Deaerator Pressure and Level Control System in Power Plants

In this article, you will learn about the deaerator pressure and level control system in power plants.

Some General Control Loop Information

1. The Make-Up control loop and valve and the other one is Surplus control loop and valve are two Deaerator level control loops.
2. The set point valve of both makeup and surplus control loops are set to 150 mm and 225 mm, respectively.
3. The actual level of the Deaerator is set to a range of 0 mm to 355 mm.
4. Make-up water is indirectly supplied to the de-aerator i.e. the feed water initially flows through the condenser and condenser level loop and the surplus goes back to the condenser.
5. According to the loop design, the relation between level control error and the valve’s position must be linear.
6. For the level error range of 0 mm to 150 mm the position of the corresponding makeup valve must lie between 0% to 100%.
7. But the controlling action must initialize only when the deaerator level falls below 150 mm.
8. The makeup water valve must get opened fully when the level reaches 0 mm.
9. The surplus valve position must lie between 0% to 100% to maintain a range of 225 mm to 310 mm.
10. Here in this case the controlling action must initialize only when the deaerator level rises above 225 mm.
11. The surplus valve must get opened fully when the level reaches 310 mm.
12. Note that both make-up control loop valves and surplus control loop valves must be in a closed position when the level lies between 150 mm and 225 mm.
13. Due to the above requirements, both these makeup and surplus control loops are implemented in Distributed Control Systems DCS using Proportional (P) Controllers.
14. The gain for the makeup control loop is – 2.5 and for the surplus control loop is + 4.17

Deaerator Level Control

In this level control technique

• For each unit, the two condensers are provided to operate in parallel but the same hot well level must be maintained at both the condensers.
• Interlock actions must be ruled by the signals generated from control instrumentation provided on these condensers.
• In low level three level switches LSW-401, LSW-402 and LSW-403 are provided in 2 out of 3 (2oo3) logic for annunciation & BFP trip.
• In addition to three level switches LSW-401, LSW-402, and LSW-403 three electronic level transmitters LT-401, LT-402, and LT-403 are used in 2 out of 3 (2oo3) logic for controlling, alarm & interlocks.
• These 2 out of 3 (2oo3) signals can be used to control the storage level of feed water tanks under Distributed Digital Control.
• Monitoring of limit value is done through (DDCMIS) Digital Distributed control Monitoring and Information System on the 2 out of 3 signals to generate alarm, interlocks & controls for Low level (LL), High level (HL), & High High-level (HHL).
• The Deaerator level in normal low level is maintained with (3E) three-element control circuit by regulating the control valves CV 401 or CV 402 on the main condensate line to De-Aerator with a selection facility from UCB which is mentioned separately in auto control schemes.
• In the 3E control circuit the feed water signal forms.
• one is Feed water flow
• second is Condensate flow
• third is Deaerator flow
• The feed water control signal predicts the change in the Deaerator tank level and acts through the controller, and positions the condensate flow control valve CDV-401 to the new position.
• The extraction steam flow, and condensate flow signal along with drain flow of HP heaters to the Deaerator is a feedback signal in the control circuit which shall confirm that In case the level reaches the high-level set point, an alarm is given in the control room.
• High level: When the Deaerator level raises to 100 mm above the High-Level set point, the Deaerator overflow control valve DRV-48 is opened to dump out excess condensate from Deaerator to LP drain the flash tank
• This overflow valve is now closed when the level falls 100 mm below the high-level set point.
• The maximum passing capacity of the Deaerator overflow control valve DRV-48 should be 10% (BMCR) Boiler Maximum Continuous Rating.

Deaerator Pressure Control

• Variable pressure operation of the Deaerator has been predicted in the cycle.
• Normally the steam to the Deaerator is supplied from turbine extraction.
• The Deaerator pressure depends upon turbine extraction pressure from 3.5 Kg/cm² at a load of 55% to 6.8 Kg/cm² at a load of 100% approximately.
• The Deaerator must have a minimum pressure of 3.5 Kg/cm² by pegging steam from turbine extraction.
• At lower load conditions under turbine bypass operation when turbine extraction pressure is less than 3.5 Kg/cm² the steam to Deaerator is supplied from Cold Reheat Line and pressure in Deaerator is maintained at 3.5 Kg/cm².
• When turbine extraction pressure falls below 3.5 Kg/cm² the steam to Deaerator can be supplied from a low-temperature auxiliary steam header from where steam to the Deaerator is supplied during the start-up of the boiler.
• There are 2 pegging control valves, one is on the Cold Reheat Line and another one is on the Auxiliary steam line.
• The pegging control valves are modulated by two pressure transmitters using (1002) 1 out of 2 logic to maintain the Deaerator pressure to 3.5 Kg/cm²

Gap Level in Deaerator

1. The gap level in Deaerator is controlled using two proportional controllers.
2. The set points are located at the center of the controller throttling ranges.
3. The Deaerator level follows the operating lines as a function of load.
4. The set point value for LIC- 1 and LIC -2 must be maintained to 71 mm 1 and 258 mm to obtain these operating lines.
5. The makeup valve must have a Fail Open or open failure position meaning that the controller must be the reverse acting type with negative gain.
6. The bias (b) must be adjusted to 50 % when only (P) Proportional controller is turned on.
7. This makes the valve opening for 50% to provide normal flow such that the level meets the set point value.
8. If the load water flows shift away from 50 % the P controller is unable to alter the flow to return the level to the set point.
9. The error signal (e) is required for the P controller to change the output signal (m) when the error (e) becomes zero the output signal “m” equals the 50% bias (b).
10. The output is calculated by relation m = e + b.
11. When the load changes an error called offset must develop.

Note: The process parameters and operating values may depend on your plant design. It may vary from plant to plant.