Overview of Boiler Drum Level Measurement

TATA CONSULTING ENGINEERS LIMITED COVER PAGE

AN OVERVIEW OF BOILER DRUM & ITS LEVEL MEASUREMENT & CONTROL


Author: Mr. Arun Kumar Maiti (M5M124)

Guided ByMr. Sugata Bnadyopadhyay (M4B329) Mr. Amitava Sengupta (S1S837) Mr. S.B. Mukherjee (M6M227)

Date: 02-02-2011 TCE Office Code: DK

SHEET i OF i

TATA CONSULTING ENGINEERS LIMITED CONTENT SHEET


SECTION

TITLE

SHEET i OF i

NUMBER OF PAGES

1.0

INTRODUCTION

1

2.0

PURPOSE OF DRUM IN BOILER

2

3.0

DRUM INTERNALS & FITTINGS ARRANGEMENTS

4

4.0

CODES & STANDARDS

5

4.1

PG-60 Requirements for Miscellaneous Pipe, Valves and Fittings

6

5.0

DRUM DISHED END INSTRUMENT TAPPING POINTS

9

6.0

METHODS OF DRUM LEVEL MEASUREMENTS

11

6.1

Direct Reading Devices

11

6.2

Indirect/Remote indication

14

7.0

DENSITY ERROR CORRECTION IN DRUM LEVEL INDICATION

16

8.0

DRUM LEVEL CONTROL SYSTEM

21

8.1

Measurement

22

8.2

Control

22

9.0

INSTALLATION PROCEDURE

32

10.0

CONCLUSION

33

11.0

REFERENCES

33

TATA CONSULTING ENGINEERS LIMITED AN OVERVIEW OF BOILER DRUM & ITS LEVEL MEASUREMENT & CONTROL

1.0

PAGE 1 OF 34

INTRODUCTION Boiler steam drum water level is one of the most important power plant parameters to measure and control. If the level is too low, boiler tubes will be damaged by overheating. If the level is too high, the super heater tubes and the turbine may be damaged by moisture or water treatment chemical carryover. As boiler operating pressures and boiler drum wall thickness have increased, many boiler steam drums have become smaller. This reduced boiler drum volume demands even more accurate level control. A variety of instruments is available and approved by the ASME for power boiler level indication. However, simply specifying one or more of these instruments will not guarantee that the boiler level will be indicated accurately. So it is required to thoroughly understand the operating principles of each instrument, instrument installation requirements and the boiler operating scheme. A difference in the water density between the level instrument and the boiler is the major source of level error. The National Board of Boiler Inspectors has repeatedly reported that low water conditions in boilers is the number one cause of boiler related accidents.

Fig. 1. Steam Drum.


2.0

PAGE 2 OF 34

PURPOSE OF DRUM IN BOILER The boiler water/steam drum, or steam drum, is an integral part of the boiler’s design.

This vessel has three specific purposes. 

Provide a volume space to hold the boiling water in the boiler.



Provide enough water volume to allow for good thermal mixing of the cooler bottom drum water with the hotter surface interface water.



Provide surface area and volume for the efficient release of the entrained steam bubbles from the boiler water.

The boiler water/steam drum also provides a logical location for 

Addition of feedwater,



Addition of chemical water treatment



Surface blowdown, which helps reduce the surface tension of the water/steam interface to allow better steam release.

The steam drum (fig. 1) is a cylinder, located at the top of the boiler. It runs length wise from the Left side to the Right side of the boiler front side. The steam drum provides a space for the saturated steam generated in the tubes and for the separation of moisture from the steam. (Saturated steam is steam that has not been heated above the temperature of the water from which it was generated). The steam drum also serves as a storage space for boiler water, which is distributed from the steam drum to the downcomer tubes. During normal operation, the steam drum is kept about half full of water. The steam drum either contains or is connected to many of the important controls and fittings required for the operation of the boiler. Made from high Carbon Steel with high tensile strength and its working involves temperatures around 390oC and pressures well above 350 psi (2.4MPa). The separated steam is drawn out from the top section of the drum and distributed for process. Further heating of the saturated steam will make superheated steam normally used to drive a steam turbine. Saturated steam is drawn off the top of the drum and re-enters the furnace in through a super heater. The steam and water mixture enters the steam drum through riser tubes; drum internals (fig. 2) consisting of demister separate the water droplets from the steam producing dry steam. The saturated water at the bottom of the


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steam drum flows down through the down-comer pipe, normally unheated, to headers and water drum. Its accessories include a safety valve, water-level indicator and level controller. Feed-water of boiler is also fed to the steam drum through a feed pipe extending inside the drum, along the length of the steam drum.

