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AarnoudO (Mechanical) (OP)
20 Jul 11 7:31
Dear All,

Is is allowed to make 150lbs and 300lbs lap jojnt flanges from 150lbs and 300lbs slip on flanges by simply remomving the raise face?

My flange sub-vendor is telling me that this is allowed, but why is it then that the ASME B16.5-2009 lists different tf thicknesses for slip on flanges and lap-joint flanges in table 8, column 3&4 and table 11 column 3&4)?

Currently when we make LJ flanges completely ourselves we use the tf from the "lap joint" column 4. But when we order out for LJ flanges 150 & 300 lbs we get slip on flanges with the raised face removed and tf according to column 3.

I would like to keep my flanges uniform, so if tf from column 3 is allowed, I would like to use it, but how can I convince inspectors with the ASME B16.5-2009 in my hand, when the value is smaller then the tf "lap joint" value in column 4?

Your input would be highly appreciated.


Aarnoud van Ommen
Helpful Member!  SnTMan (Mechanical)
20 Jul 11 9:59
AarnoudO, I'm lookin at an older edition, but it does not appear to me that conversion from SO to LJ is specifically addressed. See para. 6.3

It appears to me that the two flanges are the same thickness THROUGH, you can see that the difference in the tf dimension is equal to the raised face height. The LJ flange then gets the raised face machined out of this thickness. If it then has the raised face machined off it is now thinner THROUGH.

If you could machine the RF off and maintain tf as shown for LJ flanges, it seems to me would meet the requirements, otherwise it is too thin by the RF height.

Suppose the RF were NOT machined off. I wonder if your inspectors would buy that?


AarnoudO (Mechanical) (OP)
20 Jul 11 10:30
Thank for the input SnTMan,

We have supplied (and have had inspected and apporoved) the lapjoint flanges as supplied by our sub-vendor (maaass flanges)

But there is no formal consensus on what is right. Maass flanges machines the raised face of 150 & 300lbs flanges to get lap-joint flanges and claims this is right. We use them as such, but I never lost the nagging feeling that this is not confirm B16.5. Can anyone confirm?


Helpful Member!(2)  moltenmetal (Chemical)
20 Jul 11 13:24
Do not remove the raised face.  Merely radius the raised face to bore transition per the required dimensions.  Done it many times and never had a complaint or a problem.

DO NOT use hubless blind flanges for this purpose- those are too thin.
Helpful Member!  Shmulik (Mechanical)
28 Jul 11 16:42

Your suspicion is justified as this operation is not permitted, and I disagree with the honorable guys above.
 As you've mentioned, Lapped Flange requires higher tf as per column 4 (vs column 3), higher Length Through Y as per column 8 (vs column 7), larger bore dia. (column 12) and Corner Bore Radius (column 14).

Mike:  tf Min. thickness and Y through length have nothing to do with RF height.
See at B16.5 6.4.1: Classes 150 and 300 pipe flanges and companion flanges of fittings are regularly furnished with 2 mm (0.06 in.) raised face, which is in addition to the minimum flange thickness, tf.

Moltenmetal:  please see at B16.5 6.4.3 Lapped Joint Flanges: Lapped joint flanges shall be furnished with flat faces as illustrated in Tables 8, 11, 14, 16, 18, 20, and 22.

The only conversion permitted on B16.5 flange is raised face machining off. See at 6.3.2

Making a lapped flange out of a blind is prohibited not for the reason it is too thin, but because a B16.5 lapped flange shall be a hubbed one.
The only flanges that may be produced of blind are some of reducing flanges. See at B16.5 Table 6 note (1).
moltenmetal (Chemical)
9 Aug 11 8:31

A star and my thanks to you for forcing me to re-read the standard!  Indeed you are correct:  the minimum thickness tf and the length through the hub Y are greater for the 150 and 300# lap joint flanges than for the corresponding slip-on flanges.  The difference in Y is large beyond 12" (10" for 300#), rendering the use of a modified slip-on flange beyond these sizes a clear violation of the standard.  One presumes the hub IS there for a reason for both the slip-on and lap joint designs.  

You should note that the thickness of the raised face is 2mm, and in every case in the table in B16.5 for both 150# and 300# classes, the difference in these two dimensions is less than 2mm (up to 12" for 150# and up to 10" for 300#).  Hence my recommendation not to remove the raised face. That the difference is less than 2mm is not surprising, since both types of flange are machined from the same forgings.

While you correctly point out section 6.4.3 indicating that lap joint backing flanges shall be "furnished" with flat faces, we're not talking about furnishing flanges for sale here:  we're talking about modfiying an existing B16.5 flange (i.e. one that can be had from stock) to a different use.  While you are correct that such a modified flange cannot in the strictest sense be considered a B16.5 flange, I can see no reason that the modified flange would not perform properly.  
Shmulik (Mechanical)
10 Aug 11 8:05
Thank you moltenmetal,

Yes, it makes sense that both (small NPS) lapped-joint and slip-on flanges are made of same raw forged ring, but actually when you make a slip-on flange you are going to get a ~2 mm thinner tf. Remaining additional 2 mm raised face does not play a role in flange resistance.

Indeed, I can agree with the sense says that a thickness of tf that is sufficient for a slip-on should sufficient for a lapped one as well, BUT, this is a kind of interpretation that we are not allowed to make.
Even though this is not a "clear violation" of the standard, you still have to "trust" your inspector to not be aware of these details, or trust him be tolerant enough to close one eye for not seeing that.

I wouldn't...
moltenmetal (Chemical)
10 Aug 11 8:17
"Remaining additional 2 mm raised face does not play a role in flange resistance"

I beg to differ with you there.  Saying that is equivalent to saying that losing 2mm off the Y dimension doesn't matter either.

As I said, I've never encountered a client inspector who saw this minor modification to be a problem.  But of course we'd always clear this type of thing with them first, rather than assuming that they'd be OK with it.  Given the difference in availability and delivery time LJ backing flanges versus RFSOs, most client inspectors are more than willing to overlook this modification in an effort to save schedule.
Shmulik (Mechanical)
10 Aug 11 12:14

No, it isn't.
B16.5, B16.47 and app.2 of BPVC VIII-1 define the Minimum Required Thickness of a flange as the thickness of beyond the raised face area since this thickness resists tang. stress on flange at bolting up cond. (moment acting for gasket seating condition).
Duwe6 (Industrial)
10 Aug 11 13:49
Shmulik is correct about the Code thickness, and moltenmetalis correct about the commonly-accepted practice of machining SO's into LJ backing flanges.  

Since the minimum thickness is not being violated, 3rd-Party inspectors are supposed to accept this practice.

And, yes the Raised Face metal does contribute to the strength of a RF flange, the Code just ignores that fact.  Probably so we can do things like making LF backers from SO's.
SnTMan (Mechanical)
10 Aug 11 16:21
Or so you can make a FF out of it. As we all know facings are additive for 400# and up.


Shmulik (Mechanical)
10 Aug 11 16:42

Making a FF is straightforward. See at B16.5 6.3.1 & 6.3.2.
Facings are additive for all classes at 2009 edition.
SnTMan (Mechanical)
10 Aug 11 17:45
"Facings are additive for all classes at 2009 edition."

Now if they go doing everything the same way it's just going to confuse people:)

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