BOILER CONTROL PROCESS OVERVIEW

Process overview

Put simply a boiler is vessel containing water and steam under pressure it allows maximum fuel-burning efficiency while continually providing steam energy for space heating. Exchangers and turbine dives. There are several types of boilers, all of which operate according to the same basic principle.

In its simplest state, boiler operation involves two simultaneous and interactive cycles. The fuel-air cycle consists of fuel and air entry and hot flue gas exit, figure treats the fuel-air cycle discussed below.

FUEL AIRCYCLETwo firing methods predominate in boiler operations one is suspension firing. In which fuel is burned during a brief period of suspension in the boilers furnace. Bed firing in contrast, requires the pushing, Dropping or throwing of fuel on to a gate or bed at the base of the furnace. Where combustion occurs.

A variety of fuels are used suspension firing fuels in clued liquids such as oil gases such as natural gas and pulverized fuels such as coal ground to consistency of talcum powder suspension firing fuels mix well with air in the case of liquids and gases they are usually easy to meter as well. Bed firing systems burn relatively arge pieces of solid fuel such as coal and wood waste. These materials are difficult to meter and may vary considerably in consistency.

Large industrial and utility boilers typically re capable burning two or more fuels. A paper mill for example may burn all the waste wood it produces and also fire a combination of oil and gas to supplement the mills total heat requirements. In other cases the ration of fuels burned may be a function of fuel costs fuel availability of permissible levels of exhaust gas pollutants.

For combustion to occur air must be added to supply needed oxygen. The volume of air is critical because too little air will not urn all the available fuel and too much air will waste heat in bringing the excess from ambient temperature to final flue gas temperature. Science fuel and air can not be mixed perfectly in practice, a small margin of excess air theoretically unneeded is always supplied.

The air required for combustion is forced into the furnace by a forced draft fan. Some furnaces operate with a positive pressure, which requires a fan of sufficient size and power to supply air to the furnace and to push air and combustion gases through the boiler and out the stack. Other boilers including all bed fired units, operate with a slight negative pressure. They use a forced draft fan to supply air and an induced draft fan to pull the gases through the boiler and out the stack.

In both approaches, hot combustion gases circulate to heat the water and steam, and then are exhausted through a smokestack,. Pollution control devices such as scrubbers precipitators or bughouses often are installed to clean the combustion gases before they are exhausted into the atmosphere.

The balance of fuel and combustion air must equal the hot gases exhausted plus or minus the net effects of any air leaks into or gas leaks out of the boiler. The fuel air balance affects the boilers overall energy balance because the energy in the boiler at any given time is the sum of the enthalpy (inherent energy) of the air and fuel at room temperature plus the thermal energy released in burning the fuel. This value minus the energy loss by boiler watt heat radiation into the environment and minus the energy of the exhaust stack gases equals the energy available to produce steam.

WATER CYCLE: The primary purpose of most boilers is to heat water to steam. In doing so all boilers must replenish steam output with feed water input. Two basic types of boilers fire tube and water tube are currently in use. Water tube boilers include once through and drum type designs. Here the focus will be on the drum type water tube boiler since it is the most common type in industrial and utility power houses.

The drum type boiler takes its name from the drum located at its top which is connected to the tubes in the boiler both down comers and risers. The drum provides the water steam interface with the steam drawn off the top of the drum and the water distributed to the down comers. To maintain the drums water level, feed water is added to the drum. At the same time a low volume of water continually drawn out of the drum to keep the concentration of impurities to a minimum. This procedure is called blow down.

The boiler tubes can be kept full of water by maintaining a consistent liquid level in the drum. Through either natural or forced circulation, the downcomers deliver cool water without steam bubbles to the lower water wall headers. From there the water is distributed to the risers which covey to the boiler drum a water stem mixture of lower density than the water in the downcomers. The feedwater water steam circulation path thus is established.

CONTROL OBJECTIVES:Boiler control has three primary objectives: Maintain proper mass flow balance of feedwater and steam flows. Vary the boiler firing raters to meet the plants steam needs Ensure complete combustion while minimizing excess air over the full operating range of the boiler

For simplicitys sake, this discussion of boiler control schemes will treat a typical drum type boiler fired by a single fuel. More complex systems are frequently required in industrial applications, but they involve the same basic operating principles.

