I am installing a small plunger pump (CAT 230 with variable speed drive) to pump water at low flowrates into a pressurised steam line in our R&D test facilities.

Reading the installation manual it states that the inlet line size must be a minimum of one size larger than the pump inlet fitting (1/2") to avoid cavitation.

How critical is this? I have bought all my fittings in the same size as the pump inlet (1/2"). Will I get cavitation damage by having the inlet piping the same size as inlet. My flow range is 40-200lph (pump capacity 500lph) and pressure range 5-50barg (max capacity 105barg).

Also is it OK to fit a non return valve 1 meter downstream of the pump (outlet line size 1/2").

Its always a bit dangerous when you don't follow mfgr recommendations for one reason or another.  If you don't actually cause real damage, it may be exactly the excuse the distributor needs to avoid warranty work at a later date.  That aside, NPSH is a critical variable as well as entrance velocity to the pump.  If you prefer not to use a larger suction fitting, at least guarantee that you have well more than adequate NPSH available. 

A downstream check valve should be OK.

The only reson I can think that the manufacturer limits the suction on a PD pump is that the pump case between the suction (check)valves and the nozzel are not rated for the pressure.  As long as you do not exceed that, then the higher the pressurre, the less chance of cavitation and vapor locking the pump.  On a PD pump, I always try to place the non return valve close to a point where the fluid is entering the next process, be that a heater(boiler), vessel, mixer, ect..  There are already two check valves in the pump. 

All that really matters in this case is that the pump has adequate NPSH in order to avoid cavitation.  However, with a positive displacement pump, the acceleration head is often overlooked when evaluating the NPSH.  We recently had severe problems with a set of diaphragm pumps installed to pump water from a boot on an overhe account accumulator.  The engineering company that designed the project forgot to accpim for acceleration head and the pumps were cavitating severely.  We had to add accumulators on the pump suction and discharge to resolve the problem.  Cameron Hydraulic has some good examples and all the information needed to evaluate NPSH for a reciprocating PD pump.  Because so many of the pumps we work with are centrifugal, it is easy to forget that the length of the suction line can be just as critical as the diameter.  In our case, it was the suction line length that was the root problem.  No matter how large a suction line we had installed, the pump would have cavitated from the acceleration in the very long suction line.  The accumulator effectively de-coupled the pump from the suction piping and eliminated the cavitation.

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