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Stress Ratio for Calculation of Hydrotest Pressure

Stress Ratio for Calculation of Hydrotest Pressure

Stress Ratio for Calculation of Hydrotest Pressure

As per UG-99, hydrostatic pressure is = 1.3 x MAWP X (Sall @ Tamb / Sall @ DT).
I have been presented an argument that the presence of vents and drains c/w blind flanges on SS vessels limits the hydrotest stress ratio, "Sall @ Tamb / Sall @ DT" to "1.0".

The vendor is stating that the bolting on the vents and drains (NPS 2 - 300# RFLWN) on the 316L SS vessel is the component that has the lowest hydrotest stress ratio.

I don't buy this argument as bolting has not been checked for the DP or MAWP and is a part of a standard assembly, which design is per ADME B16.5. Also, it is stated there that the hydro for piping components should be at 1.5x MAWP. Therefore, I believe that this bolting SHOULD NOT be taken into the account for determining lowest hydrotest stress ratio.

However, I believe if this was a S&T heat exchanger with a girth flange, that girth flange bolting could limit the ratio to 1.0.

Your comments would be appreciated.

RE: Stress Ratio for Calculation of Hydrotest Pressure

I would use the following procedure:
-determine the hydrotest pressure for all components other than standard fittings per UG-99 (=1.3 x MAWP x ...)
-determine the hydrotest pressure for standard components per B16.5: if I recall correctly, it is in a table and is 1.5 times the room temperature rating
-the hydrotest pressure is the minimum of the values determined above

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RE: Stress Ratio for Calculation of Hydrotest Pressure

My thinking exactly.

RE: Stress Ratio for Calculation of Hydrotest Pressure

You will probably hear that industry practice is to consider the bolting.  It is quite common the limiting component.  With vents and drains you are within the scope of Section VIII Div 1 so they would be considered pressure retaining components and subject to the requirements of VIII-1.


RE: Stress Ratio for Calculation of Hydrotest Pressure

I've seen it done both ways for exchangers. Client specifications sometimes do not allow the use of bolting to check the stress ratio. I know of no limitation in the Code. When I design exchangers, I do not consider bolting as I believe this to be the norm.
Why do Vendors fight this? Its simply the test pressure and not like you're asking them to re-order materials!! Good luck

RE: Stress Ratio for Calculation of Hydrotest Pressure

I'll echo alan7272's comment: Why do vendors fight this? I've been victimized by this argument once. The only response I would have is that the vendor who weasels out of the stress ratio argument in this manner is one who does not have much confidence in their own work product. I avoid (though not always sucessfully) dealing with weasels.


RE: Stress Ratio for Calculation of Hydrotest Pressure

I am not really sure why vendors fight this.

IMHO, their interpretation of the Code (with blinkers on) is erroneous.

a) It stems from their argument that low pressure SS vessel w/o any vents and drains WOULD have hydrotest stress ratio > 1.

b) However, if you add a single NPS 2 - 600# RFLWN c/w blind to the vessel, now the ratio is 1.0 and your hydrotest pressure goes down to 1.3 x MAWP even though the single vent you added has a rating that far exceeds even the hydrotest pressure, let alone MAWP. I can't believe that this was the intent of the Code.


RE: Stress Ratio for Calculation of Hydrotest Pressure

Interpretation VIII-1-89-287:
Q: Under the provisions of UG-99(b), may the ratio (S test temp./S design temp.) for the bolting material be used in calculating the required hydrostatic test pressure if it results in the lowest ratio for the material of which the vessel is constructed for:
(1) bolted flange connections per App.2?
(2) bolted flange connection which comply with an ANSI product standard or a Manufacturer's standard as permitted in UG-11.
A: (1)Yes. (2)No; the limiting hydrotest pressure shall be established in accordance with the applicable standard.

RE: Stress Ratio for Calculation of Hydrotest Pressure


Thank you for your pointer!!!  

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