INFORMATION
A. The purpose of the main feed system is to receive condensate and make-up feedwater and deliver it to the boiler at sufficient pressure, temperature and quantity to maintain a safe water level in the boiler for all load conditions. Provision is also made for injecting chemicals into the boiler to maintain proper boiler chemistry. The following are the components that comprise this system in the order in which the water flows to the boiler.
1. Deaerating Feed Tank
2. Main Feed Booster Pumps (most ship classes)
3. Main Feed Pumps
4. Automatic and Manual Feed Check Valves
5. Economizer
6. Internal feed pipe
B. System Components, Location and Function (Figure 1)
Main Feed System
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Figure 1
1. Deaerating Feed Tank (DFT) serves four basic functions. Its design is to (in this order) preheat, deaerate and store feedwater. The fourth function, by virtue of its location, also provides a net positive suction head for the main feed booster pumps or main feed pumps as applicable. Preheating and deaeration is accomplished utilizing auxiliary exhaust (primary heat source) and high pressure drains (secondary heat source). It is mandatory that the dissolved oxygen content in the feedwater be reduced to no greater than 15 ppb (parts per billion). The lower part (or storage area) of the DFT stores enough feedwater to provide a surge volume for immediate use in the boiler. The capacity depends on the ship's boiler water capacity at normal steaming level, (i.e. LHA-1: 3,100 gallons). The DFT is normally located on the upper level of most firerooms or main machinery rooms (MMR). However, in some ships such as AE's, it may be located outside of the main machinery spaces; up in the superstructure thereby eliminating the need for main feed booster pumps.
2. Main Feed Booster Pumps (MFBP) - The main function of this pump is to provide the main feed pump with a required net positive suction head. If installed, these pumps are below the DFT. The reason for this placement is that these pumps are of the centrifugal design and must be provided with a positive suction head. The weight of the water in the storage area of the DFT combined with a typical pressure of 15 psig (30 psia) within the shell provides the adequate pressure required. The MFBP's discharge pressure will depend upon the steam plant's design operating pressure. For example, average discharge pressure ranges for a 600 psi plant is 40-60 psig. The number of pumps installed in a system will depend upon the given steam plant's configuration. Typically, there are two MFBPs per 600 psi main space. MFBPs can be either steam or electric driven. However, in a two pump configuration, one of these pumps will be steam driven while the other will always be electric. The initial purpose of the single electric pump is for the warm up of the DFT on initial plant start-up. A recirculation line from the pumps discharge to the DFT is fitted with a pressure break-down orifice to prevent over pressurization of the DFT. In addition to this, an electric driven MFBP can also be used for establishing boiler water level for light-off or provide a hot deaerated backfill lay up to a secured boiler.
3. Main Feed Pumps (MFP) - Located on the lower level of most 600 PSI steam plants. The main feed pumps are centrifugal pumps and cannot take their own suction. They are high powered turbine driven, multi-stage pumps taking suction from the discharge side of the MFBPs, developing a discharge pressure which will provide sufficient feedwater to the boiler.
a. Two types of controls are used, one to control pump speed in order to maintain a set discharge pressure (setpoint) in the discharge line, and the other is to provide recirculation at low flow conditions. These systems will be discussed in more detail in a future lesson.
b. The main feed pumps are fitted with low suction safety trips that will stop the pump if there is a loss of booster pressure. This prevents the pump from overheating. An example of the sequential operation of these pumps is:
(1) "A" MFP is set at 23 PSIG
(2) "B" MFP is set at 25 PSIG
c. The main feed pumps are also fitted with a discharge relief valve, a suction relief valve and a recirculation system to protect the piping from excessive pressure and prevent the pump from overheating respectively.
4. Feed Control Valves - Regulate the flow of feedwater (according to demand) to the boiler. The automatic control valve is controlled by a pneumatic signal from the three element feedwater control system and manual control valve is used by the checkman to regulate feed flow when a failure occurs on the automatic system.
5. Chemical Injection Tank - is used to inject boiler water treatment chemicals into the boiler to treat a freshly filled boiler and to control chemical casualties. This is accomplished by placing the chemicals into the boiler feed line just before it enters the steam drum. In a Chelant system, chemicals are injected directly into the DFT to maintain normal control of boilerwater chemical limits.
C. System Operation
1. Hot deaerated feedwater is pumped from the storage compartment of the DFT to the boiler(s) by the main feed booster pumps and the main feed pumps. The main feed booster pumps take a suction from the DFT and raise (boost) the pressure to 85 PSIG, providing the required net positive suction head (NPSH) for the main feed pumps. The NPSH for the main feed booster pumps is provided by physically locating the DFT at the necessary elevation above the main feed booster pumps.
a. The main feed booster pumps discharge into a common suction header for the main feed pumps. The main feed pumps raise feedwater pressure to setpoint, using multi-staging to assure feedwater will flow into the boiler under all conditions.
b. Main feed pump discharge pressure is regulated by a feed pump control system which maintains a constant discharge pressure at the main feed pump. Each main feed booster pump and main feed pump has a recirculation line from the discharge side of the pump back to the DFT. The recirculation lines provide sufficient flow through the pumps to prevent overheating during conditions when there is no or low flow to the boiler.
c. The feedwater is discharged from the main feed pumps into a common header from which feed lines branch out to each boiler. The feed line to each boiler contains an automatic feed regulating valve and a manual feed stop check valve (commonly called the check valve).
