High temperatures and danger of impeller cavitation is the major challenge of condensate pumping
Often its necessary to pump generated condensate from heat exchangers and other consumers widely distributed in the plant, back to the condensate receiver in the boiler house. A special challenge with hot condensate, which is often close to 212oF (100oC), is cavitation of the pump and the pump impeller.
Centrifugal pumps generates lower pressure behind the wheels, and the hot condensate temporarily evaporate and expand on the back side of the vanes - before it implodes and condensate. Over time this will erode and destroy the pump impeller.
To avoid the problem there are two alternative solutions:
- Add pressure to the suction side of the pump
- Use a pressure powered pump instead of a centrifugal pump
Add Pressure to the Suction Side of the Pump
If the absolute pressure exceeds the vapor pressure at the actual temperature of the fluid entering the pump, then the Net Positive Suction Head (NPSH) is positive and its theoretical possible to avoid cavitation.
A NPSH above the manufacturers specification is important to avoid the water start boiling behind the impeller. The NPSH can be expressed as:
NSPH = 144 / ρ(pa - pvp) + hs - hf (1)
ρ = density of water at the appropriate temperature (lb/ft3)
pa = absolute pressure in the condensate receiver supplying the condensate pump. This is the same as atmospheric pressure if the receiver is vented (psi)
pvp = absolute pressure of condensate at the liquid temperature (psi)
hs = total suction head in feet. Positive for a head above the pump and negative for a lift to the pump.
hf = friction loss in the suction piping
According to (1) the NPSH can be increased by
- increasing the difference in the pressure in the receiver and the condensate pressure, and/or
- extend the static difference hs by lifting the receiver or lowering the pump, and/or
- increasing the piping dimensions for minimizing the friction loss hf in the suction pipe
If it's not possible to increase the suction pipe and lowering the pump regarding the receiver, it's possible to reduce the absolute pressure of the condensate Pvp, by reducing the condensate temperature with a cooling exchanger on the suction pipe.
Use a Pressure Powered Pump
A pressure powered pump use steam or air pressure to push the condensate from the receiver back to the boiler room. In principle its a simple mechanical construction working in a cycle where a receiver is filled with condensate before the condensate is pushed out and back to the boiler room.
The pump don't need external power, it can use the available steam (or pressurized air), and there is no danger for cavitation.
Other Liquids as LPG
The pumping of other boiling liquids - as LPG (-43oC in normal atmospheric pressure) - offers the same challenges to the manufactures and users. LPG is stored at exactly its boiling point (at the actual pressure in the tank) and any increase of temperature, as well as any decrease in pressure, will cause the product to boil and form vapor. In many installations, the suction friction head is equal or larger than the static suction head, making the available NPSH a negative value. The pressure drop due to the flow restrictions in the inlet piping system, e.g., excess flow valve, control valves, fittings, strainer, etc., will induce the LPG vapor formation at the pumps suction port.