APPLICATION NOTE

1 CCYS-G-05A03-99

OVERVIEW

Steam drum level control isnecessary to add makeup water assteam is delivered into the headerand to the associated processequipment. The system shouldcontrol the drum level at a specificset point while compensating forvarying steam demands and drumpressures. For a given volume ofsteam and blowdown leaving thesteam drum, an equal amount ofwater should replace that inventory.Discussions here include the primarysensing devices and the variousstrategies used to control steamdrum level and feedwater rate. TheYS1700 Dual Loop ProgrammableLoop Controller can perform any ofthese strategies. Multipleanalog/discrete inputs and outputsallow maximum versatility within onecontrol device.

LEVEL MEASUREMENT

The drum level is measured using adifferential pressure transmitter.Refer to the drawing on the right.The output of the instrumentincreases as the differential pressuredecreases. A typical range is 30inches of water column. Acondensing reservoir is installed toallow the high side of the D/P tomeasure the steam pressure plusthe hydraulic pressure in thereference leg. The low pressureside senses the boiler drumpressure, the weight of the waterabove the low pressure tap and theweight of a column of saturatedsteam from the high pressure tap tothe water level. Being a differentialpressure device, the boiler drumpressure is canceled out of themeasurement, leaving only thewater column pressure difference.

The level measurement is accurateonly at a single drum pressure. Ifneeded, a pressure measurementcan compensate for varying drumpressures by applying a gain andbias to the drum level signal.

SWELL & SHRINK

Under steady-state conditions, bothwater and steam bubbles residebelow the water surface. Theaverage mixture density is constant.Should steam demand increase, thesteam bubbles expand under thewater surface, increasing theaverage mixture density. Thiscauses an increase in steam drumlevel without the addition offeedwater. This increase in levelproportional to an increasedsteaming rate and decreased drumpressure is called swell.

Inversely, as the steam loaddecreases, the steam bubbles in thesteam/water mixture decrease insize and volume. This causes adecrease in drum level, although themass of water and steam has notchanged. This phenomenon iscalled shrink.

These apparent changes in drumlevel can be compensated for byimplementing a pressure transmitterand drum pressure correctionfactors as a part of the feedwatercontrol strategy. The YS1700controller can accept this input andprovide the necessary corrections.These natural occurrences are notpreventable, but a properly designedcontrol system can minimize theadverse effects.

SINGLE ELEMENTCONTROL

A single element control strategyuses the drum level signal as theprocess variable. After performing aP+I computation on the leveldeviation from set point, the outputof the controller is sent to thefeedwater flow control valve. Thisconfiguration is used on small boilerswith a relatively large water volumeand steady loads.

TWO ELEMENT CONTROL

In addition to the drum levelmeasurement, a two element designuses a steam flow transmitter as afeedforward input to the feedwatercontroller. This feedforward plusfeedback configuration allows thecontroller to anticipate a need foradditional feedwater flow before thisis sensed as a lowered drum level.The steam flow signal ischaracterized for gain and applied toa summer modifying the levelcontroller output. The SAMAdiagram on the following pageshows the two element configurationwith an optional drum pressure input.This design is typically used on smallto medium boilers with moderatechanges in steam demand.

Condensate Pot

Steam

Water

H

LT

L

Reference Leg

YS1700 Steam Drum Level Control

APPLICATION NOTE

2 CCYS-G-05A03-99

The effects of shrinking and swellingare minimized in this design.However, a disadvantage is the lackof compensation for varyingfeedwater pressure. This problem iscommon in multi-boiler installationswith a common feedwater system.These pressure variations changethe material balance between steamflow and feedwater flow. If variablefeedwater pressure is a problem, athree element control configurationcan correct the difficulty.

THREE ELEMENT CONTROL

In addition to adding a feedwaterflow input, two P+I controls are usedin a cascade configuration. Refer tothe SAMA drawing at the right. Thecomputed output of the levelcontroller is software linked to theremote set point of the feedwatercontroller. The slow reacting levelcontrol is usually tuned with amoderate proportional band settingand a long integral time.

As with two element control, thesteam flow measurement is used asa feedforward input to anticipate

steam demand changes. Adding thefast acting feedwater controller allowquicker reactions to load changes.Feedwater pressure variations donot adversely affect the controlaction, since the feedwater flowrateis metered.

The YS1700 controller can beprogrammed to perform threeelement control with optionalpressure compensation, all in oneinstrument. Up to five analog inputsand three analog outputs areavailable. Up to six discreteinputs/outputs is standard. Thecascade control strategy is astandard arrangement that can beeasily implemented.

YS1700 DISPLAY FEATURES

The YS1700 Dual Loop Controller hasa high resolution backlit LCD displaywith LOOP and TREND screens. Ofspecial interest is the TRENDdisplay, allowing operator to observethe level and feedwater rates over aselectable time base.

SUMMARY

The YS1700 Dual LoopProgrammable Controller is a costeffective means of implementing anyof the boiler feedwater controlstrategies discussed here. Themultiple input/output capability, mathand characterization functions anddual P+I+D computation ability allowmaximum versatility. An optionaldigital communications card permitsYS1700 controllers to be integratedinto a PC-based boiler controlsystem. Third party Windows-basedgraphical software packages areavailable to allow the boiler operatoraccess to the YS1700 control modesand all operations can be performedfrom the PC.

KFeedforward Gain

LT

√

Drum Level

FT

Steam Flow

K ∫

A T A

Σ

Valve

X

PT

Drum Pressure (Optional)

f(x)

K FeedforwardGain

LT

√

Drum Level

FT

Steam Flow

FT

√

Feedwater Flow

K ∫

A T A

Σ

K ∫

Valve

Set Point

X

PT

Drum Pressure

f(x)