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B16.5 over pressure issue

B16.5 over pressure issue

B16.5 over pressure issue

Hi all,

I have an older corporate standard that talks about the design pressure that is to be used for flanges, and that this pressure is 110% of the fluid system design pressure. It states that "B16.5 does not allow for variations in pressure in excess of the rated pressure..., e.g. relief valve operation". And that you must consider any over pressures separate, and spec the flange to envelope the condition.

I can't find where this is explicitly addressed in the codes (B31.1, B31.3 and B16.5)...can anyone comment on the correctness of the above standard?

RE: B16.5 over pressure issue

You may find more responses in the Chemical Engineers Safety Relief Valve forum. I would consider ASME Section VIII for starters.  This is also addressed in API RP 520.

Don't forget that the systems are hydrostatically tested at 150%.  A bit more reading.

RE: B16.5 over pressure issue

The corporate standard is ambiguous.  

Relief valve setting is provided for by the design code, not B16.5.  The design code B31.3 sets maximum allowable pressure and the the maximum allowable overpressure and subsequent relief valve setting as a percentage of system design pressure.  System design pressure is defined as the minimum rated pressure of the lowest rated component.  Rated pressure is as defined in B16.5, but that is not an overpressure.  However, as the design code allows certain overpressure of a system's based on design pressure, whih is based on the limiting component's rated pressure, and B16.5 flanges are componets, the design code allows certain overpressures of B16.5 flanges.

"If stupidity got us into this mess, then why can't it get us out?" - Will Rogers (1879-1935) ***************

RE: B16.5 over pressure issue

Piping is frequently designed such that it its MAWP is limited by the B16.5 flange rating for the class of flanges used.  Though this is frequently done, there is no requirement that flanges be the limiting component of the MAWP- ANY component may limit.

It is required that this piping be tested to 150% of the relief device set pressure, which may be set as high as the MAWP.  That means of course that for piping designed for low temperatures and with MAWP limited by flanges, the flanges will experience 150% of their rated maximum pressure during the hydrotest.

It is permissible to reduce the calculated hydrotest pressure per the applicable design code to avoid putting components into yield (due to big stress ratios between the test and service temperatures for instance).  But there is no requirement to limit hydrotest pressure to no more than 100% of the B16.5 flange rating!

This relief device is allowed to "accumulate" as much as 110% of the relief set pressure at its full rated capacity for typical relief events, and up to 121% for fire relief events.  But since this is less pressure than the piping experienced during the hydrotest, this should not be of design concern.

Your corporate standard is therefore spewing BS in this particular regard.  As owner though, your firm can feel free to assign whatever conservatism beyond code in design that they are willing to pay for.

Normally if a flanged joint leaks under hydrotest at 150% of its rated pressure, that would be considered a failure of the test.  With new gaskets, flange faces and bolting, B16.5 flanged joints shouldn't leak even at 150% of their rated pressure.  Leaks at the body flanges or packings on valves?  That's another matter entirely!

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