Pump nozzle loads

A vessel nozzle only has to withstand the load from the piping. A pump has to keep running while it does. Forces and moments on the suction and discharge nozzles distort the casing and displace the pump relative to its driver. Even small displacements disturb the alignment of shaft and coupling – the consequences are increased vibration, premature bearing wear and leaking mechanical seals.

For this reason the allowable nozzle loads of a pump are usually well below what the nozzle could carry as a pressure part. The limit protects function, not strength.

The governing figure is the value stated by the pump manufacturer for the specific size. It is normally based on a standard: API 610, published as ISO 13709, for centrifugal pumps in the process industries and ISO 5199 for chemical process pumps contain tables of allowable forces and moments graded by nozzle size.

The values apply to a defined coordinate system and reference point, and they assume a sufficiently rigid installation of the pump. Anyone comparing loads from a pipe stress analysis with the table has to observe both – a sign or axis error makes the comparison worthless.

The highest load rarely occurs in normal operation. The cold condition after installation, operation at maximum temperature and every intermediate state the plant is intended to pass through have to be checked.

In a duty and standby arrangement this is above all the case in which one pump runs hot while the standby pump is cold: the header expands, the cold branch does not – and that is often when the nozzle loads on the idle pump are highest.

The remedies are a matter of design. They cost little at the design stage and a great deal in an existing plant.

An expansion joint at the pump nozzle looks like the simplest solution. An unrestrained bellows, however, transmits the pressure thrust – internal pressure times effective bellows area – in both directions. Without an anchor immediately behind it, this force acts on the pump nozzle and can be larger than anything thermal expansion would have produced without the joint.

Expansion joints at pumps therefore need either tie rods or hinges that contain the pressure thrust, or a support that transfers it into the structure.