The ultimate object of a control system is to provide steam at a constant pressure, safely and at minimum cost. The control scheme must be flexible enough to react to changes in the volume of steam required, adjusting feedwater to offset precisely the water drawn off as steam and lost as blowdown. At the same time the system must provide enough fuel for the heat needed to convert the water to steam, and the these functions must be coordinated to provide maximum efficiency and safe operating conditions. The control schemes discussed here are typical, and incorporate central concept of control.

Figure 1. fuel-air cycle and water cycle on typical drum boiler

FIGURE 2. Drum level control

FEED WATER CONTROL:

The three basic purposes of the steam drum are to separate the steam from the water- steam mixture to hose the equipment used to purify saturated steam. And to provide a reservoir to feed the boiler tubes. Maintaining the proper drum level is critical to producing steam of the proper quality and to the safe operation of the boiler.

The key control variables are the drum liquid level measurement and the feed water flow valve control. The obvious strategy would be to like the drum level measurement to a level controller. The output of which would regulate the feed water control valve. The strategy illustrated in figure 2 is called single-element control. But this solution is in adequate because the large volume capacity of water in the boiler makes level a slow responding variable that tends to integrate the results of drum pressure variations feed water flow in and steam flow out.

Likewise changes in steam flow will affect the drum level and in turn the feed water valve. But only very slowly an effective control strategy must sense a change in steam flow and quickly make a corresponding change in feed water flow. By making the steam flow measurement feed forward signal to the level controller, the control strategy con ensure that a stable drum level is maintained. This approach also shown in figure 2 is called two-element drum level control.

Boiler drum level control requirements are complicated by a phenomenon known as shrink & swell, which results from the transient effects of changes in steam pressure this effect is felt over the course of two related events in boiler operation. When the demand for steam increases a reduction in drum pressure results in turn causing the vapor bubbles in the boiler to expand. This displaces more water causing the drum level to rise or swell in response a two element drum controller would force the feed water valve to reduce the flow to the drum at precisely the time it should increase feed water flow to compensate for the higher steaming rate. The converse occurs when steam demand decreases and steam pressure increases turning existing vapor bubbles into water. This causes the drum level to lower or shrink.

Shrink and swell can be overcome by a three-element drum level control strategy (figure 2). Feed water flow to the boiler is measured directly and controlled on the basis of a demand signal that incorporates both the steam flow feed forward signal and the drum level controller. As steam flow increases feed water flow increases the converse is true as well the drum level controller maintains the correct level by adjusting the feed water flow demand to compensate for losses and minor changes. Three-element drum level control effectively overcomes the effects of shrink and swell but it may not be adequate for applications in which pressure variations are common or tight adherence to the drum level setpoint is necessary. To meet these requirements control systems may incorporate compensated measurements of the drum level and steam flow.

Since feed water controls attempt to balance the mass flow of the incoming feed water with that of the out going steam, measurement accuracy may be critical. The mass density of steam is affected by pressure and (in a superheated state) temperature. Hence steam header pressure and temperature measurements can be used to correct the steam, measurement for deviations from normal operating conditions. More accurate indication of the mass flow of steam is there by obtained incorporating drum pressure provides a means to compensate the drum level measurement in the face of deviations from normal operating conditions.

The feed water control system, which consists of the elements shown in figure 2 can be considered independent of the rest of the control system. The other major function of boiler control is to ensure adequate heat to turn the water to steam and maintain the steam supply at a constant pressure regardless of demand.

COMBUSTION CONTROL

As maintained a wide range of fuels may be used in a boiler. Liquids and gases tend to be easier to measure and control than solid fuels. But the control strategies for all fuels are similar in principle. This discussion will take suspension fired liquid and gas fuels as its case in point.

FIGURE 3. Plant header pressure control

All combustion control strategies must begin with a definition of the firing rate demand signal. In a few instances boilers are fired by waste fuels in which case fuel availability often defines the firing rate. Generally however combustion control strategies begin with a definition of the firing rate demand signal on the basis of a measurement of steam demand.

Steam pressure is the most critical process parameter and hence the most critical measured variable for the control system to maintain. Changes in steam pressure usually result from changes in lead demand and the accompanying variations in steam flow. The plant master pressure controller establishes the firing rate demand and provides the set point signal to the boiler master controllers. When steam pressure changes the plant master calls for a total firing rate change. Which the boiler masters distribute to the boilers available to respond accordingly.

