Welding pipe fittings?

By jon20013 (*****)

Date 01-23-2007 17:18

A VERY GENERAL rule of thumb; if you want full penetration welds you would normally have some gap. The amount of the gap depends on the welding process. The designer probably isn't the best one to specify weld joints. Weld Engineering is all too often overlooked as a separate discipline and lumped in with mechanical, process or civil engineering when it should be stand alone. You should have a person knowledgeable in welding working with your group.

By Stallsmith (*)

Date 01-23-2007 23:39

I agree. I worked at a Fab shop with a welding engineer on staff and that made things so much easier. A considerable amount of time can be saved simply by specifying proper joint fit up. If a full penetration weld is desired but no root opening is left the welder wastes a lot of time and resources to create a root opening with a grinder. Grinding between the tacks, feathering the bevel, etc to allow access to the root. This is particularly important on UT or RT quality welds. To an mechanical engineer or designer it seems logical that a good tight, precise fit up would be desireable but that is not really the case when a weld needs to fully pentrate the joint. And of course the root opening is determined by the welding process used. Too large of root opening is also costly in the speed in which a weld can be made and the extra amount of welding needed to fill the joint. Having knowledge of welding and joint dimensions could be very helpful to engineers and designers

Just my opinion!!!

By medicinehawk (**)

Date 01-24-2007 10:40

If you go by that logic, you'd have to include another 1/8" for the gasket to your flange........right? I have been working pipe for a few years now and really it is a matter of opinion as to whether you add the gap(for welding)or not. You need a gap for weld penetration, but also for squaring up the fitting to the pipe or run. With say thin wall stainless steel piping, you don't really need a gap to get complete penetration.
Personally, I include gap as a dimension especially when building a by-pass, but I also include take-out for gaskets as well and usually don't have any problems, but maybe I'm just lucky, I don't know.

By Stallsmith (*)

Date 01-24-2007 11:14

Your right it really depends on the particluar job and specifications. Thin wall tubing can be welded successfully w/o a gap but sch. 80 which is a little under 3/8" really does need a gap for penetration. Again there are no hard and fast rules but I do feel knowledge of welding processes and desired weld results would help a designer or engineer when drawing these fittings and projects up. The amount of gap can be a matter of welder preference also so again there are no hard and fast rules. And I would rather have a joint with too little gap than too big of a gap because I can increase a gap easier that shrinking it. Your are correct also with needing a gap sometimes to square up joints unless you are lucky enough to mill or machine the end of every connection which is no always feasable. I still contend that welding knowledge would make the engineer or designer more aware of the needs of the joint. Of course the welder is usually the last one to get his opinion heard. Of course unless there are strict tolerances the preposed root opening will vary from the optimal by the actual dimensions after cutting and preperation so there is always variables but in the end a good welder can make a joint work whatever the circumstances.

By TimGary (****)

Date 01-24-2007 14:23

An experienced pipe fitter knows how to make allowances for working conditions. If you give him an ISO drawing that details what the spool needs to be after fabrication, then he / she can take it from there. If you make allowances on the drawing for root gaps, you need to clearly detail these allowances in order to prevent problems down the line. The pipefitter should know that he needs to figure in grinding lands on bevels, root gap required for the root pass welding method, weld shrinkage, gaskets, offsets and the slight variances encountered in the actual fittings to be used.
Now I'm the first to admit that good pipefitters are a rare breed. If you are not so lucky as to have one available, then yes, the design department needs to work very closely with fabrication supervision in order to assure that all of the necessary allowances have been accounted for.

