May I request some one to kindly advise what is the permitted minimum distance between two successive C Joints for a pipe line as per B 31.1 and 31.3 and also the applicable clause of the code.

Best regards

Venugopal

I don't think there is a specific Code limit. You need to avoid the HAZ's over-lapping. This may be a few mm for thin wall or more than 100mm for heavy wall. Commonly 1D is used, or a factor on the wall thickness.

Have a look at Company standards.

Many thanks Mr Moverz, sorry for the delayed acknowledgement. What is HAZ?

I am from Paper Industry, in our sector we rarely encounter code piping, very few instances of steam piping engineering works are oursourced .

Hence may I request others who routinely follow code to advise what least distance they permit between two C Joints.

Best regards

Venugopal

It's the heat affected zone ..... where the parent metal structure is changed by heat from the welding process. It's good practice to avoid HAZ's intersecting to avoid local hardening and other problems.

The dimension of the HAZ is a function of weld process and pipe diameter and thickness, so it varies. Best to allow around 50mm between average size welds.

AVenugopal,
MoverZ has given you the right advice but I would like to add a few remarks that may help you in the future.

First, In "Piping" there are "Codes" and "Standards" and "Common Practices" and Individual Preferences".
The ASME B31 "Codes" do not address this subject. The many hundreds of "Standards" also do not cover this subject.

So, we are down to "Common Practice" and "Individual Preferences". Both of these should be based on "Common Sense". In many cases they are not.

For instance there are two "Common Practices" but only one seems to be based on "Common Sense".
(A)- Some people "claim" that you should ALWAYS use the Rule that "Two welds shall be two pipe diameters apart".

(B)- Other people "state" that the minimum distance between two welds shall be not less than 5 times the thickness of the pipe material.

In method "A" if you had a 3" pipe then the distance between the two welds would need to be 6". Okay this does not seem too bad. But, If you apply the Rule to other sizes then "Common Sense" falls right on it's face. Welds in larger pipe sizes such as 12" would need to be 24 inches apart and 18" the welds would need to be 36 inches apart. This is just not logical.

On the other hand in method "B" the rule of 5 times the wall thickness is far more logical and is more cost effective when designing a process plant. With this method 3" Schedule 160 pipe welds would need to be farther apart (5 x.438 = 2.190" )than the welds in 12" Standard Weight pipe (5 x .375 = 1.875").The choice of which of these methods to use is most often an arbitrary decision made by some individual in the Client Company or the Engineering Company.

The piece of pipe that fills this gap between the two welds is commonly called a "Pup" or "Pup Piece".


So what if you NEED to have the welds closer than 5 times the wall thickness? A thought from another writing:

The only restriction for the length of a "Pup" piece is common sense or a company imposed minimum length. This minimum length is defined in different ways by a Lead Design Supervisor or company and there is no absolute Code restriction or commonality. Individual pipers or a company minimum may consider the pipe wall thickness, the weld bevel, the heat effected zone, or the real or perceived ability of the shop to make a weld. There is no magic rule and there is no common industry standard.
In referring to the proximity of butt welds in pipe. This question has been asked many times. The answer is the piping Codes do not provide specific guidance. In this situation, you must use engineering judgment such as, locate the butt welds as far apart as possible to avoid overlapping weld “heat affected zones” and stress concentration effects. For girth weld spacing, a criterion that has been used for many years is: the greater of 2 inches or 4 times the wall thickness.
You might also want to talk to a good Welding Engineer as it could be dependent on the actual welding process. If this is for proximity of weld joints to one another there could be other concerns--again the Welding Engineer could help you.

(Just for interest’s sake, Australian Standard AS 4458 specifies the following: "The distance between the edge of two circumferential butt welds shall be not less than four times the pipe wall thickness or 30 mm, whichever is the greater, unless the first weld is post weld heat treated (PWHT) before the second weld.")

This means that if you do local or Oven Post Weld Heat Treating of the welds to "Normalize" the weld Heat Affected Zone then you can reduce the distance between the welds.

Because Jop put so much work into his post, I took a look through old B31.3 Interpretations.

I find Interpretation 14-03(b):

Question (4): Does ASME B31.3-1993 Edition, Addenda b, have requirements for the minimum lengths of pipe sections between welds?

Reply (4): No.

Thank You Dave

It is touching to recieve such nice advise.Thanks alot Mr Moverz, Mr JOP and Mr Dave Diehl for the trouble you took on my request.

Best regards

Venugopal

as we follow, minimum distance is 0.7mtr. if less than that both joint required radiography.

If you specify 0.7m between welds, you will end up with some wierd piping layouts. It is way too much.

NDT (Non destructive testing) has to be also considered when placing welds. Issue can get complicated so the best is to talk to a good technician on this issue.

Jouko,
The fact that welds, All welds, are subject to "NDE" has nothing what-so-ever with the minimum distance between welds.

And yes the proper term is NDE (Non-Destructive Examination) not NDT.


See the following website for an article (American Welding Society)with more details on NDE:
http://www.ndt.net/article/0698/hayes/hayes.htm

industry standard=50mm
the minimum a fitter can cut off without hurting his thumb.

(yes i'm serious....)

Brothers and Sisters, there ain't no such thing as an "industry standard" that covers this issue. That is why we constantly have this discussion.

"The minimum a fitter can cut off" Why not train the welder to bevel the end of a piece of pipe and weld it to the fitting (or whatever) then make the cut for the other end of the "Pup"?

Meh,

Piping designers throught the world use 50mm.
Its (normally) used to space a flange coming of an elbow or tee. Specifically, for slip on flanges, anything less than 50mm gets a bit difficult to handle.

Industry std, unwritten rule, rule of thumb, call it what you like, but its there.

Tj.