The steam pressure measurement is a slow responding parameter since large changes in heat input or output take a long time to be sensed as significant changes in steam pressure. the first indication of an impending pressure change is a change in steam flow hence by using steam flow as feed forward signal to the firing rate demand the control strategy causes corrective changes in firing rate without waiting for significant changes in steam pressure readings.

Boiler masters distribute the change in firing rate demand to the boilers feeding the plant header system. A number of parameters must be considered in designing load distribution priorities. The most common load distribution strategy is operator adjustable bias loading figure3 illustrates a plant master control strategy with steam flow feed forward and operator adjustable boiler master control.

Pressure and thus steam supply are maintained by the transfer of heat to water. Two key factors in this transfer are the firing and burning rates. Hence the secondary controlled variables are fuel and air flow to the boiler (s) servicing the steam demand. For the sake of combustion efficiency, air flow must be controlled to a point at which all fuel is burned and excess air which also decreases combustion efficiency is minimized.

In practice 100% efficient burners do not exist. Variables such as fuel type, burners design. Burner load and operating condition all influence burner effectiveness. Thus some amount of excess air-theoretically unneeded for complete combustion- is required to ensure that all the fuel is burned. At any given time the specific level of excess air required depends on the status of the variables listed above.

If the excess air level is below optimum some fuel is left unburned. This lowers efficiency and increases costs. Unburned fuel also may cause smoking or may build up and create an explosion hazard. If more excess air is supplied than required the additional air is warmed from ambient temperature and expelled from the boiler at the elevated final flue gas temperature. This represents a heat loss that (like unburned fuel losses) reduces overall boiler efficiency.

Under all conditions, however the slope of the heat loss line is more than six times steeper for losses due to unburned fuel than for losses caused by beating excess air. Hence total losses are minimized by erring on the side of excess air. The excess air above the 100% theoretically required level is called percent excess air. Figure 4 contains a graph showing heat loss in dollars scaled to percent theoretical air levels. Scales of this sort will vary based on fuel costs fuel type burner design and operating conditions.

FIGURE 4. COMBUSTION LOSS CHARACTERISTICS

FIGURE 5.BASIC FUEL-AIR CONTROL

To manage the combustion process a control system must meet varying steam demands at a constant steam pressure by controlling heat release over an acceptable range and at optimum efficiency. Figure 5 illustrates the basic control strategy used for air and fuel flow control. The boilers fuel demand as represented by the boiler masters output is used in a conventional flow control loop to remove any disturbances that may occur in the fuel supply boilor to its entry to the burner manifold.

It is equally important to control for proper airflow. To event process disturbances causes by airflow disruptions originating outside the boiler a cascade system must be established. Such an airflow control loop would consist a flow transmitter measuring airflow and sending its signal directly to an air flow controller as the measured variables. The boiler master thus originates the signal used as flow demand; it becomes the set point of the air flow controller in turn, the air flow controllers output goes to an inlet line or a speed control on the forced draft fan, establishing combustion air flow.

Since excess air must be supplied to ensure complete combustion a fuel air ratio multiplier is used to define the process air level. The fuel air ratio, which adjust the air flow controllers set point to desires excess air level, may be manually adjusted by the operator or automatically manipulated through the control system. By means of excess trim control strategies based on flue gas analysis.

One further control refinement required for safety purpose is fuel air cross limiting or lead leg control. This ensure that fluctuations in fuel flow or air flow are accumulated by concomitant changes in the other variable, so potentially explosive quantities of fuel con not build up the boiler when a sudden or marked increase in fuel depend (or a sudden decrese in air demand ) occurs.

The lead lag system consists of signal selectors placed in the set point signals to the fuel and air flow controllers a low signal selector provides the fuel flow setpoint, which will be either the firing rate demand signal as determined by the boiler master or the measured air flow as corrcted by the fuel air ratio controller which ever is lower. Conversely the high signal selector provides an air flow controller set point choosing either the boiler firing rate demand signal or the measured fuel flow whichever is higher. The air flow controller set point then is corrected for the optimum level of excess air. This approach ensures four outcomes:

Fuel demand can never exceed measured air flow. Air demand can never drop below measured fuel flow; Fuel flow lags air flow in responding to load increases; Fuel flow leads air flow in responding to load decreases.

One further improvement in control strategy design involves compensating for the variability in optimum fuel air ratios at different fuel flow rates. (this variability be comes most dramatic as fuel flow becomes low) to do this the excess air level first is determined by measuring the oxygen in the exhaust gas. This signal is used in a controller to provide an output that adjusts the fuel air ratio device for the correct level of excess air an index of load such as steam or fuel flow can adjust the oxygen set point for optimum levels throughout the entire operating range of the boiler.

