derating a vessel because of B7M bolts

[vesselguy]

All,

For sour service, MR0175 and MR0103 practice is to use B7M grade bolts if it is an exposed service. No problem; I've been practicing that for long time. In fact, in some projects, I've seen B7M bolts use all across the board regardless of whether it is an exposed service or not. The reason is uniformity so field guys don't screw it up by mixing up bolts/nuts.

I just read this sentence from Fisher Industries, "It must be remembered, however, that use of lower strength bolting materials such as B7M often requires pressure vessel derating." This is news to me.

What do they mean when they say derating a vessel?

 

[ApC2Kp]

vesselguy,

Depends if the bolting of flanged joint is the limiting component for pressure design. The wall thickness usually would be the limiting item of pressure vessel. However, if the flanged joint design of bolted body (as for filter) is the limiting feature, and then the difference in bolting strength B7=25 ksi vs B7M=20 ksi could result in the reduced pressure rating of flanged joint to require the pressure vessel derating, unless the original design was made with B7M bolting strength. (Larger or more bolts needed of B7M bolts to get equal B7 strength)

Similar reduced strength occurs with B8M=18.8 ksi stainless steel bolting vs B7 alloy steel bolting. Maybe the B7M was a typographical error in place of B8M bolting?

This de-rating would most often occur with valves having a bolted bonnet. The standard bolting would likely be B7. If the B7M bolting was an optional construction, certainly the valve bonnet flange would not be re-designed with more bolting to compensate for the reduced strength of the B7M bolting. Theoretically the bolt diameter could be increased to compensate for reduced bolt strength, but in reality it does not happen. The robust bolted bonnet designs would have full body rating with B7M bolts, while having extra margin when assembled with standard B7 bolts.

 

[vesselguy]

ApC2Kp,

Great explaination. I understand and roger that. Thanks.

 

[dcasto]

Here is how I had it explained to me one time. The B7 bolts on flanges are strong enough that only 1/2 are required to make the flanges hold together, But you need more bolts because of the distance between the bolts is a problem. So the B& (modified which means they are stress relieved and impact tested) still have enough strenght. The same goes for B8 SS studs.

I really got into it when a plant changed its studs out to a lower strenght stud B16 I think, but they had a higher strenght at 600F than a standard B7. Now what happens before the heat is applied?

I've taken advantage of this an done hot bolt change outs before and no problem as long as the flange had 8 or more bolts.

 

[Xianglu]

If you check ASME VIII Appendix 2, you will find that the type of flange gaskets governs the bolting material. From my experience, subject to the design temperature, B7M is good for spiral wound gaskets at most of the temperatures.