The use of expansion (or flexible) pipe joints in a pump system that has no excessive temperature differential problems is only necessary when the pipe fitters installing the piping are unable or unwilling to install the piping without imposing any strain on the pump nozzles.

When they are used, it should be identified why they’re being used. The most common reason is to protect the pump against pipe strain that may be imposed by the movement of the piping during system operation. In such case, the side of the flexible pipe joint closest to the pump needs to be independently supported and secured against movement. This allows all the movement of the piping to be absorbed by the flexible joint, and none of it transferred to the pump.

Another thing to note is that an expansion joint will not necessary accommodate radial movement of the piping. Some of them are designed to accommodate only axial movement where the pipe and pump flanges are trying to get closer together or further apart. These are of no value against poor piping misalignment or thermal movement of the piping.