Tim

By 11echo Date 01-24-2007 15:23

Ok, but pipe fittings normally come out of the box with bevels and lands already added. I had thought that all you did was mate up the fittings (land to land) and during the welding process that the width of the lands was thin enough that it was fused together by the arc pool making a full penetration weld!?? Obviously it's not as important when you have a "pup piece" or a couple sticks of pipe between fittings, then you do what you have to do to make sure the center line dimensions is correct. But when it's fitting to fitting make-up, and in real tight areas where your fighting 1/16"s it can be very important ...hence my need to fully understand what is needed.
PS ...Want to thank all here who have participated! It may be like getting an old dog to do new tricks but I appreciate the effort. *G*

By TimGary (****)

Date 01-24-2007 16:56

Hi again 11echo,

OK, if you take fittings out of the box and fit them up land to land, you'll most often not wind up with a complete full penetration weld that will be acceptable to the ASME code. This can be OK if your not doing pressurized pipe and not claiming the work is being done to code. The reason a root gap is used is to help the welder ensure even, full penetration. The root gap size can be adjusted according to the size of the fitting and the welding process being used.
For example, if I were doing 4" pipe and TIG or SMAW welding the root, I would use a 3/32" root gap and 1/8" filler metal. If I only had smaller or larger dia filler metal available, or was restricted against 1/8 dia by the WPS, then I would adjust the root gap accordingly. Also, if I was fitting a larger dia fitting then I would adjust the root gap and filler metal sizes to accommodate.
Now, you've got to figure in weld shrinkage as well. You can normally bet on the finished product shrinking at least half of the root gap dimension, after the pipe completely cools. The amount to allow for shrinkage depends on the welding process being used. The more heat input that the pipe is subjected to, the more shrinkage to allow for.
So, this means that if you are welding together 2, 4" pipe fittings, and you are burning the weld in hot, not allowing to cool between passes, and you have fit up with a 3/32" root gap, then after shrinkage, the 3/32" root gap will have all but disappeared.
I'm not sure if I'm explaining this very well...
Do the rest of you guys concur?

Tim

By jon20013 (*****)

Date 01-24-2007 17:42

Tim, in general I agree. I instruct my welders to use a fit-up gap equal to the filler diameter, most of them have no problems at all in doing so. In my work dimensions are critical so I have spent quite a bit of time researching shrinkage (still not claiming to be an "expert"). As I'm sure you're aware, there are some very fancy forluma's for calculating weld shrinkage. In my own opinion, these are simply fancy rule of thumb calculations. There is actually no way to set a shrinkage formula in stone for a manual process. In further researching, I have found concurrence from a few welding society's outside of AWS.

If I were welding carbon steel fittings, I would say my root gap PLUS some fraction of shrinkage would occur as a result of welding. That fraction depends on the speed of the weld metal deposited and a whole host of other variables. For stainless I would say the same, except would expect even greater shrinkage.

Sorry for the long-winded agreement to what you said already! :)

By ssbn727 (*****)

Date 01-24-2007 23:54 Edited 01-25-2007 00:17

Well said ther Jon!
I agree with Tim also, and would just like to add that "fittings out of a box' will certainly have some distinct variations in their specific dimensions, bevel/land shapes & configurations or even chemical compositions depending on the supplier, and depending on their country of origin, the accuracy of the fittings dimensions within their listed tolerances will vary greatly!!!

Why is this something to look out for? Well. both Jon & Tim work to certain codes that would reject some of these fittings because of the discrepancies found that do not conform within the specifications listed for certain codes or standards even though some of the fittings may have the standards stamped on the fittings as to say that they are in compliance but, when scrutinized further - they are found not be in compliance. This is why inspection upon delivery of these items must be done at shops that manufacture components that are to be delivered to the Customer/Client who demands such exacting specifications as per contract docs.

Case in point; 1.) Hi-low conditions are constistently occurring when using company "X" fittings but, no such conditions are the norm when using "Y' company's fittings, and the work required, loss of time, added labor costs do not justify the continued use of the fittings being supplied by "X" company especially if the pricing between the two companies are so close that to continue using "X" company's fittings would end up almost eliminating our margins!

2.) Some fittings were delivered with chemical compositions very different than what their MTR's had listed or what their stampings represented, and Jon has probably ran into this from time to time with the type of work he's involved with so, if you want to add your own experience - please do Jon!

3.) O.D.'s/I.D.'s & wall thicknesses vary on pipe & tubing coming from different countries so. it's important to have reference books available to you so that you can cross reference from let's say UNS numbers, DIN numbers or even JIS number equivalents because of the slight differences in these dimensions.

