COMPENSATION METHOD IN BOILER DRUM LEVEL …

1 COMPENSATION METHOD IN BOILER drum LEVEL MEASUREMENT ANDCONTROLS undarasekaran D1, C21 Research Scholar, Sathyabama University,2 Electronics & control dept., Sathyabama : Industrial BOILER drum LEVEL control loop is to maintain the drum LEVEL within the prescribed limit. Simplyregulating the BOILER Feed Water control Valve regulates the BOILER drum LEVEL . The drum LEVEL is measured byusing LEVEL transmitter. The LEVEL signal is compared with fixed set value of LEVEL controller. Whenever there is asudden rise in the steam flow the drum pressure decreases. Due to decrease in the pressure there is a temporaryrise in the drum LEVEL .

2 This is called as swelling effect. The shrinking effect is opposite to the swelling effect. Toavoid these problems this paper suggests the METHOD of pressure temperature compensating INTRODUCTIONThe main important control loops are drum LevelControl, Combustion control , Steam TemperatureControl and PRDS. The steam flow is measured andthe signal of steam flow is added to LEVEL controller soutput as a feed forward signal to take care of suddenload changes of BOILER . The output of a feed watercontroller is given to feed water control valve toregulate the feed water to maintain the BOILER drumlevel.

3 Conventionally two control valves are provided infeed water line, one for 30% and on for 100%. The 30% valve is used in single element control . Where as100% is used for three-element control . This paperconfined to only drum LEVEL Temp. Pressurecompensated METHOD . Typical BOILER Process Diagramshown in BOILER drum LEVEL MEASUREMENT WITHOUT COMPENSATIONFeed water is fed into the drum is a criticalcontrol, either a series of valves in parallel with a seriesof constant-pressure feed water pumps. If the feedwater LEVEL in the drum goes too high, water canbecome entrained in the steam going to the turbineand can cause problems.

4 If the drum feed water levelgoes too low, the drum itself can become overheated,possibly resulting in water control has two modes of automaticoperation: single- and three-element control . Theoperator sets the drum LEVEL set point for both single-element control the difference between thedrum LEVEL and the drum LEVEL set point provides theerror signal that is used by the single-element controllerto control the rate of water entering the drum bymodulating the feed water flow control valve. TypicalDrum LEVEL Measurement and control with outCompensation shown in control governs the threevariables, or elements, that are used in this controlscheme: drum LEVEL , steam flow, and feed water LEVEL control uses a cascaded controller schemeconsisting of an outer and an inner controller.

5 Steamflow is an indication of the rate at which water is beingremoved from the drum . A function of steam flow is56 International Journal on Intelligent Electronic Systems, Vol. 4, , July 2010 Fig. 1. Typical BOILER Process DiagramFig. 2. drum LEVEL Measurement and control without Compensationused as a feed forward to the outer controller. Theouter controller then operates the drum LEVEL error output of this controller is the feed water flow setpoint. The inner controller then operates the differencebetween this set point and the feed water flow on. Theoutput from this controller is then used to modulate thefeed water flow control control is much more stable androbust than single-element control .

6 The reason that weuse single-element control at all has to do with thenature of the instrumentation. Typically, feed water flow,and occasionally steam flow, is developed by using aflow-measuring device like an orifice plate or a flownozzle, where flow rate is proportional to differentialpressure. However, a problem occurs at low flow rates(low BOILER load), where differential pressures are notas solidly proportional as we would like and thereforeuntrustworthy for BOILER control . Consequently,single-element control is used at low well-tuned drum LEVEL control can be placed inautomatic as soon as a pump is started.

7 By the timesteam flow has passed 25% of the total range, we canconsider steam flow signals to be reliable. That is agood point at which to switch to three-element , tuning for the single-element controllerconsists of big proportional and very small integral gainsettings. Tuning for the three-element controller hassome additional requirements. As in any cascadedloop, it is absolutely crucial that the inner controller betuned as tightly as time will allow. The inner controller,the feed water controller in this case, must have anintegral action that is faster than that of the outer, ordrum LEVEL , controller (Figure 3).

8 This is true for allcascade 3. Typical Controller action GraphA. Rapid ControlA typical oil-fire BOILER with a properly tuned drumlevel control will respond very quickly to a substantialload increase (top) or load decrease (bottom). Thedynamic response of other key variables in BOILER drumlevel control system is also illustrated. Source: TimLeopold. You may notice that as the load decreases,the drum LEVEL sags downward, and as the loadincreases, the drum LEVEL is slightly elevated. Thismeans that the steam flow feed forward is just a tadtoo strong. A minute adjustment to the feed forwardsignal can add stability to the control loopFig.

9 4. drum LEVEL Charactertics for Rapid LoadChangesThe BOILER drum LEVEL responds in an integratingmanner to the steam and feed water flow, but theresponse reverses shown in When the amount ofsteam supplied increases rapidly. When the steam flowincreases rapidly, drum pressure falls and the liquid inthe drum boils 5. Schematic Process Diagram ofThree-Element ControlThe single loop programmable controller for drumlevel control can perform cascade control for drum leveland feed water, reduce fluctuations in feed waterSundarasekaran et al: COMPENSATION METHOD in BOILER drum ..57pressure, improve feed water charactertics and alsoperform three element control - drum LEVEL , feed waterflow and main steam flow-using the main steam flowsignals as a feed forward signal.

10 In addition thiscontroller includes another drum LEVEL control functionthat permits drum LEVEL feed water control to beselected during system start up. Advanced single loopcomputational function enable drum LEVEL Three Element ControlThe drum LEVEL is affected by fluctuations in themain steam flow. Typical three-element control shownin To make the feed water flow rapidly follow themain-steam flow, three-process signals- drum LEVEL , feedwater flow and main steam flow signals are used. Thisis called three element control of deviation signal isgiven byH Fw Fs K (1)WhereFw Feed water signal,Fs Main steam flow signal,K Constant,Lo drum LEVEL control unit output,The equation 1 represents that if a deviationbetween the feed water flow and main steam flowoccurs a corrective action is executed before a drumlevel fluctuatesC.