Pressure from Thermal Expansion

[Wilg10]

Hi, I am looking at a pipe (Contents are fuel oil) with a design pressure of 5 bar, the pipe also has a thermal relief valve set at 19 bar. I need to re-rate the pipe, can I use the fact that the thermal relief valve has lifted in lieu of a hydrotest ?, if the thermal relief has lifted is the system likely to have been subjected to 19bar?, why are TRV's sometimes set at higher pressures than design ?
Thanks Wilg10

 

[KenA]

It is likely that the thermal relief valve set pressure will be at or marginally below the pressure limit set by the ASME flange class. At 19bar it sounds like it is a class 150 flange.

The design pressure is a value which is selected (usually by the Process engineer) and usually relates to the maximum operating pressure + a design margin. All components in this system should be designed for at least this pressure. In this case the pipe is "good" for 19bar which far exceeds the design pressure of 5 bar so is just fine. I don't know your setup but there could (in theory) be other components in the system which are only "good" for 5bar.

What was the original system hydrotest pressure? Was it 7.5bar? Or did they test to 28.5bar?

 

[Wilg10]

Original test was 7.5bar which is the concern when rerating to B31.3, original weld quality is the other concern so the suggestion is to use the thermal relief case of 19 bar as a test pressure and justification for a waiver.

 

[chicopee]

My question is how can you have a relief valve set pressure at 19 Bars when the pipe has a design pressure at 5 bars?