Since oxygen measurements may be affected by flue gas stratification errors and by air leakage into the furnace duct work, or air heater, the oxygen measured will not always be an accurate index of the excess are at the burner move over poor burner performance resulting from burner degradation or operation at low loads will yield poor fuel air mixing characteristics and cause unburned fuel to be lost up the stack. To circumvent these problems. The control strategy should include a carbon monoxide measurement to indicate incomplete combustion in essence to monitor the effectiveness of the combustion process. The signal form a carbon monoxide analyzer then can be used to trim the oxygen set point according to the on line operating conditions. A basic combustion control strategy incorporating cross-limiting (lead-leg) and advanced fuel air ratio trim is illustrated in figure 6.

FIGURE6. fuel-air cross limiting combustion control with excess air trim

FIGURE 7. Furnace pressure control

A final requirement for boiler control is furnace pressure control. As shown in figure 7 the basic loop consists of a furnace pressure transmitter that provides the input to a controller, which in turn sends a signal to the induced draft fan damper Positioner device. This basic strategy can balance the furnace pressure to its desired set point to make control more stable in the face of varying loads, a feed for ward signal from the forced draft fan damper position may be added to obtain faster response based on changes in air flow.

EFFICIENCY OPTIMIZATION

Because of todays high-energy costs, every opportunity to enhance boiler efficiency should be examined. Combustion control as described in this data sheet will help to assure maximum efficiency for all types of boilers using all types and combinations of fuel, but there is still a limit to a boilers basic capacity for efficient operation. In an effort to enhance overall efficiency, peripheral devices such as air heaters, economizers, continuous boiler blowdown control, and flue gas analysis may be added figure8 illustrates the basic instrumentation requirements of a typical drum-type water tube boiler.

AIR Heater: air heaters take heat from the flue gas and transfer it to the cool air from the forced draft fan to provide preheated combustion air. The flue gas that subsequently leaves the air heater is relatively cool, having given up substantial Btu that otherwise would have been lost up the stack.Economizer: Economizes take heat from flue gas and transfer it to the feed water before it enters the drum causing higher feed water temperatures and lower flue gas temperature.

Economizer: economizers take heat from flue gas and transfer it to the feed water before it enters the drum causing higher feed water temperatures and lower flue gas temperatures.

Boiler Blowdown: As boiler produces steam pure water is evaporated. Any impurities from the feed water stay in the water that remains in the boiler. If these solids are nor removed by draining water form the drum through a pipe (the blow down line) they eventually become so concentrated that scale forms on the inside of the tubes. This scale lowers the rate of heat transfer, reduces boiler efficiency, and can cause tube failures.

A boiler blowdown system can save operating dollars in fuel as well as in chemicals by ensuring that the appropriate quantity of water is blown down, keeping the boiler tubes clean internally. Efficient blowdown reduces chemical expenses for boiler water treatments and minimizes the discharge of heated water.

Excess air control: A variety of measurements such as capacity, carbon monoxide, and oxygen are essential to efficient fuel burning at minimum excess air levels. These gas analysis serve as feedback in various control strategies.

The boiler scheme discussed thus far incorporates the more common strategies and devices used to enhance efficiency. The result is economical boiler operation with a control system that is sophisticated, safe, and more than adequate for most applications.

FIGURE.8 Typical boiler instrumentation

Boilers are highly complicated pieces of equipment that present unique control requirements and unique opportunities to reduce energy costs. Without steam, most processing and power plants could not function. Hence reliable boiler control is one of the key priorities in industrial operations.

Rosemount system 3 control processes the flexibility and reliability to meet present requirements and the expansion capability to meet those of the future a small system could be installed to meet immediate needs and expanded over time. For example:

Phase1: Oxygen carbon monoxide trim on existing analog controls Phase2: Complete direct digital control of the combustion process Phase3: Three-element feed water controls Phase4: Blow down and superheat controls Phase5: monitoring various pressure, flows, and temperature points Phase6: optimizing load Phase7: tie-line management

System3 control capabilityOne of the key priorities in system 3 design was to provide maximum control power with minimum hardware. To ward that end, Rosemount system 3 was built to handle continuous control, advanced calculational and logic requirements, and simple monitoring and alarming functions. A menu-style format allows users to select from a multitude of standard functions in the system3 algorithm library. In addition, users can develop custom control, calculation, and logic functions through standard mathematical notations. This user-configurability allows the solution of sophisticated equations and the development of optimizing control strategies- all tailored to the requirements of the particular boiler operation.

