Loose type flange (lap joint) or slip-on with Titanium and C.S. 1

[Peregrino7]

This is a vessel made with Titanium plates (no cladding) of 13mm with two forged covers at both sides. Nozzles necks are with Titanium as well.
Vessel ID = 1800mm (70”) design pressure = 13 bar (188psi) and Design Temp = 65ºC (150ºF),
Vendor is proposing that all main flanges and nozzles flanges to be C.S. and in order to avoid that C.S. to stay in contact with the fluid or any welding between C.S. and Ti, the vendor is proposing a loose type flange (lap joint) also named slip-on, as per ASME VIII app. 2 - Fig. 2-4 (1a).

I have the following questions:

1) What are the disadvantages of this type of flanges? These types are not usually specified for Vessels in refinery services (anyhow this equipment is for an LNG plant)
2) Is there any other option or a better suggestion to solve the problem of C.S. flange and Titanium cylinders and nozzle necks?
3) Bolts for these flanges are to be SA-320 (S.S.). Is this ok for this combination of materials?
4) Any suggestion with the material and type of gasket for tubeside is much appreciated.

Kind regards, and thanks for your help..!



PGh.

 

[vesselfab]

well, a slip on is not a lap joint. They are used in High-Alloy fabrication because of HIGH cost of flanges and have same temperature/pressure ratings as any other B16.5 flange

If it was me, I would specify the Lap joints with radius on inside corner and not chamfered. The stub ends (the Ti part) should be manufactured to match factory radius, but may be made longer to include the whole neck to eliminate a butt weld in pipe.

If the service is LETHAL there are rules for welding the flat ring to neck as well, although I would weld all of them as if it were Lethal Service.

Bolts and gaskets are up to you as you know what the service is...

 

[moltenmetal]

Unless something has changed recently, commercially pure Ti and its alloys are NOT ANSI B16 flange materials. So if you could get the solid Ti flanges in ANSI dimensions (and you can...), what are their ratings?!

Sure, you can design Appendix 2 flanges with ANSI dimensions in solid titanium, but why would you unless the flange was wetted by the corrosive service on the EXTERIOR?

Lap joint flanges and clad blinds are standard for high alloy materials.

CS or SS backing flanges can be used depending on your external corrosion conditions. Proper backing flanges do exist and can be bought. In a pinch you can take a slip on flange and machine the correct radius at the face/ID. Failing to do this will over-stress the lap joint stub end.

On any vertical nozzles, a couple flange stops (little pieces of Ti plate etc.) plug welded to the nozzle necks will keep the loose backing flanges from slipping down and hurting anybody when the studs are removed.

 

[SnTMan]

Peregrino7, I would agree that lap-joint flanges are a practical solution for your design.

I am not sure I would use the SS bolting however, you should review this choice. Maybe you have a low temperature requirement, but I would avoid bolting that has a larger coef. of thermal expansion than the balance of the joint.

Metallic gaskets, except TI, may be a poor choice also due to preferential corrosion of the gasket.

This is going to be one expensive vessel.

Regards,

Mike

 

[SnTMan]

Meant to echo moltenmetal's point about ASME B16.5 pattern flanges. You can buy them made of all kinds of stuff, but that doesn't make them genuine B16.5 flanges, and therefore suitable for Code work.

They are just a thing you can buy. Ran into this in the past with 410SS and more recently with Cu-Ni exchangers.

Regards,

Mike

 

[MJCronin]

I agree with the comments by all of the fine gentlemen above about the use of lap-joint flanges in this case.

However, with regard to your flange bolting materials, you may want to consider PTFE coated studs/nuts for this service.

Be careful that you purchase (and test) quality hardware from a QUALITY supplier.

http://hghouston.com/dcforum/DCForumID1/384.html#1

Ultra-premium bolting materials are available from:

http://www.lawsonproducts.com/webap...001&catalogId=10001&langId=-1&epage=Solution3

Please finish this thread and get back to us on your final decisions..

Regards

-MJC

 

[unclesyd]

We have 5 large Ti vessels 12' x 60' 60 psig, FV @ 150C, with extensive Ti piping and all have Ti Lap Joint flanges with SS backup flanges using ASTM A193 B8 Cl 2 studs.
Our process material is extremely corrosive so we have standarized on SS backup flanges flanges and studs on all piping in this process area both SS and Ti. It is a very expensive proposition to change out a corroded backup flange or studs.

 

[Peregrino7]

Thanks for all your replies,

It seems that CS loose Lap joint Flanges are ok with the Ti vessel. I'll take in account the recommendations on this forum. Also the bolts seem to be ok in SS.
In fact this is a heat exchanger with sea water inside the channel (tubeside) and its fixed tubesheet.

On the other hand… does anybody know why some users prohibit this type of flange in refinery service? They always prefer Welding Neck type..!

Regards,


PGh.