Fig. 2. Steam Drum Internal Arrangements.

A steam drum is used without or in the company of a mud-drum/feed water drum which is located at a lower level. A boiler with both steam drum and mud/water drum is called a bi-drum boiler and a boiler with only a steam drum is called a mono-drum boiler. The bidrum boiler construction is normally intended for low pressure-rating boiler while the mono-drum is mostly designed for higher pressure-rating.


PAGE 4 OF 34

Low water levels affect the internal thermal recirculation of the boiler water resulting in cold spots in the boiler water and steam collapse. This lack of circulation also reduces the effectiveness of the chemical water treatment and can cause precipitation of the chemicals as chemical salts or foams. High water levels raise steam exit velocities and result in priming or boiler water carryover into the distribution system. Priming results in wet dirty steam while carry-over can result in dangerous water hammer and pipe or equipment damage.

3.0

DRUM INTERNAL & FITTING ARRANGEMENTS The internal fittings in the steam drum help distribute the water evenly throughout the drum, separate the generated steam from the water and remove moisture from the steam before it leaves the boiler (fig. 3). a) Lower baffle plates or apron plates separate the incoming feed water and generated steam and direct the steam to the separators. b) Primary separators (cyclone separators) separate most of the water from the steam by giving it a cyclone or rotary motion so that the water particles are expelled from the steam by the centrifugal forces. These separators are vertically mounted in the steam drum so that the steam rises out the top and the water falls back into the steam drum. c) Secondary separators (chevron dryers) remove additional moisture from the steam by changing the direction of steam flow several times. The steam passes on but the moisture cannot make the direction change with the steam. These separators are mounted above the primary separators and direct steam to the dry box which collects the steam at the top of the steam drum, directing it to the steam outlet piping to the super heater. d) Feed water leaves the economizer and enters the boiler through the internal feed pipe and becomes "boiler water." Perforations along the side of the feed pipe allow water to be distributed evenly throughout the steam drum.


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e) Since suspended solids may accumulate on the surface of the water in the steam drum, there must be means of removing them. The surface blow pipe is used to remove these light suspended solids from the surface of the water and to reduce the total dissolved solid content of the boiler water. Suspended solids usually consist of oil, salt contaminants, or excessive treatment chemicals which can cause foaming on the water surface. Dissolved solids usually consist of salt contaminants and treatment chemicals that are in solution

Fig. 3. Arrangement of Internal fittings in a Single Furnace Boiler.


4.0

PAGE 6 OF 34

CODES & STANDARDS The requirements for power boiler drum level indication are clearly stated in the ASME Boiler and Pressure Code Section I, Paragraph 60.1.1. In addition to the direct reading gauge glass, remote liquid level indicators of various types are permitted under certain design conditions. These guidelines give the minimum level of protection considered suitable for safe equipment operation.

4.1

PG-60 Requirements for Miscellaneous Pipe, Valves and Fittings 60.1 Water Level Indicators (Refer Fig.4.a,b,c) 60.1.1 Each boiler, except forced-flow steam generators with no fixed steam and water line, and high temperature water boilers of the forced circulation type that have no steam and water line, shall have at least one water gauge glass. Except for electric boilers of the electrode type, boilers operated at pressures over 400 psi shall be provided with two water gauge glasses which may be connected to a single water column or connected directly to the drum. Two independent remote level indicators may be provided instead of one of the two required gauge glasses for boiler drum water level indication in the case of power boilers with all drum safety valves set at or above 900 psi. When both remote level indicators are in reliable operation, the remaining gauge glass may be shut off, but shall be maintained in serviceable condition. When the direct reading of gauge glass water level is not readily visible to the operator in the area where immediate control actions are initiated, two dependable indirect indications shall be provided, either by transmission of the gauge glass image or by remote level indicators. A gauge glass image transmitted to the operator’s working area by means of a fibre optic cable, with no electrical modification of the optical signal is considered to provide direct reading of the gauge glass water level. The lowest visible part of the water gauge glass shall be at least 2” (50mm) above the lowest permissible water level, as determined by the boiler manufacturer. When remote level indication is provided for the operator in lieu of the gauge glass, the same


PAGE 7 OF 34

minimum level reference shall be clearly marked.