Moreover a high degree of interaction between continuous control, calculational, and logic functions helps to ensure safe operating conditions through such functions as rate limiters, data validity determinations. And the setting of outputs and set points to pre-defined or calculated values. System 3 logic capabilities can even be set to prevent the operator form making unsafe changes in a control loop.

System 3 control can be configured to accommodate various operating conditions, including start-up and shutdown sequences, low fire conditions, safety overrides, and fuel-limited conditions. The system also can accommodate control strategies designed to ensure the overall operating efficiency of the powerhouse, such as oxygen carbon monoxide fuel-air ratio trim, turn down optimization, load leveling, load allocation, and tie-line optimization.

In Rosemount system 3 , all control blocks are identical. Each is able to perform multiple functions, including standard and advanced control and logic functions, as well as control and calculation functions defined by the user through standard mathematical notations. Using information generated within itself by other control blocks can modify a control action or override a control output. This extensive mathematical and logic capability allows a standard system 3 controller to perform that require a supervisory computer in other control systems. The result is a simple, cost effective control system.

FLEXIBLE, RELIABLE HARDWARE:Designed for maximum flexibility, system 3 control can accommodate a broad range of input output mixes and control strategies without forcing users to buy unneeded excess capacity, moreover system hardware is reliable with high integrity components and user-selectable one for one redundancy to minimize the effects of hardware failures. In designing the control file / flex term, Rosemount engineers sought to provide maximum flexibility in defining the input output interface. Each control file, for example can hold as many as eight controllers. The se of two multi loop controllers, one multiplexer and one contract controller actually would provide up to 116 analog inputs, 16 analog outputs and 96 discrete inputs and outputs. In this design moreover all controllers could be made redundant.

This arrangement would be more than adequate to meet the control requirements of two dual fuel fired boilers with the following features carbon monoxide and oxygen excess air trim three element feedwater control with pressure compensation of the steam flow and drum level measurements furnace pressure and superheat control miscellaneous motor and valve control and monitoring and alarming functions. In this scheme, all controllers are redundant, and communication on a highway is not required. Floor space is kept to minimum as well. Since the control cabinet measures only 22 x 36 (front and rear access)BOILER DRUM PRESSURE CORRECTION

FIGURE 1. four-element drum level control

INTRODUCTION:This application data sheet continues the discussion of the boiler drum level transmitter calibration application data sheep (ADS 3055) which describes the initial calibration of a drum level transmitter.

Boilers are highly dynamic and rarely operate at steady state. Changing load conditions variations in fuel quality variations in combustion air quality and physical changes in the boiler heart transfer surfaces represent only a few of the conditions that can cause a boiler to deviate from its expected design performance standards.

Measurement accuracy of the process variable is not always a function of the transmitter applied to the measurement. All too often inaccuracies are blamed on the transmitter when, in fact, the process has changed significantly enough to cause an inaccurate measurement.

There are two acceptable ways to correct for the error. The first is to recalibrate the transmitter to new process conditions. However, this method keeps a technician busy doing little else, and requires the feed water control system to be in manual.

The second method is to correct the measurement error in the control system. This is easy to do and produces quite satisfactory operating results.

SOURCES OF ERROR:

Various in the drum pressure alter the density of the steam and water mixture and thus represent the major cause of error in the drum level measurement. Boilers that are subjected to wide load swings experience swings in the drum pressure, ideally, the combustion control system should respond quickly enough to minimize pressure deviations by altering the firing rate of the boiler. But this can result in constant over firing or under firing of the boiler. Such conditions may cause thermal stresses that shorten the life of boiler components particularly superheaters.

In extreme cases, drum temperature is closely monitored to ensure that boiler loading does not exceed the allowable rate of temperature change with respect to time. Combustion control strategies and drum level control systems incorporate this feature.

MULTI ELEMENT LEVEL CONTROL:

A review of drum level control system operation provides some background for correcting the level measurement. The most common drum level feedwater control system is the three element system. Drum level steam flow and feedwater flow are the measured variables. level measurement corrections add a fourth element drum pressure.

Figure1 shows a SAMA (scientific apparatus manufacturers association) diagram of a common four-element feed water control system. The system uses a cascaded feedforward control strategy.

