Socket Weld vs Threaded 1

[JABoomer]

Hi,

I am wondering what the pros and cons are for socket weld vs threaded connections for drain and vent branch connections (so sockolets vs threadolets) in an oil and gas application. Specifically I am working on a sweet separator package but I'm also wondering what effect sour service would have on this decision.

I'm curious:
- Which is cheaper
-Which is safer / more reliable
- Which is easier to fabricate
- Which is more operator friendly

Any insights would be appreciated.

Thanks.

 

[moltenmetal]

In a typical oil and gas application, drain and vent valves are socket weld by threaded: socket weld at the root to eliminate the potential leakage location, with a thread at the discharge so they can be fitted with a removable plug when not in use.

Both SW and threaded joints are potential crevice corrosion initiation points, so no difference there.

I'm a big champion of threads, but even I would spend the extra few bucks for the ONE socket weld versus the threads in and O&G application. If you use a long-body valve, you only need one socket weld, versus two if you use a nipple and a SW x thrd valve.

 

[LittleInch]

There are many views on this site for similar questions if you look them up with proponents for each side. IMO, like MM, you should weld your first connection off the process pipework. Especially for small drains and vents, the risk of leakage outweighs anything else. out of your questions

1) threaded if you only look at components, but if you include any issues with installation, then cost is probably pretty neutral
2) sockolet
3) roughly equal overall. welding in some of the drain locations isn't always easy, but do it before placing into final location
4) Depends - removal for repair is difficult, but so is repairing leaks.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[racookpe1978]

Into "open air" (a free drain or to a nozzle or hose or something moveable) a threaded connection (downstream of the first joint) is faster and cheaper to connect and fabricate.

But, each threaded joint is "variable length" - you CANNOT tell ahead of time exactly how long each threaded joint will be when it is finally made up tight and torqued down, and what angle each threaded joint will be at when it is finally that tight. Hence the extra fittings needed such as flanges and couplings to disconnect and re-connect that final joint between any two fixed length sections - sections of pipe assemblies that absolutely have to exactly go between any two (or three) fixed locations in space. A SW joint cannot, however, be unscrewed; but it will always be the same length every time between any two flanges or couplings; and those SW connections on a SW pipe section will not leak. SW joints, once fitted up in place and tack welded t size and angle and offsets, can be bench-welded cheaper than in-place welded.

Socket weld joints are easier and more accurately made up and welded when fittign into such point-to-point pipe runs. (The long socket length at every SW joint allows a LOT of "flexibility" and "misalignment" in fitup to make up for today's all-too-common errors and bad cuts.) SW fittings can be made up or corrected locally by the pipefitter and welder, compared to a threaded fitting which has to be unthreaded at each end (disturbing other threaded joints), taken down from the scaffolding, hand-carried over the threading/cutting machine, re-cut and re-threaded, hand-carried back to the scaffolding and lifted back into place, re-threaded back 9again disturbing already-made-up joints), and test-fitted again. Rinse, wash, repeat at every joint.

Hence, SW fittings almost always are a little cheaper and faster in the net project budget.

 

[osoloco]

I agree with Racookpe. Moltenmetal has good points. I avoid threaded whenever possible. The last fitting on vents, drains, wash stations, etc. use threaded.

 

[moltenmetal]

racookpe: your mileage may vary. Threaded piping is way faster and cheaper to construct than socket-welded piping in our environment. Based on lots of project history, even when using very efficient pipe fab shops to do 80-90% sub-spool fabrication, threading to 2" is significantly cheaper in total labour and materials installation cost than socket weld. And that's for bare pipe: if you compare galvanized threaded versus durably painted socket weld, the threaded is vastly cheaper. With the right thread sealant system (high density Teflon tape PLUS anaerobic paste), a decent fitter using sharp threading dies etc., leakage is rare.

Avoid unions which don't have the brass seal rings in them- no seal ring (i.e. most of the 3000# unions), we skip unions entirely and use flanges instead (in both SW and threaded too). At 2.5" + for non-steam utility services, we use Victaulic roll-grooved, which offers similar benefits to threaded.

O&M cost? Depends on your service and corrosion environment (internal and external), design life, and whether or not the piping needs to be changed/altered. If periodic alterations to the piping are required, threaded wins again by a country mile. Fixed piping systems that never, ever need to change? Leakage that might realistically cost a large fire or a life? Forget about threaded entirely. By realistically, I mean that it's perfectly acceptable to use threaded for 1/4 psi natural gas- the risk of leakage leading to fire is credible but very, very low probability.

All that said, on a flammable O&G service other than LP nat gas, the root valve should be welded for vents and drains, to avoid leaks that can't be isolated without shutting down the entire line. After the root valve, the rest can be threaded, because you can close the valve to repair a leak should one occur later.

 

[weldstan]

Numerous Oil & Gas industry specs call for seal welding the threads, in which case, there will be no gain over socket welded connections.

 

[moltenmetal]

There are occasions where threaded and seal welded is advised, but they're rare. If you're going to weld anyway, socket welding is definitely better. The notches introduced by threading plus the shrinkage from the welding are potentially a bad combination.

 

[Duwe6]

Notice a meme in the above excellent advice? If future leaks are not going to be a problem, like with the root valve welded, threads are kinda acceptable. Except that you may be required to backweld the threads to get rid of stress-risers from the sharp roots of those threads. [and they left out "never thread Sch40, as the remaining wall very thin"].

