Fabricated slip-on flange 1

[sbnz]

How do I calculate the thickness (and other requirements) of a fabricated (from plate) slip-on flange, to comply with piping code (B31.1) requirements

For specific reason, we can not use a standard Cl-900 flange to B16.5 in this particular case and need a special flange made.

thanks

 

[C2it]

You need to read and apply Appendix 2 of ASME VIII Div. 1, or the similar section in ASME VIII Div. 2.

 

[11echo]

I worked at a small refinery, and we routinely got blind flanges, in the rating we were working, on and took them to the machine shop to bore a center hole (making a slip-on flange)to fit and certain size pipe we wanted to mate up. We did this a number of times.

 

[sbnz]

11echo - interesting to note your practice. May be it is ok in refinery piping design

In power stations, for high pressure piping as per code, that would not be acceptable though. The SO flange thickness is higher than the blind thickness and need to be full hub thickness for fabricated flange.

I am looking for support/ justification as per ASME pressure vessel and power piping code.

thanks

 

[moltenmetal]

11echo: I can't quote you chapter and verse (there are many here who no doubt can and WILL!), a blind flange with a full-bore hole machined through it to turn it into a slip-on will NOT have the same rating as a real slip-on with hub. The hub is actually there for a reason!

There's a certain point where you can get away with machining a reducing slip-on flange out of a blind, but machining out a blind to the full OD of its nominal pipesize is a no-no at ANY flange size unless it is to be no longer used and treated (and rated) as an ANSI B16.5 flange.

You'll also (sometimes) have trouble re-rating an ANSI flange to its full rating using Section VIII appendix 2. ANSI flanges tend to be "over-bolted", such that appendix 2 requires the flange to be thicker with the same number of bolts and the same other dimensions.

 

[ColonelSanders83]

If the small hub on the back side of a slip on flange is indeed considered a hub (I was unsure at first glance but as moltenmetal says its there for a reason) then per ASME Section VIII div 1 App 2 sections 2.2(d)1 and 2.2(d)2 you cannot fabricate a hubbed flange from plate. There are further requirements, I sugest you read and familiarize yourself with them before proceeding.



A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

 

[11echo]

Well this is interesting, have any of you guys been out in the field and seen where they have welded a coupling, thread-o-let, or weld-o-let to a blind flange? …So how is this different?

 

[ColonelSanders83]

11echo,

There are code allowanced for what you describe as far as couplings, thread-o-lets, or weld-o-lets in a blind flange however these come with limitations on the hole size allowed (usually percentage of line size). The larger the hole in the blind the weaker the blind becomes. This is substantially different because most of the blind material has been reduced and the resistance to the radial and tangential stresses has been reduced.

A key variable in the design of flanges is the bore diameter for this reason. Again, take a look through App 2.

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

 

[11echo]

SO ...as long as the size doesn't limit the connection, then drill/machining a blind flange is ok!??

 

[doct9960]

sbnz,

Aren't slip-on flanges limited to Class 300 in ASME B31.1? I guess what you need is an "integral type" ring flange similar to Fig. 2-4 Sketch (7) in Appendix 2 of ASME Section VIII, Division 1.

To calculate for the flange thickness, ASME B31.1 says to design in accordance with Appendix 2 of ASME VIII Div. 1, except that the notations for P, S_a, S_b, and S_f shall be as described in paragraph 104.5.1(A) of ASME B31.1.

 

[jte]

11echo-

As moltenmetal stated, there is chapter and verse on this. Here 'tis: See ASME B16.5 2009 Table 6 on page 70. Same table exists in previous editions. This table provides the smallest size hole which requires a hub. Holes smaller than the size noted can be punched into the center of a blind and still maintain compliance with B16.5 without a hub.

When this size criteria or central location is not met (i.e. the not unusual 3/4" drain opening near the bottom of a blind on a horizontal pipe), then the blind is no longer a B16.5 compliant component. Not a problem yet - all you have to do is qualify the flange to whatever code is being used - these usually wind up pointing to Section VIII. But you can't use the flange as a "standard component" anymore.

jt

 

[moltenmetal]

ColonelSanders: when you weld a slip-on properly per code, you weld it with two fillets- one at the face to bore joint, and the other at the hub to pipe OD joint. The welds are the same size as the single fillet which would be used if the flange were socket-welding type, where there is only one weld used. Hence the flange can be made from plate, because the potential laminations in the plate will never be challenged in tension.

That said, the hub DOES contribute thickness to the flange and cannot be done away with- otherwise the standard flange wouldn't have it.

A star to jte for pointing to the correct table in B16.5.

11echo: perhaps now you understand better what you observed in the field. I would not be surprised if you observed a few locations where these rules were violated- I certainly have. Sometimes this is done by design (i.e. in piping whose design is limited by components other than flanges) and sometimes it is done out of ignorance. Seeing things in the field is valuable experience, but only if it is properly interpreted and understood in its context.

 

[ColonelSanders83]

moltenmetal,

I am curious about the technical justification you have laid out from a code compliance standpoint.

ASME B16.5 sect. 5.1 clearly states that "plate materials shall be used only for blind flanges and reducing flanges without hubs". BPVC sect. VIII div.1 app 2. also does not allow fabrication from plate unless the plate has been forged into a ring.

Since we both agree that slip-on flanges have hubs and they are necessary this leads me to the conclusion that if you machine a hubbed slip-on flange from plate without forging it into a ring first you will not have a B31.1 compliant flange.

I am well aware of the delamination concern with thick plates; however the code does not have a caveat for double welded hubbed slip-on flanges. I also have am curious about you assertion that since the tensile failure is not a concern with a double welded slip-on it will be ok. I agree that this is a true statement from a strictly tensile sense, however the shear failure through the plate thickness at the flange to hub transition is also a concern in slip-on flanges due to ring rolling effects because of low hub stiffness.

Just my two cents worth.


A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!

 

[moltenmetal]

ColonelSanders: It appears your strict interpretation is correct, and I retract what I said before.

As you've stated, if you machine away material from a plate flange to produce a hub reminiscent of a B16.5 slip-on flange, and rely on the resulting hub in your App 2 calculation, you're in trouble unless you first forge a ring from the plate. If you do the calculation assuming no hub and do not machine the plate to produce one, you're OK to use plate.