Density Compensation for level Measurement in Boiler Drum

Requirement of Accurate measurement:

The drum level must be controlled to the limits specified by the boiler manufacturer. If the drum level does not stay within these limits, there may be water carryover. If the level exceeds the limits, boiler water carryover into the superheater or the turbine may cause damage resulting in extensive maintenance costs or outages of either the turbine or the boiler. If the level is low, overheating of the water wall tubes may cause tube ruptures and serious accidents, resulting in expensive repairs, downtime, and injury or death to personnel. ( Reference : InTech Magazine by ISA, July 2010).

When a differential pressure transmitter is used to measure drum level and the instruments used are sensitive to density variation, density compensation techniques must be employed.


Why Density Compensation ?

Here we are measuring level using Differential Pressure (DP) transmitter with two different densities (of Water and Steam). Moreover density also changes with different pressure in drum. So density compensation is required for accurate level measurement.

Refer to below diagram.

Here,
Hm --Compensated drum level.
Dw --Density of water in gm/cm3
Da --Density of water at ambient temperature (34°C) = 0.994 gm/cm3.
(For water in Condensate pot)
Ds --Density of Steam in gm/cm3
H --Distance between tapping points.

So,
Head acting on LP side = H* Da     and
Head acting on HP side = Hm* Dw + (H-Hm)*Ds

Therefore,
Delta P = HP-LP = Hm*Dw + (H-Hm)*Ds–H*Da

So Level Hm = {(Delta P)+H (Da-Ds)}/(Dw-Ds)

Delta P is the differential pressure value from drum level transmitters.

The values of Dw and Ds will be taken from following look-up table depending upon the current drum pressure signal.




So, this way more accurate level can be measured with changing densities.