Can’t get NPT thread to seal.

[Big_eddy]

The outlet on the new pump I put on my splitter has a 3/4" NPT thread. I have a 90 degree elbow mounted there that then connects to the 3/4" hose to the valve. I gooped it up well with pipe dope and tightened it and after a day or two, it was leaking. I cleaned it, re-doped it, tightened it again, a full turn more, and now a month later it's leaking again. I know you never use Teflon tape on hydraulic fittings, but is there something better than what I'm using? I hate leaks!!

[21incher]

I use loctite 545 on hydraulic fittings. Clean with IPA first and let it cure 24 hours before pressurizing. Supposed to be good up to 50mm fittings. 

[Wlmedley]

I always use pipe tape on NPT no matter what it is used on.I've never heard not to use it on hydraulic applications.I always used it for over 40years and I've did a lot of hydraulic work.Put pipe tape on it and pay attention which way you put it on.

[Magicman]

I had a situation similar to what you are describing on an electric tankless water heater.  Same thread count but no taper.  It actually was some sort of compression fitting rather than NPT.

[YellowHammer]

NPT shouldn't need any sealer at all, it's a tapered thread, so I would assume the new pump threads are fine and the elbow fitting is the problem, especially since you are blowing out sealant. Since it's been previously used into the other pump, the tapered threads in the elbow may have been formed or even galled and so are not sealing under high pressure. 

I assume the pump isn't a straight thread with O ring?

Anyway, I would try swapping the elbow fitting.

I also use Teflon tape on hydro fittings to keep them from galling, stainless to stainless, etc.  The key is to make sure the tape starts a thread or two past the first engagement on an NPT, and so there is no way a fragment of tape can peel off and get into the system. 

[Old Greenhorn]

NPT threads are grossly misunderstood by many. These are fine points, but they can matter. There are two basic types of NPT threads, one is simply NPT with basic dimensions (think 'plumbing'), the other is NPTF, also called 'dry seal' or 'oil seal'. This is the same thread, however the NPTF has much tighter tolerances and is designed to seal without any sealant. It is often very hard to know what you are actually buying, many manufacturers are confused by this (yes, that should boggle your mind, it did mine). Let's just say I worked a lot with these in high pressure applications over what they should be used in, like 8,000 psi or so and had to learn, understand, and teach our suppliers how to make these threads properly.  At those pressures, it really matters to get it right.
 No matter. It sounds like, for whatever reason, you have a thread problem. I would change out the old fitting as suggested by Robert because that is the old part and these threads are not designed to be used more than once. The threads seal by mating up when torqued. They can't make that adjustment many times before they just can't anymore. We are assuming the pump threaded hole is good because it is new and yes, it should be BUT sometimes they are not. If they were cut with a tap there will be 'quality excursions' from time to time. If they were milled threads, they should be right on the money, statistically.
 Change out the fitting maybe gently run a tap in the pump hole before you put the new fitting in But don't drive the thread any deeper, just clean it out. Pipe dope doesn't do much to hold back pressure, just seepage.

[Big_eddy]

It was a new elbow with the pump. The old pump had a smaller outlet.  I'll clean and inspect the threads again and get another elbow ordered. 

Pump data sheet says outlet 3/4" NPT (no F)
The elbow is described as 3/4" NPTM to 3/4" NPTF swivel 90

I always thought the F and M were female and male. 

[fluidpowerpro]

I use loctite 545 on hydraulic fittings. Clean with IPA first and let it cure 24 hours before pressurizing. Supposed to be good up to 50mm fittings.

I second this recommendation. Locktite 545 is all we used at our company on NPT and NPTF threads. It is a good product. There are other brands. The main thing is that it's an anaerobic type sealer that dries with the absence of air. I have used 545 on even damaged threads up to 5000 psi with success.

[21incher]

I use loctite 545 on hydraulic fittings. Clean with IPA first and let it cure 24 hours before pressurizing. Supposed to be good up to 50mm fittings.

I second this recommendation. Locktite 545 is all we used at our company on NPT and NPTF threads. It is a good product. There are other brands. The main thing is that it's an anaerobic type sealer that dries with the absence of air. I have used 545 on even damaged threads up to 5000 psi with success.

If you want to put it in service faster or are using it on aluminum there's a primer accelerator that should be used first. It is also a lubricant that keeps stainless fittings from galling. No particles that can enter a hydraulic system and cause damage. Works great on air lines also. Expensive but worth it with the poorly cut threads on many offshore produced fittings. 

[Big_eddy]

Thanks for the recommendation. Sourcing some today. It's syrup season, and I'm currently surviving on caffeine and steam, so having to take a day or two off splitting for it to cure would be good.

[moodnacreek]

I could not operate without teflon tape on hydraulic pipe threads. If that leaks clean it up and use liquid teflon, just a little, and then wrap it with the tape. Tapered pipe threads cannot be used dry with oil. diesel or gasoline. I have read that the tape should not be used in hyd. work. Don't believe that. The problem with pipe fittings is that over tightend they can split the pump etc. housing.  In a perfect world we would have O ring fittings on everything. 

[Old Greenhorn]

........

I always thought the F and M were female and male.

