Boiler Feedwater System
S Shariq Ahmed
Asst. General Manager
Function
•
Boiler feedwater
is water used to supply ("feed") a boiler to generate
steam or hot water. At thermal power stations the feedwater is usually
stored, pre-heated and conditioned in a feedwater tank and forwarded
into the boiler by a boiler feedwater pump
•
The function of the
Feedwater System
is to supply high-pressure water to
the boiler during start-up, normal, and emergency operations.
•
The System automatically maintains the proper flow to and water level in
the boiler drum.
Function
•
The Feedwater System also supplies water for
Desuperheater Sprays
that
control the Superheat steam temperature.
•
The feed water used in the steam boiler is a means of transferring heat
energy from the burning fuel to the mechanical energy of the spinning
steam turbine.
•
The total feed water consists of
re-circulated condensate water
and
purified makeup water.
•
The makeup water in a
500 MW
plant amounts to perhaps
20 US gallons
per minute
(1.25 L/s) to offset the small losses from steam leaks in the
Basic System Description
•
Feedwater is supplied by the Deaerators, which
also provides the necessary
Net Positive Suction
Head
(NPSH) to the Boiler Feed Pumps.
•
The
Boiler Feed Pumps
(BFP) must supply a
constant flow necessary to replace water in the
boilers that has been changed to steam.
•
The Boiler Feed Pumps (BFP) must also develop
the required pressure to overcome head and
drum pressure.
Deaerators
Boiler Feed Pumps
Feedwater Heater
Feed Water Regulation Valves
Boiler Economizer Inlets
Feedwater System Block Diagram
Basic System Description
•
Boiler Feed Pumps
on Units supply the
Feedwater System High Pressure Heaters and
Header.
•
The
Feedwater header
supplies heated,
high-pressure water, through level control valves, to
the boiler drums on Units
•
The Feedwater Header supplies Desuperheater
Sprays to control the steam temperature.
Deaerators
Boiler Feed Pumps
Feedwater Heater
Feed Water Regulation Valves
Boiler Economizer Inlets
System Flow Path
•
The Feedwater System starts at the
bottom of the
Deaerator
.
•
The Feedwater exits the bottom of the
Deaerator to the suction of the Boiler
Feed Pumps
(BFP).
•
The
Deaerators
are high enough to
supply the Net Positive Suction Head
(NPSH) to the Boiler Feed Pumps.
System Flow Path
•
Boiler Feed Pumps
, are motor driven and
turbine driven pump.
•
The pumps can provide Feedwater to any
of the
High Pressure
(HP) Feedwater
Heaters.
•
After the Feedwater is heated in the HP
heaters it is then sent through drum
level
control valves
on the boilers and from
the drum level control valves to the
System Flow Path
•
The Feedwater System starts in the
Deaerator Storage Tank; water exits the
Deaerator Storage Tank and is high
enough to supply the NPSH (Net Positive
Suction Head) for the suction of the
Boiler Feed Pumps.
•
The
Boiler Feed Pumps
can also pump
water to any of the HP heaters and
boilers.
System Flow Path
•
The Deaerator Storage Tank provides the
NPSH
to the Boiler Feed Pumps suction.
•
The Boiler Feed Pumps pump the
Feedwater through the drum level
control valve to the Low Temperature
Economizer.
•
There is a
tie line
between the Deaerator
on Units.
System Flow Path
•
With this tie line Feedwater from Units
can supply the Boiler Feed Pumps on
Unit and vice versa.
•
The Feedwater System also supplies
feedwater to the De-Superheater Spray
nozzles to cool the steam between the
465 pound
steam system and the
165
pound
steam system.
•
The
De-Superheater Sprays
are also used
to control steam temperatures from the
Boilers.
System Flow Path
•
In the Feedwater System is a
shell and
tube Feedwater Cooler
, which uses
cooling water from the Cooling Tower to
cool the feedwater.
•
This cooler takes a portion of the
feedwater from the High Temperature
Feedwater Heater off boiler, this water
passes through the cooler and discharges
the cooler feedwater to the Deaerator
Storage Tank. At the present time this
cooler is not in service.
System Major Components
•
The Feedwater System consist of the following major components:
– Boiler Feed Pumps– Feedwater Heaters
– Feedwater Regulation
Boiler Feed Pumps (BFP)
•
Boiler Feed Pumps are
motor driven
and
turbine
driven.
•
Although steam turbines cannot be used for
start-up unless a separate source of steam is
available for there operation.
