What is the difference between welded pipes and threaded pipes? 2

[abc9]

I am a mechanical engineer, for the first time involved with a LPG (Liquefied Propane Gas) station design. On piping between the storage vessel and the pump and dispenser I am not sure to use welded or threaded pipes. Please let me know what the differences are in their applications, and what you recommend. Thank you.

 

[Latexman]

Who do you think I am, the Amazing Kreskin?

Okay, now that that is out of my system, can you provide more details please? Material of construction, operating pressure and temperature, line size, pulsating service or not, liquid or gas or 2 phase service, etc.

Good luck,
Latexman

 

[stanier]

THe LPG industry has used threaded connection for many years at the distribution end. Whereas in the refinery welded piping is used as it is inherently safer and the pioing is generally larger diameter.

In Australia the LPG industry has its own piping code specifically tailored to the user requirements and general practice. hence deisgn is not to ASME B31.3 or AS 4041. perhaps there are other industry specific standards in your country.

The risk with a threaded joint comes from fatigue. As LPG is relatively low pressure and at ambient temperature threaded fittings and equipment are common place at gas stations and even truck load out facilities. Fatigue is not usually an issue.

The risk in a refinery is far greater than a truck load out or gas station because the consequences of failure are higher.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[Duwe6]

Trust me, welded is better.

Think about the connections as a Mech Engr; threads reduce the wall thickness of sch 40 pipe by half. The thread roots are a sharp 60° reentrant angle, and thus a huge stress-riser. This stress-riser is located at the thinnest point of your pipe [root of threads] The threads provide an uninterrupted leak path.

Specify sch40 pipe using 3000# socket-weld fittings, all welds to be made using E-7018 [low-hydrogen] filler, or 'tigged' GTAW using ER-70S6 [or S2]. There's a reason why no petroleum or flammable products are run in threaded lines at good refineries or chemical plants.

 

[stanier]

Duwe6,

You are correct about welded piping but the industry is mature and has used threaded connections for decades with little incident. Many of the manufacturers like Corken & Blackmer (pumps and compressors) & robertshw controls (valves and instruments) make low cost threaded fittings in ductile iron and non ferrous materials. The distribution companies are not going to spend additional money when there isnt a problem.

Get the training manual

http://www.blackmer.com/cbpdf/cb254.pdf.

http://www.corken.com/media/brochure/lpg100.pdf

There are plenty depots dispensing gasolene, LPG and even LNG with piping with threaded fittings. It is preferred because it can be maintained in a cold condition.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[moltenmetal]

Duwe6: threaded pipe is not a problem when specified, designed and fabricated properly. It is permitted for "normal fluid service" under B31.3, so it is permitted for use with materials a lot more hazardous than LPG.

Of course there are plenty of examples of where threaded pipe has NOT been specified, designed and fabricated properly. A few experience like that tends to colour the perceptions of many people, to the point that some companies will spend thousands of dollars to avoid a single NPT joint. Whether or not that is actually justified depends entirely on the service, and what is at risk. But in my experience, I've seen a lot of people who had NPT-o-phobia rather than a proper understanding of the acceptable service limitations of the method.

Welded piping, particularly socket welded piping, has its disadvantages as well, not least of which being that any time you need to maintain or alter the piping you require a saw. Welding under field conditions can be a very dodgy business indeed.

 

[stanier]

There is probably more chance of getting a dodgy weld than a dogy threaded joint. Teh weld has a great deal of human involvement where a thread is cut using taps and dies which are inherently accurate.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[Duwe6]

Gotta disagree gent's; I have seen 10 to 20 times as many leaking threaded small-bore connections than socket-welded. Ask any chem/refinery *field* engineer or maintenance supervisor.

In theory stanier may be right, but it just hasn't happened that way in practice.

Additionally, the welded stuff is much more robust. If my 300# self is tied off to a run of 3/4" socketweld pipe and falls, I will probably ruin the pipe due to deformation, but there won't be any leaks. Try that with threaded fittings. Leaks galore, and a good probability of a pipe break.

 

[SEP87]

I agree that welded connections are less likely to have leaking issues than threaded connections. Even with the worst possible welder its pretty difficult to have such a poor weld that leaks occur.

Threaded connections are completely reliant on the sealant, which can often be used too economically or to excess. Both of which can cause problems. Furthermore, the sealant becomes very brittle and can easily crack if force is applied to the nearby pipework or joint itself.

Having said that, welded joins are normally more expensive than threaded, but you get what you pay for.

 

[waterpipe]

I don't know which is better for your application but would like to point out that welding is a permanent joint while threaded joints are not permanent. Welded joints should be cut if needed but threaded joints can be opened.
This might be a consideration sometimes.

 

[Duwe6]

Threaded joints being readily opened seems to argue against their use in Liquified Propane service. I have a great aversion to Fuel-Air Explosions.