BOILER DRUM LEVEL CONTROL SYSTEM

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  • Boiler is the heat source of steam for the turbine.
  • In boiler water is a cooling medium in the boiler pressure part by absorbing the the heat. 
  • If too little water is available in the boiler then inadequate cooling of metal parts and chance of damage to the pressure parts.
  • If too much water is supplied to the boiler then steam can carry the water or water particles to the turbine & causes turbine damage.


 In boiler amount of water and levels are controlled by a station called as Feed Regulating Station (FRS) or Feed Control Station (FCS).
  1. In FRS/FCS there are three control valves, which is arranged in parallel. In it two valves can handle 100% BMCR load and one can handle 30% BMCR load.
  2. Two valves of 100% are provided for redundancy.
  3. 30% capacity used for startup and shutdown condition.
  4. Each control valve consist its own motorized isolation valve.
  5. If feed water flow is not controlled by a single valve then any of 100% valve can operate with 30% valve.
  6. Both 100% valve can failed during failure of air supply.
  7. All these valve can operate in manual and auto mode.
  8. Normally only one valve is opened and other are closed.
  9. 30% load control valve closed after main valve start opening.


  • Before the study of boiler level control it is important to know about twot things (a) swellin & (b) Shrinking in boiler drum
  • both are related to boiler drum level variation.
  • Swelling is a transient increase in boiler level which accompanies a rapid increase in steam flow.
  • Shrinking is a transient decrease in boiler level which accompanies a rapid decrease in steam flow.
  • So during load variation on turbine there are swelling and shrinking occur in boiler.
  • If the steam flow from a steam generator increases, the pressure in the steam generator drops.
  • This results in flashing of water to steam in the riser section of the steam generator.
  • This increase in volume in the tube bundle forces water to back up the downcomer and, since downcomer level is what is measured as steam generator level, this results in an up swelling of boiler level which is called swell. 
  • On a down power transient, the pressure in the steam generator increases.
  • This results in a collapsing of steam bubbles in the tube bundle.
  • This decrease in volume in the tube bundle causes a drop in downcomer level which is called shrink.

  • On swelling boiler drum level goes high with out addition of feed water in boiler while shrinking boiler drum level goes low without reduction of feed water.
  • Swelling and shrinking are the function of  rate of change of steam flow in which boiler drum level fluctuates. 
  • Rate of change of steam flow is the function of rate of change of load on turbine.

  • If feed water added to the boiler drum then this cold feed water can collapse the voids in the riser and the level droop and when feed water cuts then level increases.
  • Boiler trip is provided at high & low boiler drum levels. 
  • Turbine trip is also provided at high drum levels. This trip value is lower than boiler drum level trip value.

BOILER DRUM LEVEL CONTROL
In boiler steam continue generated so continue feed water is supplied to the boiler.  Boiler feed pump continue supply feed water to the boiler drum. 
In this condition it is important to maintain the boiler drum levels within specified limit or safe limit.
In boiler drum both water and steam exist.
The purpose of boiler level control system is to maintain the water level at a specified set point in the boiler.
When load changes on turbine the drum level fluctuates.
So it is important to maintain the balance among the feed water flow, main steam flow and boiler drum level.
There are three types of level controller as given
(A) single element controller
(B) Two element controller
(C) Three element controller

When steam taken out from drum the boiler drum level goes down.
When drum level goes to low then automatic signal is generated and transfer to the feed control valves foe level controlling.
The control valve is pneumatic operated.

Details of drum level controlling are as discussed:- 

Single element control level:- 
(1) In single element drum level controlling only drum level is controlling.
(2) when drum level is less than the desired level then signal is generated and transfer to control valve for opening of control valve as feed water goes to the drum.
(3) This type of controller is used in small boiler or when unit run at low load or during startup & shutdown.
(4) In large boiler it is used when boiler load is below 30 % of the MCR of boiler.
(5) If load is suddenly increased on the turbine then pressure in side the boiler decreases and due to swelling drum level increases.
(6) while suddenly load is decreased or load throw off on the turbine then pressure inside boiler increased and due to shrinking effect drum level goes to low.
(7) These type of shrinking and swelling are not computed in single element level controller.
Two element control level:- 
(1) In this controller swelling and shrinking effect is taken in care.
(2) In boiler drum pressure varies according to steam flow.
(3) If load increased steam flow increased and drum pressure drops.
(4) In this system steam flow and drum level is measured.
(5) In it steam flow may be given incorrect feedback if it is not compensated with pressure and temperature of steam.
(6) So in this system steam flow measurement is compensated by. Pressure and temperature.
(7) this type of system is used in medium size of boiler.

Three element control level:- 
(1) Three element control system is widely used in modern thermal power plant.
(2) In this controller can sense any type of variation.
(3) In it feed water flow , steam flow, pressure & temperature of steam is compensated.
(4) In it feed water flow main steam flow and boiler drum level is measured continuously.

Other points regarding boiler drum level:- 
An annunciation is provided foe high level, very high level, low levels and very low level.
Steam flow measurements:- 
Steam flow measurement is used in boiler level control when boiler load is above 30% of BMCR.
At low loads there are inaccuracy and noise are present in the measured value.
There are no  any alarm is provided for steam flow.
Feed water flow measurement
Feed water flow is measured before economizer.
For measurement  measuring element is provided.




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