Flow
Changes with the speed: from 50 to 60 Hz the speed, and with it the flow, rises by a factor of 1.2.
Reference tool — not a selection
A pump at another frequency, before it goes in an RFQ.
Pump speed change
Frequency, flow, head.
Enter the frequencies or speeds and the pump duty. The drawing and the RFQ line update with every change.
All 130 engineering tools
A pump built for a 50 Hz supply runs faster on a 60 Hz ship, and everything moves: flow, head, power and the motor loading. Enter the duty from the plate and the two frequencies or speeds, and the calculator works out the new duty by the affinity laws, shows the motor loading, draws the duty point moving and writes the RFQ line.
The affinity laws are an estimate: they assume the same efficiency and the same fluid. The pump maker’s curves decide.
How it is worked out
The speed, its square and its cube.
Head
Changes with the square of the speed: by 1.44 from 50 to 60 Hz. The duty point moves up and to the right along the affinity parabola.
Power
Changes with the cube of the speed: by 1.73. That is why the motor is the first thing to check on a change to 60 Hz.
| Speed ratio | Example | Flow | Head | Power |
|---|---|---|---|---|
| 0.7 | 0.7 | 0.49 | 0.343 | |
| 0.8 | 0.8 | 0.64 | 0.512 | |
| 0.833 | 60 → 50 Hz | 0.833 | 0.694 | 0.579 |
| 0.9 | 0.9 | 0.81 | 0.729 | |
| 1.1 | 1.1 | 1.21 | 1.331 | |
| 1.2 | 50 → 60 Hz | 1.2 | 1.44 | 1.728 |
Flow is the speed ratio, head its square, power its cube; for the same fluid and an unchanged efficiency.
Sources
Affinity laws at constant impeller diameter: Wikipedia ("Affinity laws"), impeller.net and EASA; synchronous speed changing with the supply frequency (50 to 60 Hz by 1.2): impeller.net. The law for a smaller impeller differs between the sources (Wikipedia gives flow with the cube of the diameter for geometrically similar pumps, trimming practice the first power), so no trim is worked out.
Before you send the request
Questions about pumps at another frequency in an RFQ
What happens to a 50 Hz pump on a 60 Hz ship?
Without a frequency converter it runs a fifth faster: flow rises by about 20 %, head by about 44 % and shaft power by about 73 %. A motor sized for the 50 Hz duty can then be overloaded, so check its loading.
How accurate are the affinity laws?
They assume the same efficiency and the same fluid, so they are an estimate over a moderate speed range. In a real system the duty point moves along the system curve; the pump maker’s curves decide.
Does it work out an impeller trim?
No. The sources disagree on the law for a smaller impeller, so only the speed change is worked out. A trim to restore the original duty is the pump maker’s to specify.
What should the request include?
The pump maker, model and serial number, the impeller diameter, the duty and the speed on the plate, the ship’s supply frequency and voltage, and the motor rating.
Duty clear?
Send the line with photographs of the plates.
A calculation is an estimate, not a pump selection. Attach photographs of the pump and motor plates, and Vessel Core checks them before an offer goes back.
Buyer tools
