Pump Nozzle Loads Under API 610: Checking Allowable Forces and Moments in CAESAR II

A centrifugal pump is only as good as the alignment between its shaft and its driver, and piping is one of the few things that can quietly destroy that alignment. This post explains how API 610 limits pump nozzle loads, how to model a pump in CAESAR II, and how to check and fix a hot-line case using a worked example.

Why Pump Nozzle Loads Matter

Every force and moment from the suction and discharge piping lands on the pump casing. Excess load distorts the casing, shifts the casing feet against the baseplate and moves the shaft relative to the driver. The result is misalignment, worn seals and bearings, coupling damage and shorter pump life. API 610 sets nozzle load limits so that shaft-end movement stays small, roughly 0.25 mm (0.010 in), once thermal growth and installation error are added. For background on how these machines develop head and flow, see our note on turbomachinery and pumps.

Base-mounted end-suction centrifugal pumps: every pipe load at the flanges is carried by the casing and baseplate. Photo: Bitjungle, CC BY-SA 4.0, via Wikimedia Commons.

API 610 Table 5 and Annex F

The nozzle loads table (Table 5 in the 11th edition; confirm the numbering in your edition) lists allowable forces and moments, Fx, Fy, Fz, Mx, My, Mz and their resultants, for each nozzle size up to NPS 16 on steel casings. Values depend on nozzle size and orientation, and API 610 defines the coordinate system to use. The pressure casing is designed to withstand twice the Table 5 loads. That margin allows Annex F to qualify loads above the table, but only with purchaser approval, and the standard warns that misalignment can be up to 50% greater than with Table 5 loads. For a horizontal pump, Annex F requires:

• F.1.2.1: each individual force and moment component on a nozzle must not exceed 2 × its Table 5 value.
• F.1 and F.2: F_RSA / (1.5 F_RST) + M_RSA / (1.5 M_RST) ≤ 2 for suction, and the same form for discharge. A is the applied value and T is the Table 5 value.
• F.3: at the pump centre, F_RCA ≤ 1.5 (F_RST + F_RDT).
• F.4: M_YCA ≤ 2.0 (M_YST + M_YDT).
• F.5: M_RCA ≤ 1.5 (M_RST + M_RDT).

The pump-centre values come from translating the nozzle loads to the pump centreline using the nozzle coordinates and the API 610 sign convention.

Modelling the Pump in CAESAR II

• Place nodes at the suction and discharge flange faces and connect each to the pump centreline with rigid elements representing the casing.
• Anchor the centreline node, with imposed displacements for casing thermal growth in the operating case.
• Use the API 610 equipment compliance check, map each nozzle to the correct API axes and set the allowable load multiplier. Use 1.0 for a strict Table 5 check, or up to 2.0 where Annex F is accepted with vendor and purchaser approval.
• Evaluate suction and discharge in the same load case, because the Annex F equations and pump-centre checks combine them.
• Run the operating, sustained and standby cases. In standby, the casing is cold while the connected lines may be hot.

Treat the pump check as part of the wider workflow in our overview of piping design and stress analysis.

Thermal Growth, Cold Spring and Springs Near the Pump

Both the casing and the connected pipe grow with temperature, following ΔL = α × L × ΔT with α near 12 × 10^-6 per °C for carbon steel. An OH2 pump is supported at the shaft centreline, so the casing grows up and outward from that line. With a nozzle 450 mm above the centreline and ΔT = 200 K, the nozzle rises about 1.1 mm. A 3 m vertical discharge riser over the same ΔT grows about 7 mm. If the support under it resists that growth, the difference goes straight into the nozzle.

Cold spring, where a pipe is deliberately fitted short to offset growth, is rarely used at pumps. It relies on fit-up accuracy that is hard to verify, and codes restrict the credit allowed. Prefer routing flexibility and well-chosen supports. A variable spring hanger near the nozzle carries pipe weight without loading the casing and without resisting thermal movement. For selection, spacing and guide placement, see our guide to pipe supports in process piping.

Closed cooling water pumps at Gebze power plant, where large-bore suction and discharge lines need flexible routing and supports to keep nozzle loads within limits. Photo: Harald the Bard, CC BY-SA 4.0, via Wikimedia Commons.

