An old mojonnier turned up on a site to be re installed & has some pipework not seen by us before.

It has a standard surge drum with liquid supply & return for plates.
On the bottom of the liquid leg into plates, just above oil drain, is a line with a non return valve.

This line is piped back upstream of high pressure liquid line makeup solenoid valve.

The suction line has standard back pressure regulator.

We can't see any obvious reason for this line with non return valve, any ideas?