Boiler Feed Water System

Boiler Feedwater System

S Shariq Ahmed Asst. General Manager Allied Energy Systems Pvt. Ltd. S Shariq Ahmed

INTRODUCTION

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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.

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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 system. S Shariq Ahmed

Basic System Description • Feedwater is supplied by the Deaerators, which also provides the necessary Net Positive Suction Head (NPSH) to the Boiler Feed Pumps.

Deaerators

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.

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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.

Deaerators

Boiler Feed Pumps

• The Feedwater header supplies heated, highpressure water, through level control valves, to the boiler drums on Units • The Feedwater Header supplies Desuperheater Sprays to control the steam temperature.

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Feedwater Heater

Feed Water Regulation Valves Boiler Economizer Inlets Feedwater System Block Diagram

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.

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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 Economizer inlet valves. S Shariq Ahmed

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.

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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.

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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. S Shariq Ahmed

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. S Shariq Ahmed

SYSTEM MAJOR COMPONENTS

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System Major Components • The Feedwater System consist of the following major components: – – – –

Boiler Feed Pumps Feedwater Heaters Feedwater Regulation De-Superheating Spray

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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. S Shariq Ahmed

Boiler Feed Pump

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.

• Variable speed pumps are more expensive but the increase in cost tends to be offset by the operational cost savings. S Shariq Ahmed

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.

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Boiler Feed Pump

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.

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Boiler Feed Pump

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

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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.

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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.

Boiler Feed Pumps

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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.

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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 S Shariq Ahmed

LP Closed Feed Water Heater

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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.

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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

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High Pressure Heater

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HP Closed Feed Water Heater

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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. • Installation and removal may be more difficult than for horizontal heaters.

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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.

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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.

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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.

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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.

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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.

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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

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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

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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 exits the top of the heater.

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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. • Heaters also use flue gas to heat the feedwater. S Shariq Ahmed

Feedwater Heaters • Feedwater Heater Controls – The inlet and outlet feedwater valves to the Extraction Steam Feedwater Heaters on Units are manually operated valves.

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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 interactions are smaller or greater at various points in the boiler’s load range.

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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.

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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.

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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.

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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.

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Feedwater Regulation • Single Element Control – The situation arises due the following effects: • Swell • Shrinkage

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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.

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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 temperature is also lowered and the boiling rate therefore increases. S Shariq Ahmed

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.

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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. S Shariq Ahmed

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.

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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. S Shariq Ahmed

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 indication provided by the drum level transmitter.

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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.

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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.

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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.

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Single Element Control

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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. S Shariq Ahmed

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 nonlinear function within the level controller to compensate for the differences across the range.

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Two Element System

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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.

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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 addition of a third element to the system- a measurement of feed-water flow.

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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. S Shariq Ahmed

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.

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Three Element System (1st Method)

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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.

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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.

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Three Element System (2nd Method)

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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 during the pressure changes.

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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.

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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.

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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.

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Feedwater Regulation • The feedwater controller then compares actual feedwater flow to the desired feedwater flow signal sent from the drum level controller. • This arrangement provides immediate response in feedwater flow to changes in steam flow. Feedwater Control Valve

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SYSTEM OPERATION

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System Startup • All system components that have been tagged out for maintenance have had tags cleared and removed.

• All control and indication instrumentation on which maintenance or calibration has been preformed has been returned to service. The Instrument Air System is in service. • The system should be walked down to verify that all pump and motor lubrication are at their proper levels.

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System Startup • That the system’s valves have been placed in their proper startup positions with all drains closed except where otherwise noted.

• That all AC and DC electrical power supplies has been racked in and ready for service.

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Normal Operation • All systems should be monitored for pressures, temperatures, flows, and levels.

• Pump and motor lubrication are monitored and filled as required. • Any abnormal conditions should be reported and corrective action taken.

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System Shutdown • The Feedwater System is normally in operation. • All pumps should be taken out of service and breakers racked out. • Close valving to isolate the system and open drain valves where system needs draining. • Any part of the Feedwater System is to be worked on, proper clearance and tagging should be obtained.

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