Applications & Industries
Pumps and compressors
Two questions decide almost everything here. How critical the machine is — whether it has an installed spare, and how fast and flexible its shafts are — determines which standard governs and how much documentation follows. Whether its torque is smooth or pulsating determines the service factor, and often the coupling technology.
- Author
- Priyansh Thummar, Editor
- Dates
- Published · Last updated
- Reading time
- 3 minutes
1. Criticality decides the standard
A spared pump in a process train is general-purpose service — the standby starts, production continues, the failed machine is repaired at leisure. An unspared compressor on a single train is special-purpose: the plant stops, and the cost dwarfs the price of any coupling.
Sparing is the usual deciding question on a pump train, but not the only one: ISO 14691 also points high-speed, flexible-shaft and unbalance-sensitive machines to ISO 10441. Together these are what separates ISO 14691 from ISO 10441, and they decide how much analysis, testing and traceability the order carries. Get it wrong in either direction and you pay: paperwork with no reliability gain, or a coupling that fails a documentation review at the end of the project.
2. Smooth or pulsating
This separates two machines a power figure makes look identical. A centrifugal machine applies torque essentially steadily. A reciprocating machine applies it in pulses at shaft frequency or a multiple — a triplex pump three times per revolution, every revolution, for its whole life.
The coupling on the second sees a peak-to-mean ratio well above unity continuously, and the same coupling that runs indefinitely on the first can wear progressively on it. That is what the service factor exists to separate.
3. Why pump couplings have spacers
More often maintenance than alignment. A spacer long enough to be removed lets the pump rotating element be withdrawn without disturbing the driver or breaking the pipework — the back pull-out arrangement process pump standards are built around.
The length is set by the maintenance envelope. The extra offset capacity is real, but usually a side effect rather than the reason.
4. Pipe strain is the alignment problem
The alignment problem on a pump train is rarely the alignment. Pipework pulled into position to meet a nozzle applies that load to the casing for its whole service life, moves the shaft centreline, and keeps moving it as the line heats and cools.
It presents as wear concentrated at one mesh — which correctly reads as misalignment at that end, and incorrectly leads to shimming the feet. The test takes minutes: set indicators on the shaft, slacken the flange bolts, watch whether anything moves.
5. Where a gear coupling stops being the answer
| Machine | Torque character | Sparing typically | Coupling consequence |
|---|---|---|---|
| Centrifugal pump | Smooth | Spared | General-purpose; gear coupling with a spacer is the default |
| Reciprocating / triplex pump | Pulsating every revolution | Often spared | Higher load class; interference fit worth considering |
| Centrifugal compressor, moderate speed | Smooth | Varies | Gear coupling viable; check sparing and speed first |
| Reciprocating compressor | Pulsating, with reversal content | Often unspared | Highest load class; torsional analysis likely |
| High-speed unspared turbomachinery | Smooth but fast | Unspared | Dry element — the grease film is the limit, not the teeth |
Where the machine is spared, moderate speed and inspected on a sensible interval, a gear coupling remains the densest torque per unit of envelope available — which is why it is still the default on general pump and compressor service and the exception on a special-purpose train. The full trade is in gear versus disc.
Frequently asked
- What decides whether a pump coupling is general-purpose or special-purpose?
- Criticality, which ISO 14691 frames in three ways. It recommends the special-purpose standard, ISO 10441, for large or high-speed machines that run continuously, are often unspared and are critical to the plant; for flexible-shaft machines, whose first lateral critical speed is below the maximum operating speed; and for rotors particularly sensitive to coupling unbalance. On a pump train sparing is usually the deciding question: a spared pump in a process train is general-purpose service, while an unspared compressor on a single train usually is not.
- Why do reciprocating compressors need a higher service factor?
- Because their torque is not steady. A reciprocating machine applies torque in pulses at shaft frequency or a multiple of it, so the coupling sees a peak-to-mean ratio well above unity on every revolution, for its whole life. A centrifugal machine of identical rated power applies torque essentially smoothly. Nothing about the average power distinguishes them; only the load class does.
- Why does a pump coupling usually have a spacer?
- Mostly for maintenance rather than alignment. A spacer long enough to be removed lets the pump rotating element be withdrawn without disturbing the driver or breaking the pipework — the back pull-out arrangement process pump standards are built around. The extra offset capacity that comes with the length is a genuine benefit, but it is usually a side effect rather than the reason.
- My pump coupling keeps wearing at one end. Is the coupling wrong?
- Probably not. Wear concentrated at one mesh means misalignment concentrated at that end, and on a pump the usual cause is pipe strain — pipework pulled into position to meet a nozzle, applying that load to the casing for the whole of its service life and moving the shaft centreline as the line heats and cools. No amount of shimming at the feet resolves it, and a larger coupling only survives it a little longer.
References
- ISO 14691:2008 and ISO 10441, general-purpose and special-purpose flexible couplings for petroleum, petrochemical and natural gas service.
- API 610, Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries, for the back pull-out arrangement in §3.
- API Standard 671, Special-Purpose Couplings for Petroleum, Chemical and Gas Industry Services.
- Correction, 15 September 2026: earlier editions said sparing alone, not size or speed, separates ISO 14691 from ISO 10441. ISO 14691’s own scope and introduction also name speed, shaft dynamics and sensitivity to unbalance; §1 and the first answer have been corrected.
- Editorial note: no service factors, spacer lengths or speed thresholds are published here. The general- to special-purpose boundary is a decision about the machine's criticality made by the project, not a number this article can supply. Duty-specific selections and the field evidence behind them are to be added from Super Mech Industries records and confirmed at technical review.
Related articles
Standards
ISO 14691
General-purpose and special-purpose, and where the boundary falls.
Coupling Types
Between shaft ends
Spacer length, the back pull-out arrangement and what length costs.
Selection & Sizing
Service factor guide
Why a triplex pump and a centrifugal of equal power differ.