hope this info helps, found somewhere unkown:
Steam drum level control: Three element system apply to boilers where greater capacities are accompanied by high pressures and temperatures,
and where the drum and other water capacitiy sections of the boiler are relatively small. A change in steam demand will create: considerable
swell on increasing load and vice versa, shrinkage of the drum level on decreasing load. Swell caused by an increase in steam mas flow rate within the water due to an instantaneous reduction of pressure in the drum results in an increase in drum-level controller to call for a reduction in feedwater valve opening. This directly opposes the needs of the boiler. To counteract this effect, the three element system
employs the ratio of steam flow and feedwater flow as the source of principal action. Readjustment of the drum-level controller provides compensation for the steam flow water-impulse, should the actual variation in drum level remain. Steam flow is ratioed to feedwater via proportional action (two input controller).This output signal is one of two inputs to the feedwater controller, its second input is the output of the level controller. The feedwater controller is P+I. In the past, these controlling actions were achieved via controllers designed for boiler control. An important point to remember is the calibration of the level XTR. It must be done based on design operation condition, because the large decease in water SG as temperature increases. For starting from cold, one may equipe the drum level with several XTR's calibrated at various operating temperature or correct for the level reading. This is no problem anaway because the procedure for cold start calls for manual reading of the level through glass gage. Your Bailey system will do the job providing you reproduce the original analog tuning parameters, pertaining to your boiler. You say high pressure, what operating pressure ? Is it constant? Near critical point becomes delicate. We do not enter in that today.
Since drum level us typically measured using DP, if the drum swelled from 50% to 60% when the pressure dropped, the level measured should still be about 50%, as the column of water is taller, but since it has a lower density the pressure on the DP should be the same, still sensing 50%. Now if the lower tap is not at the bottom of the drum, then some of the water that used to be below the tap would rise above the tap, adding to
the level measured. Three element control does not fix this problem, proper location of the tap, or pressure compensation is the way to fix it.
Three element control is helpful, as it allows the level control to react to load changes so that inflow matches outflow as the flow changes, while the PID on level control can be tuned slow - and stable. But this is unrelated to drum swell.
Mark Blunier
Any opinions expressed in this message are not necessarily those of the company.
