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Key takeaways
- Keep suction lines short, simple and free of air pockets, with a straight run into the pump.
- Use eccentric reducers with the flat side on top on suction lines so gas and vapour can’t collect.
- Support the piping so the pump nozzles see only small loads; the pump is not a pipe support.
Why suction piping matters
A centrifugal pump needs liquid arriving smoothly and with enough pressure above vapour pressure, known as net positive suction head (NPSH). Long, winding suction lines add friction and turbulence, which reduce the available NPSH and lead to cavitation, noise and seal failures.
Suction rules of thumb
- Short and direct. Place the pump close to the suction vessel and keep fittings to a minimum.
- Straight run. Give the pump a straight length before the suction nozzle so the flow is even. A common guideline is at least five pipe diameters, but follow the pump vendor and your project specification.
- Elbow orientation. For double-suction pumps, an elbow close to the nozzle in a plane parallel to the pump shaft can feed the two sides of the impeller unevenly. Use a straight run or turn the elbow plane perpendicular to the shaft.
- Eccentric reducers, flat side up. A concentric reducer on the suction leaves a pocket at the top where gas can collect and then be swallowed by the pump.
- No high points. The suction line should fall continuously to the pump, or rise continuously to it from a vessel below, never forming a hump.
- Strainers. A temporary strainer protects the pump during commissioning when debris is still in the system. Leave room to remove it.
Discharge rules of thumb
The usual sequence after the pump is: concentric reducer to line size, check valve, then isolation valve. Put the pressure gauge between the pump and the check valve. Leave room for a minimum flow recirculation line if the process needs one.
Nozzle loads
Pump vendors and API 610 set limits on the forces and moments the piping may put on the nozzles. Heavy piping or poorly placed supports twist the pump casing and misalign the coupling. Support the line close to the pump with adjustable supports, and check thermal loads in the stress analysis.
Onshore
Onshore pumps usually sit in a row under the pipe rack, with suction lines dropping from the rack or coming from nearby vessels. Access for pulling the pump and the motor is easy to plan.
Offshore
Offshore pumps are often skid-mounted with very short suction lines. The skid, the deck and vessel motion all move, so flexibility and support design need extra care, and removal routes for pumps and motors have to be planned with lifting beams.
Code reference
API 610 Centrifugal pumps for petroleum, petrochemical and natural gas industries, including allowable nozzle loads.
Hydraulic Institute (ANSI/HI 9.6.6) Rotodynamic pumps: guideline for pump piping.
What to take back to your desk
Before you finalise a pump layout, walk the suction line in the model from the vessel to the nozzle and ask: could gas collect anywhere? Is the flow straight when it arrives? If not, fix it before the vendor asks.
Educational content. Always design to your project specification.
