Flange Rating vs Pressure Test on Flange

[LearnerN]

My question pertains to steel gas transmission pipelines with flanges being hydrotested. If it's 600# class, then the flange rating is 1480psig. However, the pressure test often exceeds 1480psig, sometimes to 150% of the weakest component on the pipeline (like the flanges).

My question is conceptual. So during the pressure test, the flange can handle more than 1480psi because, I assume, it's treated almost like pipe for purposes of calculating the max allowable hydrostatic test pressure (unless limited to 150% MOP)? But for the longevity concern, the flange is rated to 1480psi, which is probably due to it being a connection with bolts and so forth (don't want it to leak, and its stress distribution is different than on pipe). I understand why flanges are rated lower - my question is how they can be tested higher than the rating. Am I on the right track conceptualizing this? Thanks!

 

[XL83NL]

If this question pertains to ASME B16.47 flanges, read paragraph 2.6. The flanges are allowed to go to 150% of their maximum working pressure dy allowingthe flanges to go to yield. Review this current topic which deals with the same matter to a certain extent.

 

[LearnerN]

Thank you very much! That answered my question.

 

[LittleInch]

Because they discovered a long time ago that metal components, especially in hazardous service, need to operate with a safety factor over max design pressure / MOP.

B 31.3 actually permits up to 30% above MOP for limited periods ( 5 hours per year I think).

Therefore you cannot design the flange / pipe to work at yield at DP/MOP because practice has told everyone that sometimes things go above the design pressure / MOP and when it does that you don't want the steel pipe / flange to break.

The balance as ever is between making something ridiculously strong, heavy and costly and making something efficient. A safety factor of 1.5 has stood the test of time for piping components (pipelines often use 1.25).

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[GD2]

LearnerN,
Always remember one simple rule. The test pressure in any piping system should NEVER exceed the yield strength of the material at temperature. You can cross-check this by the simple hoop stress equation. Because you will have different components with different materials, you will need to consider the component with the lowest yield strength. Different construction codes specifies different safety factor for hydro-testing as LI had pointed out.
ASME codes usually set the basic allowable stress at 2/3 of SMYS. If you multiply this by 1.5, this will equate to SMYS of the material.



GDD
Canada

 

[gwalkerb]

GD2,

I posted the other thread linked here by XL83NL. In that thread, I asked a question about hydrotest pressure vs yield strength, but the response from TGS4 would appear to contradict your point. While flange pressure rating isn't based on hoop stress, the test pressure for group 2 & 3 materials does actually slightly exceed published yield strength.

 

[TGS4]

Right - there's nothing particularly special about engineering yield. It's an arbitrary limit. Slightly exceeding "yield" is not catastrophic. Excessive deformation, however, is bad.