drum level control

1
CLOSED LOOP CONTROL

• Ashvani shukla

2
BOILER DRUM LEVEL CONTROL
3
Prs
LT-1
LY-1
LY-1
LY-1
2 00 3
2 00 3
Steam Drum Pressure
Steam Flow Compensation
F
L
AVERAGE DRUL LEVEL(TWO OUT OF THREE) SIGNAL
AVERAGE(TWO OUT OF THREE) MAIN STEAM FLOW SIGNAL
4
2 00 3
1 00 2
2 00 3
Compensation Block
Compensated Feed water Flow
Main Steam Pressure
Main Steam Temperature
AVERAGE TWO OUT OF THREE FEED WATER FLOW
5
R.S.P LIC O/P STEAM FLOW -50 BIAS OF STEAM
FLOW
Q 2
SET POINT 50
Q 3
COMP MAIN STEAM FLOW
PV
FEEDWATER FLOW
PV
Q 1
FIC 1
DRUM LEVEL
LIC-1
OUT PUT SCALED BETWEEN 0 -100 OF FEEDWATER FLOW
R . S .P
1 30 FCV 101
1 100 FCV 102
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Introduction of Closed loop Control/Drum level
Control

• OBJECTIVE-
• Drum is a closed vessel which is filled with
  Treated water during the Normal operation of
  Boiler or it act as a water storage tank for
  boiler tubes. Water is used to Generate the Steam
  and Cooling of water tube for protection from
  Departure Nucleate Boiling. So we have to
  maintain the adequate level of water during the
  normal operation of boiler. In steam drum there
  are some physical phenomenon are happening during
  the load change of boiler. Due to load change
  Pressure goes suddenly up and sudden down So
  level will fluctuate and will not maintain
  properly at desired level. It is called shrink
  and swell effect. When sudden load increase then
  stem flow increases and pressure drops inside the
  drum and level stars swelling vice versa when
  steam flow decreased the pressure will rise then
  and level will goes down called shrink effect. So
  we are not able to maintain the adequate level of
  water level inside the drum. Formerly we are
  using the single element and two element level
  control philosophy to control the level of steam
  drum.

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• But during the sudden load change above
  technologies are failed to maintain the adequate
  water level in steam drum. So due to these
  constrains we are using the three element
  control philosophy for steam drum level control.
• It has some important physical system and
  parameters which are used to control the process.
• 1.FEEDWATER CONTROL
• The BFP Discharge pressure controller is
  responsible for maintaining enough pressure in
  the BFP Discharge Header.
• The objective of the feed water control system is
  to balance the boiler steam and feed water flows
  in proportion to steam out flow to maintain a
  safe water level in the steam drum at any boiler
  load condition

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• Drum level control is typical three-element
  control consisting of drum level, feed water flow
  and main steam flow parameter. It is used to
  maintain drum level by sensing steam load
  variation in main steam flow and by regulating
  Feed water flow through feed water control
  valves.
• The feed water controls valves that both
  typically have 100 capacity can be substitute
  each other with one 30 capacity control valve
  (start up control valve).The feed water start up
  valve controller represents the single element
  portion of the feed water control loop that is
  used during the start up of the boiler. When the
  boiler generates steam load of 30 MCR, the feed
  water control is switched to three element mode
  (1E to 3E switch over is manual) and controls
  shall be done with 100 control valves.
• The control loop is designed in such a way that
  100 CV can be operate in single element by
  Operator selection

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• Feed water Start up Valve Control 30 ( FCV-101)
• The controller objective is to maintain drum
  water level at a required set point during the
  start up of the boiler. A pressure compensated
  drum water level measurement is used as the
  process variable for the controller.
•  
• The feed water start up valve controller
  (LIC-101) compares the drum water level
  measurement to the operator adjustable set point
  and generates an output that sets the position of
  the feed water start up control valve( 30 )
  resulting in the change in feed water flow rate
  and drum water level.
•  
• Controller (LIC-101) action is reverse type.
•  

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• Feed water Main Control Valve (FCV-102)
•  
• The controller objective is to maintain drum
  water level at a required set point during the
  operational load i.e. (gt30) of the boiler. A
  pressure compensated drum water level measurement
  is used as the process variable for the
  controller.
• The feed water main valve controller (LIC-101)
  compares the drum water level measurement to the
  operator adjustable set point and generates an
  output that add or subtract with the (e.g. load
  changes) the balance between the steam and feed
  water flow rates and final value sets the
  position of the feed water Main control valve
  resulting in the change in feed water flow rate
  and drum water level.
•  

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• Drum Level Control
•  
• Drum level is measured using three nos. of level
  transmitters. The 2 out of 3 level signals (
  LT-1,2,3) is compensated for drum pressure
  individually before the 2 out of 3 block. Drum
  pressure is measured with the help of drum
  pressure transmitter.
• As the characteristics of drum pressure Vs drum
  level is non- linear. Drum pressure is passed
  through function generator (for generation of
  non- linear curve) the output of which is
  multiplied with drum level signal to generate
  compensated level signal.
• The values (as per enclosed annexure) for the
  curve are to be fed in PLC.

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Continue.

• For Boiler protection, level switches very high
  and very low will be derived in PLC using level
  transmitters 2 out of 3 logic for high-low alarm
  (LAHH-01 LALL-01) will be generated in PLC,
  very high and very low drum level will be used
  for trip logic.
•  
• The remote water level indication from Hydra step
  will be displayed near by the Feed water flow
  control station.
•  
• The steam drum level controller (LIC-101 )
  objective is to maintain drum water level at a
  required set point during the operating of the
  boiler. If for any reason (e.g. load changes)
  the balance between the steam and feed water flow
  rates is altered, a change in the boilers water
  inventory is affected that is indicated by the
  change in the amount of water stored in the drum.
• So at that point we summate the LIC-101 output
  with steam flow and subtract the bias of steam
  flow to generate the remote set point for
  FIC-101.Depends upon the error generated by
  FIC-101 we control the position of FCV-102 to
  maintain the feed water flow.