pressure

600 psia

Inlet steam temperature

700ºF

Outlet steam pressure

2 psia

Turbine efficiency

75%

Brake horsepower required

1000 hp

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Calculate: •

Theoretical steam rate

•

Actual steam rate (water rate)

•

Steam outlet condition −

temperature

−

% moisture

Solution: Use the Mollier chart for steam (Elliot Bulletin H-37B, inside back cover); see Figure 9 for a graphic illustration of this problem. 1.

Locate the Inlet Steam Temperature and Pressure on the Mollier diagram. Read inlet enthalpy, h1

2.

1350 Btu/lb

Move vertically downward, along a line of constant entropy, to the outlet pressure of 2 psia. Read the outlet enthalpy, h2


923 Btu/lb

21



Figure 9. Use of Mollier Diagram for Steam Turbine Calculations


22



3.

Calculate the isentropic (ideal) ∆h ∆his

= h1 - h2 = 1350 − 923 = 427 Btu/lb

4.

The conversion factor from heat to work is: 2545

Btu hp − hr

Therefore, Theoretical Steam Rate, TSR =

2545 Isentropic ∆h

Btu hp − hr Btu 427 lb

2545 TSR

=

= 5.96 5.

lb hp − hr

Actual Steam Rate, ASR (Water Rate) ASR

=

TSR Turbine Efficiency

=

5.96 lb hp − hr 0.75

= 7.95 lb/hp/hr


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6.

Calculate Steam Flow Rate Steam Flow Rate

= hp x Actual Steam Rate = 1000 hp × 7.95

= 7950 7.

lb hp − hr

lb hr

Outlet Steam Condition: Calculate actual outlet enthalpy Actual ∆h

= ∆his x Turbine Efficiency = 427 Btu/lb x 0.75 = 320 Btu/lb

Actual h2

= h1 − Actual ∆h = 1350 − 320 = 1030 Btu/lb

Locate the outlet steam condition on the Mollier chart, at h

= 1030 Btu/lb and 2 psia

Read Outlet Moisture Content

= 8.4%

Read Outlet Temperature

= 130ºF

NOTE: Since the outlet steam is saturated, and the pressure is known, you can also obtain the temperature from a steam table.


24



Example Calculation - Efficiency of an Operating Turbine This type of problem is also solved using the Mollier Chart. The method is illustrated by the following sample problem: Given, from plant operating data: Inlet Steam Pressure

400 psia

Outlet Steam Pressure

40 psia

Inlet Steam Temperature

650ºF

Outlet Steam Temperature

320ºF

Steam Flow Rate

28,000 lb/hr

Calculate: •

Turbine Thermodynamic Efficiency

•

Brake Horsepower, bhp

Procedure: Work Aid 2 is a calculation form for this type of problem. See Figure 10 for a graphic illustration of this problem. Turbine Efficiency

1.

Actual Enthalpy Drop Isentropic Enthalpy Drop

Locate steam inlet condition on the Mollier chart at 400 psia and 650ºF. Read h1.


1335 Btu/lb

25



2.

Calculate isentropic ∆h from P1 to P2. Follow a constant entropy line vertically downward to P2, 40 psia. Read h2 is ∆his

1127 Btu/lb = h1 − h2 is = 1335 − 1127 = 208 Btu/lb

3.

Calculate Actual ∆hact. On the Mollier chart, locate the actual outlet condition at P2 = 40 psia, T2 = 320ºF. Read h2 act

1196 Btu/lb

Calculate ∆hact: ∆hact

= h1 − h2 act = 1335 − 1196 = 139 Btu/lb

4.

Calculate Turbine Efficiency. Efficiency

=

∆hact ∆his

=

139 208

= 0.67


26



Figure 10. Efficiency of an Operating Turbine


27



5.

Calculate Brake Horsepower Water Rate

=

2545 ∆hact

Btu hp hr Btu 139 lb

2545

=

= 18.3 lb/hp−hr

bhp

=

Steam Flow Rate Water Rate

=

28,000 lb hr 18.3 lb hp hr

= 1530 hp


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Turbines With Saturated Exhaust Steam

Note that the five-step method above cannot be used if the exhaust gas is saturated, because it is not possible to measure the moisture content of the exhaust steam. In this case, the turbine efficiency can be calculated only if the brake horsepower of the driven machine, for example, the compressor, is known. The procedure is as follows: Calculate ∆his as above, using P1, T1, and P2. 2545 ∆his

TSR

=

ASR

=

Actual Steam Flow, lb hr Actual bhp

Efficiency

=

TSR ASR

Efficiencies of Steam Turbines for Use in Calculations

Use efficiency curves provided by manufacturers, if they are available. If they are not available, estimates can be made from references. For multistage turbines, GPSA Figures 15-12, 1513, and 15-17 give the basic efficiency of turbines operating at full power. For factors to estimate the efficiency at less than full power, use GPSA Figure 15-11. For single-stage non-condensing turbines, efficiencies can be obtained from Work Aid 4.