•
With any pump, the pressure tends to fall as the
throughput increases rises, on the other hand
due to the effect of friction, the resistance
offered by the boiler system to the flow of water
increases as the flow rate increases.
Boiler Feed Pumps (BFP)
•
Therefore the pressure drop across the valve will be highest at low flows.
•
It is wasteful to operate with the pressure drop which is significantly
above that at which effective control can be maintained, both because it
entails an energy loss and also because the corrosion of valve internals
increases with high pressure drops.
•
With fixed speed pumps there is nothing can be done about this but an
improvement can b made if variable speed pumps are used.
Boiler Feed Pumps (BFP)
•
Such savings are increased if the plant operates for
prolonged periods at low throughputs and are most
apparent with the larger boilers.
•
From the engineer’s view point, variable speed
pumps are an attractive option because of the
following reason:
– They enable the control system to be linearized over a wide range of flows, leading to improved
controllability.
Boiler Feed Pumps (BFP)
•
All of the pumps are centrifugal pumps.
•
The pumps are equipped with recirculation valves.
•
When operating the BFP's for some time at
reduced capacity, much of the pump horsepower is
transferred into the liquid in the form of heat.
•
A recirculation line prevents the liquid in the pump
from becoming hot enough to vaporize and causing
cavitation and possible BFP impeller damage.
Boiler Feed Pumps (BFP)
•
The recirculation lines are piped from the
discharge line of each of the BFP's prior to
the discharge check valve.
•
The recirculation flow is routed through a
manual block valve, air-operated valve,
orifice and another manual block valve to
the suction line of the BFP’s.
Boiler Feed Pumps (BFP)
•
The recirculation valve automatically opens when the BFP discharge
decreases to approximately 45,000 pounds per hour flow to assure a flow
above the minimum flow required. At 40,000 pounds per hour an alarm is
sounded in the Control Room.
•
Each pump has a recirculation valve which recirculates water back to the
suction line.
•
Each pump has a recirculation valve which recirculates water back to the
condensate inlet of the Deaerator. These BFP’s only supply feedwater to
Low Temperature Economizer inlet.
Boiler Feed Pumps (BFP)
•
Boiler Feed Pump Controls
– All the Boiler Feed Pumps are controlled and monitored from the DCS displays in the Control Rooms.
Feedwater Heaters
•
A feedwater heater is a power plant component used to pre-heat water
delivered to a steam generating boiler.
•
Preheating the feedwater reduces the irreversibility involved in steam
generation and therefore improves the thermodynamic efficiency of the
system.
•
This reduces plant operating costs and also helps to avoid thermal shock
to the boiler metal when the feedwater is introduced back into the steam
cycle.
Feedwater Heaters
•
Pressure classification
– Low pressure heater• A heater located between the condensate pump and either the boiler feed pump or , if present , an intermediate pressure (booster) pump. It normally extracts steam from the low pressure turbine.
• It normally extracts steam from the low pressure turbine
Feedwater Heaters
•
Pressure classification
– Intermediate pressure heater
• A heater located between the booster pump and the boiler feed pump.
• Usually the tubeside pressure is within 200-300 psi of the low pressure heaters , and the steam is extracted from the intermediate pressure turbine.
Feedwater Heaters
•
Pressure classification
– High pressure heater• A heater located downstream of the boiler feed pump.
• Typically, the tubeside design pressure is at least 1500 psig, and the steam source is the high pressure turbine.
• With a similar purpose to the low pressure feed heaters, the high pressure feed heaters are the last stage of feedwater heating before the feedwater enters the boiler system at the economizer
Feedwater Heaters
•
Orientation
– Vertical, Channel Down
• Although these conserve floor space, the amount of control area available for liquid level fluctuation is less.
• Disassembly is by shell removal.
Feedwater Heaters
•
Orientation
– Vertical, Channel Up
• These are the least frequently used.
• Disassembly is by means of bundle removal.
• If a subcooling zone is present, it must extend the full length of the
bundle, since the water must enter the bottom and exit at the top end of the heater.
Feedwater Heaters
•
Orientation
– Horizontal• Most heaters are of this configuration.
• These are the most stable in regard to level control, although they occupy more floor space.
• Disassembly is by means of either shell or bundle removal.
• Most are floor mounted, although some are mounted in the condenser exhaust neck.
Feedwater Heaters
•
Zones
– Zones are separate areas within the shell in a feedwater heater
• Condensing Zone
– All feedwater heaters have this zone. All of the steam is condensed in this area , and any remaining non condensable gases must be
removed. A large percentage of energy added by the heater occurs here.
