Socket Weld vs Threaded
Socket Weld vs Threaded
(OP)
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.
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.





RE: Socket Weld vs Threaded
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.
RE: Socket Weld vs Threaded
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
RE: Socket Weld vs Threaded
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.
RE: Socket Weld vs Threaded
RE: Socket Weld vs Threaded
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.
RE: Socket Weld vs Threaded
RE: Socket Weld vs Threaded
RE: Socket Weld vs Threaded
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.
RE: Socket Weld vs Threaded
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.
RE: Socket Weld vs Threaded
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
RE: Socket Weld vs Threaded
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.
RE: Socket Weld vs Threaded
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.
RE: Socket Weld vs Threaded
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.