 

[MikeMet]

I agree with unc reply, we too have numerous titanium vessels and the extra cost to use stainless steel lap joint flanges and bolting is well worth the little cost increase over carbon steel. I recommend Class 2 stainless bolting. Having had to change out corroded bare carbon steel and galvanized carbon steel lap joint flanges and carbon steel bolting on titanium, nickel alloy and zirconium equipment, the cost of those repairs greatly exceeded the initial cost of stainless steel flanges and bolting.

Teflon coated carbon steel bolts are good, but not fully corrosion resistant due to damage that occurs to the Teflon coating during handling and installation. Not a problem in most atmospheric conditions, however, if exposed to CT drift or sea coast salt conditions, they don't last as long as one would like, again from many years of experience in TX Gulf Coast

As for gaskets you give no process data. However, a very good gasket we use is stainless steel encapsulated in Teflon. Good rigidity and it does not have the cold flow problems of solid Teflon. We also you Grafoil gaskets with good success. The caution is if your process has chlorides you have to beware of crevice corrosion on the titanium gasket surface, and use a gasket that extend all the way to the ID of the nozzle pipe, if possible.

 

[SnTMan]

Peregrino7, at least part of the answer is that weld neck flanges are (should be) always full-penetration welded to the neck and the joint can be RT'd. A lap to neck weld unless for lethal service would not necessarily be full penetration and could be harder or impossible to full RT.

Regards,

Mike

 

[moltenmetal]

SnTMan: either I'm confused by what you're saying, or you're confused about what a lap joint flange is.

A lap joint stub end is a pressure-retaining component just like any other flange or fitting, subject to code-prescribed materials- and weld integrity testing which depends on how the part is fabricated.

A lap joint stub end is ALWAYS full penetration butt welded to the pipe (or nozzle neck or vessel shell) it is connected to. That weld joint can be fully or spot RT'd per code. The lap joint backing flange is LOOSE- not welded to the neck at all.

 

[vesselfab]

I think he is speaking on how the lap ring (the part that is the raised face) is welded to the pipe itself.

Most Ti lap joint stub ends are not just called up and ordered like lower alloy materials.

Most shops that are fabricating the pipe also fabricate the lap joint stub ends .

The point he was trying to make was how the lap ring is welded to pipe and not how the stub end was welded to pipe. see fig uw-13.5

 

[SnTMan]

moltenmetal, sorry for the confusion, I was speaking of a fabricated stub end, thanks, vesselfab. Anything outside pipe dimensions is most likey to be fabricated.

Every one of these I have seen have indeed been full penetration (through thickness) welded, although a partial penetration detail can be imagined. And as a corner joint they are not well suited to RT.

Regards,

Mike

 

[moltenmetal]

Good- thanks for the clarification.

What you're saying is that you don't feel that the code-required inspections on this joint are adequate to detect all potential flaws. Given that the exotic materials we're talking about for the stub ends are usually GTAW welded through and through, and both sides of the weld can be readily inspected, I'm not that concerned.

 

[SnTMan]

Peregrino7:

On the other hand... does anybody know why some users prohibit this type of flange in refinery service? They always prefer Welding Neck type..!

moltenmetal, I wasn't really speaking to the original question, for which I think a lap joint is fine, but attempting at least a partial answer to the above.

Regards,

Mike

 

[Outlaw500]

Can anyone shed some light on where the calculations for lap ring thickness are located?

I need to design a 36" lap joint flange connection for 304SS.

I found the sketch UW-13.5 for lethal service but i don't see where it is referenced anywhere.

I need to calculate the minimum ring thickness after machining.

The welding looks pretty straight forward.. full pen.

Thanks

 

[vesselfab]

generally speaking, the lap ring is the same thickness after machining as the schedule of pipe.

i say generally because I have never seen a calculation done for that.....but with this crowd....ya gotta cover you butt

 

[MJCronin]

I have never heard to term "lap ring" ... can you give us any internet links for this ?

We should be careful with correct use of terms here

We should use the term "lap joint stub ends" for the component butt welded to the end of the pipe. The dimensional standard used is MSS SP 43 for the stub ends. But the lap joint flange itself is dimensioned to ANSI B16.9.

The tables in these standards only go to 24 inch NPS.

Another ANSI dimesional standard covers the lap joint flanges above 24 inch NPS.....that is ANSI B16.47.

I do not know what standard covers stub ends in sizes above 24 inch....

Anyone ?

-MJC

 

[vesselfab]

The term LAP RING is what fabricators call the flat plate welded to end of pipe for shop fabricated lap joint stub ends

I don't know if there is an internet site for this and really don't care if there is one.

Would you care to re-name it to something else that is more clearly descriptive of what is being built?

 

[doct9960]

I agree with vesselfab that "lap ring" is a term for custom-designed lap joint stub ends.

I have seen Process industry Practices used the term "lap ring" in their vessel specs. VECV1001 and VESV1002 for custom-designed lap joint flanges. KR Rao also used the term in his book. See page 52 on the link below...

http://books.google.ca/books?id=AOC...mZ6a8J&sa=X&oi=book_result&ct=result&resnum=2