There are other discrepencies to look out for but for now, I'll just point out the more prevelant ones.
I know for sure that many of us have found more than once, some fittings that just were'nt acceptable, yet when questioned as to why we would'nt accept them - were told otherwise by the vendors, and ultimately had to make the call to reject those fittings & use alternative suppliers that would indeed meet specs.

Hope I did'nt open up another can of worms here.
Respectfully,
Henry

By 803056

Date 01-25-2007 02:16

My experience has been that the forged fitting are not "ready to weld" as received. The bevels and root face are "rough". The welder is expected to grind the fitting as necessary to produce the proper root face and root opening as required in their WPS. That's right, if you review the welding requirements of the ASME piping codes, they refer you to the WPS for the proper fitup requirements. They are not provided by the ASME codes.

Integrally reinforced branch fittings are typically manufacturered to fit "several" diameters of the run pipe. Again, the welder/fitter is expected to hand grind the fittings to provide for a proper root opening and bevel angles around the circumference of the fitting.

While we are on the subject, I have found that I get more consistent results if I rework the grooves to be U-grooves with a 1/32 root face (when welding with GTAW). More fitup time, but fewer rejects due to incomplete joint penetration. I find the U-grooves work especially well with welders that have marginal pipe welding skills. With the 1/32 root face, I do not use a root opening. This also improves the purging operation because there is less gas leakage when the root opening is 0 inches. One cautionary note: some of the piping codes require the manufacture/contractor/erector to qualify the groove detail if it varies from B16.25, which depicts the "standard" groove details for piping and fittings. I usually use the U-groove detail when I qualify the procedure to Section IX so that I'm covered.

For those of us that review WPSs, the construction code, i.e., B31.1, B31.3, requires the joint detail to be in accordance with B16.25 and they (joint details) should be shown in the WPS. Statements such as "all grooves and fillets" listed under joint design does not fulfill the requirements of the construction code. Why, because, if the joint detail is not per B16.25, it is not qualified unless you qualify the detail via a PQR.

That's my interpretation and I'm sticking to it.

And Henry, that's why we're here, to open up those cans of worms and make everyone's life more interesting.

Best regards - Al

By ssbn727 (*****)

Date 01-25-2007 04:40

I agree Allan but, that was'nt my point! My point is that the dimensions pertaining to the alignment of the fittings to the pipe/tubing are sometimes not even close to where it's acceptable and alot of work has to be done to correct the alignment which is what takes more time than originally alloted for!

I agree with your preference of using U grooves also - provided that the U groove is qualified via PQR.
B31.1 & B31.3 are the two piping codes that I work to most of the time so I already know the note with respect to B16.25... The U grooves work especially well when welding stainless.

I've worked with some operations managers & production supervisors that have said to me more times than I care to remember that I could disregard alignment or even the fact that forged fittings are NOT ready for welding, that I could weld them as is and still be working to code... Afterwards, I was told that if I did'nt weld them up to the pipe ASAP? I'd be out of a job ASAP!!! Been there-Done that!!!

I can also tell you that I've said "Sayonara" to quite a few shop's that did'nt do the right thing!!!
No Regrets!!! The only thing that really burns me up is when someone tries to be a "hero" and cut corners when they should know better!!! This is also my interpretation and I'm sticking to it!!!

And Al, I consider myself to be very greatful to be blessed with the "Gift of Life" and living a very interesting life, thank you! However this second time around, and I say this because I was considered dead more than once on the operating table during my transplant - I like to keep my life from becoming too boring while at the same time, I do'nt mind it when it's boring because there are less unwelcome surprises and, life seems safer - Capish?

Respectfully,
Henry

one1_2months220lbslighteraftertransplant.jpg (32k)

By 803056

Date 01-26-2007 05:11

I know where you are coming from.

I do believe we're singing the same tune, just a little out of tune.

Best regards - Al

By Shane Feder (****)

Date 02-10-2007 02:17

Allan,
Greetings from "downunder".
With all due respect I have to disagree with the following statement.

" For those of us that review WPSs, the construction code, i.e., B31.1, B31.3, requires the joint detail to be in accordance with B16.25 and they (joint details) should be shown in the WPS. Statements such as "all grooves and fillets" listed under joint design does not fulfill the requirements of the construction code. Why, because, if the joint detail is not per B16.25, it is not qualified unless you qualify the detail via a PQR."

