Suction piping requirements


You must do three things to avoid problems:

  • Eliminate air entrainment in the liquid,
  • Minimize friction loss, provide straight and uniform flow at the pump inlet
  • Avoid excessive forces due to pipe strains at the pump.

All inlet (suction) fitting joints should be tight, especially when the pressure in the piping is below atmospheric, to preclude air leaking into the fluid.

  • Any valves in the inlet (suction) line should be installed with stems horizontal to eliminate the possibility of air accumulation. For pumps operating with a suction lift, the inlet (suction) line should slope constantly upwards toward the pump, with a minimum slope of 1 percent.

As liquid travels through a piping network, entrained air tends to rise to the highest point. If the pipeline slopes upward, then the velocity of the liquid will move the air bubbles towards this high point.

  • If the pipeline is fairly flat and the inside surface of the pipe is very rough, or the pipeline slopes downward, the fluid velocity may not be sufficient to keep the air bubbles moving.
  • It is possible for a pocket of air to collect at high points and gradually reduce the effective liquid flow area, which can create a throttling effect similar to a partially closed valve.

The suction pipe should be at least as large as the pump suction nozzle.

  • A larger size is desireable. Make sure the reducer is not installed upside down.
  • Valves and other flow-disturbing fittings located in pump inlet (suction) piping should be at least one pipe size larger than the pump inlet (suction) nozzle, with the exception of continuous-bore, 100 percent open valves (such as full-ported ball valves).
  • The maximum velocity at any point in the inlet (suction) piping is 8-ft/s. For fluids close to the vapor pressure, the velocity must be kept low enough to avoid flashing (cavitation) of the liquid in the piping, especially when fittings are present.

The most disturbing flow patterns to a pump are those that result from swirling liquid that has traversed several changes of direction in various planes.

  • Liquid in the inlet (suction) pipe should approach the pump in a state of straight steady flow.
    • When fittings, such as tees and elbows (especially two elbows at right angles), are located too close to the pump inlet (suction), a spinning action, or swirl, is induced.
  • This swirl may adversely affect pump performance by reducing efficiency, head, and NPSH available, and potentially causing noise, vibration, and damage.
  • It is recommended that a single uninterrupted section of pipe be installed between the pump and the nearest fitting to allow the flow to straighten itself.

The suction pipe design must also be designed and built to minimize forces or strains on the pump suction nozzle.

  • The pump must not be used as an anchor to close gaps due to construction errors or to withstand forces from pipe expansion due to temperature changes during operation.