Compensating drum level with drum pressure


Thread Starter


what are the details to compensating drum level with drum pressure, i need formulas, curves, etc or where to find them. i have the general idea of the curves it's just a matter of where to apply them, i have a measurement of 15 to -15 inh2o with 0 to 1500 psig drum pressure

Béla Lipták

In a steam drum application a drop in pressure results in an APPARENT rise in level, because the steam bubbles grow within the boiling water. Therefore, feedforward-based control is preferred, based on substracting the steam
leaving from the feedwater flow entering. Take a look at Figure 8.5tt on page 1059 of the Process Control volume of my handbook (Instrument Engineers' Handbook).

Béla Lipták
The attached is a paper I wrote to present drum level measurement to operators and technicians
> The attached is a paper I wrote to present drum level measurement to operators and technicians Hi, Do you have any literatures on three elements drum level control?Thanks Regards, C.P.Ku
Sorry, I don't have anything at hand that I can e-mail but I can recommend highly "Control of Boilers" by Sam Dukelow published by ISA. This book is about $50 for ISA members and very worthwhile if you are new to boiler controls (even if you are an old hand for that matter. Joe
Bela is correct about the APPARENT rise in drum level. Anyone who has had to load a turbine-generator in a hurry has seen this happen. Unfortunately, that rise is enough to trip a turbine. It's hard (if not impossible) to write control strategies that give stable control for normal changes and even harsh situations, while also allowing for "Complete Insanity".
On our three-pressure HRSG with reheat turbine, we have had (limited) luck with a control scheme that, during start-up, "pulses" the block valve open from the Intermediate Pressure steam drum to the cold reheat of the turbine, while simultaneously closing down on the IP drum vent. (without the closing of the drum vent to "squelch" the loss of pressure, the IP drum goes high). These "pulses" of the block valve are based on seconds of duration. The closing of the vent is based upon the "Pulse Number" of the block valve. The problem is that at SOME point, the IP drum will go HIGH. It might be at "Pulse 4", or it might be at "Pulse 8". The operator doesn't know. And it can result in a high-high level trip.

So...the question remains the same?
How does one anticipate the drum level shift?