Density Error and its Correctionin Boiler Drum Level Indication

INTRODUCTIONBoiler steam drum water level is one ofthe most important power plant para-meters to measure and control. If thelevel is too low, boiler tubes will bedamaged by overheating. If the level istoo high, the superheater tubes andthe turbine may be damaged by mois-ture or water treatment chemical car-ryover. As boiler operating pressuresand boiler drum wall thickness haveincreased, many boilers have becomesmaller. This reduced boiler drum vol-ume demands even more accuratelevel control.

A variety of instruments are availableand approved by the ASME for powerboiler level indication. However, simplyspecifying one or more of these instru-ments will not guarantee that the boil-er level will be indicated accurately.The user must thoroughly understandthe operating principles of each instru-ment, instrument installation require-ments and the boiler operatingscheme.

A difference in the water densitybetween the level instrument and theboiler is the major source of level error.The purpose of this evaluation was to

© 1996 Yarway Corporation

Presented at the InternationalSociety for Measurement andControl (ISA) Conference,October, 1995

ABSTRACTThe requirements for power boiler level indication are clearly stated in the ASME Boiler and Pressure Code SectionI, Paragraph 60.1.1. In addition to the direct reading gage glass, remote liquid level indicators of various types arepermitted under certain design conditions.

Regardless of the instrument selected, consideration must be given to the level error caused by connecting anexternal device to the boiler drum. The error is inherent because the level measuring instrument is at a lower tem-perature than the boiler.

Various mechanical and electronic solutions are available to minimize the level indication error. The purpose ofthis evaluation was to determine the actual level error under various laboratory conditions for three types of waterlevel gages and for an electronic level gage column. Lab measurements were compared with both design theoryand field observations. All this information was then used together to improve product design, application andinstallation procedures that would minimize boiler level indication error.

determine the actual level error of sev-eral instruments under various con-trolled laboratory conditions. Thesemeasurements would then be com-pared with both design theory andfield observations to develop installa-tion requirements and an understand-ing of specific application limitations.

LEVEL INDICATIONREQUIREMENTSThe requirements for boiler level indi-cation are clearly stated in the ASMEBoiler and Pressure Vessel Code,Section I, Paragraph 60.1.1. A combi-nation of gage glasses and remotelevel indicators are required as follows:

Each boiler (except forced flowsteam generators with no fixed steamand waterline, and high temperaturewater boilers of the forced circulationtype that have no steam and waterline)shall have at least one water gageglass. (Figure 1a).

Boilers operated at pressures over400 psi (except electric boilers of theelectrode type) shall have two watergage glasses (Figure 1b).

Boilers with safety valves set at orabove 400 psi may operate two inde-pendent remote level indicators

instead of one of the two requiredgage glasses (Figure 1c). When bothremote level indicators are in reliableoperation, the remaining gage glassmay be shut off, but shall be main-tained in serviceable condition. (CodeCase 2109)

REMOTE LEVELINDICATORS Several devices have been developedover the years to take advantage ofthe option offered by the Boiler Codefor two independent remote level indi-cators instead of one of the tworequired gage glasses. These devicesinclude:

1. Differential Pressure Indicator.These are available in both themechanical and electronic type(Figure 2). Both measure the differ-ential pressure between a refer-ence water level and the boilerlevel.

2. Electronic Water Level Gage(Figure 3). This device measureswater level through conductivityprobes installed in a water columnconnected to the boiler.

DENSITY LEVEL ERRORWater level gages and all types ofremote level indicators are affected bydensity level error. This level measure-ment error occurs because the watermeasured by the device is colder thanthe boiler water, creating a density dif-ference. This system can be explainedand modeled by a U Tube Manometer(Figure 4).

The manometer contains fluid withthree different densities. On one side issaturated water with boiler water den-sity Dd. On the other side are two flu-ids: water in the gage with anunknown density Dg, and saturatedSteam with density Ds.

A pressure balance at point “A” in thissystem produces the following rela-tionships:

Equation 1.

HdDd = HgDg + HsDs

Equation 2.

Hs = Hd - Hg

Equation 3.

Hg =Hd(Dd - Ds)

Dg - Ds

Equation 4.

Hd =Hg(Dg - Ds)

Dd - Ds

The use of these formulas to deter-mine the actual drum water level, Hd,can be explained with the followingexample.

Example:

Boiler operating pressure 3000 psia.

Average gage temperature is 20°Fbelow saturated steam temperature.

Hg, water level in the gage is 20 inch-es above the water connection.

Using the steam tables:

Ds = 11.90 lbm/ft3

Dd = 31.94 lbm/ft3 (Assume theaverage water temperaturein the drum is 10°F belowsaturation)

Dg = 33.89 lbm/ft3

Hd = 20 inches(33.89 - 11.90)

31.94 - 11.90

Hd = 22.0 inches, or a 10% error.