PG-60.2 Water Columns PG-60.2.1 The Water Column shall be so mounted that it will maintain its correct position relative to the normal waterline under operating conditions. PG-60.3 Connections PG-60.3.1 Gauge Glasses that are required by PG-60.1 shall be connected directly to the shell or drum of the boiler or to an intervening water column. PG-60.3.2 The lower edge of the steam connection to a water column or gauge glass in the boiler shall not be below the highest visible water level in the water gauge glass. There shall be no sag or offset in the piping which will permit the accumulation of water. PG-60.3.3 The upper edge of the water connection to a water column or gauge glass and the boiler shall not be above the lowest visible water level in the gauge glass. No part of this pipe connection shall be above the point of connection at the water column. PG-60.3.4 Connections from the boiler to the water column shall be at least NPS 1. Connections for gauge glasses connected directly to the boiler shall be at least NPS 1/2. Connections from the boiler to the remote level indicator shall be at least NPS 3/4 and including the isolation valve and from there to the remote level indicator at least 1/2 in. O.D. tubing. These connections shall be completely independent of other connections for any function other than water level indication.


PAGE 8 OF 34

Fig. 4. ASME Boiler and Pressure Vessel Code Requirements.


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Some typical specifications have been provided relating to various different boiler pressures and types of trim equipment. Each of these specifications is followed with a short list of issues relating to the system design or equipment selected (refer Table-1). This section might be worth considering when choosing a specific design concept.

Table-1.

5.0

Sl. No. 1. 2.

Drum PSI 250 1100

3. 4.

1500 1500

5.

1500

6.

3000

7.

As Reqd.

Indication & Tripping Concept 2 gauge glasses + whistle alarms + Float switches 2 Magnetic level gauges for remote indication & Alarms + 1 gauge glass 2 gauge glasses + probe type alarms 2 vertical probe columns with remote Indication & Alarms + 1 gauge glass 2 horizontal probe columns with remote Indication & alarms + 1 gauge glass 2 horizontal probe columns with remote Indication & alarms + 1 gauge glass 3 element control with triple redundancy D/P

DRUM DISHED END INSTRUMENT TAPPING POINTS Fig. 5.a & 5.b shows the details Instrument tapping point for measurement and control of a typical single furnace Boiler Steam Drum dished taken from Left side and Right side. The capacity of Steam Generator is 805T/Hr, Drum Pressure is 200 Kg/ Sq. Cm (g), Super heater Stem Pressure and Temperature is 155 Kg/ Sq. Cm (g) and 540°C respectively. Drum Internal Diameter is 1778mm. Drum Thickness is 135mm (min) & 170mm (max). The Normal Water Level (NWL) of Steam Drum is normally considered 250mm below the Centre Line of Drum.

Left Side P1-P1 P5-P5 P2-P2 P3-P3 P4-P4

Right Side P10-P10 P6-P6 P9-P9 P8-P8 P7-P7

INSTRUMENTATION DETAILS Type of Instruments Measuring Range D/P LEVEL TRANSMITTER (+)147mm, (-)622 mm of C.L. DWLG EWLI D/P LEVEL TRANSMITTER

(+)512mm, (-)771 mm of C.L. +512mm, -675 mm of C.L


Fig. 5.a. Drum Dished Left Side Instrument Tapping Points.

1. 2. 3. 4.

TRIP & ALARM LEVEL SET POINTS HIGH Level TRIP 225mm above NWL HIGH Level ALARM 125mm above NWL LOW Level ALARM 125mm below NWL LOW Level TRIP 225mm below NWL

PAGE 10 OF 34


PAGE 11 OF 34

Fig. 5.b. Drum Dished Right Side Instrument Tapping Points.

6.0

METHODS OF DRUM LEVEL MEASUREMENTS There are numerous techniques used for determining level. These can be simplified by dividing into a few categories.

