High temperatures and danger of impeller cavitation are the main challenge for pumping condensate in steam systems.
It is often 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 usually close to 212 O F (100 O C) , is the cavitation of the pump and pump impeller.
Centrifugal pumps generate less pressure behind the wheels and hot condensate temporarily evaporates and expands at the rear of the vanes – before imploding and condensing. Over time, this corrodes and destroys the pump impeller.
To avoid the problem, there are two workarounds:
- Add pressure to the suction side of the pump
- Use a pressure driven 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, the net positive suction ( NPHS ) is positive and it is possible to avoid cavitation.
One NPHS above the manufacturer's specification is important to prevent water from starting to boil behind the impeller. O NPHS can be expressed as:
NSPH = 144 / ρ(p a – p vp ) + h s – h f (1)
Where
r = density of water at the proper temperature (lb/ft 3 )
p a = absolute pressure in the condensate tank that feeds the condensate pump. This is the same as atmospheric pressure if the receiver is vented (psi)
p vp = absolute condensate evaporating pressure at liquid temperature (psi)
h s = total suction height in feet. Positive for a head above the pump and negative for an elevation to the pump.
h f = friction loss in the suction pipe
According to (1) NPSH can be increased by
- increasing the difference in pressure on the receiver and condensate pressure, and/or
- extend the static difference h s raising the receiver or lowering the pump and/or
- increasing the dimensions of the pipe to minimize friction loss h f in the suction tube
If it is not possible to raise the suction line and lower the pump under the sump, it may be possible to reduce the absolute evaporating pressure in the condensate P vp reducing the temperature of the condensate with a cooling exchanger in the suction tube.
Use a pressure driven pump
A pressure-driven pump uses steam or air pressure to push condensate from the receiver back into the boiler room. In principle, it is a simple intermittent mechanical construction running on a cycle, where a receiver in the pump is filled with condensate before steam or pressurized air pushes the condensate out and back into the boiler room.
External power is not required as available steam or pressurized air is used as power. There is no danger of cavitation.
Other liquids like LPG
Pumping other boiling liquids – such as LPG ( -43 O C at normal atmospheric pressure) – offers the same challenges for manufacturers and users. LPG is stored exactly at its boiling point (at actual tank pressure) and any increase in temperature, as well as any decrease in pressure, will cause the product to boil and form vapour. In many installations, the suction friction head is equal to or greater than the static suction head, making the available NPSH a negative value. Pressure drop due to flow restrictions in the inlet piping system, for example, excess flow valve, control valves, fittings, filter, etc., will induce the formation of LPG vapor in the suction port of the pumps.
Source: https://www.engineeringtoolbox.com/condensate-pumping-d_280.html
