FEEDWATER HEATERSIN THERMAL POWER PLANTS SHIVAJI CHOUDHURY
FEEDWATER HEATERS A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler In a steam power plant (usually modeled as a Rankine cycle), feedwater heaters allow the feedwater to be brought up to the saturation temperature very gradually.
PURPOSE OF FEEDWATER HEATERS Feedwater heaters serve three purposes in the power plant. They provide efficiency gains in the steam cycle by increasing the initial water temperature to the boiler, so there is less sensible heat addition which must occur in the boiler, They provide efficiency gains by reducing the heat rejected in the condenser, and they minimize thermal effects in the boiler. Steam is extracted from selected stages in the turbine to shell and tube heat exchangers or to open feed water heaters where the steam and feed water are in direct contact.
FEED WATER HEATERS In shell and tube or closed type feedwater heaters the feedwater flows through the tubes and the extracted steam condenses on the shell side. The condensed steam from each feedwater heater drains successively to the next lower pressure heater and is returned to the feedwater through a heater drain pump or through the condenser.
PRESSURE CLASSIFICATION Low Pressure Heater: A heater located (with regard to feedwater flow) between the condensate pump and either the boiler feed pump . It normally extracts steam from the low pressure turbine. High Pressure Heater: A heater located downstream of the boiler feed pump. Typically, the tube side design pressure is at least 100 KG/CM2, and the steam source is the high pressure turbine.
HEATERS TO POWER PLANT CYCLE The heating process by means of extraction steam is referred to as being regenerative. The feedwater heaters are an integral portion of the power plant thermodynamic cycle. Normally, there are multiple stages of feedwater heating. Each stage corresponds to a turbine extraction point. The presence of the heaters in the cycle enhances the thermal efficiency of the power plant.
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. 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. 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.
ZONES Condensing Zone: All feedwaters 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 the energy added by the heater occurs here. Subcooling Zone: The condensed steam enters this zone at the saturation temperature and is cooled by convective heat transfer from the incoming feedwater. Desuperheating Zone: The incoming steam enters this zone, giving up most of its superheat to the feedwater exiting from the heater.
TUBE MATERIAL Both copper alloys & non-ferrous alloys are used for the LP Heaters & HP Heaters tubes. Copper alloys are used extensively in the LP Heaters tubes. These alloys have got excellent thermal conductivity but on the other hand these alloys have problems of copper carry over & ammonia attack, which may require a complex boiler cleaning after short intervals. To avoid all the above problems, the stainless steel tubes are invariably used for LP Heaters. Stainless steel is unaffected at all operating conditions, except that, it is susceptible to chloride induced stress corrosion. Most common materials used for HP Heaters are carbon steel, stainless steel & monel metal.
FEEDWATER HEATER EFFICIENCY Two variables are used to monitor a feedwater heaters efficiency. The heater Terminal Temperature Difference or TTD is a measure of how close the outlet feedwater temperature is to the feedwater heater saturation temperature. The heater Drain Cooler Approach or DCA is a measure of how close the heater drain outlet temperature is to the feedwater inlet temperature.
IMPACT ON THERMAL PERFORMANCE The impact of a 1°F (0.56°C) increase in the TTD of the top heater is approximately a 0.016% increase in heat rate. For the other feed water heaters, a 1°F (0.56°C) increase in TTD increases the heat rate by approximately 0.013. If there is a 1°F (0.56°C) Increase in DCA, the corresponding increase in heat rate is 0.005%. The impact can be less at part load.
ITEMS AFFECT PERFORMANCE OF FEEDWATER HEATERS x Improper heater level can cause flashing in the drain cooler section and tube damage. — Check operation of automatic controls and level instrumentation. — Check for possible tube leaks in feedwater heater. — Vent valves may not be set up properly. x Improper extraction line pressure drops. — Possible problem with extraction line check valve. x Tube fouling due to corrosion affects the heat transfer in the heater .— Clean tube bundles x Continuous vent orifice plugging. x Channel pass partition/gasket leak.
Heat balance diagram -660 mwSuper critical thermal power plant Feedwater heaters in red
Efficiency Improvement as anEmissions Control Strategy Feedwater heaters are designed into the turbine cycle to improve efficiency (lower heat rate). As the heat rate decreases (heat rate improves), the amount of fuel for the same generation also goes down. Of course with less fuel burned, emissions are lowered.
BOILER OPERATION WITHOUT HPHEATERS Some plants have been designed for peaking by allowing the removal of HP heaters out of service but there is a significant heat rate penalty for this. Even if the boiler was designed for a low feedwater temperature condition,the increased gas temperature and weight through the superheater from higher firing to compensate for the lower feedwater heat input increases tube and support temperatures. The high temperature ferritic tubes are life limited by creep, as are ferritic to austenitic dissimilar welds and support or alignment casting attachment welds. Low feedwater temperature operation results in closer approach to tube "alarm" temperatures, and may require load reduction to stay within temperature limits.
Safety Issue during Tube Leaks As per HEI (14), section 6.1.2 (b), a feedwater heater shell-side safety valve shall be sized to pass the flow from a clean break in one tube resulting in the flow from two tubes. If a heater is not isolated promptly when a tube leak occurs, more tubes may be damaged resulting in a higher flow into the shell than the safety can relieve. Obviously as the heater becomes more susceptible to tube leaks, the risk of this scenario increases.
TUBE PLUGGING The majority of feedwater heaters are U-tube type. Consequently, it is very difficult to replace damaged tubes unless they are located on the periphery of the tube bundle. The most common practice is to plug both the inlets and outlets of damaged tubes. Once tube plugging begins, a feedwater heater’s performance starts to decline.
AFFECTS OF TUBES PLUGGED Tubes plugged in HP feedwater heaters will increase the pumping power requirements. On a unit that has a steam driven boiler feed pump, the turbine extracts more steam which is typically exhausted into the condenser. These turbines are generally less efficient than the main turbine, add extra heat loading to the condenser and therefore decrease efficiency. A motor-driven boiler feed pump will use more power, again increasing heat rate. In some cases, this extra power is not available so maximum generation will be reduced. Tubes plugged in LP feedwater heaters will also increase pumping power requirements.
HP HEATER PARAMETERS(210 MW)S.N PARAMETER UNIT DESIGN DESIGN HPH 5 HPH 61 BLEED STEAM PRESS KG/CM2 17.03 37.522 BLEED STEAM DEG C 431.4 336 TEMP3 INLET FEED WATER DEG C 164.2 197.8 TEMP4 OUTLET FEEDWATER DEG C 197.8 240 TEMP5 DRAIN TEMP DEG C 170.9 2076 DRIP IN TEMP DEG C 199 --7 SATURATION TEMP DEG C 203.1 2458 FW TEMP RISE DEG C 37 42.29 TTD DEG C 3.1 5.00
FEEDWATER HEATERS IN 500 MW To increase the cycle efficiency, the condensate and feed water is preheated in stages of low pressure & high pressure heaters . Extraction no. 1 is taken from LP turbine casing and connected to LP heater no. 1 which is under vaccum at full load. Extraction no. 2 and 3 are taken from LP turbine casing and connected to LP heater no. 2 and 3 respectively. Extraction nos. 4 and 5 are taken from I. P. Cylinder and supply steam to the deaerator and H. P. heater no. : 5A & 5B respectively. Extraction no. : 6 is taken off the cold reheat line and supplies steam to H. P. heater no. : 6A & 6B.