6.1

Direct Reading Devices These are instruments which provide for the determination of the level by direct observation such as gauge glasses. This device provides for the direct viewing of the liquid level. Various designs are available providing reflex, transparent or refraction (BiColour) images. (Fig. 6.1.a.). The gauge glass is the final verification method of drum level if all other methods are unavailable. For this reason the ASME Code mandates that all boilers have at least one gauge glass at all times.


PAGE 12 OF 34

Fig. 6.1.a. Direct Reading Gauge Glass.

Gauge glasses made from tubular glass or plastic are used for service up to 450 psig and 400°F. If it is desired to measure the level of a vessel at higher temperatures and pressures, a different type of gauge glass is used. The type of gauge glass utilized in this instance has a body made of metal with a heavy glass or quartz section for visual observation of the liquid level. The glass section is usually flat to provide strength and safety. Fig. 6.b illustrates a typical transparent gauge glass.

Fig. 6.1.b. Gauge Glass.

Another type of gauge glass is the reflex gauge glass (Fig. 6.1.c). In this type, one side of the glass section is prism-shaped. The glass is moulded such that one side has 90-


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degree angles which run lengthwise. Light rays strike the outer surface of the glass at a 90-degree angle. The light rays travel through the glass striking the inner side of the glass at a 45-degree angle. The presence or absence of liquid in the chamber determines if the light rays are refracted into the chamber or reflected back to the outer surface of the glass.

Fig. 6.1.c. Reflex Gauge Glass.

When the liquid is at an intermediate level in the gauge glass, the light rays encounter an air-glass interface in one portion of the chamber and a water-glass interface in the other portion of the chamber. Where an air-glass interface exists, the light rays are reflected back to the outer surface of the glass since the critical angle for light to pass from air to glass is 42 degrees. This causes the gauge glass to appear silvery-white. In the portion of the chamber with the water-glass interface, the light is refracted into the chamber by the prisms. Reflection of the light back to the outer surface of the gauge glass does not occur because the critical angle for light to pass from glass to water is 62-degrees. This results in the glass appearing black, since it is possible to see through the water to the walls of the chamber which are painted black. A third type of gauge glass is the refraction type. This type is especially useful in areas


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of reduced lighting; lights are usually attached to the gauge glass. Operation is based on the principle that the bending of light, or refraction, will be different as light passes through various media. Light is bent, or refracted, to a greater extent in water than in steam. For the portion of the chamber that contains steam, the light rays travel relatively straight, and the red lens is illuminated. For the portion of the chamber that contains water, the light rays are bent, causing the green lens to be illuminated.

6.2

Indirect/Remote indication In the event of the non visibility of the gauge glass to the operator, or as a part of an alternate design philosophy, remote level indicators can be used in place of the gauge glass. It is important to consider the level of integrity such a device provides since it is being used in place of a direct reading device. Indirect level monitors do not offer a direct observation of the level, however they provide additional benefits such as control contacts or analog signals that can be used in the alarm, tripping or control systems.

Electronic Water Level Gauge This device measures water level through conductivity probes installed in a water column connected to the boiler (Fig. 6.2.a). Electronic Gauging System is designed as an electronic alternative to water level gauges on boilers, giving a more reliable and safer water level indication than conventional visual gauges. It uses the significant difference in resistivity of water and steam in temperatures up to 370°C (698°F) to determine the water level. A vertical row of electrodes is installed in the water level column attached to boiler and typically aligned so that half the electrodes are above and half below the normal water level The resistance measurement is made between the insulated tip of each electrode and the wall of the column.


PAGE 15 OF 34

Fig. 6.2.a. Electronic Water Level Gauge.

Differential Pressure Indicator The most popular device for level monitoring on the boiler drum is the Differential Pressure transmitter. This device infers the level of the water within the drum by comparing the head pressure generated by the water in the drum to a reference level. Typically this reference is derived from a chamber attached to the drum and filled with condensate derived from the sub cooling of steam. Connections are made from this reference and from the drum itself to the high and low inputs of the level transmitter. It is important that the two inputs to the transmitter be kept as close together in temperature as possible to prevent errors resulting from differing specific gravity resulting from temperature differences in the two sampling legs and erroneously interpreting this to be a level difference. This device is frequently associated with pressure and temperature transmitters that are used to correct for errors throughout the operating cycle. This type device is subject to calibration shifts or sensor failures. Typically these devices will be installed in a redundant or triplicate configuration which is intended to help provide for a more secure system through an averaging or mean select scheme. This device provides a continuous analog representation of level and an integral component in the analog control system.