The process variables consist of drum level, drum pressure, steam flow and feed water flow. The other or master loop functions as the drum level controller. The inner or slave loop functions as the feed water flow control loop.

The setpoint for level is usually a manually set value entered by the operator. The level measurement is compensated for variations in drum pressure.

The output from the drum level controller fixes the setpoint for the feed water controller. The measurement for the feed water controller is difference between steam flow and feed water flow. This difference provides the feed forward index for manipulating the flow controller.

If the system works properly, the feed water flow controller does most of the work, while the drum level controller merely trims the feed water flow to maintain the drum level at the required setpoint.

Some systems incorporate boiler blow down flow as a fifth control element when boiler blow down varies enough to warrant using this measurement.

In the case of boilers with widely varying fuel beating content, such as black liquor recovery units in craft paper mills, a sixth element, black liquor solids flow, is added to the feed water control system. Comparing solids flow against steam flow demand provides a feed forward gain on the drum level measurement that effectively anticipates changes in drum level due variations, and not just steam flow.

TRANSMITTER CALIBRATION

A review of the basic transmitter calibration procedure leads to the final step in determining the corrections for drum pressure variations. Figure 2 shows the process.

FIGURE 2. drum level transmitter calibrationProcess definitions

Po= drum pressure (lb/ in2 absolute)PH=static pressure at the high side of the transmitterPL=static pressure at the low side of the transmitterTo=temperature of the water (0F) in the reference leg (low pressure side of the transmitter)Ts=density of the saturated steam (lb/ft3)TW=density of the saturated water in the drum (lb/ft3)To= density of the water in the reference leg (lb/ft3)H=distance between the high and low drum tapsh=Drum water level (measured from the bottom tap)Vs=Volume of the saturated steam(ft3/lb)Vw=Volume of the saturated water(ft3/lb)

General Equations

Use the following general equations to determine the calibration:PH:The maximum differential pressure occurs at the minimum level(hmin). At this level the transmitter output equals 4ma.Thus,

Ph=(H(Ts-To)+hmin(Tw-Ts))/To

PL: The minimum differential pressure occurs at the minimum level (hmin). At this level, the transmitter out put equals mA thus=(H ( TS TO)-hmax(TW TS))/TO

Example

For this example assume:

H=30 in. (76.2 cm)hmax=21 in. (53.3 cm)hmin=5 in. (12.7 cm)

The desired instrument readout range is 8 to +8 inches.Po = 2,300 psig =2,313 Pisa (at mile-high elevation)To = 1200F (49OC)From saturated steam tables at 2,313 psia:Vs = 0.1512 ft2/lb TS = 6.61 lb/ft3VW = 0.0273 ft3/lb TW =36.63 lb/ft3From saturated water tables at 1200F(490C)VO =0.0162ft/lb TO = 61.73 lb/ft3PH = Pmax = (30(6.61- 61.73)+5(36.63- 6.61))/61.73 = -24.36 inches PPL=Pmin = (30(6.61- 61.73)+21(36.63- 6.61))/61.73 = -16.57 inches PTo determine the instrument span:Pmin- Pmax

(-16.57)- (-24.36)=7.79inches PIf hmax= 30inches and h min = 0 inches, the instrument readoutRange is 15to+15 inches:Pmax = -26.79 inches P (also the ZERO elevation)Pmin = -12.20 inches P to determine the instrument span:Pmin-Pmax:(-12.20) (-26.79)=14.59 inches PThe example above calculates the calibration for the design condition of 2,300psig. Suppose that the actual drum pressure varied from 1,800 to 2,500 psig. What are the effects of the pressure variation on the required transmitter span if hmax =30 inches and hmin = 0inches

Table 1 shows the required transmitter span at increasing drum pressures with all other variables remaining constant.

Pressure span(Psig)(in.)1,80017.452,10015.772,30014.592,40013.962,50013.27

Table 1. Required span at increasing drum pressure

Since the original calibration assumed a pressure of 2,300 psig. Correction factors can be determined by plotting a curve of the ratios of transmitter spans at various pressures various the span at the design condition. Figure 3 plots the correction factors as a function of pressure. The curve can be entered into the function generator in the feed water control system to correct the level measurement.

Figure 3. Correction factors vs. drum pressure

REFERENCES

Application data sheet 3011, level measurement , Rosemount inc.Application data sheet 3055, boiler drum level transmitter calibration, Rosemount inc.