29



Use of Hand Valves to Maximize Efficiency

Turbines should always be operated at maximum efficiency to reduce the amount of steam required. If a turbine is operating at full power, the efficiency is determined only by the turbine design. At reduced power, there are other ways to improve the efficiency. At reduced power, the main steam valve, called the governor valve, is partially closed. This means that a steam pressure drop will be taken through the governor valve. This pressure drop does not provide any power; therefore, it reduces the efficiency of the turbine. Downstream of the governor, the steam passes through several nozzles. Most turbines have a means of closing off some nozzles when the turbine is not fully loaded. This increases the steam flow through the nozzles that remain open. The result is a higher pressure drop through the nozzles. The steam governor valve will then have to open. The net result is a shift of pressure drop from the governor valve to nozzles, which increases efficiency. On large turbines, the multiple valves that block off some of the nozzles are all controlled by the governor mechanism. They open and close automatically at the proper time. On smaller turbines, only a single main valve is controlled by the governor. The others are hand valves. The hand valve must be operated manually in order to achieve maximum efficiency. It must be either fully open or fully closed; it is not designed to be a throttle valve. Note: If hand valves are supplied on steam turbines in auto start or critical applications (lube/seal oil pumps), all hand valves should be open to ensure that maximum power will be available if required.


30



THEORETICAL STEAM RATE TABLES Theoretical steam rate tables are an alternative method to calculate steam rates without using the Mollier diagram. The GPSA Manual Figure 15-15 is such a table. Using a table of this type, you can calculate the steam rate required for a given horsepower, if the steam inlet conditions and the outlet pressure are known. Interpolation is required. With steam rate tables, you cannot calculate the outlet steam temperature or steam quality. Also, it is not possible to calculate the efficiency of an operating turbine from plant data. Mollier charts must be used for these two calculations.

Performance Curves Figure 11 is a typical performance curve for a condensing turbine. The curves for backpressure turbines are similar in format. Steam Conditions: Inlet Exhaust

600 psig, 750°F 4 in Hg Absolute

Figure 11. Typical Turbine Performance Curve


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Figure 12 is a performance curve for an extraction turbine. This is a family of curves to cover the variable of extraction rate. Curves for extraction turbines are usually plotted for one speed only. Steam Conditions: Inlet Exhaust Extraction

600 psig, 750°F 4 in Hg Absolute 250 psig

Figure 12. Typical Performance Curve - Extraction Turbine


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COMMON OPERATING PROBLEMS Work Aid 5 summarizes the major process operating problems such as: •

Insufficient power developed

•

Low efficiency

•

Erosion of blades

•

Exhaust too hot

•

Vibration

•

Failure to start quickly on automatic cut-in


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WORK AID 1:

CALCULATION OF THEORETICAL AND ACTUAL STEAM RATES AND OUTLET TEMPERATURE

Steam Conditions: P1:

psia

T1:

ºF

P2:

psia

Turbine Efficiency

(from manufacturer's curve or GPSA, Figures 15-11, 12, 13, and 17)

bhp required

hp

1.

h1 (from Mollier)

Btu/lb

2.

Move vertically on Mollier from P1, T1 to P2 h2 is (from Mollier).

Btu/lb

3.

∆his

= h1 - h2 = (

)−(

)

= 4.

5.

Theoretical Steam Rate: =

2545 2545 = = ( ) ∆his

=

TSR = Turbine Eff .

lb hp − hr

Actual Steam Rate: ASR


( (

)= )

lb hp − hr

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6.

Steam Flow Rate

= hp × ASR = (

)×(

)

= OUTLET STEAM CONDITIONS Actual ∆h

= ∆his × Turbine Eff. = (

)×(

Btu/lb

=

Actual h2

)

= h1 − Actual ∆h = ( =

)−(

) Btu/lb

On Mollier, locate P2, Actual h2, and read T2

=

% Moisture

=


ºF

35



WORK AID 2:

CALCULATION OF TURBINE EFFICIENCY AND HORSEPOWER FROM STEAM CONDITIONS

P1:

psia

T1:

ºF

P2:

psia

T2:

ºF

Steam Flow Rate:

lb/hr

1.

h1 (from Mollier at P1T1)

2.

Move vertically on Mollier from P1T1 to P2.

Btu/lb

h2 isentropic

=

Btu/lb

3.

h2 actual (from Mollier at P2T2)

=

Btu/lb

4.

∆h is

= h1 − h2 is = (

)−(

Btu/lb

= 5.

∆h act

= h1 − h2 act = (

)−(

Turbine Efficiency


) Btu/lb

= 6.

)

=

∆h act ∆h is

=

( (

)= )

36



7.

Water Rate

=

=

bhp


2545 ∆h act

(

2545

)

=

=

Steam Flow Rate Water Rate

=

( (

) lb / hr ) lb / hp − hr

lb/hp-hr

=

hp

37



WORK AID 3:

CALCULATION OF EFFICIENCY FROM INLET STEAM CONDITION AND BRAKE HORSEPOWER

P1:

psia

T1:

ºF

P2:

psia

bhp:

hp

Steam Flow Rate:

lb/hr

1.

h1 (from Mollier)

2.

