Full encirclement on fittings

[Kokevi]

Hi All,

I am designing some piping according to B31.4 and this includes a number of places where supports are welded onto the pipe. I am using full encirclement as per the code at these locations, as the stress is > 20%. However, I am not quite sure what to do at a location where I need to support under a tee. I can't do full encirclement because of the outlet, but do need to weld the support on here. With the high loads I don't have any flexibility to move the support location.

My options seem to be:

- Don't try and put encirclement in on the basis that the code applies to pipe, not fittings, so it is not a requirements.

- Put a half pipe or cradle type encirclement in, on the basis the the code does apply, and as much encirclement as possible should be used. The problem with this approach is that the tee is not cylindrical, being a wrought fitting, so this is not practical unless a thin plate is used (9.5 mm versus the tee of 25 mm) so it is not ideal.

- Use pads for the supports on the basis that the reason for the additional encirclement material is to reduce local stresses that may exceed the yield, so use pads to distribute the load more evenly.

I would appreciate any input on how the code should be interpreted for this application and comments on the options or any other options that I haven't considered.

Thanks

 

[BigInch]

The code DOES apply to fittings and pipe. Fitting stresses are usually worse due to stress concentrations. Theae are not reinforcement pads. These are wear pads and will also help in preventing corrosion from reaching the pipe.

Put a full encirclement hold-down on one side, or on both sides of the fitting.

Independent events are seldomly independent.

 

[Kokevi]

Thanks for the reply

This is for attachments welded to the pipe, there is no wear aspect to this requirement. The full encirclement is required so that welded attachments "will not cause ... excessive localized bending stresses, or harmful thermal gradients" in the pipe; from section 404.3.4 and 421.1.

The fittings will be worse do to stress concentrations, but the design of the fittings does account for this already to some degree. Pipe and fittings are treated separately under the pressure calcs, so why must it be assumed that this specific clause applies to both pipes and fittings?

I am afraid I don't understand what you mean by a full encirclement hold-down. Could you please elaborate?

 

[Kokevi]

Ah, you meant a full encirclement support on each side of the tee. Sorry, a bit slow... If I do that the stresses in the tee are too high.

 

[BigInch]

Sorry I didn't understand you were talking about adding metal reinforcement for area of steel removed when making a branch connection. BTW, that is a "branch connection", not a tee. A tee is a fitting. A branch connection is made by joining two pipes more or less perpendicular to one another. Since you referred to a tee, I thought you didn't need area of steel replacement reinforcemant and were making a simple wear pad.

For reinforcement of the branch to replace area, correct, you will need full encirclement reinforcement.

To put a support at this location, or any other location with that stress level, regardless of if it is a branch connection, or a tee fitting, you will need a full encirclement "clamp" type attachment around the pipe to the support. Since you say you need a support there, I assume you can put one there .. ie. physically there is enough space and the only problem is the outlet pipe that's getting in the way of your U-clamp.

Option 1) Do not worry. Move off the tee just enough to fit the support in and just put it there.
Option 2) Build a T support, encircle both sides of tee.
Option 3) Build a T-Offset, encircle one side, or extend and do both.

Independent events are seldomly independent.

 

[Kokevi]

Hi BigInch,

We are still not entirely on the same page. I am using a tee, not a branch connection. The tee is a B16.9 XS welding fitting. The requirement for reinforcement is not based on any pressure or material replacement requirement, but purely on the code requirement I quoted, which requires full encirclement for welded attachments. That was the origin of the pipe v fitting distinction. If you think I am misinterpreting the requirement please let me know.

I need a support on the tee on the opposite side of the outlet. So the encirclement would then wrap on either side of the outlet, but be over a very small portion of the tee. This would be fine for a fabricated branch connection, but as I am using a tee it is not exactly cylindrical so any wrapping round becomes impractical, unless I use a much thinner plate.

 

[BigInch]

Yes, I think you are misinterpreting the requirement. The paragraph you quote applies to a fabricated tee, or branch connection, but DOES NOT normally apply to a MANUFACTURED, FORGED, fitting that you bought ready made to a B16.9 spec.

If you are using a manufactured tee, made to ASME B16.9 spec, I DO NOT think that you need any reinforcement. It would be very unusual. For example, if you want schedule 40, or 80 pipe, the manufacturer provides sufficient wall thickness in a schedule 40, or 80 fitting, in many cases a bit extra wall thickness than the attaching pipe, in order to guarantee a strength of the fitting equal to, or slightly greater, than the strength of an adjoining straight pipe of the same schedule, without any reinforcement needed to be added at the weld.

If you are making some kind of a custom connection in more or less "T" shape, fabricated, maybe from two pieces of pipe, by cutting out a hole in the mainline pipe and welding on a perpendicular pipe (attaching around the hole), you may and probably DO need reinforcement. Any support placed under a gas pipeline, operating at your indicated stress level, requires a hold-down clamp-type. Inverted "U" band, strap, or "U-bolt" type, holding the pipe to the support, but that is not considered "reinforcement" for the weld. The strap, band, or U-bolt is not welded directly to the pipe.

Independent events are seldomly independent.

 

[Kokevi]

Thanks for the input, it is very much appreciated. I wondered if I was trying to apply something that wasn't meant for the case.