My take on this advice is to only use threaded joints where nobody will mind the occasional leak. And my personal experience is that a GOOD pipefitter and two welders can keep up with most fitters running screw-pipe. This is because for most Code pipe guys, screw-pipe is rare enough that they are not very good, or very fast with it. Now, if the screw-pipers are sprinkler fitters, the 3-man team making socket welds will get badly 'smoked' - these guys are screw-pipe masters. But my experience has shown that almost all new sprinkler systems have a few leaks. And it is very rare to find leaks on socket-welded pipe.

 

[weldstan]

Duwe6,
I must disagree with your statement, "And it is very rare to find leaks on socket welds." Due to welder, welding management and inspector incompetence, I have seen numerous leaks in socket welds during hydrotest and during the warranty period. Those competently made and competently repaired do comply with your statement.

 

[hacksaw]

Seal welded, threaded fittings is prohibited in many firms and the legacy installations that I've seen leak, where corrosion or high temperatures are involved.

They do not constitute an engineered solution

 

[moltenmetal]

Duwe6- again, as I said, your mileage may vary, but in our environment we can run at least twice as much threaded pipe in a day with a fitter and a helper than a trained fitter/welder pair can socket weld. I can't imagine we're alone in that experience.

Even when 80-90% of the welds are done by an efficient outside pipe shop with our crew doing only the final fit-up welds, versus the threaders building the whole system from an iso, our threaders can easily keep pace. On a dollars per foot installed basis, there's no comparison at all. If you add blasting and painting with a durable multicomponent paint system which comes close to the practical in-service corrosion resistance offered by hot-dip galvanizing, if a customer won't accept threaded galvanized we'll actually substitute stainless because it's cheaper in installed cost terms and more important than the cost in our business, it's WAY faster. Everybody forgets about the paint, because nobody can estimate it properly...and paint can eat your lunch.

We thread sch40 all the time. For the services we're using threaded in, there's plenty of wall thickness left. And we use 150# MI fittings too, and have no problems with the fittings splitting etc. Quality of the fittings varies, but I haven't seen a split MI fitting in at least a decade and we've run tons of pipe. I have in that time seen exactly one 3000# forged stainless SW fitting which cracked at the forge line when the welder was halfway through the weld, so bad material can happen to any system.

Leaks are a matter of installation skill (i.e. training) and the selection of a good sealant system used within its design limits. That's where most people fail with threaded joints in our experience- wrong sealant system or right one used outside its limits.

I've seen socket welded pipe leak too, though leaks are definitely rare. Welder start/stop locations are the culprit normally.

I don't like threaded and seal welded for a variety of reasons. It's obviously useless for galvanized, which is most of the threaded we run. We use it only where we have small control valves or delicate instruments that have come threaded but are operating above the safe temperature limits of a thread sealant, or where axial alignment is important.

I've seen a lot of systemic prejudice against threaded that has cost a lot of companies a load of money and time with limited benefit. Even so, I'd weld the root valve of a vent or drain on a large line carrying flammable or highly toxic commodity- it's just the right thing to do.

 

[JABoomer]

I’m still working on the package, and still having trouble deciding how to handle the many 1” lines. There’s gas, emulsion, oil, water, and drain 1” lines.

I have thought of the following:

• Socket welding the entire lines (I don’t think this is necessary especially since it could warp the valves).
• Threading the entire lines (I think this would increase the risk of leaks, especially in the gas line???).
• Socket welding the branch connections and threading everything else.
• Socket welding everything accept the valve connections.
• Socket welding the branch connection on all lines and socket welding the gas lines but threading everything else.

What do you think would work best and more importantly WHY? What should I be concerned about here?

Many thanks if you could take a minute and let me know your thoughts.

 

[moltenmetal]

Temperature, pressure and corrosion risk MATTER to this decision. Note that both threaded and SW are both a crevice corrosion risk point. Note also that if you're using stainless, the cheap CF8M commodity valves which are threaded and then have the threads removed to form socket weld connections, do not give you low carbon content in the HAZ once welded. If 316/L SS is already a marginal selection to resist the corrosion you expect to encounter, avoid welding these or switch to CF3M- and good luck finding commodity valves cast in CF3M on the shelf at a distributor for a reasonable price. The market is flooded with CF8M.

2" and smaller, below 300 F, 150# class or below, for services which are not immediately dangerous to life or health if leaked, it's a no brainer- 100% threaded is the right answer UNLESS the lines need to be durably painted i.e. stainless or galvanized are not an option. LP natural gas 2" and smaller is usually threaded and not galvanized, but these lines are not usually very durably painted...

The very worst system would be one where you have threads which cannot be tightened because there are inadequate points of rotation in the design. That's possible in 100% threaded systems designed incompetently too- do it wrong and you'll have to disassemble a long section of pipe to tighten a leaking joint.

I would stay away from threads in the run of line (i.e. threaded hand valves, welded fittings) in any line which is substantially welded. Except perhaps for control valves or other instruments- you can use threaded ones in the run of line if they're greatly cheaper and just spin flanges onto them to give both points of rotation for tightening leaking threads as well as removability for calibration or maintenance.

So your options are a) all threaded or b) socket welded with threaded branches (for drains, vents, instrument or tubing connections etc.) and limited use in the run of line. Which services you put in each of those specs is up to your assessment of the service per my 1st sentence.