7.0

DENSITY ERROR CORRECTION IN DRUM LEVEL INDICATION

7.1

Density level error

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Water level gauges and all types of remote level indicators are affected by density level error. This level measurement error occurs because the water measured by the device is colder than the boiler water, creating a density difference. This system can be explained and modelled by a U Tube Manometer (Fig. 7.1.a).

Fig. 7.1.a. Level gauge and “U” tube equivalent.

The manometer contains fluid with three different densities. Dd = Saturated water with boiler water density. Dg = Water in the gauge with an unknown density, and Ds = Saturated Steam density. A pressure balance at point “A” in this system produces the following relationships and the actual Drum water level can be determined by the use of these formulas HdDd = HgDg + HsDs ---------------------equation 1. Hs = Hd – Hg

---------------------equation 2

Hg = Hd (Dd - Ds) / (Dg – Ds) --------------equation 3. Hd = Hg (Dg - Ds)/ (Dd – Ds) --------------equation 4.

7.2

The factors that determine the magnitude of the level error are the operating pressure,


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the gauge sub-cooling, and the boiler drum level above the gauge return connection. Fig. 7.2.a, b. shows the effect of operating pressure and gauge sub cooling on gauge error. As pressure increases, the density of saturated liquid and saturated steam converge (Fig. 7.2.a.).

Fig. 7.2.a. Operating Pressure Vs. Density.

As gauge sub-cooling and Hd increases, the level error increases (Fig.7.2.b.).


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Fig. 7.2.b. Steam Pressure Vs. %Level Error.

Hd can be minimized by returning the water pipe from the gauge horizontally to the drum, and by designing the drum water connection to be a minimum distance below the lowest gauge view port.

7.3

Level Gauge Dynamics Standard level gauges and electronic level gauges connected to the boiler are not in a static condition. If they were, the water temperature in the gauge would be near ambient. Referring to Fig. 7.3.a, steam condenses in the supply piping and upper gauge to the gauge waterline. This saturated water raises the gauge water level slightly above the equilibrium point. The excess water flows back into the boiler drum. The circulation flow depends upon the condensing steam rate and the volume in the gauge and water return piping. The condensing steam enters the gauge at the saturation temperature. The condensate continues to cool until it re-enters the boiler.


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Fig. 7.3.a. Level Gauge Water Flow Dynamics.

A profile of this temperature gradient is shown in Figure 7.3.b. Many level gauges are connected to the boiler with a “tie bar” or “water column” because the visibility of the gauge may be adversely affected by excessive water flowing over the glass. The tie bar short circuits the excess water from the steam supply piping. The only flow through the gauge is from steam condensing in the gauge itself and the gauge piping. Gauges and remote level indication devices will indicate incorrectly if boiler pressure decreases suddenly. The temperature of the water in the gauge or reference column is normally within a hundred degrees of the saturation temperature. If the boiler pressure decreases rapidly, the water in the gauge will flash to steam. The steam bubbles will cause the level in the glass to rise and will often obliterate the meniscus. Indication will return to normal when thermal equilibrium is re-established. Systems utilizing a reference column will indicate high as water in the reference leg flashes to steam. Indication will return to normal very slowly as steam condenses to refill the reference leg. The reference leg can also be manually filled to establish proper indication more quickly.


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Fig. 7.3.b. Gauge Water Temperature Profile.

7.4

Gauge Correction Options Every level measurement device must be corrected in some manner to compensate for density level error. A number of techniques have evolved over the years to address each individual device. Water Level Gauges, direct reading and electronic type The primary difficulty in calculating the level error for devices of this type is to determine the average gauge water temperature and corresponding water density. When this is calculated or measured, then the gauge glass is physically installed in a lower position to compensate for this error. Steam heated level gauges are available that significantly reduce the density error. But these may be especially susceptible to boiler pressure transients. The electronic gauge can be corrected by locating each probe individually to correct for the error at that level.

