ASME Sec VIII Vessel ultimate bolting selection

[rmw]

If a Sec VIII Div I vessel is designed, stamped and submitted to a certifying agency with a paticular size and grade of bolt for its flanges that connect to external piping, is the ultimate installer and/or user of the vessel compelled to use the bolt size and grade that is shown on the vessel mfg. dwg submitted for certification?

In other words, can a smaller diameter, but stronger material bolt be used, or the opposite, if a smaller bolt is used for Compress calcs, but the hole is drilled larger, can a larger bolt with a lesser pedigree be used, if it is (or even if it is not) a bolt listed in SEC II part D?

rmw

 

[metengr]

rmw;

If a Sec VIII Div I vessel is designed, stamped and submitted to a certifying agency with a paticular size and grade of bolt for its flanges that connect to external piping, is the ultimate installer and/or user of the vessel compelled to use the bolt size and grade that is shown on the vessel mfg. dwg submitted for certification?

No. This would be outside the scope of Section VIII, Div 1 based on your stated conditions.

 

[rmw]

Metengr,

Thanks for your quick response. Can I ask further then, what relevence does the bolt size and grade have in Compass on the code calculations? I recognize that size determines hole size which goes right to ligament width and hence flange strength, which dictates thickness, but why would Compass care if that was a Gr X or GR Y or GR Z bolt? The ASME allowables for bolt stress are so low anyway that it seems that brass bolting might be strong enough - see the tongue in cheek on the last remark.

rmw

 

[metengr]

rmw;
For some pressure vessel designs, a flange can be used to bolt shell sections together. In this case, bolting is considered within the scope of Section VIII.

For bolting used to join a vessel flange to a pipe flange, the face of the vessel flange is where Section VIII jurisdiction ends (see UG-1 (e) 1 (c)).

Regarding compress, I have no answer. However, bolt selection exclusive of rules in Section VIII, Div 1 still requires good engineering judgment regarding selection.

 

[rmw]

Thanks Metengr. And that is the rub. A technically qualified high strength non ASME listed bolt has been selected for the pipe attachment that is a good technical pick, but is not an ASME material and that along with the fact that the dwg submitted to the certifying agency specifies the bolt size and the Compass documentation specifies the bolt grade; which was, of course an ASME material which is all that Compass will allow has caused a lot of confusion.

rmw

 

[metengr]

rmw;
I see your dilemma. I would make the case that the flange bolts fall under ASME Power Piping Code, and here you have some flexibility.

 

[rmw]

Metengr,

Referring to U-1(e)(1) (c) quoted above he wording of the code that we have is "the face of the first flange for bolted, flanged connections" not the face of the vessel flange as you noted.

A colleague who is more code versed than I am (by a long shot) is interpreting that to be the face of the attaching flange which would then make the bolting that straddles that flanged connection within the scope of Section VIII as well as the gaskets which are also used in the calculation for flange thickness.

Other connections within the whole paragraph would indicate the first sealing surface of the attached part while some would make it seam that it was the sealing surface of the vessel itself.

Do you have any enlightenment to share on that apparent conflict of interpretation?

 

[metengr]

rmw;
The face of a flange means this, the face of the flange attached to a vessel nozzle or appurtance. When you order a pressure vessel, bolted connections are supplied with a flange open face unless as part of the pressure vessel engineering specification, the fabricator is provided appurtance hardware to install as an extra.

The flange face of the pipe system that mates to the vessel nozzle or appurtance flange is not part of Section VIII, Div 1. There has to be some well defined pressure boundary where welding and pressure retaining material end under Section VIII, Div 1 Code space. Sorry, but your colleague is not correct on this interpretation.

This sounds to be more like a contractual issue and not a Code issue.

 

[rneill]

I agree with metengr. The face of the first flange would be the vessel flange as when you move out from the vessel shell, this is the first flange. I would treat the bolting as being part of the associated piping system (e.g., B31.3, B31.1). These design codes also have listed bolting material but do provide latitude to use unlisted materials providing you do your homework on these materials.

However, in a case like this I would clarify with the certifying authority as they may have some local rules which take precedence over the ASME Code.

 

[rmw]

Thanks for the followup rneill. In this case it is the certifying agency that is raising the issue. The issue of the non ASME bolting has been settled because it meets neither Sec. VIII nor B31.1 (which is the appropriate code for this application.

The interesting thing is that if this had been a standard off the shelf 'store bought' flange, the issue of bolting wouldn't have arisen because the flange thickness would not have needed to have been calculated, but since it was a custom designed flange and a bolt material and size had to be selected to make the flange thickness calculation the certifying agency is making an issue of deviations from the size and grade used for the certifying calcs.

rmw

 

[rneill]

Didn't realize it was not a standard flange. in that case, I think you would still have the opportunity to use alternative bolting but it would require another analysis to verify the flange design with these new bolting materials.

As long as the bolt holes are unaltered, and the new bolts are only slightly smaller (within reason as you have to have enough surface under the nut to transfer the load), then all the flange cares about is the load per bolt. If you can achieve the same bolt load by reducing the diameter but increasing the allowable stress, it should not change any other aspect of the flange design.

Not sure about the tolerance limitations as to how small a bolt is permitted in a flange bolt hole; that would need to be checked.

 

[unclesyd]

It has been our design philosophy for many years to use many small alloy fasteners in lieu of fewer large ones on alloy and mainly SS flanges. The limit on the number of small fasteners is guided by the calculations and clearance for a heavy wall socket + 3/32" min.
This approach to flange bolting has served us well with the added benefit that the mechanics love this approach as they don't have to use the heavier wrenches.

 

[rmw]

rneil,

Thanks again. The flange has been analyzed for every conceivable size and grade of fastener and with one exception (large size, high strength), passes muster.

I am a few inches short of 6' tall, and let's just say that I can walk through this flange with my hard hat on without ducking. The reason for having to seek alternate bolting is due to hole misalignment caused by a variety of reasons.

We will use a good grade of hardened washer for any smaller diameter bolt than the one for which the hole was drilled.

We are setting the torque for the undersized bolts to be the equivalent of that for the standard size bolting in order to obtain the same clamping force on the gasket.

rmw

 

[MartinSr00]

Another item

As a matter of safety, you cannot use allowable stress numbers alone as the final arbiter of safety. Note that the Code does this for approved materials -- but the fact that those bolts are approved means that they are sufficiently ductile, and their quality sufficiently controlled for use in pressure vessels.

For example, unless it's been changed recently, I don't believe that ASTM A325 structural bolts are suitable for pressure vessel flanges. You can also find very high strength aerospace fasteners that would easily meet strength requirements, but their lack of ductility would make them unsuitable for use.

I made this mistake once on the bonnet of a large valve using H900 condition 17-4 PH bolt to replace ones made from ASTM A193-Gr B7 ones. Mechanics were working on the valve while in service at 200 psi, removing one bolt at a time, replacing it with this higher strength bolt. The flange was about 6 foot in diameter as I recall. After installing about 20 bolts successfully, the head popped off a bolt being installed. A little while later, the same happened again. Had a progressive failure then resulted, we would have emptied the water from several miles of 12 foot diameter pipe in an around a strip mall. Perhaps a few of us would have survived.

Usually customers figure they can get a bolt of much higher strength and substitute it wily-nily just because the allowable stength exceeds the design limit. When I was stupider than I am today, I've followed this line of reasoning.

Just some advice -- stick with code approved fasteners and code compliant flange designs. Advise your clients that if they depart from this, they do so at their own peril. Maybe there's a legal loophole in the code somewhere that you could skirt legal responsibility, but there's two issues in play -- (1) code rules, and (2) intelligent action.