I am presuming that you are talking about the intent of PG-58.3.2 of Section I.
Two stop valves are required when two boilers are connected to a common steam header or when a single boiler is connected to a steam header having another steam source. In this case boiler(s) should have at least one manhole. For two stop valves, Manhole opening is a condition but 15 psig is not a condition.
I am sure two stop valves cannot be replaced by one check valve and one stop valve.
Please see also the following interpretation;
Interpretation: I-82-09
Question: PG-58.3.2 requires, for two or more boilers connected to a common steam header, that the connection from each boiler having a manhole be fitted with two stop valves having an ample free-blow drain between them. Does this requirement apply if neither of the two boilers has a manhole?
Reply: No
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Please see another interpretation
Interpretation: I-92-90
Question: Does the presence of pressure reducing valves in a common steam header waive the requirements of PG-58.3.2 for two stop valves having an ample free blow drain between them?
Reply: No.
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Please see another interpreation on subject matter;
Interpretation: I-86-12
Question: When the steam piping of a boiler is to be connected to that of an existing boiler, is this arrangement considered to be multiple boilers on a common header and therefore subject to the provisions of PG-58.3.2?
Reply: Yes.
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Hope it helps a little.
Nasir
Welding Engineer
DESCON ENGINEERING LIMITED
PAKISTAN