Allowable bends and formings in piping 1

[VPbris]

Hi,

I was looking at the internal piping of a Liquid Oxygen truck (picture in annex). I noticed these formings and bends of the piping and some questions came to my mind.

(

https://files.engineering.com/getfi...f-b4cc-b9a18f9c9785&file=20181009_084626.jpg)

1- What are the limits of these bends and forming? Any part of code to reference (We can assume to be working on ASME B31.3)?

2- I think this question is probably more suitable at the pressure vessel forum but maybe I can get an answer in this same forum. At the connection with the head, there is a small machined piece of piping welded to the vessel (english is not my 1st language, do not know if there is a technical name for this piece), and there is a socket at this piece where the out piping connects and is fillet welded, is this attachment covered by ASME B31.3 or by pressure vessel code? I assume the small piece connection is by pressure vessel and this latter piece by ASME B31.3, any references?
I attached a sketch of these pieces.
(

https://files.engineering.com/getfi...8-ad1d-23c0da481bd4&file=20190212_202243.jpg)

 

[chicopee]

Are you doing some reverse engineering on this LOX tank? If so it will be cheaper for your company to buy new and used LOX tanks then reverse engineer this tank for manufacturing reasons.

 

[VPbris]

Hi

Not trying to reverse engineer this. I actually have all the drawings and full acess to manufacturing site during production. I am trying to have some code reference, as these piping are made that way by 'experience' and the is no documented paper on the company, so I'm trying to evaluate the design by code

 

[StressGuy]

What kind of pressure does this operate at? It's possible that it operates low enough to not be a code vessel. Though, being an on road vehicle, I wouldn't be surprised if DOT regs had something to say about it.

But, to address your specific questions:

1 - Yes, B31.3 does address formed pipe bends. Looking at the 2016 edition, 304.2.1 is a good place to start regarding terminology and minimum wall requirements. 306.2 will address and link to requirements for design and fabrication.

2 - It's hard to see enough detail in the picture, but what you are describing sounds like a sockolet Bonney Forge Sockolet.

I'm only familiar with them as piping branch connections. For a vessel connection, I'd flip through the catalog here to see if something matches up Forged Vessel Components. Though, I think their stuff is pretty much all flange connection fittings, which is what I'm used to dealing with as the connection to a pressure vessel.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[LittleInch]

What are the limits - The design like this really should be stress analysed using some sort of analysis program like Ceasar or similar ad stresses should not exceed the maximum allowable in B 31.3 (which might be quite low)

sockolets are not fittings designed to ASME B 16.9, but are fittings as designed to MSS-SP97. They may fall under the designation of allowable fittings in ASME VII, but I also don't think this thing is a Pressure vessel.

That pipework doesn't look great to me. The two loops are clearly there to reduce stresses and allow for contraction as do the strange U shape piping, but I don't like the 10 or 15 degree mitre cut on the inner 2" pipe.

I highly doubt any of this has ever been analysed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

So this is all austenitic SS. The bends are both to allow thermal expansion/contraction and vibration isolation.
I am a little surprised that there are as many welds as there are, they could have formed these in larger sections.
It is probably related to multiple uses for bent portions.
There are special tank truck rules in most countries.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[VPbris]

Hi,

Thank you for your replys, Answering some general questions:

Yes it is a pressure vessel, operates at 17 barg, temperature of inner vessel ranges about -140 to -180 degrees celsius. Material of piping and vessel is Austenitic SS. I also agree most bends are to allow expansion/contraction, the bends on the bottom part serve also as a gas trap to avoid excessive heat transfer from the liquid on the bottom part.

@StressGuy

It really looks like what you call a socketolet, the idea is the same but geometry just a bit different, in this case the outer face is a cylinder, in the vessel design it is designed as a nozzle without a flange, and just above the reinforcement limit area the socket is made to receive the piping which is then fillet welded​

@LittleInch

Does not look good to me either, thats why I'm looking into it to improve the design, I agree these shape of pipe should be better analyzed. And you are right to the point, these have never been analyzed. What would you suggest to be the key points to pay attention on this analysis?​

@EdStainless

The reason of so many welds is lack of adequate design and drawings, requiring a lot of adjustments during assembly which would be more difficult with larger sections. A lot of room to improve.​

 

[gwalkerb]

Is the mystery fitting a socket-weld coupling or half-coupling? Those are basically just a cylinder with bores sized to match pipe, that's kind of what it looks like from what I can see. They're commonly used on pressure vessels, and they're a standard fitting, defined in ASME B16.11.

http://www.steel-forged-fittings.com/socketweld-coupling.html

 

[LittleInch]

Use of socket weld anything on a system which clearly see high stresses from contraction doesn't look like a great plan to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[VPbris]

Thank you for the clarification of the name of the piece in english, thats an exactly it.

@LittleInch

Could you elaborate on why you think this is not a good idea? On Cryogenic vessels all connections are usually welded for two reasons one is that the inner tank is not acessible once fully assemblied, hence it would be not possible to change or repair any sealing. Also, due to low temperature and fluids some sealing types are just not applicable​

Regarding pipe stress analysis, I'm not really familiar with analysis of piping but I do have some experience with FEA analysis of Pressure Vessels and since at the moment I'm carrying this analysis for study purposes and not for production I'm willing to give it a try, I thought of the following procedure to be applied. Fix the end that is connected to the vessel, fix the part that passes through the outer vessel head (not shown in picture as outer head is not in place),apply pressure and temperature and look at the stresses, is this acceptable? Or there are specific code rules for this type of analysis (Again Im not familiar with B31.3)

Once again, thank you all for your replies

 

[LittleInch]

Welding is good, butt welding is much better in high stress environments than socket welding

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[matschka]

I see just ( 2 )smooth hand made or machine bends. The rest of the bends are formed from welding (2) 90deg. fittings back to back. If you increase your magnification to 'max' on your computer you will see these welds in greater detail. They look good. The 'machined' fitting welded into a vessel like this should be a 'full' penetration weld. Your drawing does not show the vessel weld. You need something like a weldolet where you can get a full penetration weld at the vessel fitting interface and the pipe to fitting interface. I am not a 'fan' of a socket weld at that location.I do not have access to the code but always check it to keep you & others safe.