FEEDWATER SYSTEM

Feedwater heaters

Open type—deaerators.

(1) Purpose.

Open type feedwater heaters are used primarily to reduce feedwater oxygen and other noncondensable gases to essentially zero and thus decrease corrosion in the boiler and boiler feed system. Secondarily, they are used to increase thermal efficiency as part of the regenerative feedwater heating cycle.


(2) Types.

1) There are two basic types of open deaerating heaters used in steam power plants—tray type and spray type. The tray or combination spray/tray type unit will be used. In plants where heater tray maintenance could be a problem, or where the feedwater has a high solids content or is corrosive, a spray type deaerator will be considered.

2) All types of deaerators will have internal or external vent condensers, the internal parts of which will be protected from corrosive gases and oxidation by chloride stress resistant stainless steel.

3) In cogeneration plants where large amounts of raw water makeup are required, a deaerating hot process softener will be selected depending on the steam conditions and the type of raw water being treated (Section IX, paragraph 3-38 and 3-39).


(3) Location.

The deaerating heater should be located to maintain a pressure higher than the NPSH required by the boiler feed pumps under all conditions of operation. This means providing a margin of static head to compensate for sudden fall off in deaerator pressure under an upset condition. Access will be provided for heater maintenance and for reading and maintaining heater instrumentation.











(4) Design criteria.

1) Steam pressure to the deaerating heater will not be less than three psig.
2) Feedwater leaving the deaerator will contain no more than 0.005 cc/liter of oxygen and zero residual carbon dioxide. Residual content of the dissolved gases will be consistent with their relative volubility and ionization.
3) Deaerator storage capacity will be not less than ten minutes in terms of maximum design flow through the unit.
4) Deaerator will have an internal or external oil separator if the steam supply may contain oil, such as from a reciprocating steam engine.

5) Deaerating heater will be provided with the following minimum instrumentation: relief valve, thermometer, thermocouple and test well at feedwater inlet and outlet, and steam inlet; pressure gauge at feedwater and steam inlets; and a level control system (paragraph c)







Closed type.

a. Purpose.
along with the deaerating heater, closed feedwater heaters are used in a regenerative feedwater cycle to increase thermal efficiency and thus provide fuel savings. An economic evaluation will be made to determine the number of stages of feedwater heating to be incorporated into the cycle. Condensing type steam turbine units often have both low pressure heaters (suction side of the boiler feed pumps) and high pressure heaters (on the discharge side of the feed pumps). The economic analysis of the heaters should consider a desuperheater section when there is a high degree of superheat in the steam to the heater and an internal or external drain cooler (using entering condensate or boiler feedwater) to reduce drains below steam saturation temperature.

b. Type.
The feedwater heaters will be of the Utube type.

c. Location.
Heaters will be located to allow easy access for reading and maintaining heater instrumentation and for pulling the tube bundle or heater shell. High pressure heaters will be located to provide the best economic balance of high pressure feedwater piping, steam piping and heater drain piping.

d. Design criteria

(b) Each feedwater heater will be provided with the following minimum instrumentation: shell and tube relief valves; thermometer, thermocouple and test well at feedwater inlet and outlet; steam inlet and drain outlet; pressure gauge at feedwater inlet and outlet, and steam inlet; and level control system.
c. Level control systems.
(1) Purpose. Level control systems are required for all open and closed feedwater heaters to assure efficient operation of each heater and provide for protection of other related power plant equipment. The level control system for the feedwater heaters is an integrated part of a plant cycle level control system which includes the condenser hotwell and the boiler level controls, and must be designed with this in mind. This paragraph sets forth design criteria which are essential to a feedwater heater level control system. Modifications may be required to fit the
actual plant cycle.
(2) Closed feedwater heaters.
(a) Closed feedwater heater drains are usually cascaded to the next lowest stage feedwater heater
or to the condenser, A normal and emergency drain line from each heater will be provided. At high loads with high extraction steam pressure, the normal heater drain valve cascades drain to the next lowest stage heater to control its own heater level. At low loads with lower extraction steam pressure and lower pressure differential between successive heaters, sufficient pressure may not be available to allow the drains to flow to the next lowest stage heater. In this case, an emergency drain valve will be provided to cascade to a lower stage heater or to the condenser
to hold the predetermined level.
(b) The following minimum instrumentation will be supplied to provide adequate level control at
each heater: gauge glass; level controller to modulate normal drain line control valve (if emergency drain line control valve is used, controller must have a split range); and high water level alarm switch.
(3) Open feedwater heaters-deaerators. The following minimum instrumentation will be supplied to provide adequate level control at the heater: gauge glass, level controller to control feedwater inlet control waive (if more than one feedwater inlet source, controller must have a split range); low water level alarm switch; “low-low” water level alarm switch to sound alarm and trip boiler feed pumps, or other pumps taking suction from heater; high water level alarm switch; and “high-high” water level controller to remove water from the deaerator to the condenser or flash tank, or to divert feedwater away from the deaerator by opening a diverting valve to dump water from the feedwater line to the condenser or condensate storage tank.

