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.

Boiler three element understanding.

Hello Friend...

Many times, when i look for instrumentation questions,  i frequently came across this topic.
I have worked in power plant and have also developed logic and scada for 10MW biomass power plant.
You can find many articles online regarding this but here I'll show you actual implementation.


In this control philosophy, there are three process variables.
1. Boiler Level,
2. Feed water flow and
3. Steam Flow
to control boiler Drum Level.

Understanding of diagram :

Here, LT1,LT2 and LT3 are three different Level transmitter. reason for using three level transmitters is simple that, in case of failure of any transmitter(s), control wont be affected. LT is average of three LTs.
Water density changes with pressure. So density compensation is there for every level transmitter.

LIC is first PID block with LT as process variable.

FT1 is the steam flow leaving the steam drum. Here we have done pressure and temperature correction.

Output of LIC and FT1 goes to one calculation block. Output of this block is our remote set point for Flow controller (FIC).

FT2 is feed water flow to the boiler drum and process variable for FIC.

SS is selector switch. By this controlling philosophy can be selected either single or three element.

FCV is feed flow control valve.


Control:

Single Element:
During lower boiler loads or <30% steam flow, drum level signal LT and the fixed local set point LSP are
compared in LIC and the controller output is fed to feed water control valve FCV

Three Element:
The steam flow signal sensed by the steam flow transmitter FT1 acts as a feed forward signal and takes care of the shrink & swell effect.The steam flow transmitter is connected across flow nozzle, and the signal is then compensated for pressure and temperature.
LIC is the primary controller in the three element level control function. When the steam drum water level is below the set point, controller LIC will further increase the remote set point of the feed water flow controller to increase the feed water flow. When the level is too high the reverse action will take place.

The Level controller LIC output signal is added with the compensated steam flow signal at calculation block.

The following equation is implemented in summing block
Remote SP for (FIC) % = (LIC) O/P + Steam Flow (FT1) PV in % - 50%

FIC is the secondary controller in the three element level control. When the feed water flow is below the set point, controller FIC will further increase the feed water flow by opening the feed water control valve. When  the flow is too high the reverse action will take place.


I'll give overview about few other points like Density compensation for level, Temperature and Pressure compensation for flow, shrink and swell effects in my upcoming blogs.