Specifying valves for a oil centre.
For lethal gas, the ASME standard simply mentions that the valve should have enhanced stem seals.  No further definition or reference to any standards.
My opinion, ISO 15848 does not prove that the shaft seals will not leak once the gate valve is in the plant.  It is just an expensive, time consuming and overly complex lab. experiment that at best measures the permeability of the seal material.  It does let you know if the seal will remain tight after a thermal cycle.  But thats about it.
Once the valve is in the plant, outside factors will detemine if the shaft seal leaks.  Is the operator mounted properly or are there side loads on the valve shaft?  Is anyone monitoring for leaks and properly adjusting the packing load?  etc.  Do not rely on certificates from a lab. experiment as proof that a seal is going to remian tight in service.  Especially with lethal media.  You need to rely on experience and ensure you have the best seal possible for the application as proven through actual use in a plant.

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