3-30. Boiler feed pumps.
a. General. Boiler feed pumps are used to pressur- 3-41 TM 5-811-6 ize water from the deaerating feedwater heater or deaerating hot process softener and feed it through any high pressure closed feedwater heaters to the boiler inlet. Discharge from the boiler superheated steam in order to maintain proper main steam ternperature to the steam turbine generator.
b. Types. There are two types of centrifugal multi-stage boiler feed pumps commonly used in steam power plants—horizontally split case and barrel type with horizontal or vertical (segmented) split inner case. The horizontal split case type will be used on boilers with rated outlet pressures up to 900 psig. Barrel type pumps will be used on boilers with rated outlet pressure in excess of 900 psig.
c. Number of pumps. In all cases, at least one spare feed pump will be provided.
(1) For power plants where one battery of boiler feed pumps feeds one boiler.
(a) If the boiler is base loaded most of the time at a high load factor, then use two pumps each at 110-125 percent of boiler maximum steaming capacity.
(b) If the boiler is subject to daily wide range load swings, use three pumps at 55-62.5 percent of
boiler maximum steaming capacity. With this ar- rangement, two pumps are operated in parallel between 50 and 100 percent boiler output, but only one pump is operated below 50 percent capacity. This arrangement allows for pump operation in its most efficient range and also permits a greater degree of flexibility.
(2) For power plants where one battery of pump feeds more than one boiler through a header system, the number of pumps and rating will be chosen to provide optimum operating efficiency and capital costs. At least three 55-62.5 percent pumps should be selected based on maximum steaming capacity of all boilers served by the battery to provide the flexibility required for a wide range of total feedwater flows.
d. Location. The boiler feed pumps will be located at the lowest plant level with the deaerating heater or softener elevated sufficiently to maintain pump suction pressure higher than the required NPSH of the pump under all operating conditions. This means a substantial margin over the theoretically calculated requirements to provide for pressures collapses in the dearator under abnormal operating conditions. Deaerator level will never be decreased for structural or aesthetic reasons, and suction pipe connecting deaerator to boiler feed pumps should be sized so that friction loss is negligible.
e. Recirculation control system.
(1) To prevent overheating and pump damage, each boiler feed pump will have its own recirculation control system to maintain minimum pump flow whenever the pump is in operation. The control system will consist off
(a) Flow element to be installed in the pump suction line.
(b) Flow controller.
(c) Flow control valve.
(d) Breakdown orifice.
(2) Whenever the pump flow decreases to minimum required flow, as measured by the flow element in the suction line, the flow controller will be designed to open the flow control valve to maintain minimum pump flow. The recirculation line will be discharge to the deaerator. A breakdown orifice will be installed in the recirculation line just before it enters the deaerator to reduce the pressure from boiler feed pump discharge level to deaerator operating pressure.
f. Design criteria.
(1) Boiler feed pumps will comply with the latest evisions of the following standards:
(a) Hydraulics Institute (HI).
(b) American National Standards Institute (ANSI).
(2) Pump head characteristics will be maximum at zero flow with continuously decreasing head as flow increases to insure stable operation of one pump, or multiple pumps in parallel, at all loads.
(3) Pumps will operate quietly at all loads without internal flashing and operate continuously without overheating or objectionable noises at minimum recirculation flow.
(4) Provision will be made in pump design for expansion of
(a) Casing and rotor relative to one another.
(b) Casing relative to the base.
(c) Pump rotor relative to the shaft of the driver.
(d) Inner and outer casing for double casing pumps.
(5) All rotating parts will be balanced statically and dynamically for all speeds.
(6) Pump design will provide axial as well as radial balance of the rotor at all outputs.
(7) One end of the pump shaft will be accessible for portable tachometer measurements.
(8) Each pump will be provided with a pump warmup system so that when it is used as a standby
it can be hot, ready for quick startup. This is done by connecting a small bleed line and orifice from the common discharge header to the pump discharge inside of the stop and check valve. Hot water can then flow back through the pump and open suction valve to the common suction header, thus keeping the pump at operating temperature.
(9) Pump will be designed so that it will start safely from a cold start to full load in 60 seconds in
TM 5-811-6 an emergency, although it will normally be warmed before starting as described above.
3-31. Feedwater supply
a. General description.
(1) In general terms, the feedwater supply includes the condensate system as well as the boiler
feed system.
(2) The condensate system includes the condensate pumps, condensate piping, low pressure closed heaters, deaerator, and condensate system level and makeup controls. Cycle makeup may be introduced either into the condenser hotwell or the deaerator. For large quantities of makeup as in cogeneration plants, the deaerator maybe preferred as it contains a larger surge volume. The condenser, however, is better for this purpose when makeup is of high purity and corrosive (demineralized and undeaerated). With this arrangement, corrosive demineralized water can be deaerated in the condenser hotwell; the excess not immediately required for cycle makeup is extracted and pumped to an atmospheric storage tank where it will be passive in its deaerated state. As hotwell condensate is at a much lower temperature than deaerator condensate, the heat loss in the atmospheric storage tank is much less with this arrangement.
(3) The feedwater system includes the boiler
feed pumps, high pressure closed heaters, boiler feed suction and discharge piping, feedwater level controls for the boiler, and boiler desuperheater water supply with its piping and controls.
b. Unit vs. common system. Multiple unit cogeneration plants producing export steam as well as electric will always have ties for the high pressure steam, the extraction steam, and the high pressure feedwater system. If there are low pressure closed heaters incorporated into the prime movers, the condensate system usually remains independent for each such prime mover; however, the deaerator and boiler feed pumps are frequently common for all boilers although paralleling of independent high pressure heater trains (if part of the cycle) on the feedwater side maybe incorporated if high pressure bleeds on the primer movers are uncontrolled. Each cogeneration feedwater system must carefully be designed to suit the basic parameters of the cycle. Level control problems can become complex, particularly if the cycle includes multiple deaerators operating in parallel.
c. Feedwater controls. Condensate pumps, boiler feed pumps, deaerator, and closed feedwater heaters are described as equipment items under other headings in this manual. Feedwater system controls will consist of the following
(1) Condenser hotwell level controls which control hotwell level by recirculating condensate from the condensate pump discharge to the hotwell, by extracting excess fluid from the cycle and pumping it to atmospheric condensate storage (surge) tanks, and by introducing makeup (usually from the same condensate storage tanks) into the hotwell to replenish cycle fluid.
(2) Condensate pump minimum flow controls to recirculate sufficient condensate back to the condenser hotwell to prevent condensate pumps from overheating.
(3) Deaerator level controls to regulate amount of condensate transferred from condenser hotwell to deaerator and, in an emergency, to overflow excess water in the deaerator storage tank to the condensate storage tank(s).
(4) Numerous different control systems are possible for all three of the above categories. Regardless of the method selected, the hotwell and the deaerator level controls must be closely coordinated and integrated because the hotwell and deaerator tank are both surge vessels in the same fluid system.
(5) Other details on instruments and controls for the feedwater supply are described under Section 1 of Chapter 5, Instruments and Controls.




