Weld size requirement on a slip on flange 1

[Muckerj]

Hello all,

I have a questions and I am hoping some of you will have an answer....

We are using a lot of slip on flanges on our cargo oil lines (Consider our facility as an oil shuttle tanker). According to the building specs, we have both front and back weld (front being welded to the hub and back being on the inside). I know ASME B31.3 Figure 328.5.2B has some information but it applies only to the what I call extreme services. Cargo lines being hazardous-ish fluid, low pressure, what's the weld size requirement on the front weld (We are good on the back weld)? It would be good to know the reference too. For your information, currently for some sizes, the weld size can be below pipe wall thickness.

Also, I hear a lot about slip on flanges only have 2/3 pressure retaining strength and 1/3 fatigue resistance compared to a weld neck flange. Out of curiosity, I would like to know more about it, is there any backup calculation or reference that i can read up on? Purely for my interest.

Many thanks and have a good day.

 

[racookpe1978]

Yes, you need to specify the minimum size of that inside weld. There is no reason to specify anything different than the outside weld, since ANY difference between inside and outside will be confusing and will lead to errors.

I deliberately said minimum size of the inside weld and outside weld, because any size weld specified larger than the pipe wall thickness is useless and contradicts the requirement for a 45 degree bevel on the fillet. In practice also, specifying something like " equal to the size of the pipe wall" will in many cases lead to wasted time and effort and money because on very heavy-walled pipe, the weld does not need to be as thick as the pipe.

 

[Muckerj]

Thanks racookpe1978 (how can I tag people when I reply?)

If you look at ASME B31.3 Figure 328.5.2B, it does call for higher than thickness weld size.

I have some cases where the weld size is below the pipe wall thickness. Not sure if they are acceptable...

 

[Ripz]

308.2.1 Slip-On Flanges
(a) A slip-on flange shall be double-welded as shown
in Fig. 328.5.2B when the service is
(1) subject to severe erosion, crevice corrosion, or
cyclic loading
(2) flammable, toxic, or damaging to human tissue
(3) under severe cyclic conditions
(4) at temperatures below −101°C (−150°F)

.....

If you do not fall under these these categories then any weld size would do. In fact, no weld would do.

If you do fall under these categories, then you'll have to fulfil the requirements of B31.3 Fig 328.5.2B. This being, the inner weld is to be Pipe thickness or 6mm, whichever is lesser.

 

[racookpe1978]

Thanks racookpe1978 (how can I tag people when I reply?)

I thank you for your compliment. On this site, one way to compliment a writer is to add a "star" to his post - as done above in recognition of Ripz (Mechanical) for his specific research in the tech manual about weld sizes and weld requirements. Below each reply is a small question: Do you like this post? Star it!" Press that link, and the writer is rewarded a brownie point.

And, having raised several brownies, brownie points are much less fattening than brownies, cookies, cakes, or pies.

 

[XL83NL]

@Ripz

If you do not fall under these these categories then any weld size would do. In fact, no weld would do.

Please, explain me how the pipe would hold the flange in place.

 

[racookpe1978]

XL83NL (Mechanical)

Quote:
If you do not fall under these these categories then any weld size would do. In fact, no weld would do.

Please, explain me how the pipe would hold the flange in place.

Reconsider the design of a slip-joint flange:
First is the raised face of the flange -> 1/16 inch above the bolt face of the flange,
Then the flange itself, with an opening the the OD of the pipe,
then the end of the pipe - recessed about 3/8 of an inch inside the flange opening.
The first weld is at this point, a fillet weld around the end of the pipe wall joining the end of the pipe and the interior opening of the flange hole.
On a proper slip joint flange, the second weld is on the outside of the pipe, on the back side of the bolt flange reinforcement, between the outside of the pipe and the bolt flange reinforcement ring.

What Ripz was saying, is that theoretically you could eliminate the inside weld and just use the outside weld. But don't do that.

 

[Ripz]

I meant the inner weld. This weld is not mandatory for all applications. Only the outer weld is mandatory.

 

[Muckerj]

Thanks all,

On the outside weld, the leg length / thickness should surely need to be verified by designers? (Can someone answer this?)

I showed my drawings to one of our inspectors yesterday and he confidently replied that the thickness of the outside weld was ok. He added the drawings must have been approved by a third party. He also referred to AWS structure guideline.... And this is what I found

http://www.twi.co.uk/technical-know...-determine-the-minimum-size-of-a-fillet-weld/

Interesting discussion so far (at least for me)

 

[Ripz]

According to 328.5.2B, your outer weld leg length has to be the lesser of 1.4 X Pipe thickness or the thickness of your hub.

Your designers would have specified the weld size (but anyone can calculate, it's not that hard), and your inspectors are to verify that it's according to design.

 

[Ripz]

Last point, the value calculated is your minimum leg length, not an absolute value.