I’m pressure testing a new build rn and am spending more time than I’d like getting NPT fittings to not leak. A few parts in particular (HFS lids) that I got for my configuration are poorly made and aren’t tight even after 6 full turns with 8 wraps of blue monster thread tape. Currently using calipers to get a few hundred micron on some larger hex nips to make it fit tighter and it’s working well enough. Still makes me swear off NPTs in general (compression ftw).
During one of my fits of rage, I wondered if you could just thread your NPTs without tape and just weld the joint shut so you would have a joint that could stand up to very high pressures and would be just as sanitary as an NPT fitting if you hit it on the outside. Since you aren’t relying on the thread to make a seal, it would be clockable as well.
Is there any reason you couldn’t do this? Maybe not near valves since the PFTE could get wrecked, but is there any flaw in my logic? The only things I can think of is you wouldn’t get great penetration since contact between the threads may be insufficient, but going right at the seam might do the trick. I never take NPTs apart but I know sometimes this needs to happen so increased cost of replacement could make this non-viable. The last would be if you’re going to weld it, just ditch NPTs altogether. I figure since most off the shelf parts have NPTs, it allows you to cut the cost of the build or offers more flexibility.
Not an experienced fabricator (and I can’t weld) just curious if this is possible from a pressure and sanitary standpoint.
No dont weld it unless completely necessary…
Go get some dope/pipe compound, filled with PTFE.
This product is made for this exact purpose.
NSF rated for incidental contact, just do a rinse/wipe over the joints after tigetening with ISO/ETOH and itll be good to go.
If you want, you can ultrasonic clean the parts if you really wanna get rid of all excess.
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Is that rated to work with butane/propane etc? That’s actually quite nice in that case.
I’ve absolutely used this approach. Usually to get to compression fittings. It does make changing your mind later more problematic, but that’s what the compression fitting is for.
I’ve also removed the threads from npt fittings so they could be welded into non-threaded holes.
Both of those were taught in university machine shops
Offering a physicist npt on their vacuum apparatus is a non-starter…
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Haha, that actually hits close to home. Vacuum deposition of metals is 0.1-0.01 micron; didn’t know about fittings other than KF; oh how spoiled I was.
Yes…
Your home gas lines were likely assembled using a similar/same product by your local gas company.
“Pipe dope remains pliable for future disassembly and repositioning of parts after application, even in cold temperatures
High viscosity, slow drying pipe sealant prevents leakage from vibration, temperature cycling and extreme pressures
Contains naturally occurring oils and lubricants, making it ideal for new, poorly cut or damaged threads
Recommended uses: acid, air, butane oils, cutting oils, diesel fuel oil, ethylene glycol, freons, gasoline, heating oils, kerosene, LPG gases mineral oils, natural gas, nitrogen, petroleum solvents, potable water, propane, propylene glycol, steam lines and water
Temperature range: -100°F to 600°F
Pressure range: 10,000 psi (fluid), 3,000 PSI (gas)”
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0.01microns is only ~1x10-5 mbar
These guys go to 11 (actually 10^-12)
What is the definition of high, ultra and extreme high vacuum?
The pressure range of XHV is usually defined as 10-12 mbar and lower, while UHV is between 10-7 and 10-12 mbar, and HV between 10-7 and 10-3 mbar. XHV is associated with the levels found in outer-space in the form of geo-stationary orbiting satellites, UHV with high-energy physics and nuclear research, such as that being conducted at CERN and KATRIN, and HV for industrial and research applications.
The Fundamentals of High, Ultra & Extreme High Vacuum
Still only -15psi…but leaks are verboten
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We were on the edge of HV and UHV. 10^-7 mbar I think is where we were for depositing silver but I’m not sure, no one wants to pick a unit of pressure it could have been as high as 10^-7 torr (10^7 sticks out in my memory); the chamber was an Angstrom system, insanely nice. It likely could have gotten to 10^-10 mbar just an insanely long pump down time; why bother if 10^-7 works 
. Pardon my poor conversion and/or memory.
The upper edge of UHV to XHV is insane. Not interest in that kind of work; seems easier to launch yourself into deep space and do these experiments than it is to leak hunt on the ground lol.
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I wouldn’t think this is a great idea for one reason: You’re basically destroying the resell value of this equipment when you’re done using it. I wouldn’t buy something that had had customer welding of fittings all over it, fittings I might not even want.
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Fair enough, I do scavenge a lot of parts so that is a downside
Meh, resell value on a leaking vessel should be lower than a sealed welded vessel in my opinion, and with the price of used equipment these days i wouldnt worry about recouping much.
I’d much prefer a working vessel that makes me money now than one that might resell better
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Sure, if the vessel really was completely unworkable with NPT that could be the case. But i’d still try the pipe dope first. At least you can rip that out later.
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Haven’t seen anyone mention it yet so here’s my unfortunate experience: a lot of low quality Chinese import parts marketed to the cannabis industry has “NPT” threaded parts that don’t actually match the NPT spec for whatever size it is. Usually these are threaded straight, which is to say that a 1/4" NPT/pipe fitting should be 18 TPI (threads per inch) with a .055" pitch taper that gets smaller as the threads get deeper (opening side) and smaller towards the end (pipe side).
Usually what happens - and I suspect this is because metric threads effectively never have a taper and often metric fittings that need to seal tightly do so with a dovetailed O-ring or face seal - is they get cut on a CNC and instead of cutting 1/4-18 with a taper, they just cut the 1/4"-18 threads straight down with no taper, which means that your fitting will never get tighter as it threads in, and you will be reliant on the taper of only one half to do all the sealing work - and on some fitting there simply isn’t enough threads to get big enough to do so effectively.
Your options are then to either drill a larger hole, tap it correctly (if in stainless, this will take a much better tap and lube combo than most people are willing to buy) and then either use a reducer bushing or buy a new fitting to get from the new hole size to whatever you’re trying to connect to. Other option is welding.
I’m out of country til end of the month but if you’re near Denver I could certainly help with retapping it to the correct taper. Nothing should take 8 turns of quality tape, and Blue Monster is good stuff - nothing like the thinner-than-rice-paper-thin white garbage that gets thrown in with most import equipment.
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Yep, definitely seen my share of that nonsense.
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Have you checked it against other standards? NPS or BSP?
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Not often. But I’m my defense things have usually devolved too far for that by the time someone asks for my help…
I do check for the taper out of habit now….
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I agree. NPT does NOT Stand for national pipe thread it stands for national pipe taper!!
Per a quick google :
NPT is defined by ANSI/ASME standard B1.20.1.[4]
The taper rate for all NPT threads is 1 inch of diameter in 16 inches of length (3⁄4 inch per foot or 62.5 millimeters per meter) measured by the change of diameter (of the pipe thread) over distance of thread. The half-angle (between the taper surface and the axis of the pipe) is arctan(1⁄32) ≈ 1.7899° ≈ 1° 47′.
The standard for Nominal
Dry seal :
National Pipe Taper Fuel (NPTF , also called Dryseal American National Standard Taper Pipe Thread , defined by ASME B1.20.3) is designed to provide a more leak-free seal without the use of PTFE tape (often referred to by the popular brand name “Teflon”) or another sealant compound. NPTF threads have the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path.
NPT or any thread for that matter will never be considered a sanitary joint if it’s potentially product contact…
Yes you can weld it, done it tons of times. Will be fine.
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