INFORMATION SHEET

 

                                                            MAIN FEED SYSTEM

                                                   Information Sheet Number 62B-221

 

INTRODUCTION

 

            The Main Feed System is a very important cornerstone of the basic steam cycle.  The feed system receives condensate and make up feed water into the DFT.  Once in the deaerating feed tank the condensate is preheated and deaerated to become feed water.  The feed system delivers the feedwater to the boiler at sufficient pressure, quantity and chemical make-up at all operating conditions.  Typical components are the deaerating feed tank (DFT), main feed booster pumps (MFBP), main feed pumps (MFP), feedwater control valves (FWCV), and a chemical injection system.  Lastly, an understanding of the feedwater system’s operation is necessary to anticipate necessary actions in the event of a casualty.  (Refer to Figure -1)

 

REFERENCES

 

            (a)        Main Feed Pump NAVSEA 0947-LP-107-8010

            (b)        Deaerating Feed Tank NAVSHIPS 0955-LP-010-1010

            (c)        Propulsion Plant Manual NAVSEA 0941-LP-051-6010

 

 

INFORMATION

 

A.                 DFT

 

1.         The Deaerating Feed Tank (DFT) performs three basic functions: preheats, deaerates and stores feedwater.  Operation of the DFT is based on the principle of inverse solubility, which in general, means that as temperature increases the capacity of the water to retain dissolved oxygen approaches zero.  The temperature at which water cannot hold oxygen in solution is the saturation temperature for the given pressure.  Therefore the first step in the process of deaerating feed water is to raise the temperature of the water to approximately the boiling temperature for the particular pressure (246-2520F 15 psig). This renders the entrained gas insoluble in water but does not automatically remove all the gas molecules from the water mass as some molecules cannot escape the bulk of the fluid.  To accomplish the final deaeration, steam is used to scrub condensate of the gas molecules. The atomization process is necessary in that it more readily allows gasses to come out of solution and does not promote the solubility of gasses in solution.

 

B.                 System Components, their Functions, and Safety Devices

           

 

 

MAIN FEED SYSTEM

 

 

Figure 1

 

1.         Deaerating feed tank (DFT) (Refer to Figure -2)

 

a.         Heating serves two functions.  It is essential to the deaeration process and it improves overall plant efficiency by recovering energy from the auxiliary exhaust (AE) system.   As condensate enters the DFT, it is directed to an annular distribution ring containing spring loaded spray nozzles.   A pressure differential of approximately 2.5 - 3 psig across a nozzle will cause the spring loaded valve to open ejecting the condensate from the spray nozzle as a fine spray into the upper section of the DFT. This section of the DFT is filled with auxiliary exhaust steam at roughly AE system pressure and temperature (350-4000F at 15 psig ).   As the condensate comes into contact with the steam it is heated and some of the dissolved oxygen and non condensable gases are released and removed from the system through the air outlet into the auxiliary gland exhaust system.   The steam which condenses (preheated feed) falls down to the conical water collecting baffle and drains through the atomizing valve and into the lower section of the DFT.

 

CUTAWAY OF A DFT

 

FIGURE 2

 

b.         The atomizing valve requires at least .5 psig differential pressure across the valve to cause it to open.   AE and high pressure (HP) drain steam open the valve and impinge upon the condensate draining from the conical baffle.  This impingement and the ensuing direction changes complete the deaeration and heating of the condensate.  The lower section stores the deaerated, preheated feedwater and provides a net positive suction head (NPSH) to the MFBPs.

 

c.         To protect the DFT from over or under pressurization a vacuum breaker and a shell relief valve are installed on the DFT shell.

 

(1)      The vacuum breaker is designed to admit air into the DFT when shell pressure drops to 13 psia.  It protects the DFT from implosion while cooling down from normal shut down. 

 

(2)      The shell relief valve is typically set to open at 30 psig and is installed to protect the DFT from over pressurization.

