5188373,

Refer to your applicable piping code; I'm most familiar with ASME B31.3.

You need to keep your flanges ratings within the design conditions as defined by your code. B31.3 allows you to exceed the rated pressure at the coincident temperature for short durations. The duration times and pressure limits are clearly defined in B31.3.

Another word of caution, there has been a number of changes to the rating pressures in ASME B16.5 and ASME B16.34, especially with alloys at higher temperatures. Make sure you are using the latest flange ratings for new work and the codes of record for existing systems.

NozzleTwister
Houston, Texas

Use the design pressure and design temperature along with the correct material group to set the flange class.  Perhaps the maximum pressure and temperature would not occur at the same time.  If the maximum temperature is for one case and the maximum temperature is for a different case, consider the maximum pressure and temperature for case1 and the max pressure and temp for the other case  --  then select the higher to set the flange class.

Pressure-Temperature Ratings for Pipe Flanges and Flanged Fittings from ANSI B16.5-1996F Pressures are in pounds per square inch, gauge (psig)


CLASS      150    300    400    600    900    1500    2500
Temp F
–20 to 100 285    740    990    1480   2220   3705    6170
200        260    675    900    1350   2025   3375    5625
300        230    655    875    1315   1970   3280    5470
400        200    635    845    1270   1900   3170    5280
500        170    600    800    1200   1795   2995    4990
600        140    550    730    1095   1640   2735    4560
650        125    535    715    1075   1610   2685    4475
700        110    535    710    1065   1600   2665    4440
750         95    505    670    1010   1510   2520    4200
800         80    410    550     825   1235   2060    3430
850         65    270    355     535    805   1340    2230
900         50    170    230     345    515    860    1430
950         35    105    140     205    310    515     860
1000        20     50     70     105    155    260     430


Material Group 1.1 (Carbon Steel)
A105 (1), A216?WCB (1), A515?70 (1)
A516?70 (1) (2)
A350?LF2 (1), A537?C1.1 (3)

There are many tables.  One has the highest pressure and temperature rating for those meterials.

What are the materials that connect to the system?

What is the set pressure of the relief valve?

Material group 1.1 includes A 216 Grade WCB.  Group 1.2 for A 216 Gr. WCC or A 352 Gr. LCC are rated a bit higer - up to 1500 psig at 100 degree F.  Grade 2.3 for A 182 or A 240 Grades F316L and 304L are much lower.  It is common to have flanged stainless steel instruments connected to the piping or vessel.

Material group 1.2 includes 1500 psig at 100 degree F.  
This would apply to the A 203 Gr. B or Gr E plate for the vessel if you follow notes 1 and 2.  However it would be unusual for the complete system protected by the pressure safety relief valve to comply with group 1.2.  It might be possible to comply with group 1.1 rated 1480 psig at 100 degree F.  I would expect some "non specification" Class 900 flanges for stainless, etc.  The problem with these ratings is that too few people actually read them.

The process engineers often establish the relief valve settings to accommodate the operating upset conditions.  This establishes the maximum pressure for the system.  They also determine the maximum temperature and add some 50 degree F for the temperature.