I have a SLPC controller for boiler drum level controller and a separate unit called as SPLR, which calculates the corrected steam flow rate (pressure and temp compensation).

The o/p of SPLR i.e. steam flow signal is fed to SLPC controller. The SLPC controller o/p is control signal for controlling feed water flow. I have noticed that steam flow is used as feedforward signal. Presently it is not connected, but i want to know how it will affect drum level, if it is connected.

I can share SLPC and SPLR program if somebody wants more info, mail me: [email protected]

 

Your question is not very clear. At one point you have mentioned that the O/p of SPLR Steam flow)is connected to SLPC (drum level). And at another you have said Steam flow which is feed forward signal is not connected.

But any how to compensate the swelling and sinking effects in a large boiler you need to have 3 element control.

This may be done by using two single loop controllers of Yokogawa. I have scene some consultants like in Seimens they are using the technique with one PID controller in DCS.

 

Maintaining water level in drum one must use ONLY feed FWD logic. Or you will caught in a situation where you will find your drum empty/full with heavy deviation of boiler feed water flow.

Now most probably you will be bothering how could -
When the speed of the turbine increased, it will snatch higher steam from drum & this sudden exit of steam will bring up the drum water level to a higher level (this level is false as its a vacuum type situation) & it will close your feed water valve.

While actually this fictitious water level will come down in some time & your drum will be running out of water at that time your feedwater valve will open but it might be too late.

That's why steam flow calculations are always based on feed forward control so that during false surge of water, PLC is knowing that since steam flow is increasing it must open feed water valve more to compensate & maintain your drum level.

I hope this clarification will solve your problem.If you have not connected your steam flow into ckt than your feedwater valve is very vulnerable to failure as it will go for max opening & max closing. While if you apply feed fwd logic most of the time you will maintain FCV at almost 40-60%.

Regards,
Sandy