 

(3)      To begin DFT operation, the DFT must first be warmed up to the normal temperature (246-2520F 15 psig).  This is done by recirculating water from the MFBP back to the DFT and by aligning the 150/15 psig reducer utilizing certified shore steam.   If no MFBPs are installed, the emergency feed pump may be used.  Once in the normal operating band, a temperature differential of no more than three degrees between the upper and lower sections is allowed.  Greater than three degrees is indicative of a component malfunction and should be investigated.

 

2.         Main Feed Booster Pumps - MFBP

 

a.         The MFBP takes a suction from the DFT and provides a positive suction head to the MFP.  Since the MFBPs are centrifugal pumps and require pressure at the impeller to function properly, they are located below the DFT.  The weight of the water in the storage section of the DFT plus the shell pressure provide the necessary suction head during normal operations.  Some ships do not have MFBP's because the DFT is located high enough above the MFPs.

 

b.         The flow rate of the MFBP is greater than that of the MFP to accommodate recirculation flow and to prevent a loss of suction head to the MFPs during high feed flow conditions.  If the MFP had a greater capacity than that of the MFBP then they could create a suction at their inlet which would cause the MFP to trip off the line.

 

c.         On some ships, an auto start feature will start an additional MFBP on a low discharge pressure condition to prevent the MFP from tripping off the line due to low suction pressure.

 

d.         Some ships provide a suction to the MFBP from either an emergency tank or a DFT bypass line in the event of a casualty to the DFT.  This method is used only long enough to safely secure the boilers, thereby limiting the amount of cold, non-deaerated feedwater sent to the boiler.

 

e.         MFBPs are protected from overheating and pump damage by recirculating pump discharge to the DFT.  The recirc line is orificed to ensure sufficient recirculation during low demand conditions.

 

f.           MFBPs on some ships are equipped with a low discharge pressure alarm to warn operators of a possible loss of main feed control casualty.

 

3.         Main Feed Pumps - MFP (Refer to Figure -3)

 

a.         MFPs are steam driven, centrifugal, multi stage pumps.  Similar to the MFBP the MFP requires a pressurized inlet for proper operation (provided by the MFBP).  A discharge pressure greater than that of boiler pressure is maintained by altering turbine speed through a pneumatic/hydraulic control system.  All MFPs discharge to a common header which branches into a feed line for each boiler.

 

MAIN FEED PUMP

 

 

FIGURE 3

 

b.         Each boiler feed line contains an automatic feedwater control valve (FWCV) to control feed flow to the boiler (refer to figure-4).

 

c.         An electric lube oil pump (ELOP) allows proper cool down of the machine while securing, and provides lubrication during start-up.  An automatic feature starts the ELOP to back up the attached lube oil pump in the event of a failure by starting the ELOP upon a low lube oil pressure condition, typically at 45 psi.

 

d.         Trips and Safety devices

 

(1)      A speed limiting governor restricts the turbine operation to a safe speed of 105 - 107 % of rated speed.  Refer to your applicable technical manual.

 

(2)      An overspeed trip mechanism trips the MFP if turbine speed is excessive and attains the trip speed of 110 % of rated speed.  Refer to your applicable technical manual.

 

(3)      A low lube oil pressure alarm warns operators of a loss of turbine lubrication which may lead to failure (typically at 5 psi).

 

(4)      The low suction safety trip shuts the MFP throttle valve when the pump suction pressure drops to near the minimum net positive suction head (NPSH) required to prevent cavitating the pump.  The low suction trip settings are staggered to prevent all MFP's from tripping simultaneously.  For a system with three MFP's, typical trip settings are:

 

(a)             1A MFP - 25 psig

(b)             1B MFP - 27 psig

(c)             1C MFP - 29 psig

 

(5)      The high speed and close tolerances of the MFP design requires maintaining sufficient NPSH to prevent cavitation and subsequent thermal expansion.  Thermal expansion can quickly lead to wearing ring seizure and/or impeller to casing contact.