The factors that determine the magni-tude of the level error are the operating

Figure 1

ASME BOILER AND PRESSURE VESSELCODE REQUIREMENTS

2

REMOTE LEVEL INDICATOR—DIFFERENTIAL PRESSURE

Figure 2

ELECTRONIC WATER LEVEL GAGE

Figure 3

Mechanical Electronic

3

LEVEL GAGE AND “U” TUBE EQUIVALENT

Figure 4

GAGE OPERATING PRESSURE AND SUBCOOLING EFFECT ON LEVEL ERROR

Figure 5

HdDd = HgDg + HsDs

4

LEVEL GAGE WATER FLOW DYNAMICS

Figure 6

pressure, the gage subcooling, andthe boiler drum level above the gagereturn connection. Figure 5 shows theeffect of operating pressure and gagesubcooling on gage error. As pressureincreases, the density of saturated liq-uid and saturated steam converge(Figure 5a). As gage subcoolingincreases, the level error increases(Figure 5b). As the drum level Hdincreases, the level error increasesproportionally. Hd can be minimized byreturning the water pipe from the gagehorizontlly to the drum, and by design-ing the drum water connection to be aminimum distance below the lowestgage view port.

However, a significant questionremains unanswered. What is theaverage gage temperature and corre-sponding density Dg?

LEVEL GAGE DYNAMICSStandard level gages and electroniclevel gages connected to the boiler arenot in a static condition. If they were,the water temperature in the gagewould be near ambient. Referring toFigure 6, steam condenses in the sup-ply piping and upper gage to the gagewaterline. This saturated water raisesthe gage water level slightly above theequilibrium point. The excess waterflows back into the boiler drum. Thecirculation flow depends upon thecondensing steam rate and the vol-ume in the gage and water return pip-ing.

The condensing steam enters thegage at the saturation temperature.The condensate continues to cooluntil it re-enters the boiler. A profile ofthis temperature gradient is shown inFigure 6.

Many level gages are connected tothe boiler with a “tie bar” or “water col-umn” because the visibility of the gagemay be adversely affected by exces-sive water flowing over the glass. Thetie bar short circuits the excess waterfrom the steam supply piping. Theonly flow through the gage is fromsteam condensing in the gage itselfand the gage piping.

Gages and remote level indicationdevices will indicate incorrectly if boilerpressure decreases suddenly. Thetemperature of the water in the gageor reference column is normally withina hundred degrees of the saturationtemperature. If the boiler pressureshould decrease rapidly, the water inthe gage will flash to steam. Thesteam bubbles will cause the level in

GAGE WATER TEMPERATURE PROFILE

the glass to rise and will often obliter-ate the meniscus. Indication will returnto normal when thermal equilibrium isre-established. Systems utilizing a ref-erence column will indicate high aswater in the reference leg flashes tosteam. Indication will return to normalvery slowly as steam condenses torefill the reference leg. The referenceleg can also be manually filled toestablish proper indication morequickly.

GAGE CORRECTIONOPTIONSEvery level measurement device mustbe corrected in some manner to com-pensate for density level error. A num-ber of techniques have evolved overthe years to address each indivicualdevice.

1. Differential Pressure Indicator,mechanical type (Figure 2).Thedevice is installed with a tempera-

ture compensated reference col-umn. The purpose of this equip-ment is to heat the water in the ref-erence leg to a value that is halfwaybetween saturation and ambient.This corrects the indication if thelevel is at the mid scale position,regardless of pressure. An addi-tional pressure sensing device isadded to correct the pointer forwater levels that are above orbelow the mid scale value.

2. Differential Pressure Indicator, elec-tronic type (Figure 2). The mostsophisticated of the devices mea-sures the differential pressure ofthe reference and variable legs withboth at ambient temperature. Theambient temperature and the boil-er operating pressure are alsomeasured. An analog or digital pro-gram then corrects the apparentdifferential for boiler water densityto indicate the boiler level.

5

Less sophisticated devices are cal-ibrated to read correctly at onlyone pressure, usually the normaloperating pressure. They will indi-cate improperly when operated atpressures above or below the cali-bration pressure.

3. Water Level Gages, direct readingand electronic type. As discussedearlier, the primary difficulty in cal-culating the level error for devicesof this type is to determine theaverage gage water temperatureand corresponding water density.When this is calculated or mea-sured, then the gage glass is phys-ically installed in a lower position tocompensate for this error. Steamheated level gages are availablethat significantly reduce the densityerror. But these may be especiallysusceptible to boiler pressure tran-sients. The electronic gage can becorrected by locating each probeindividually to correct for the errorat that level.

