Proper Pump Suction Piping to Prevent Deprivation of Medium

1. Type of pump?

2. The input suction head available (is it positive or negative ie the pump is below the water level of lagoon or above it?)

If I understand correctly - You have one inlet of 150mm to the pump (let us forget about standby pumps and consider only the one in operation)

There are 4 outlet pipes (each 4" NB)

a) If I calculate the crossection area the

150mm pipe has 17671 sqmm

4" Pipe has 8107 sqmm each (x4 = 32429 sqmm)

(I am neglecting the pipe thickness effect that would make it precise, but in absense of schedules etc it is not possible to calculate exactly. But for this order of difference even this approximation is enough)

The almost half crossection area at the outlet may not only give suspicion towards insuffucient inlet piping size (which can only be confirmed if we know the LPM of the pump. You can calculate the usual LPM should translate to a maximum 3 to 5 meter per second.

The other effect is extremely low velocity at the outlet and that would reduce the efficiency of the heat exchanger. You do not want a stream lined flow after all in the cooling circuit.

b) calculate the Head Loss in the pipe - there is a good one here or even this may be enough. But then the equivalent pipe length you must calculate by taking care of The tees and elbows

Check up the Head available at pump. It should not be below the vapour pressure by quite a few degrees if you do not want cavitation.

This sounds odd; heat exchangers would more typically be piped in the dicharge of the pump.

A sketch with dimensions would help.

basic design for pump suction piping is that the net positive suction head (NPSH) of the installation should not be less than the net positive suction head of the pump (design). NPSH is a function of its suction static head, friction and vapor pressure. Thus, try to check these parameters. you might really need to redesign your suction line.

You have to calculate net postive suction head (NPSH) by calculating friction losses in the suction pipe and the static lift which the pump is required to work with. If the NPSH is not within the design NPSH of the pump, it will not function. In your case since the pump is running but stops after a while, please examine if there are any high spots in piping where air can get accumalated causing an air lock and starve the pump of water. Some times a leakage in the suction line can also permit air to be sucked in and cause airlock and loss of prime.

you said your pumps suffer a breakdown... what is the breakdown?

Before you start taking your pipework to bits and re-designing it all, look at the breakdown/failure of the pump, analyse that first, then find out how to fix it.