Differential Pressure Indicator, electronic type The most sophisticated of the devices measures the differential pressure of the reference and variable legs with both at ambient temperature. The ambient temperature and the boiler operating pressure are also measured. An analog or digital program then


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corrects the apparent differential for boiler water density to indicate the boiler level. Less sophisticated devices are calibrated to read correctly at only one pressure, usually at the normal operating pressure. They will indicate improperly when operated at pressures above or below the calibration pressure.

8.0

DRUM LEVEL CONTROL SYSTEM The benefits of boiler drum level control are: 

Maximizes steam quality



Maintains proper drum level to prevent damage to boiler

Drum level control is complicated by the inverse response in level to a change in the firing rate. This phenomenon is known as swell and shrink. When the firing rate increases, vapour bubbles form at a faster rate and that causes the drum level to rise or swell. A decrease in firing rate causes drum level to shrink. The problem associated with the swell and shrink phenomenon is that a standard feedback control loop measuring level cannot correct for load changes without wide swings in drum level. When drum level swells, feedwater flow decreases in order to correct for level. However, feedwater should be increased in order to match the higher steam demand. The level control loop does not start to correct for the increase in steam flow until after drum level has fallen below set point. The risk is that drum level can significantly drop before the control system finally adjusts to the change in steam load. There are three typical control strategies: single-element, two-element, and threeelement drum level control. 

Single-element control measures drum level and regulate the feedwater valve in order to maintain drum level.



Two-element control measures drum level and steam flow rate. This is a feedforward control strategy. Feedwater flow tracks steam load while the drum level control loop trims it.



Three-element control measures drum level, steam flow, and feedwater flow. It is a refinement over 2-element control that improves drum level control and responds quicker to variations in feedwater flow rate.


8.1

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Measurement 

Boiler Drum Level: The boiler drum level has a typical span of 750mm H2O, often with some suppression required depending on the physical location of the transmitter. The boiler will be pressurized so the transmitter must be able to operate with a static pressure of up to several thousand PSI. In addition, chemicals injected into the feedwater need to be considered when specifying the transmitter’s materials of construction.



Drum Pressure: Since the density of steam and water at the saturation temperature change with pressure, the drum level calibration will only be accurate at a single boiler drum pressure. The drum level signal can be compensated for all pressures by using a drum pressure transmitter. Pressure compensation is typically employed with utility boilers that operate at much higher pressures than industrial boilers, but this strategy is useful for either application.



Steam Flow [required for 2 & 3 element control]: Steam flow is a mass flow measurement typically in T/Hr. This measurement can be made using a differential pressure transmitter across an orifice plate, venturi meter, or nozzle. In order to calculate mass flow, steam density compensation is required. With saturated steam, density can be determined by measuring the upstream static pressure. Now-a-days for 200MW and onward units, steam flow is derived from HP Turbine 1

st

stage

steam pressure. 

Feedwater Flow [required for 3 element control only]: Feedwater is pumped from the deaerator tank. The deaerator heats feedwater to boiling to remove dissolved oxygen, carbon dioxide and other gases. Dissolved gases contribute to boiler drum corrosion. Feedwater flow is a mass flow measurement and is typically measured using a differential pressure transmitter and primary element.


8.2

PAGE 23 OF 34

Control Typically one of three control strategies is employed to control the boiler drum level. Single-Element Drum Level Control The single-element system is the simplest type used for controlling packaged fire tube and watertube boilers. In this strategy, control is based on the boiler drum level measurement only. This does not allow for compensation of shrink or swell and, therefore, is an acceptable control strategy only for fairly constant steam demand applications; for example, small boilers with slow load changes. Single-element control is the minimum feedwater control system and shall be used for the following applications: 1) During start-up or at low-load operation, when flow measurements are generally not accurate. 2) When steam flow rate of change is nominal and feedwater supply pressure is essentially constant. Pressure compensation: - If the instruments used to measure drum level are sensitive to density variations, then density compensation technologies shall be employed.


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PAGE 25 OF 34

SWELL NWL DRUM LEVEL

SHRINK

100

% STEAM FLOW

*

75 STEAM FLOW

FEEDWATER FLOW

50 * 25

0 0

TIME

* INTERACTION WITH FIRING RATE CONTROL DUE TO IMBALANCE BETWEEN STEAM FLOW AND FEEDWATER FLOW.

