Repad length?? 2

[CM06]

I am trying to determine the required length of a repad that I have requested to be added to a stub-in tee (12" run and branch dia). I concluded that I needed some reinforcement on tee during the pipe stress analysis.

There is not enough room to fit a better quality fitting (welding tee). Our standard generally calls for repad to be made from the same material as pipe (i.e. same WT). However, I don't know how to determine the length of the repad. that is, the distance from the branch OD on either side.

I appreciate any help/guidance.

 

[StevenHPerry]

The applicable piping code probably gives you guidance, though it won't give you an exact number.

For B31.1 see paragraphs 104.3.1 and 127.4.8 plus reference mandatory appendix D.

Appendix D does not consider the repad OD (what you call length) in calculating the SIF.

- Steve Perry

 

[pennpiper]

In general a Reinforcing Pad for a standard piping "Stub-In" branch will be a ring shaped plate. The I.D. will be equal to the O.D. of the Branch pipe and the width of the Pad will be equal to one half the nominal diameter of the branch pipe.

Set that for your dimensions then have your Stress Engineer do the calcs.

 

[CM06]

Steve,

I can't find anything in the code (B31.3) regarding the dimension I am looking for. Most of the reference that I've found on this matter pertain to the thickness (height) of the pad. The SIF calculation in Caesar is the same. Only the thickness is entered.

The one reference in B31.3 that I have found on this issue is 304.3.5(b) This paragraph states:

"Branch connections made by welding the branch pipe directly to the run pipe should be avoided under the following circumstances.
1. when branch size approaches run size, particularly if pipe formed by more than 1.5% cold expansion , or expanded pipe of a material subject to work hardening, is used as the run pipe
2. where repetitive stresses may be imposed on the connection by vibration, pulsating pressure, temperature cycling, etc.

In such cases, it is recommended that the design be conservative and that consideration be given to the use of tee fittings or complete encirclement types of reinforcement."

So in my case, we have a stub-in tee with run 12" and branch 12". Because of the high SIFs associated with this unreinforced tee, as calculated in Caesar, the stress failed until a pad was added to the fitting (only pad thickness is required input). I would use a better quality fitting if possible but space doesn't allow it unless a much larger modification is completed. I am trying to 'avoid' using this type of fitting as stated in the code and will revert to a larger modification if I can't get any satisifaction with the option we are proposing. But obviously if I can make this option satisy code requirements, I will use it.

C

 

[CM06]

Penn,

Is the 1/2 the OD of branch for width measurement a rule of thumb? Or is it defined in a code or standard?

C

 

[MJCronin]

CMO6..

No chance you can use a weldolet instead of a pad ?

Sometimes it is less work to install a weldolet rather than a pad.

The pad would have to be of a "split" design to get it on the stub-in.... is that correct ?? (more welding ?)

What is the pipe material and schedule ?

-MJC

 

[CM06]

MJC,

Pipe material is A106 Gr B, sch XS (12.7mm WT).

no chance for a weldolet.

C

 

[StevenHPerry]

C, that's why I said originally that the code wouldn't tell you a number.

For pressure containment, B31.3 will specify a reinforcement zone within which you can take credit for a repad. The OD (length) of this zone is 2*d_2, which is generally speaking 2*OD_branch. In my experience, using a wider (longer) reinforcement has always been preferable to a thicker (taller) reinforcement. I've never had your space constraints though.

For example: use a 25.5" OD (length) reinforcement and the minimum thickness (height) required to satisfy B31.3 304.3.3 and your flex analysis considering B31.3 appendix D.

Is there no support arrangement that could force the growth away from this size on size stub in? That might be a good idea even if you add reinforcement.

- Steve Perry

 

[pennpiper]

CM06,
Rule of Thumb backed up by yeeeeaaaaarrrrsss of experience.