Heater Drain Pumps

Service Conditions Condensate drains from closed heaters can be flashed to the steam space of a lower-pressure heater or pumped into the feedwater cycle at some higher-pressure point. Piping each heater drain to the heater having the next lower pressure is the simpler mechanical arrangement and requires no power-driven equipment. This "cascading" is accomplished by an appropriate trap in each heater drain. A series of heaters can thus be drained by cascading from heater to heater in the order of descending pressure, the lowest being drained directly to the condenser.

This arrangement, however, introduces a loss of heat because the heat content of the drains from the lowest-pressure heater is dissipated in the condenser by transfer to the circulating water. It is generally the practice, therefore, to cascade only down to the lowest-pressure heater and pump the drains from that heater back into the feedwater cycle, as shown in Figure 38. Because the pressure in that heater hot well is low (frequently below atmospheric even at full load), heater drain pumps on that service are commonly described as on "low-pressure heater drain service."

In an open cycle, drains from heaters located beyond the deaerator are cascaded to the deaerator. Although the deaerator is generally located above the closed heaters, the difference in pressure is sufficient to overcome both the static and the frictional losses. This difference in pressure decreases with a reduction in load, however, and at some partial main turbine load it becomes insufficient to evacuate the heater drains. They must be switched to a lower-pressure heater or even to the condenser, with a subsequent loss of heat. To avoid these complications, a "high-pressure heater drain pump" is generally used to transfer these drains to the deaerator. Actually, this pump has a "reverse" system head to work against; at full load, the required total head may be negative, whereas at light loads, the required head is at its maximum.

BOILER FEED PUMP FIGURE 38 Typical arrangement for heater drain pumps

High-pressure drain pumps are subject to more severe conditions than boiler-feed pumps encounter:

1. Their suction pressure and temperature are higher.

2. The available NPSH is generally extremely limited.

3. They are subject to all the transient conditions to which the feed pump is exposed during sudden load fluctuations, and these transients are more severe than those at the feed pump suction.

Types of Heater Drain Pumps In the past, heater drain pumps were often horizontal, either single-stage or multistage, depending upon total head requirements. In the single stage type, end-suction pumps of the heavier "process pump" construction (Figure 43) were preferred for both low- and high-pressure service. Current construction features the vertical can-type pump (Figure 30) on heater drain services. As previously described, the advantages of the vertical can pump are lower first cost and a built-in additional NPSH because the first-stage impeller is lowered below floor level in the can. Against these advantages, one must weigh certain shortcomings. A horizontal heater drain pump is more easily inspected than a can pump. The external grease- or oil-lubricated bearings of the horizontal pump are less vulnerable to the severe operating conditions during swinging loads than the water-lubricated internal bearings of the can pump. If vertical can heater drain pumps have a bearing in the suction bell, consideration must be given to the fact that the water in the immediate location of that bearing is at near saturated pressure and temperature conditions (high temperature and low pressure). To keep the water in the bearing from flashing, additional water should be piped back to the bearing from a higher stage.

Heater drain pumps should be adequately vented to the steam space of the heater. Because heater drain pumps and especially those on low-pressure service may operate with suction pressures below atmospheric, it is necessary to provide a liquid supply to the seal cages in the stuffing boxes. Low-pressure heater drain pumps use cast iron casings and bronze fittings if no evidence of corrosion erosion has been uncovered. On high-pressure services, stainless steel components are generally mandatory and 12% chrome stainless steel casings are preferred.

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