Thanks for your answer, however I don't think my original question was clear enough. Working back from the boiler towards the feed pumps, B31.1 122.1.7(B.1) requires a stop valve closest to the boiler, and then a check valve. On a boiler I've seen, the line was arranged with a stop valve at the boiler, and then a stop check valve closer to the feed pumps. In accordance with the code, para. 122.1.7(B.6), this stop check valve only counts as a stop valve, and to be compliant the boiler would require an additional check valve to be installed. So... why is a combination stop check valve considered only to act as a stop valve. Why can no credit be taken for the "check" function of the valve.
Study the stop check design. It does not qualify because the stopping mechanism is bound on he check valve. Forget anything g closest to the pump, does not matter on B31.1. Usually the boiler mfr install and certifies the stop and check and are installed very closer to each other and near the boiler. Any secondary check valves are not a requirement per Code. GenBlr