Feedwater Heaters
•
Zones
– Zones are separate areas within the shell in a feedwater heater
• Subcooling Zone
– The condensed steam enters this zone at the saturation temperature and is cooled by convective heat transfer from the incoming
Feedwater Heaters
•
Zones
– Zones are separate areas within the shell in a feedwater heater
• Desuperheating Zone
– The incoming steam enters this zone, giving up most of its superheat to the feedwater exiting from the heater.
Feedwater Heaters
•
Type of feedwater heaters
– Open feedwater heater.• An open feedwater heater is merely a direct-contact heat exchanger in which extracted steam is allowed to mix with the feedwater
Feedwater Heaters
•
Type of feedwater heaters
– Closed feedwater heater.• Closed feedwater heaters are typically shell and tube heat exchangers where the feedwater passes throughout the tubes and is heated by turbine extraction steam
Feedwater Heaters
•
Feedwater Heaters are of the shell and tube, extraction steam type and
are flue gas type heaters.
•
Steam from the turbine extraction or from the steam header can supply
these heaters
•
The feedwater heaters are shell and tube, U-tube type heaters.
•
Feedwater enters the bottom of the heater and passes through the tubes,
picking up heat from the steam on the shell side of the heater; the
Feedwater Heaters
•
Steam from the steam header or from the turbine extraction, enters the
top of the shell and through baffles is directed across the tubes, as the
steam crosses the tube it gives up heat to the water passing through the
tubes and condenses.
•
As it condenses it falls into the bottom of the heater in the drain section of
the heater.
•
From the drain section of the heater the condensate (drips) is returned to
the Deaerator.
Feedwater Heaters
•
Feedwater Heater Controls
– The inlet and outlet feedwater valves to the Extraction Steam Feedwater Heaters on Units are manually operated valves.
Feedwater Regulation
•
The objective of feed water control may seem simple : it is to supply
enough water to the boiler to match the rate of evaporation.
•
But doing so is a complex mission because of the following reasons:
– There are difficulties even in the basic level drum measurements on which the control system depends.
– Interactions occurring within the boiler system and the effects of these
Feedwater Regulation
•
The Feedwater Regulation is controlled from the DCS (distributed control
system) in the Control Rooms.
•
The Feedwater Regulation maintains a constant boiler drum level by
regulating feedwater flow to the boiler drums.
•
The system provides for drum level control from startup through the
normal full operating load range.
Feedwater Regulation
•
Feedwater Flow is controlled by two (2) separate modes of control:
– Single element – The single-element system responds to the drum level to control the level control valve. The level controller compares the drum level set point, set on the DCS to the actual drum level. The controller in “AUTO” generates a signal, which regulates the level control valve to maintain the drum level. In “MANUAL” the level is controlled by the operator.
– Three-element – The three-element system utilizes measurements of steam flow, feedwater flow and drum level to control feedwater flow.
Feedwater Regulation
•
Single Element Control
– The level of water in the drum provides and immediate indication of the water contained by the boiler.
– If the mass flow of the water into the system is greater than the mass flow of steam out of it, the level of water in the drum rises and vice versa.
– As the level of water in the drum rises , the risk increases of water being carried over into the steam circuits.
Feedwater Regulation
•
Single Element Control
– If the level of water falls there is a possibility of the boiler being damaged , partly because of the loss of essential cooling of the furnace water-walls.
– The target of feed water control system is to keep the level of water in the drum to approximately the midpoint of the vessel.
– The level of water is affected by transient changes of pressure within the drum and the sense in which the level varies is not necessarily related to the sense in which the feed flow must be adjusted.
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Swell
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Swell
– Boiling water comprises a turbulent mass of fluid containing many steam bubbles, and as the boiling rate increases the quantity of bubbles which are generated also increases.
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Swell
– The mixture of water and bubbles resemble foam, and the volume it occupies is dictated both by the quantity of water and by the amount of the steam bubbles within it.
– If the pressure within the system is decreased, the saturation
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Swell
– As the boiling rate increase, the density of water decreases, but since the mass of water and steam has not changed , the decrease in
density must be accompanied by an increase in volume of the mixture.
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Swell
– By this mechanism the level of water in the drum appears to rise a phenomenon referred to as “SWELL”.
– The rise of level is misleading, it is not indicative of the real increase in mass of the water of system, which would require the supply of water to be cut back to maintain the status quo.
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Swell
– If the drop in pressure is the result of the steam demand suddenly increasing , the water supply will need to be increased to match the increased steam flow.
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Shrinkage:
– It is the opposite of swell.
– It occurs when the pressure rises.