By adding integral / reset control effect the level will come back to set point. The swell or shrink results in an increase in level on steam increase. Integral control must be tuned to be slow so the initial rise effect is reduced.

Two-Element Drum Level Control Two-element drum level control measures drum level and steam flow. It is a feed forward control loop. Feedwater tracks steam flow and responds directly to changes in steam load rather than indirectly by a change in drum level. The drum level trim algorithm uses a PID function block and adjusts feedwater flow to maintain drum level at set point. The feedwater control valve must provide a flow rate that is linear with valve position in order to track steam mass flow. This control method permits tighter drum level control and is more tolerant of load changes. Typically, steam load is measured as mass flow, T/Hr. A differential pressure transmitter across an orifice plate or venturi meter can be used to measure mass flow if steam density is taken into consideration. For a saturated steam boiler, density can be determined from the steam pressure. In 2-element drum level control, steam mass flow


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must be converted into percent (%) feedwater valve opening engineering units. The conversion is performed using a characterizer function block. This permits scaling between steam mass flow measurement and valve position. It also permits linearization of the feedwater control valve.

Drum level trim is the controlled variable from the drum level PID function block. The engineering units are % valve opening. Drum level trim is summed with the feedwater flow in order to regulate feedwater control valve position. The PID function block supports controlled variable scaling. Set the minimum scale at -100% and the maximum scale at +100%. This will enable the trim control to reduce and increase the flow control valve position.


NWL

SWELL

PAGE 27 OF 34

FEEDWATER PRESSURE DRUM LEVEL SHRINK

100 % STEAM FLOW 75 STEAM FLOW

FEEDWATER FLOW

50

25 0 0

TIME

The PID feedback signal is important because it enables integral action. In 2-element control, the feedback signal is the feedwater control valve position (the output of the A/M function block) minus the equivalent steam flow valve position (feedwater valve characterizer). When the controller is in manual mode, the drum level trim PID function block is forced into tracking mode. While the system shown will achieve all of the desired control objectives under the conditions specified, it has a serious drawback if the feedwater control valve pressure differential and thus the control valve flow characteristic are not always the same. This figure demonstrates how the performance is seriously degraded by variations in feedwater pressure. Such feedwater pressure variations change the relationship between steam flow and feedwater flow.


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Boiler drum level is then forced to develop an offset from set point in order to bring the steam flow and feedwater flow into balance. Under conditions of unpredictable or variable feedwater pressure, three-element feedwater control is necessary if the desired results are to be achieved. Three-Element Drum Level Control Three-element drum level control measures drum level, steam flow, and feedwater flow. In 2-element control, the assumption is that the feedwater valve can provide an equal amount of feedwater to replace steam lost. Deviations between the two flows are compensated for by the reset value in the drum level PID function block. However, this compensation is sluggish due to the tuning parameters required for drum level control. Variations in drum pressure affect the feedwater flow rate and the result is drum level swings. Three-element drum level control solves this by introducing a feedwater flow control loop. This separates the feedwater flow control from the drum level control, and it permits quick response to variances in feedwater flow without affecting drum level. The set point to the feedwater control loop is the sum of the steam load and the drum level trim. Typically, steam load is measured as mass flow, T/Hr. A differential pressure transmitter across an orifice plate or venturi meter can be used to measure mass flow if steam density is taken into consideration. For a saturated steam boiler, density can be determined from the steam pressure. The analog input block is scaled for full mass flow at a reference pressure. The upstream static pressure is used to measure steam density. Referring to the steam tables, a characterizer function block is configured to convert absolute pressure into density. The actual mass flow is the measured flow multiplied by the measured density and divided by the reference density. The PID function block requires the process variable and set point to be in the same engineering units. In 3-element control, the set point for the feedwater control loop is the sum of the steam flow and drum level trim. Fortunately, both steam mass flow and feedwater flow are measured in the same units, T/Hr, and this eliminates any need for flow conversions. The PID feedback signal is important because it enables integral action. In 3-element


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control, the feedback signal is the feedwater flow rate minus the steam mass flow rate. When the controller is in manual mode, both the feedwater and drum level trim PID function blocks are forced into tracking mode.


