Nozzle load

At every nozzle the piping transmits three forces and three moments to the equipment – different ones for each load case. Together they are the nozzle load. It is also an interface between two responsible parties: the piping designer calculates the actual loads, the equipment manufacturer states the allowable ones.

For both to talk about the same thing, two points have to be agreed: the coordinate system, usually referred to the nozzle axis and the vessel wall, and the reference point – the flange face or the junction of nozzle and shell.

In larger projects the equipment specification contains a nozzle load table, graded by nozzle size and rating. The equipment is designed for these loads before the piping has been analysed; the pipe stress analysis later has to stay below them.

Where there is no table, the manufacturer states allowable values, or the nozzle is verified individually for the actual loads. The rules of EN 13445-3 for local loads on nozzles, the bulletins of the Welding Research Council and finite element analysis are available for this. For nozzles in the shell of storage tanks, API 650 contains its own procedure.

Two assumptions in the calculation model decide the magnitude of the nozzle load. The first is the movement of the nozzle itself: a hot vessel expands and the nozzle moves with it. This displacement is applied in the model as an imposed movement – if it is missing, the result is wrong, and it can be wrong in either direction.

The second is the stiffness of the connection. Modelled as a rigid anchor, the nozzle yields the highest loads. In reality the vessel wall gives way. If this flexibility is included as a spring, the loads often fall considerably.

The sequence of measures follows the effort – from the support to the equipment.

Most discrepancies in a nozzle load comparison are bookkeeping errors, not strength problems.