– The mechanism is exactly the same as that for swell, but in the reverse direction.
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Shrinkage:
– It causes the level of water in the drum to fall when the steam flow decreases, and once again the delivery of water to the boiler must be related to the actual need rather than to the possibly misleading
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Shrinkage:
– The effect of swell and shrinkage in addition to being determined by the rate of change of pressure, also depends on the relative size of the drum and the pressure at which it operates.
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Shrinkage:
– If the system pressure is low the effect will be larger than with a boiler operating at higher pressure, since the effect of a given
pressure change on the density of water will be greater in the low pressure boiler than it would if the same pressure change were to occur in a boiler operating at higher pressure.
Feedwater Regulation
•
Single Element Control
– The situation arises due the following effects:
• Shrinkage:
– The simplest solution for these is implementation of a ‘two element system’ since it is based on the use of two process measurements in place of the single drum-level measurements used.
Feedwater Regulation
•
Two element system
– A valve with a linear characteristic is employed in conjunction with a transmitter that produces a signal proportional to steam flow.
– If the transmitter produces a signal which is equal to the steam flow at all loads and if the flow through the valve is matched with this signal at every point in the flow range, a controller gain of unity is will ensure that ,
throughout the dynamic range of the system, the flow of water will always be equal to the flow of steam.
– The correct gain of the controller can be determined from a knowledge of the swell and shrinkage effects within the boiler.
Feedwater Regulation
•
Two element system
– Theoretically better results can be obtained by carrying out tests to determine the swell effect at various points in the load range and introducing a
non-linear function within the level controller to compensate for the differences across the range.
Feedwater Regulation
•
Three element system
– In two element system it is assumed that the feed water valve characteristic is linear and the valve is sized to produce a fixed flow when it is 100% open.
– However the flow through the valve depends both on its opening and on the pressure drop across it.
– In a feed water system, the pressure drop across the valve varies from instant to instant, and the flow through it at any given opening will therefore vary.
Feedwater Regulation
•
Three element system
– The varying flow results in the level control becoming offset, to restore the steam flow/ feed flow balance. This offset is undesirable since it needlessly erodes the safety margin provided by the presence of the drum.
– One method of correcting this error produced by the feed valve is the
Feedwater Regulation
•
Three element system
– Ways of implementing a three element system are:
• 1st Method
– The output of the drum level controller is trimmed by a signal representing the difference between the feed flow and the steam flow signals.
– A gain block is introduced to compensate for any difference between the ranges of the two transmitters.
Feedwater Regulation
•
Three element system
– Ways of implementing a three element system are:
• 1st Method
– In most cases the steam flow and the feed flow signals will cancel out, and the drum level controller will be modulating the feed flow to keep the level at the set point.
Feedwater Regulation
•
Three element system
– Ways of implementing a three element system are:
• 2nd Method
– In this method a cascading control technique is applied.
– The drum level controller compares the measured level signals with a set value and produces a bipolar output proportional to any error.
Feedwater Regulation
•
Three element system
– Ways of implementing a three element system are:
• 2nd Method
– This trims a modified steam flow signal , which is acting as the desired value for a closed loop feed water controller.
– As previously a gain block adjusts for any range difference between the steam flow and the feed flow transmitters.
Feedwater Regulation
•
Discrepancies between drum readings
– Density errors- differences between installations can cause one instrument to be more affected by density factors than another.
– Turbulence- standing waves exists around the downcomers, affecting some measurement points more than the others.
– Flashing off- differences in the geometry of the measuring systems can cause some measurements to be more affected than the others by flashing off
Feedwater Regulation
•
Discrepancies between drum readings
– Calibration- it is vital that all the transmitters are carefully and accurately calibrated, and that any density compensation is correctly set up.
– Installation- errors or sluggish response can be the result of partial or complete plugging of impulse lines, or imperfect blow down operations.
Feedwater Regulation
•
In AUTOMATIC, the steam flow is the primary signal to the drum level
controller.
– The drum level controller then compares drum level to drum level setpoint set on the DCS.
– The output signal from the drum level controller is equal to steam flow if actual boiler drum level is equal to boiler drum level setpoint.
– An actual low boiler drum level is corrected by the drum level controller generating an output signal greater than steam flow.
Feedwater Regulation
•
In AUTOMATIC, the steam flow is the primary signal to the drum level
controller.
– An actual high boiler drum level is corrected by the drum level controller generating an output signal less than steam flow.
– Thus, the output of the drum level controller is the desired Feedwater flow, which is equal to steam flow if the actual boiler drum level is at setpoint.