DENSITY LEVEL ERRORTEST PROGRAM ANDTEST RESULTSA test apparatus was constructed inthe Yarway Test Lab, shown in Figure7, to measure the level error and gagetemperature of various gages under avariety of conditions. Probes of thesame type used in the electronic gagewere installed with 1/2" spacing in areference level column. The water inthe reference column was maintainedwithin 10°F of saturation temperature.The level was maintained at a knownpoint by throttling control valve V1. Thegage under test was connected to thereference column with typical pipingand valves.

Tests on various devices were per-formed with a variety of connectingpipe sizes, pipe lengths, operatingpressures and insulation conditions.Since most of this information is pro-prietary in nature, only general charac-teristics will be discussed.

Figure 8 shows the level error andtemperature gradient for a typicalgage as a function of operating pres-sure. As expected, the level errorincreases with operating pressure.However, the temperature differencebetween saturation and the water out-let remained relatively constant. This isbecause the steam condensing rateincreases significantly as pressureincreases. Performance graphs weredeveloped for each gage and configu-ration evaluated. Performancechanges related to installation aresummarized in Table I.Figure 8 shows the effect of connect-ing a standard level gage to an elec-tronic level gage in two different man-ners. In the first case the gage lowerconnection is attached 4" above thewater return connection. In the sec-ond case it is attached at the sameheight as the water return connection.Connection in the second mannercauses minimal change to the normaltemperature profile of the electronicwater gage.

WATER LEVEL GAGE TEST SETUP

Figure 7

6

Configuration Gage Temperature Level Error

Gage Insulated Increases Decreases

* Steam supply pipe diameter larger Increases Decreases

* Steam supply pipe length longer Increases Decreases

Gage water level lower Increases Decreases

* Only applies to the electronic gage column since all standard gages were tested with a “tie bar” installed.

TYPICAL GAGE LEVEL ERROR AND AVERAGE TEMPERATURE

Figure 8

GAGE CONNECTION EFFECT ON ELECTRONIC LEVEL GAGE PERFORMANCE

7

TABLE I

180 6M 396 Printed in U.S.A.

CONCLUSIONSThe water level gage glass has beenused for many years to indicate boilerwater level and it is required by theASME Boiler and Pressure VesselCode. A number of supplementaryindicators are available that provideconvenient remote indication and levelcontrol. All of these devices are sub-ject to indication error because thewater measured is external to thedrum and is not at the same densityas the boiler drum water.

A number of techniques have beendeveloped for each indicating deviceto address this problem. The elec-tronic dp indicator can be adjusted byan analog or digital program. Themechanical dp indicator uses a tem-perature compensated referencechamber with a pressure compensa-tion device. The standard level gageand the electronic level gage can becorrected by measuring or calculatingthe average gage water temperatureand installing the gage in the properlocation.

Testing confirms that the gage tem-perature is affected by a large numberof conditions: steam supply and returnpipe size, length and configuration;insulation, ambient conditions andwater level. Maintaining the gagewater temperature very close to thesaturation temperature will have anadverse effect on indication if the pres-sure drops suddenly. However, if thegage temperature is significantly lessthan the boiler water temperature, thelevel error will be significant at highpressure.

The best compromise is to deter-mine the average gage temperatureand the gage level error that will existunder certain boiler operating condi-tions. Then, the gage installation posi-tion can be corrected accordingly. Forthe electronic level gage, probes canbe located individually to indicateproperly. For the visual level gage,repositioning the unit can compensatefor the density error. It is vital that thegage installation follow the manufac-turer’s specifications. Only then will thelevel device indicate accurately andperform as designed and tested.

REFERENCESWilliams, James

“Methods of Obtaining AccurateBoiler Water Level Indication,”ASME Power Division Winter AnnualMeeting, New York, NY, December,1960

Spencer, J. D.“Design Considerations for anElectronic Water Gage,” FossilPower Systems Technical Publica-tion 1988.

“Boiler Drum Level Indication in HighPressure Boilers,” Yarway TechnicalPublication, November, 1992

Spencer, J. D.“Density Error Considerations forSteam Drum Water Gage Systems,”Nova Scotia Power Corp., NovaScotia, Canada, June 1991.

“Boiler Drum Pressure Correction,”Rosemount application data sheet3084, August 1990.

Y A R W A Y C O R P O R A T I O NA SUBSIDIARY OF KEYSTONE INTERNATIONAL, INC.

480 Norristown Road • P.O. Box 350 • Blue Bell, PA 19422 • Phone 610-825-2100 • Fax 610-825-1235

North American Regional Offices: Eastern 800-523-6248 • Southeast 800-561-5816Southwest 800-561-5819 • Midwest 800-561-5814 • Western 800-561-5813 • Canada 519-824-8500

Inquiries from other than North American Locations should be addressed to the International Department, Blue Bell, PA