Pressure Safety Valve required Surface finish/flatness for sealing Surfaces

[MechatroPro]

what is the required Surface finish and Flatness for seat surfaces of Pressure safety valves in high pressure gas service? I've seen PSVs with metal to metal seat design with perfect mirror finish and flatness, however I'm not sure if there's an established standard for it, either ASME or API. Is there any standard code about this?

 

[gerhardl]

Even if you narrow down the pressure safety valve group down to metal-seated safety valves for high-pressure gas (but still open for further specification : which high pressure, types of PSVs', and for wich gases (steam is also a gas) and temperatures?) I doubt you will find a common factor here for seat flatness and smoothness.

The reasom, in my mind, is the vast difference of construction details (also of seats), different materials and material combinations, and that all final tests goes on function and thightness/leakage rather than middle manufacturing test of seat details.

Of course any respectable factory will have some mechanical description of tooling processes and manufacturing accuracy, down to exact figures and measurement of smoothness and flatness, but the last only if necessary. In addition any metal seat with a scratch, even microscopic, might leak and by cavitation further damage the seat, so at least visual inspection for scratchfree seats is a must.

Again in my opinion flatness and smoothness will in most cases be based on detailed instructions for tooling/working processes and QA checks of results in general.

Even if below link is based one producers information and hence somewhat limited, I find this illutrative of the multitude af valvetypes, certifications and regulations we are talking about.

http://www.spiraxsarco.com/resource...rials/safety-valves/types-of-safety-valve.asp