Two different standards - They will not mate.

Anything else if defeating the objective of a standard.

*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

Is there any way to rectify these?

Cut one of them off and weld on a matching flange or just design the pipe spool this way. Not uncommon.

No other way other than a flange adaptor / small pup piece with an EN flange one end and an ASME class 150 flange the other end.

You CANNOT "adapt" one flange to meet the PCD or bolt size of the other. This is not allowed and is very poor practice.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

I have seen a thick flange ring which was tapped with holes to allow the studs to go in between each other ( assuming the number of bolt hols is the same), but it looks horrible and is defiantly a last resort.

what size flanges are these?
How many bolts in each one?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Thanks for your reply.

Its on the blower discharge end. (BS EN 1092-1 PN10) drilling & the ordered is 20" SS flange drilled to ASME B16.5 150#.
No. of holes are same.

Thank you.

So order a 20" SS flange drilled to BS-EN 1092-1 PN 10.

There is no other proper way to do this. You can only ask the supplier to give you a credit for the other flange when you swap it for the proper one. Maybe he says yes, maybe he says no. But that's life and why design checking is important, sometimes you make a guess and it turns out wrong.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.