NPSH for a Serial Configuration

http://www.mcnallyinstitute.com/11-html/11-12.html

if your second pump is very strong, you will have cavitation issues there, so be careful in calculations.

Upstream or downstream?

downstream of the existing pump.

Do you know anything about pumps?

Like, maybe their specifications?

The current pump it is a part of the last effect of an evaporator (juice side) and it has two functions:

1. recirculate the juice to the top of the effect

2. send the juice to a residence tank before another process

So, as a part of a modification the destination of point 2 will change (more piping, more fittings). So I want to install an downstream pump in the piping that sends juice to the tank. I don't want to change the current pump because the recirculation rate of the evaporator would change.

The current pump delivers a head of 2.5 bar at 10 m3/hr (required flow to the tank).

Use of pumps in serial position means that the downstream pump is the booster for the upstream one. You should take care that the NPSHA of the second pump is well over the NPSHR, so you will have to calculate the system resistance between both pumps and deduct it from the operating pressure of the 1st pump.

The remaining pressure should be over NPSHR of the 2nd pump, assuming both pumps are the same type and size.

BR, Gert

It is an iterative calculation based upon the performance curves of each pump and the system characteristic curve. If all else fails, hire a Process Engineer to do the maths.