3-3. Steam power cycle economy
a. Introduction. Maximum overall efficiency and economy of a steam power cycle are the principal design
criteria for plant selection and design. In general, better efficiency, or lower heat rate, is accompanied
by higher costs for initial investment, operation and maintenance. However, more efficient cycles are more complex and may be less reliable per unit of capacity or investment cost than simpler and
less efficient cycles. Efficiency characteristics can be listed as follows:
(1) Higher steam pressures and temperatures contribute to better, or lower, heat rates.
(2) For condensing cycles, lower back pressures increase efficiency except that for each particular turbine unit there is a crossover point where lowering back pressure further will commence to decrease efficiency because the incremental exhaust loss effect is greater than the incremental increase in available energy.
(3) The use of stage or regenerative feedwater cycles improves heat rates, with greater improvement
corresponding to larger numbers of such heaters. In a regenerative cycle, there is also a thermodynamic
crossover point where lowering of an extraction pressure causes less steam to flow through the
extraction piping to the feedwater heaters, reducing the feedwater temperature. There is also a limit to
the number of stages of extraction/feedwater heating which may be economically added to the cycle. This occurs when additional cycle efficiency no longer justifies the increased capital cost
(4) Larger turbine generator units are generally more efficient that smaller units.
(5) Multi-stage and multi-valve turbines are more economical than single stage or single valve machines.
(6) Steam generators of more elaborate design, or with heat saving accessory equipment are more efficient.
b. Heat rate units and definitions. The economy or efficiency of a steam power plant cycle is ex-less efficient cycles. Efficiency characteristics can be listed as follows:
(1) Higher steam pressures and temperatures contribute to better, or lower, heat rates.
(2) For condensing cycles, lower back pressures increase efficiency except that for each particular turbine unit there is a crossover point where lowering back pressure further will commence to decrease efficiency because the incremental exhaust loss effect is greater than the incremental increase in available energy.
(3) The use of stage or regenerative feedwater cycles improves heat rates, with greater improvement corresponding to larger numbers of such heaters. In a regenerative cycle, there is also a thermodynamic
crossover point where lowering of an extraction pressure causes less steam to flow through the extraction piping to the feedwater heaters, reducing the feedwater temperature. There is also a limit to the number of stages of extraction/feedwater heating which may be economically added to the cycle. This occurs when additional cycle efficiency no longer justifies the increased capital cost.
(4) Larger turbine generator units are generally more efficient that smaller units.
(5) Multi-stage and multi-valve turbines are more economical than single stage or single valve machines.
(6) Steam generators of more elaborate design, or with heat saving accessory equipment are more efficient.