Joining two spools of pipe together with two different types of flanges....... 3

[borobam182]

Hi everybody.

First of all - great forum and great posts in here. I feel like I've learnt a lot from reading various threads, and its my first port of call with any kind of technical issue. Thank you for that.

However, after searching the forum, I cant find any information on my current issue.

In the past, whenever I've wanted to join two spools of pipe together with a pair of flanges, I would specify 2 of the same flange depending on the conditions......2 weld neck flanges with a gasket, 2 lap joint flanges with a gasket, etc. Each flange pair would have its strengths, weaknesses, pro's and con's. However, today I was challenged about using two different flanges at the 'connection point'. Are there any rules or regulations covering this issue? Should you avoid connecting a weld neck flange to a lap joint flange? I understand that you COULD - the PCD's, hole sizes etc will match, but SHOULD you?

My specific pipework system is 425oC and 14 barg steam service, stainless steel pipework construction. The system will be used intermittently. My gut feeling was that I should be using weld neck flange pairs but I cant find any code or data to back this up.

Any information you guys may have would be greatly appreciated.

Thanks in advance.

Craig.

 

[LittleInch]

Craig,

The fact the code doesn't say something can't be done tells you it can (double negative makes a plus).

So long as the two flanges are the same (class/rating, design code, type of flange face (FF/RF/RTJ), bolt size, PCD etc) then how that flange it attached to the corresponding pipe is not relevant to the flange itself.

Hub or lap flanges are often used to allow for rotational errors or potential misalignment when mating a pipe to a fixed flange, (WN, screwed, cast items - whatever).

It's not common on a piping joint to have one or two lap joints because normally they weigh more, cost more and the pipe or flange can be easily rotated to match the other flange so two WN flanges are the default choice.

Hope that helps.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[borobam182]

LittleInch,

Thank you for the response on my issue, and the extra information regarding lap joint flanges.

Craig.

 

[ehbadger]

Most lined piping systems come with one fixed flange (i.e. weld neck) and one rotatable flange (stub end with LJ or SO). I specify one fixed and one rotating end in nearly all the piping I buy (PTFE, PFA, Tantalum, PVDF, etc). The majority of joints in these systems are rotating to fixed..

 

[Gator]

What spec was the original piping built to?

Piping Design Central

 

[XL83NL]

So long as the two flanges are the same (class/rating, design code, type of flange face (FF/RF/RTJ), bolt size, PCD etc) then how that flange it attached to the corresponding pipe is not relevant to the flange itself.

If a WNRF is joined to a SORF, the raised face surfaces wont match at their ID, and can (easily) result in a spiral wound gasket to disintegrate since the last windings at the inner diameter of the gasket are not clamped together.

 

[racookpe1978]

Why would the two not match?

What size and class of which RFWN flanges differ substantially from their equal SORF diameter?

 

[moltenmetal]

XL83NL is correct that in certain size ranges (at all sizes 1.5" NPS and smaller, but at some sizes/classes, as large as 8"), slip-on flanges whether used in pairs or in combination with any other flange facing, require SPECIAL DIMENSION spiral wound gaskets. Gaskets cut from sheet, or corrugated metal type gaskets, kammprofile type etc., do not have this restriction. Note that special dimension spiral wound gaskets are ALSO required for use on nozzle (long weld neck) flanges and on threaded flanges in some cases also.

Note that the suppliers of spiral wound gaskets, at least the people working on the sales desk, are often ignorant of this fact.

Note also that a gasket of the correct dimension may not comply with ASME B16.20. That's OK, unless you're working to a code/standard or client spec that requires your gasket to meet B16.20.

Otherwise, as long as the right gasket is specified, there is nothing to prevent you from mating any pair of flanges of the same size and class together, as long as they have the same facing, i.e. both raised face or both flat face.

If one of the two flanges is flat faced and made of a brittle material such as gray cast iron, you should not mate a raised face flange to a flat face flange. Otherwise, overtorquing the bolts may lead to a failure of the brittle flange.

 

[borobam182]

Thank you for the extra input guys.

The piping system is designed to ASME B31.3.

I now appriceate that I must specify the correct detail for the gasket. I will include this in my documentation.

I have a 4" WNRF flange mating with a 4" lap joint flange. As I understand it, the lap joint flange together with the 'lap joint stub end' will form a 'raised face' which will result in mating flanges with the same facing.

Craig.

 

[moltenmetal]

Both the RFWN and the lap joint stub end will have the same bore, which is the ID of the pipe. No special gasket is required. Special gaskets are required when flanges have bores equal to or greater than the OD of the pipe, i.e. RFSO, threaded or LWN.

 

[Gator]

I figured the system might be to B31.1 given that it's steam. My question upstream was referring to the spec used by the original design firm when the facility was built.

There won't be a gasket-gripping groove cut into the "raised face" of the lap joint stub end, will there?

Piping Design Central

 

[rconner]

[also borobam182, and while I'm not sure of any practical significance with your chosen gasketing, was curious are you saying the O.D. of the upset/lap end is precisely the same as the O.D. of the raised face on the other flange??]

 

[moltenmetal]

MSS lap joint stub ends have the same OD as the OD of the raised face of any flange of any class of that pipesize.

MSS lap joint stub ends can be machined with a serrated gasket finish (i.e. 125-250 microinch, 40-55 grooves per inch) like any other RF flange. But you can't machine "grooves" into them for use with O-rings, RTJ metal gaskets etc.

 

[borobam182]

Thank you for that confirmation moltenmetal - that was my thoughts.

 

[LittleInch]

Errr, it may not be common, but according to figure 6 of ASME B 16.5 you can machine an RTJ groove in the end of a lap flange providing thant

6.4.3.4 Ring Joint. The thickness of the lap
remaining after machining the ring groove shall be no
less than the nominal wall thickness of pipe used.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[moltenmetal]

I stand corrected, though why someone would do such a thing is a bit of a mystery to me! Thanks LittleInch.