Worked Example: OH2 Pump on a Hot Line

An OH2 end-suction pump handles hydrocarbon at 220 °C, with an NPS 8 suction and an NPS 6 top discharge. The discharge rises 3 m before turning to a header. The Table 5 resultants below are assumed for illustration only. Use the values from your API 610 edition or the vendor datasheet on a real project.

• Suction (NPS 8): F_RST = 4,500 N, M_RST = 2,300 N·m.
• Discharge (NPS 6): F_RDT = 3,300 N, M_RDT = 1,700 N·m.

First run, as routed. Suction applied loads are 5,200 N and 3,400 N·m, so the F.1 sum is 5,200 / 6,750 + 3,400 / 3,450 = 0.77 + 0.99 = 1.76, which passes. Discharge applied loads are 5,900 N and 3,900 N·m, so the F.2 sum is 5,900 / 4,950 + 3,900 / 2,550 = 1.19 + 1.53 = 2.72, which fails. At the pump centre, the limits are 1.5 × (4,500 + 3,300) = 11,700 N for force and 1.5 × (2,300 + 1,700) = 6,000 N·m for moment.

The fix. The rigid rest under the first discharge elbow was resisting riser growth. The fix replaced it with a variable spring hanger on the riser, added a 600 mm offset leg before the header and moved the first guide further from the nozzle. After the re-run, discharge applied loads fell to 2,900 N and 1,900 N·m, and suction loads to 3,600 N and 2,000 N·m.

Check	Limit	As routed	After fix
Suction, F.1 sum	≤ 2	1.76 (pass)	1.11 (pass)
Discharge, F.2 sum	≤ 2	2.72 (fail)	1.33 (pass)
Pump-centre force, F.3	≤ 11,700 N	8,400 N (pass)	6,800 N (pass)
Pump-centre moment, F.5	≤ 6,000 N·m	7,100 N·m (fail)	4,900 N·m (pass)

The F.1.2.1 component check and the F.4 moment check must also pass in the software report.

Practical Engineering Checks

• Confirm the allowables: API 610 pumps use Table 5, while non-API pumps often have lower vendor limits, or follow ANSI/HI 9.6.2.
• Check axes and signs: the loads on the pump are the reactions from the piping, and each nozzle has its own API axis orientation.
• Cover every operating mode: include start-up, upset, steam-out, standby and hydrotest weight.
• Support weight near the nozzle: avoid letting strainer, valve and elbow weight sit on the casing.
• Take care with expansion joints: pressure thrust can load the nozzle unless the joint is anchored or tied.
• Protect suction hydraulics: flexibility changes must not add friction losses that cut NPSH margin. For the basics, see viscous flow in pipes.
• Get approvals in writing: record vendor and purchaser acceptance of 2 × Table 5 or Annex F use.
• Re-check after changes: layout revisions and as-built conditions can change nozzle loads.

How Fluxiss Approaches Pump Nozzle Checks

Fluxiss is a multidisciplinary engineering consultancy. Its pipe stress analysis work treats equipment nozzle loads as part of the same model as supports and flexibility, so a fix at the pump is checked against the rest of the line.

Frequently Asked Questions

Can I always use 2 × Table 5 for pump nozzles?

No. Annex F allows loads above Table 5 only when its combined criteria are met and the purchaser approves. Vendor confirmation should be obtained early.

Do I check suction and discharge separately?

Check each nozzle, then check both together at the pump centre. The Annex F equations combine them, so both must be evaluated in the same load case.

Is cold spring a good fix for high pump nozzle loads?

Usually not. It depends on fit-up accuracy that cannot be easily verified, and codes limit the credit taken. Reroute or change supports first.

What if the pump is not API 610?

Use the vendor's allowable loads, or ANSI/HI 9.6.2 where the project specifies it. These values are often lower than API 610.

Need pump nozzle load checks?

Fluxiss provides pipe stress analysis that includes equipment nozzle load checks, support design and reporting for oil and gas, power and industrial projects. Share your pump data and line layout, and the team can review nozzle loads against API 610 or vendor allowables.

Discuss your project with Fluxiss.