Suction piping arrangement

Q - A pump vendor recommends the length of straight run of suction pipe
connected to a pump suction nozzle to be four times (4x) the suction pipe
diameter. Another vendor recommends twice as much length (8x). Which is
correct?

A - Both are correct. Which one to follow depends on the pump type and its
suction piping set-up.

Elbows, valves, and other suction pipe fittings cause flow disturbance to the liquid
as it enters the impeller eye. The effects of the flow disturbance is to increase the
suction entrance loss, reduce the NPSHA at the impeller eye, and in hydraulic
thrust imbalance.

The straight run of suction pipe allows the disturbance to settle down, and the
liquid to stabilize before it enters the impeller eye. The recommended length of
straight pipe depends on the NPSH margin, and the sensitivity of the impeller to
flow separation.

Generally a straight run of 4x the suction pipe diameter is ample for end suction
pumps where the liquids feeds directly into the impeller eye. The same is true on
pumps with double suction impeller provided the nearest elbow is piped in a
vertical plane and that there is ample NPSH margin.

If the elbow is in horizontal plane, or if the NPSH margin is tight, a straight run of
up to 8x the suction pipe diameter is preferred to avoid uneven flow into the two
sides of a double suction impeller. Uneven flow can result in hydraulic thrust
imbalance.

Q  -  The recommended minimum length of straight run of suction pipe cannot be
met because of limited space. Is there a way to compensate for this deficiency?

A  -  It will help to provide the suction with a "cross splitter". A cross (+) splitter is
simply a couple of short ribs welded inside the suction pipe. One rib is welded
horizontally on one end; the other is welded vertically on the other end of the pipe.
The metal ribs are  1/8" to 1/4" thick depending on pipe size, and at least one
diameter in length. This will help stabilize any flow disturbance caused by an
adjacent elbow, bend, valve, or other fittings.

Question:
We have a double suction, horizontal pump. A 90-degree elbow in horizontal
position is directly piped to its inlet. On start up the pump experienced cavitation in
spite of ample NPSHA and the bearing temperature was high. We believe those
were caused by flow imbalance into the double suction impeller. Will reorienting
the inlet piping such that the elbow will be in a vertical plane solve the flow
imbalance?

R-0509-SUPI
Category: trouble-shooting, installation
File: suction piping

Related topic: suction strainers
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