 

(6)      A suction relief valve protects the inlet piping to the MFP and MFBP casings should a MFP discharge check valve on a idle or standby pump leak by.

 

(7)      A MFP discharge relief valve protects the feed lines from over-pressurization should main feed header pressure control be lost.

 

(8)      MFP recirc system prevents overheating the pump end during low load conditions by recirculating feed back to the DFT.  The recirc valve opens on low feed flow and shuts on high feed flow through the MFP and is controlled by the ABC system.

 

e.         The FWCV maintains a normal steam drum water level during all steaming condition (refer to figure-4).

FEEDWATER CONTROL VALVE

 

FIGURE 4

 

(1)      The ABC system normally controls the FWCV automatically.

 

(2)      The operator may control the FWCV position locally by a handwheel or remotely from the console board when the board is in remote manual control.

 

4.         Continuous chemical injection system

 

a.         The chemical injection system provides a means to add treatment chemicals to the boiler water for maintaining proper boiler chemistry using the CHELANT system.

 

b.         The CHELANT boiler treatment system is an automatic system which continuously injects a chemical solution into the DFT.  The injection rate is proportional to feed flow based on a signal received from the feed flow transmitter.  The treatment system consists of a piercing apparatus, a mixing tank, nitrogen supply, solenoid valve, and various piping and valves.

 

c.         To operate the system, CHELANT solution is placed in the piercing apparatus which drains to the mixing tank.  The mixing tank is pressurized with 30 - 60 psig nitrogen which forces the solution through a solenoid valve and control panel where the flow of chemicals to the DFT is regulated based on feed flow transmitter output.

 

d.         A chemical injection tank for one time or batch addition of chemicals is used for adjusting boiler water chemistry prior to light-off, securing, and during boiler chemistry casualties.  The system consists of a chemical injection tank which fills the lines from the condensate and reserve feed, and feed flow lines to carry chemicals to the boiler.  System specifics and operation are covered in the boiler water/feedwater course.

 

C.                 Feed System Operation

 

1.         The main condensate pumps (MCP) discharge condensate to the DFT.   Once inside the DFT the feed is heated, deaerated, and stored until it is sent to the boiler through the MFBP, MFP, FWCV.

 

2.         DFT response to the system operations

 

a.         Maintaining the DFT steam bubble during feed system/plant operations is vital. Normally, AE steam and HP drains pressurize the DFT upper section with a  steam bubble for feed preheating and deaerating and to establish sufficient NPSH at the MFBP inlet.  Providing sufficient NPSH throughout all plant evolution’s can make the difference between controllable plant conditions and a loss of feed control casualty. An understanding of how the DFT responds to changing system conditions is necessary to anticipate necessary actions.

 

b.         During normal operations, condensate is preheated prior to entering the DFT via the main or auxiliary air ejectors.  If a cold slug of condensate is sent to the DFT it will quench the DFT (suddenly collapse the steam bubble due to a large amount of cold condensate water condensing the steam bubble of the DFT  upper section), for example,  trying to place a condenser on line quickly by opening the air ejector condenser discharge valve quickly will send a relatively cold slug of water to the DFT (relative to the DFT water temperature).

 

(1)      Quenching causes a loss of NPSH to the MFBP impeller causing cavitation and a subsequent loss of MFBP discharge pressure

 

(2)      Quenching creates a sudden loss of pressure and at times creates a vacuum in the DFT shell which can result in damage to the shell, or even implosion of the DFT.

 

c.         Flashing is a consequence of quenching.  When cold condensate contacts the steam in the upper section of the DFT, the steam bubble temperature and pressure are lowered rapidly and result in the temperature of the feed water exceeding its saturation temperature.  This causes vigorous boiling and flashing of the feed into steam.  The flashing and quenching actions can occur in succession repeatedly which increases the potential for damage to the DFT shell.