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When piping the suction side of a centrifugal pump, it is not always best practice to adapt to the size of the suction and continue back to the source. In choosing pipe size, consider a larger pipe diameter than the suction connection, particularly if the source is not near the pump.

 

A larger diameter suction line will reduce piping losses. When choosing a larger diameter, an eccentric reducer is a good choice with the flat side up to avoid an air trap in the suction. Also, it is recommended that the suction be a straight pipe for 5 to 10 times the diameter before entering the pump to reduce turbulence which can add to cavitation risks. This would mean that a pump with a 2” suction should have straight pipe from 10 to 20 inches.

 

Often piping to the suction is the source of either pre-mature failure or an underperforming pump due to what is called insufficient Net Positive Suction Head (NPSH). Centrifugal pumps must have available NPSH above the required NPSH to perform adequately and avoid damage from cavitation. NPSHa is a calculation of the positive head in the system that will cause fluid to enter the pump suction. If this number is less than the NPSHr (suction head required by the pump manufacturer) cavitation will occur, and premature failure will follow.

 

NPSHa is made up of four major components:

• Pressure
• Piping loss
• Fluid vaporization temperature
• Elevation

 

Piping loss is always a negative, so reducing it ill reduce NPSHa as well. Careful consideration of the range in elevation in relation to the piping is also critical.

 

Foot valves or check valves are often used in suction piping either to hold prime in the pump or to prevent backflow after shut-off. It is good practice to verify the head loss in the valve or the Cv number associated with the valve to determine how much head loss will need to be calculated in NPSHa. For reference, the pressure drop through a valve is (Flow/Cv) ^2 for water at 60° F, or the square of flow in GPM divided by the Cv. This head loss plus the losses through any fittings and the pipe itself all contribute to losses to be included in NPSHa.

 

Finally, you will want to make sure that the inlet to the suction piping will have enough submergence to prevent cavitation at the inlet. This will appear as a swirling on the surface of the liquid if the submergence is too small. As the volume being pumped increases, the required submergence will also increase. For instance, 30 gpm being pumped through 2” pipe requires 8” of submergence to prevent cavitation.

 

By observing the recommendations above, an efficient piping system can be put in place. A good suction system is critical to pump performance and is especially critical to the life of the centrifugal pump. A solid system with proper sizing to the best flow range of the pump will reduce vibration, noise, and turbulence in the flow.