Is a Wafer Check Valve, when subjected to reverse flow, a stop valve for the purpose of pipe design according to ASME Code 34.3?  The main purpose of this question is to determine the boundry between system design pressures. 

I don't think it's wise to depend on a check valve as your only means of isolation.  A manually operated valve should be included downstream of the check valve.

I agree that one should not rely on a check valve as a means of isolation, especially to work behind (say on a pump or control valve where you have a line break).  

But with regard to defining the design temperature and pressure of a pipe system.  That is the gist of the question.

A check valve should not be used against pressure protection.  But in defining spec break,  it is acceptable to have a spec break at upstream of check valve, ie check valve has the same rating of the downstream piping and equipment.  I am assuming the downstream equipment rating is higher than upstream equipment and you have other isolation valves at the inlet of the downstream equipment.  However, you will require a PSV upstream of the check valves. In case the check valve leak. See API RP 14E or 14J.

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