D. Component Theory of Operation
1. Deaerating feed tank (DFT) (Figure 2)
a. As mentioned earlier the deaerating feed tank performs four primary functions:
(1) Heats the condensate to saturation temperature for the shell pressure existing in the DFT.
(2) Deaerates the condensate (removes air and non-condensable gases).
(3) Stores a reserve quantity of heated, deaerated feedwater and provides a surge volume for immediate use in the boiler.
(4) Provides a Net Positive Head Suction for the Main Feed Booster Pumps.
b. Since the presence of air in feedwater can cause serious, long term damage to the boiler and accelerate boiler metal corrosion it is critical to remove as much as possible from feedwater. The first three above purposes are dedicated to that end.
c. Air in solution in boiler feedwater is usually referred to as "dissolved oxygen." The nitrogen content of the air, which is also dissolved along with the oxygen, is inert and causes no corrosion of the exposed metal surfaces. The carbon dioxide content of ordinary air represents a small percentage of the total constituents, but is of interest in feedwater conditioning because it is more soluble than oxygen or nitrogen and combines with water, lowering the pH of the condensate system.
d. The relatively pure condensate absorbs oxygen and carbon dioxide at a rapid rate when exposed to the atmosphere, and to much greater capacity than ordinary fresh shore water which already contains some mineral impurities in solution. Feedwater with a given air content (either as free or dissolved air) becomes more corrosive as the water temperature increases.
e. The primary function of the DFT, heating of the water, is essential to the second function, the deaerating process, and improves the overall plant efficiency by recovering energy from the auxiliary exhaust system. Heating the water to saturation temperature renders the oxygen insoluble and is necessary to provide a means for removal of the gas molecules entrained in the heated condensate. The DFT helps accomplish this by scrubbing the gas molecules free of the water molecules through atomizing the condensate and mixing it with the auxiliary exhaust steam.
(1) Condensate enters the DFT and is directed to an annular ring containing spring loaded spray nozzles. A pressure differential of approximately 2 1/2 to 3 PSI across the spray nozzles will cause the spring loaded valves to open. The condensate is ejected from the spray nozzles in a finely atomized spray into direct contact with the auxiliary exhaust steam in the upper part of part of the DFT. The spray of condensate is discharged up against the top of the tank and the cylindrical baffle surrounding the air outlet. As the fine spray of condensate comes in contact with the steam it is heated and some of the entrained air is released. The steam which is condensed and the preheated condensate fall down to the conical water collecting baffle, and are directed towards the center of the DFT and the atomizing valve.
(2) The atomizing valve is a spring loaded valve which requires at least 1/2 PSI pressure differential between auxiliary exhaust steam pressure and the shell pressure of the DFT to cause it to open. Auxiliary exhaust steam and high pressure steam drains (HP drains) are applied on top of the atomizing valve. When the valve opens, steam discharges at a relatively high velocity and impinges on the condensate falling down from the conical baffle. The resulting mixture of steam and condensate is directed upwards by a baffle where it strikes the inverted cone and is directed downwards. The condensate and condensed steam fall down into the storage compartment of the DFT as deaerated feedwater.
(3) The impingement of the steam with the condensate and redirection of the mixture accomplishes final heating of the condensate and scrubs it free of the remaining dissolved oxygen and entrained gases. The remaining steam and gas mixture flows up around the conical water baffle to the top of the DFT. The steam is used to continue the preheating of incoming condensate. All but a small portion of the steam is condensed in this process and a portion is vented through the air outlet, carrying with it all the non-condensable gases which have been removed from the condensate.
(4) In the air outlet line there is an orifice and a three-way vent valve. The purpose of the orifice is to control the amount of steam and non-condensable gases vented from the DFT. The orifice is sized to pass the amount necessary to obtain the best results in heating and non-condensable gas removal. The three-way vent valve allows the steam and non-condensable gases to be directed to the atmosphere or to the auxiliary gland exhaust condenser. The mixture is vented to the atmosphere only when heating up or securing the DFT. For normal operation, the DFT is vented to the auxiliary gland exhaust condenser where the steam in the vented mixture can be condensed and reclaimed.
DEAERATING FEED TANK
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Figure 2
(5) The amount of heated and deaerated feedwater stored in the storage compartment of the DFT varies by class of ship and is a function of the steam generating capacity of the boiler. (A rule of thumb has the storage capacity of the DFT to be equal to 2.5 minutes of feedwater supply to the boiler at full power).
(6) The level of feedwater in the DFT is automatically controlled by operation of the make-up feed and excess condensate dump valves. Normal water level will varies with ship type. Air operated control pilots make use of reference and variable legs of water to sense DFT level and generate pneumatic control signals to open or close the excess condensate or makeup feed diaphragm control valves to maintain desired level. When DFT level rises to the upper limit of normal operating range, the excess condensate dump valve (excess feed valve) starts to open and diverts condensate from the condensate main to a reserve feed tank. As DFT level continues to rise, the excess feed valve opens further until it is fully opened, when the level is about twenty percent greater than normal level. If DFT level continues to rise, a high level alarm sounds.