Move vertically on Mollier from P1T1 to P2

3.

Btu/lb

h2 isentropic

=

Btu/lb

∆h isentropic

= h1 − h2 isentropic = (____) − (____) Btu lb

= 4.

5.

6.

TSR

= 2545 2545 = = ( ) ∆his

ASR

Turbine Efficiency


=

lb hp − hr

=

Steam Flow Rate bhp

=

( (

TSR = ASR

) lb hr ) hp ( (

=

lb hp − hr

)= )

38



7.

Outlet Steam Condition: ∆hActual

= ∆hIsentropic × Turbine Efficiency = (

h2Actual

) =________ Btu/lb

= h1 − ∆h actual = (

8.

)×(

)−(

) = ________ Btu/lb

On Mollier, locate point at P2,h2 actual. Read % Moisture

%

T2

ºF


39



WORK AID 4:

ESTIMATING EFFICIENCY FACTORS FOR SINGLESTAGE NON-CONDENSING TURBINES

Correction Factors Condition

Efficiency Multiplier

P1 = 600 psig

= 0.80

P2 = 50 psig

= 1.12

P2 = 0 psig

= 0.90

N

= 0.68

= 1,800 rpm

Figure 13. Typical Efficiencies, Single Stage Turbines, Non-Condensing "Normal Efficiency" Type


40



WORK AID 5:

COMMON OPERATING PROBLEMS Steam Turbines Common Operating Problems

Problem

Insufficient Power Developed

Possible Cause

•

Steam pressure too low.

•

Backpressure too high.

•

Supply temperature too low.

•

Deposits in steam path.

•

Deposits in steam path.

•

Erosion of nozzles or blades.

•

Hand valves open at reduced power.

Erosion of Blades

•

Too much moisture in turbine; inlet temperature too low or outlet pressure too low.

Exhaust Too Hot

•

Low efficiency

•

Low steam flow rate

•

Deposits

•

Erosion

•

Broken blades

•

Damaged bearings

•

Misalignment of piping

•

Water in supply line; traps not working.

Low Efficiency

Vibration

Failure to Start Quickly on Automatic Cut-In


41



GLOSSARY Actual Steam Rate (ASR)

See Water Rate.

Backpressure Turbine

A steam turbine that does not exhaust into a condenser. The exhaust pressure will typically be 15 psig or higher. The exhaust nozzle can be vented to atmosphere or piped to a steam header.

Blade

A component of a steam turbine that converts steam energy to mechanical energy. Blades are mounted on rotating wheels.

Buckets

Another name for blades. Usually, the blades of a Curtis stage.

Curtis Stage

A type of steam turbine stage with one row of nozzles and one or more rows of buckets. The usual sequence of components is: nozzles, rotating buckets, stationary turning buckets, rotating buckets.

Cycle Efficiency

In a steam turbine cycle, the sum of power output plus useful heat divided by fuel input.

Extraction

The process of removing medium pressure steam between the inlet and exhaust of a steam turbine.

Governor

A device that regulates the speed of a steam turbine. It may be mechanical or electronic.

Governor Valve

The primary valve controlling the steam flow to a turbine.

Hand Valve

A valve used to shut off the steam supply to a portion of the inlet nozzles.

Impulse Blades

Rotating turbine blades in which only velocity decreases; pressure does not decrease.


42



Mechanical Drive Turbine

A steam turbine used to drive process machinery.

Nozzle

The component of a steam turbine that converts pressure energy to velocity energy.

Overspeed Trip

A protection device that senses excessive turbine speed and shuts off the steam supply.

Rateau Stage

A steam turbine stage with one row of nozzles and one row of blades. A relatively small pressure drop is taken in the rotating blade of a Rateau stage.

Stage

A section of a steam turbine containing one set of nozzles and one or more row of blades.

Reaction Blades

Rotating turbine blades in which pressure drop takes place.

Steam Chest

A chamber upstream of the first stage nozzles of a steam turbine. The area of highest steam temperature and pressure in a turbine.

Theoretical Steam Rate (TSR)

The flow rate of steam in pounds per hour required to produce 1 horsepower in an ideal turbine. TSR is determined by steam inlet temperature and pressure and outlet pressure.

Turbine Efficiency

The theoretical steam rate divided by the actual steam rate. Also, the actual work output divided by the theoretical work output for a given pressure range.

Water Rate

The actual steam rate required per unit of power. (Pounds per horsepower-hour.)


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REFERENCES Supplementary Text

Gas Processors Suppliers Association •

Engineering Data Book, Section 15

Vendor's Bulletins •

Elliot Bulletin H-37B, Multivalve Turbines

•

Elliot Bulletin H-31K, Single-stage Turbines

Industry Standards •

API 611, General Purpose Steam Turbines for Refinery Services

•

API 612, Special Purpose Turbines for Refinery Services

Saudi Aramco Materials System Specifications 32-SAMSS-009 General Purpose Steam Turbines 32-SAMSS-010 Special Purpose Steam Turbines Saudi Aramco Engineering Standards •

SAES-K-501 Steam Turbines


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Steam Turbines

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