3E DRUM LEVEL CONTROL & COMPENSATION

The aim of drum level control is to maintain the drum level at the centre line of the boiler drum.

The 3 element in this loop are the drum level, the steam flow & the feed water flow. The level is measured by two independents DPT.The stream drum pressure is measured using pressure transmitter PT. The two level signals are corrected for density using the drum pressure signal before the same is used for indication, control & high, low alarm generation.

The density compensation is done by the formula-

Actual Steam Flow = Indicated steam flow * √ {(P1+1.029)*(T2+273)} / {(P2 +1.029) *(T1+273)} …………….1

Where P1 is measured pressure signal & T1 is measured temp signal, P2 = 89kg/cm2, T2 = 515 Deg C (Designed constant).

For very low trip generation first the constants are generated from individual transmitter & then two out of two is performed on the three density compensated level signal and the output is used as PV from the drum level controller.

In case 3E the controller LIC output goes to feed forward summation block where the steam flow is added to it & generates the remote set point flow controller FIC.

Steam flow measured using steam flow transmitter FT-1 & FT-2 , two out of two is performed, whish acts as a feed-forward signals & takes care of shrink & swell effects. the steam flow transmitter are connected across flow nozzle which is square rooted and then pressure & temperature compensation done.

This compensation takes care of error arising due to density variation of stream when boiler is operating at conditions different from design conditions.

For drum level compensation by modifying eqn 1

Hm + DP + h (ra - rs) / (rw - rs)

Hm = corrected drum level, DP = diff pressure measured by level transmitters, ra= wet led density (0.992), rs = saturated steam density(refer steam table) , rw = saturated water density , H =  wet leg height (h=H)

PRESSURE IN KG/CM2

rw

rs

45.040

0.7910

0.0217

48.9

0.784

0.0237

……..

………

……..

88.644

0.7145

0.04623

94.990

0.07019

0.05225

101.68

0.6984

0.05461

                                                        

Note - Hope I shall able to share more on PPIC,next to come – steam temp control , furnace draft , combustion control , CBD tank level control , deareator level , flue gas outlet temp control etc…….keep on visiting.