Checking and Maintenance of the Dissimilar Check Valves

You have not given any information about type of valve, whether it is single flap, double flap, spring operated or with hydraulic cylinder. Also no information about orientation (horizontal/vertical) and type of fluid handled. I only can presume that back flow may be disastrous because two check valves are installed in series.

I can suggest some simple ways, but it may need some modifications, if your system is not equipped with it.

There must be an isolation valve before the first check valve (if not there, install it). Install two 1/2" vent valves between the isolation valve & first check valve and between the first check valve & the second check valve. If your fluid is liquid it should orient downwards (drain valves). For checking, depressurise upstream of isolation valve, close the isolation valve, open the 1/2" vent/drain one by one and check the leak for respective check valve. Downstream of second check valve must be at full operating pressure.

We use double check valves and have to test these regularly.

There is a need to be able to access the fluid between the valves so that each can be verified independantly. This does not need to be full pipe diameter. Our test points are 5mmID for a 200mm system.

The standard will depend on your situation. there will be huge differences relating to whether you are in petrochemical (Big bang when things go wrong), sewage (Big pong when things go wrong) or water (BigPond when things go wrong).