High temperatures and danger of impeller cavitation is the major challenge for condensate pumping in steam systems.
Often its necessary to pump condensate generated in heat exchangers and other consumers widely distributed in a 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 evaporates and expands on the back side of the vanes - before it implodes and condensates. Over time this erodes and destroys 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 possible to avoid cavitation.
A NPSH above the manufacturers specification is important to avoid that 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 evaporation 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 under the receiver it may be possible to reduce the absolute evaporation pressure in the condensate Pvp by reducing the condensate temperature with a cooling exchanger in 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 intermittent mechanical construction working in cycle where a receiver in the pump is filled with condensate before steam or pressurized air pushes the condensate out and back to the boiler room.
External power is not required since available steam or pressurized air are used as power. There is no danger for cavitation.
Other Liquids as LPG
The pumping of other boiling liquids - like LPG (-43oC in normal atmospheric pressure) - offers the same challenges to 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.