PAGE 30 OF 34


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The control loop configuration of fig. provides a feedwater controller wherein steam and water flow measurement are not recognized if they are in an equilibrium condition because the derivative action gives zero output if the difference between steam and water flow remains constant. This condition is prevalent at start up and hence, the controller provides an improved response in attaining drum level equal to drum level set point. If water flow changes relative to steam flow then that will be taken into account. This configuration implements level control by means of a fast level controller as distinguished from the previous slow prior art level controller.


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This configuration provides the benefits:

Where preference is given to fast drum level control under certain condition.



Fast correction of drum level error.



Not affected by Steam and Water flow measurement inaccuracies.

9.0

INSTALLATION PROCEDURE

9.1

Impulse lines from the Drum Dish tapping points to the instruments and the mounting fixtures should be designed in consideration with the downward & outward expansion of the boiler drum as well as the boiler with respect to the fixed structure of the Boiler. Typical 250MW PF boiler of M/s- BHEL make Boiler expansions are as follows: Boiler Drum expansion: - Downward-16mm, Outward- 33mm. W W Lower Front Header: - Downward 227mm, Outward- 25mm.

9.2

Environmental Concerns Density of the fluid whose level is to be measured can have a large effect on level detection instrumentation. It primarily affects level sensing instruments which utilize a wet reference leg. In these instruments, it is possible for the reference leg temperature to be different from the temperature of the fluid whose level is to be measured. An example of this is the level detection instrumentation for a boiler steam drum. The water in the reference leg is at a lower temperature than the water in the steam drum. Therefore, it is denser, and must be compensated for to ensure the indicated steam drum level is accurately indicated. Ambient temperature variations will affect the accuracy and reliability of level detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in the instrumentation circuitry, and, therefore, affect the calibration of electric/electronic equipment. The effects of temperature variations are reduced by the design of the circuitry and by maintaining the level detection instrumentation in the proper environment. The presence of humidity will also affect most electrical equipment, especially electronic equipment. High humidity causes moisture to collect on the equipment. This moisture can cause short circuits, grounds, and corrosion, which, in turn, may damage components. The


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effects due to humidity are controlled by maintaining the equipment in the proper environment. 10.0 CONCLUSION The water level gauge glass has been used for many years to indicate boiler water level and it is required as per the ASME Boiler and Pressure Vessel Code. A number of supplementary indicators are available that provide convenient remote indication and level control. All of these devices are subject to indication error because the water measured is external to the drum and is not at the same density as the boiler drum water. A number of techniques have been developed for each indicating device to address this problem. The electronic dp indicator can be adjusted by an analog or digital program. The mechanical dp indicator uses a temperature compensated reference chamber with a pressure compensation device. The standard level gauge and the electronic level gauge can be corrected by measuring or calculating the average gauge water temperature and installing the gauge in the proper location. Testing confirms that the gauge temperature is affected by a large number of conditions: steam supply and return pipe size, length and configuration; insulation, ambient conditions and water level. Maintaining the gauge water temperature very close to the saturation temperature will have an adverse effect on indication if the pressure drops suddenly. However, if the gauge temperature is significantly less than the boiler water temperature, the level error will be significant at high pressure. The best compromise is to determine the average gauge temperature and the gauge level error that will exist under certain boiler operating conditions. Then, the gauge installation position can be corrected accordingly. For the electronic level gauge, probes can be located individually to indicate properly. For the visual level gauge, repositioning the unit can compensate for the density error. It is vital that the gauge installation follow the manufacturer’s specifications. Only then will the level device indicate accurately and perform as designed and tested. 11.0 REFERENCES a)

Write Up on Boiler & Auxiliaries, PETS, Durgapur.

b)

Density Error and its Correction in Boiler Drum Level Indication. Yarway, Presented at ISA conference, Oct.,1995

c)

A guide to Boiler Drum Level Equipment and Control Concepts. By Clark


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& Reliance. d)

Improving Boiler Room Efficiencies. Presented by David C Farthing.

e)

BHEL 250MW Boiler Drum Dished Document.

f)

Boiler Level Control System, US Patent. Jan. 6, 1981.

g)

Control & Instrumentation, Vol-1., NTPC, Power Management Institution.

Overview of Boiler Drum Level Measurement

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