You are not alone, and this is the tip of the iceberg in the mystery of NPT threads. In fact many major catalogs will use NPTM and NPTF to delineate between male and female. THIS IS WRONG. I have bought and checked many from these catalogs and called to inform them they did not meet the standards. I forget the title, but there are government standards printed for just NPT thread forms and I used to have those memorized back in the day, it came up so often.
Back in the day, all the thread systems we used were called "AN" (American National) thread forms. Late in the 60's or very early 70's (perhaps before) a transition began with the change to "UN" threads (Unified National). this applies to almost all common threads we all use. If you buy a 1/4-20 box of screws in any store they are now called "UNC" (Unified national coarse). This change did NOT carry over into the pipe threads and they remain the same (AN). So the written standards for pipe threads has remained unchanged for many decades with only minor revisions. Even if you have the standard in your hands and read it, it is still confusing as heck. Many times I felt like a lawyer parsing out specific passages in the standard for suppliers so they could make us good threads that would pass inspection. Every time I think back on it I get a knot in my stomach. I was fighting an uphill battle, even with the engineering folks in my own company, until I got them to come around and get educated.

If you care to look it up, you will find there is no such thing as an NPTM in any standard. The closet thing is NPSM, which is straight pipe for mechanical connections and not for liquid or gas. NPTF is the dry seal thread and if if still recall the passage it is "A more precise thread intended to seal without the need for sealants or pipe dope" or something like that, the actual sizes are the same except the NPTF has a much closer tolerance band.
I really do hope and expect that you will get this fixed. I am sorry if I went off there a bit, but this is a trigger for me. I fought the knowledge battle for years as we would buy many thousands if fittings made just for us that would not seal. We had used Parker for decades without issue, but their business management practices forced us to develop a new supplier over in the EU and it was a nightmare for me for a long time trying to get them to make quality parts, the language barrier didn't help at all.
Best of luck with this issue, I hate leaks as much as you do.

[dougtrr2]

Inspect the fittings with female threads very closely.  When I was in the Air Force I spent quite a bit of time trying to fix a leak on a diesel transfer pump system.  Finally discovered a previous gorilla had overtightened the fitting and it had a hairline crack.  And in that case, ironically, the harder you tightened it, the worse it would leak.

Also, remember that since this is sort of plumbing related it will take a minimum of three trips to get it fixed.

Doug in SW IA

[Don P]

To me an AN fitting is an Army/Navy fuel line thread... a 4 is 1/4" line, 6 is 3/8", 8 is 1/2" etc 

[Old Greenhorn]

To me an AN fitting is an Army/Navy fuel line thread... a 4 is 1/4" line, 6 is 3/8", 8 is 1/2" etc 

You're right Don. for decades Army/Navy and American National were essentially the same thing, but they kept separate specs so the military could make corrections and updates to their specs without civilian standards consent.
 Those fuel lines were sized in increments of 1/16" so 4x1/16=4/16 or 1/4". It made perfect sense in those days.
 The company I retired from had a line of tools that used, as a standard, a 7/16-28 ANS (American National Special) thread, which they still make today. They wanted to keep backward compatibility for legacy attachments. This is biting them in the butt these days, because dies, gauges, and taps are only avail $pecial order. In addition, the pitch diameter, major, and minor diameters were held to very close tolerances because it was a longitudinal high load application. We had nothing but problems with suppliers when we sent out parts to be made. Our engineers refused to learn that they had to include ALL the thread data and tolerances on those prints. I found myself putting in repeated revision requests for drawings produced in the 50's and 60's to include the data so we could get good parts from our vendors, it was an argument EVERY time, even when I showed them that the machinist's handbook had dropped the thread from it's listings, so a machinist no longer had a way to look it up. I have mentioned lately that it's great to be retired, especially when I recall all the 'fun things' like this from on the job, like doing mental battle with unarmed people.

[Old Greenhorn]

Just in case anybody has a perverse sense of curiosity, I went and found an Online SPec copy of ASME B20.3 (1976). Usually you have to buy these but this old, it seems to be commonly available. You have to be stone sober and read it very carefully a few times to pick up most of the nuances. Knowledge is power.
 

[Big_eddy]

I took it all apart, cleaned it well and dried it, applied the Loctite 545 and reassembled. Let it cure overnight and I'll give it a try later today or tomorrow.  Hopefully done right this time.

For curiosity I stopped at the local hydraulic shop on the way by and asked what they do. Leaks?  We never have leaks - we just wrap a few times with Teflon plumbers tape and assemble. They don't leak.
What about the risk of Teflon tape shreds getting into valves and pumps? No problem - we're a repair shop! 

[Magicman]

Just as a side note, I use nothing but the Blue Teflon.

[21incher]

Just as a side note, I use nothing but the Blue Teflon.

I keep a roll of the yellow for gas lines also to meet code.

[Big_eddy]

Well, I ran the splitter for an hour yesterday and no leaks yet. That said, it took a month last time for the leak to start. I think the "shock" when the auto return pops off at 28gpm might be shaking the hoses each cycle, and that might be what caused the pipe dope to fail. I have the hoses secured in guides, but they still move about a bit.

The Loctite is supposedly good for vibration. Time will tell.