B31.1 Clause 127.3 states " Butt-welding end preparation dimensions contained in ASME B16.25 or any other end preparation which meets the WPS are acceptable."
B31.3 Clause 328.4.2 states " End preparation for groove welds specified in ASME B16.25, or any other which meets the WPS is acceptable."

As long the joint meets the WPS it is acceptable irrespective of whether it complies with B16.25 or not.
ASME IX QW212 states " Except as otherwise provided in QW-250, the type and dimensions of the welding groove are not essential variables".
Therefore, a WPS qualified in accordance with the essential variables of QW 256 (GTAW) can be used for any joint configuration required without requalification.
Please correct me if I am wrong or I have missed something,
Regards,
Shane

By CWI555

Date 02-10-2007 04:43

I believe your on the money Shane.

By 803056

Date 02-10-2007 19:21

You caught me! My attempt to over simplify chewed me in the butt once again. I usually tell my young design engineers to use the joint details in B16.25 to keep them from designing the impossible. You know, the V-grooves with a 45 degrees included angle because they saw it in a text book once or the 15 degree narrow groove because they saw it an advertisement for narrow groove welding.

That being said, I have stated (in past threads) that the construction code, be it ASME B31.1, B31.3, etc., may modify the requirements of ASME B&PV Code Section IX. It has been my experience that many contractors miss that important aspect of qualifying their procedures or their welders and brazers. They limit their reading to Section IX and fail to read the requirements of the construction code that may restrict or prohibit specific joint details or may change a nonessential variable listed in Section IX into essential variables.

Consider the following examples from B31.3 Process Piping (paraphrased):

Chapter IX High Pressure
Reference K328.2 Welding Qualification
Reference K328.2.1 Qualification Requirements -Qualification of welding procedures and performance of welders shall comply with the requirements of B&PV Code Section IX, except as modified herein:
(a) Impact testing shall be performed for all procedures,
(b) Test weldments shall be made using the same specification and type or grade of base metal(s) and the same specification and classification of filler metal(s) as will be used in production welds.
(c) Test weldments shall be subjected to the same heat treatment.
(d) Where tensile test specimens are required by Section IX, the yield strength shall also be determined.
(e) Mechanical testing is required for all performance qualification tests (i.e., radiography of the welders test coupons is not accepted).
(f) Qualification of pipe or tubing also qualifies for plate, but qualification on plate does not qualify for pipe or tubing (this applies to procedures and welders).
(g) For thickness greater than 51 mm (2 inch), the procedure test coupon shall be at least 75% as thick as the thickest joint to be welded in production.
Reference K328.2.3 Performance qualification by others. Welder performance qualification by others is not permitted.
Reference K311.2 lists prohibitions on the use of backing, fillet welds for pressure retaining joints, socket and seal welds.
Reference K328.3.3 if consumable inserts are to be used, the procedure must be qualified.

Chapter VIII Category M Piping
Reference M328.3 Split backing rings can't be used, removable backing and consumable inserts may be used if their suitability is demonstrated by procedure qualification.

For metallic piping not categorized as M or high pressure:
Reference 328.4.2(2) End preparation for groove welds specified in ASME B16.25, or any other which meets the WPS, is acceptable. I take that to mean the groove details included in ASME B16.25 and those not included in B16.25 should be detailed in the WPS. Those details need to be complete to the extent the welder knows what groove angles are permitted, what root face and root openings are permitted, whether backing or consumable inserts are to be used, etc.

Reference 328.4.3 (a)(1), 328.4.3(c)(1), and (3) and (4) all reference the WPS and the limitations or tolerances in the WPS, i.e., this information is more than a simple statement of "all fillets and grooves".

I have to humbly retract my comment regarding the qualification of any weld groove detail that does not meet B16.25 must be qualified. Shane, I could swear I saw that in one of the codes I was reviewing, but I couldn't find it today, so thanks for keeping me on the straight and narrow. Open mouth, insert foot!

Best regards - Al