On this page
Key takeaways
- Pumps, compressors and turbines have allowable nozzle loads set by API standards and vendors, and they are much lower than for vessels.
- Excess loads distort casings and misalign couplings, causing vibration, seal leaks and bearing failures.
- Meeting the limits usually needs flexible routing, careful support placement and sometimes spring supports, not just stronger steel.
Why rotating equipment is different
A vessel nozzle on a thick shell can take large piping loads. A pump or compressor nozzle is attached to a casing that holds precisely aligned internal parts. Even small distortion can shift the shaft relative to the casing or the driver, leading to:
- coupling misalignment and vibration,
- seal and bearing failures,
- internal rubbing and wear.
Where the limits come from
- API 610 gives allowable forces and moments on centrifugal pump nozzles as a table by nozzle size, with an alternative method that allows higher loads under conditions.
- API 617 for centrifugal compressors and NEMA SM23 for steam turbines set allowables (API 617 uses 1.85 times the NEMA SM23 values, unless the vendor states otherwise).
- API 661 covers air-cooled heat exchanger nozzle loads.
- For other equipment, use vendor allowables from the datasheet.
For vessel and tank nozzles, local stresses are checked with methods such as WRC 107 / WRC 537 and WRC 297, or finite element analysis.
How piping meets the limits
- Flexibility in the routing. A direct, short connection from a hot header to a pump nozzle almost never works. Offsets and loops absorb expansion before the nozzle.
- Supports close to the nozzle. An adjustable support just before the nozzle carries the weight so the nozzle doesn’t.
- Spring supports where the equipment nozzle moves vertically with temperature.
- Line stops and guides to direct thermal growth away from the nozzle.
- Thermal cases for all operating modes, including standby pumps, which are cold while the running pump is hot.
Alignment at construction
Even a perfect design fails if the piping is forced into place. Many specifications require that flanges at rotating equipment line up without force, and that the coupling alignment is checked before and after the piping is bolted up.
Onshore
Onshore pumps sit on concrete foundations with good access, so supports and springs can be added fairly easily.
Offshore
Offshore rotating equipment sits on skids that also move with the deck, so the skid, the piping and the deck are often modelled together.
Code reference
API 610 Centrifugal pumps, including allowable nozzle loads.
API 617 Axial and centrifugal compressors and expander-compressors.
NEMA SM23 Steam turbines for mechanical drive service.
What to take back to your desk
Get the vendor’s allowable nozzle loads before the pump layout is frozen. If you get them after, the layout will change.
Educational content. Always design to your project specification.
