A small needle type valve has a pressure rating of 3500 psi and a temperature rating of 900 F but the manufacturer (who is no longer around) doesn't indicate if this is a continuous service rating. What testing must I do to assure myself the stated psi/temp will work in a continuous 3500-psi, 900 F application?
What's the ASME/ANSI Standard I should buy to check this out?

I can't say for certain, but i would say that if the valves was designed suitable for those pressures and temperatures then it will work on continuous service.

However if there are process conditions that may cause damage to the valve, these may not be suitable for continuos duty (I.e high pressure drop and large flow rate etc..) might try putting some process figures through to a needle valve manufacturer and seeing what they suggest.

Typically the maximum pressure listed is not coincident with the maximum temperature listed and the working pressure rating at maximum temperature is usually substantially less than at ambient; based on reductions in the allowable stress values for the materials of construction at temperature.

However, it is not clear from the original post if the quoted rating was the coincident pressure temperature rating at 900 F or not?  Since it is quite typical for small needle valves to have a cold working pressure rating of 6000 psig, it is possible that the 3500 psig was the rated pressure at temperature.

Needle valves are not normally made to an ASME or API standard.  To the best of my knowledge they are typically made to MSS SP-99 "Instrument Valves" or MSS SP-105 "Instrument Valves for Code Applications".  Keep in mind that many needle valves are made to SP-99 (and not SP-105) even though they are routinely used in Code applications and unfortunately, the standard is rarely listed on the valve.

I looked at a couple of needle valve catalogues and in one the 900 F rating was only 33% of the cold rating (Anderson Greenwood) but in the other it was 83% of the cold rating (Century) so not much consistency there.

With regards to testing to determine the allowable working pressure of the valve, if you wanted to use testing to determine the ratings basis for the valve, you would want to review UG-101 of ASME Section VIII Div 1 "Proof Tests to Establish Maximum Allowable Working Pressure".

Without some traceability back to the original design/manufacturing standard, and with the manufacturer no longer around, I would rather just replace the needle valve since they are a very low cost item and replacement will be cheaper than testing.

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