You fellas might be in over your heads here. If you don't know what NPSH is, then you shouldn't be engineering projects with delicate running parameters. There is a gross lack of info here, to include lack of monitoring devices, (thermometers, flow recorders, pressure gages, etc.), unsufficient mention of psi and temp at various points of interest, whether suction pressure is positive or negative, and what is their values, the list goes on and on.
What you need to do is not replace parts, as it will possibly repeat at a cost, but find and correct the problem.
If the owner of this site is compassionate, or has deep pockets, s/he, or they might incur the cost of refinement, as it WILL be their equipment upon completion.
Just my thoughts at this point.
No sweat!
They wouldn't listen anyway! Just keep on guessing.
