Page 151 - Centrifugal Pumps Design and Application

Vertical Pumps 129

Condensate and Heater Drain Pumps

This pump type is typically installed as two 50% capacity pumps tak-
ing suction from a header connected to a condenser or heater for boiler
feed water. The available NPSH is normally only two to four feet at the
mounting floor, requiring additional NPSH to be built into the barrel.
The multi-stage bowl assembly, typically in the 1,500 to 2,500 specific
speed range, can be fitted with a special first-stage impeller to meet the
required NPSH. Figure 9-13 shows a unit with a double suction first-
stage impeller. The suction nozzle may be located either in the barrel or
the discharge head, whichever the user prefers. The shaft seal in the dis-
charge head is typically of the mechanical face type and must be water
quenched because the seal is under vacuum when on standby. A continu-
ous vent line must be provided from the top of suction side in the pump to
the vapor phase in the suction tank (condenser). A minimum flow bypass
line may be required at the discharge control valve if extended low flow
operation cannot be avoided. Induction motor drive is the most common,
but a variable speed drive offers advantages for peak loaded plants. Coti-
densate pumps normally operate in the 130°F range, and cast iron bowls
with bronze impellers and bearings are usually adequate. For applica-
tions where the peripheral vane velocity in the suction eye exceeds 80
feet per second, a stainless steel impeller should be used. Some users will
not permit bronze materials in the system because it may contribute to
corrosive attack on condenser tube welds. In these cases, all impellers
should be furnished in martensitic steel. Heater drain pumps may operate
up to 350°F and require impellers of martensitic steel and bearings of a
carbon-graphite composite. Because flashing in the first stage cannot al-
ways be avoided in this service, injecting second-stage pump pressure
into the suction case bearing is recommended.

Process Pumps

These pumps are of multi-stage construction, with a special first-stage
impeller to meet the limited available NPSH. Handling liquids near their
boiling point requires a continuous vent line from the pump suction side
back to the suction source. The mechanical shaft seal can either be
mounted internally in the discharge stream, flushed and cooled by the
pumped liquid, or mounted in an external, water jacketed stuffing box
for high temperature applications. Materials for the bowl assembly and
fabricated components are selected to suit the liquids handled, including
cavitation resistant material for the first-stage impeller, when applicable.