(7) When the DFT level falls to the lower limit of the normal operating range, the make-up feedwater valve will start to open and admit make-up feedwater to the condensate system from a reserve feed tank. The make-up feed valve is fully opened when DFT level reaches a predetermined set point. The excess feed and make-up feed valves can be manually operated should the pneumatic system fail. If the water level in the DFT continues to drop, a low DFT water level alarm sounds.
(8) In a properly operating DFT, the temperature of the water in the feed storage compartment should be about equal to the temperature of the steam in the preheating compartment. A thermometer is provided in each compartment to measure these two temperatures. Incomplete deaeration of condensate is probable if the temperature of the feed storage area is more than 30F lower than the temperature in the preheating compartment.
(9) To protect the DFT from under or over-pressurization a vacuum breaker and a shell pressure relief valve are installed. The vacuum breaker is a spring-loaded valve which will open to admit air to the DFT when shell pressure drops to 13 PSIA. The vacuum breaker is sized to protect the DFT against pressure variations that may occur during normal securing. It is not sufficiently large enough to prevent damage to the DFT in the event of maloperation, such as introducing large quantities of cold water into a hot DFT. This will result in "quenching" the DFT; the cold water condenses the steam in the DFT causing a sudden drop in pressure. If the quench is severe enough, the DFT can implode.
(10) Another condition called "flashing" may result in the event DFT shell pressure is suddenly decreased below the minimum saturation pressure, the water in the storage area will flash to steam. The flashing of the DFT will also cause a momentary disruption of flow to the MFPs causing the low suction trips to stop the pumps.
(11) Deaerating feed tanks are designed to withstand about 45 PSIG maximum pressure. A relief valve installed on the shell is normally set to lift at 30 PSIG to protect the tank from over-pressurization.
f. Main feed booster pumps (MFBP)
(1) Main feed booster pumps (MFBP) are vertical, two-stage, centrifugal pumps which are electric motor driven and some times steam. One main feed booster pump is required for each main feed pump in operation, and typically the main feed booster pump will have a rated capacity (GPM) of about 105-110 percent of the rated capacity of the main feed pump.
g. Main feed pump (MFP)
(1) The main feed pump (MFP) is a high powered, turbine driven, multi-stage centrifugal pump taking a suction from the discharge piping of the main feed booster pumps and raising the feedwater pressure sufficiently to ensure the capability for adequate flow of water to the boiler.
(2) All main feed pumps have a low suction safety trip. The low suction safety trip will shut the throttle valve when main feed suction pressure (MFBP discharge pressure) decreases to a value approaching minimum NPSH for the main feed pumps. For multiple main feed pump installations, the low suction safety trip set point, for each pump is different. Examples are (A-23 B-25 PSIG) allowing for sequential tripping of the main feed pumps as suction pressure decreases. Also, most ships incorporate an audible low booster pump discharge pressure alarm.
(3) An automatic start feature may be installed to start the standby motor driven pump when the MFBP discharge pressure falls to 60 PSIG. This is less than alarm pressure but greater than low suction trip pressure. This series of low suction pressure alarms/protection features is necessary to prevent main feed pump damage due to cavitation. The high speed, close tolerance main feed pump will overheat rapidly should cavitation occur and thermal expansion may cause pump seizure.
(4) All main feed pumps will take a suction on the MFBP common discharge header or DFT suction and discharge to a common feed header from which all boilers in the space are fed.
(5) All main feed pump installations have the following features in common some examples are listed below:
(a) Individual suction and discharge stop valves and a discharge check valve on each pump.
(b) A suction relief valve located between the suction stop valve and the main feed pump casing to protect the suction piping from over-pressurization should the discharge check valve leak through on an idle pump, subjecting suction piping to discharge pressure.
(c) A discharge relief valve located between the pump casing and discharge stop valve to protect the pump and discharge piping from over-pressurization, should the main feed pump overspeed.
(d) A recirculation line from the main feed pump discharge to the DFT, to provide flow through the main feed pump when no or low feed is being supplied to the boiler. Recirculation lines from each main feed pump have a multiple orifice type, pressure breakdown device which is sized to limit flow through the recirculation line and to reduce main feed discharge to a pressure compatible with DFT pressure.
(e) Each boiler feed line has a feed stop valve, an automatic feed regulating valve, a manual feed stop check valve, and a lift check valve. The lift check valve is installed in the feed piping between the boiler economizer and steam drum to prevent steam and water flowing out of the boiler in the event of an economizer rupture.
h. Feed regulating valve (Feedwater Control Valve)
(1) The feed regulating valve is automatically controlled by the feedwater control system to maintain steam drum water level within plus or minus one inch of normal water level under steady steaming conditions. The feedwater control system will be covered in the Automatic Boiler Controls Lesson.
(2) For normal operations, the manual feed stop check valve is left open and the flow of feed to the boiler is controlled automatically by the feedwater control system which varies the position of the feed regulating valve.