NPT Threads leaking.

[qaqc1]

I have to conduct failure analysis on some screwed fittings used in fire fighting industry. These are used to connect pipes & sprinkler heads. We have a situation where some of the fittings along with pipe nipples, sprinkler heads installed in them have been returned as leaking after six months. The peculiar aspect is that the fittings were not leaking at threads when initially installed & tested. The test pressure could have been 150 psi. After four to six months they started to drip when in service & the leak could not be stopped even after tightening.
The threads will gauge OK. There was no sign of any bi-metallic corrosion between threads. In some cases there were marks that looked like galling.
However we do not know any proper reason for such delayed leaks. The threads are NPT as per ANSI B1.20.1. These threads require a sealant to be used during assembly.
Anybody can give some clues or hints on how to get to the bottom of this matter?

 

[diamondjim]

What size NPT thread?
Are you using teflon tape to seal these?
Are both internal and external threads
tapered?

 

[ccw]

Hi qaqc1,
Here is one hypothesis: During the four months, dependent on the geographical location, a freeze thaw cycle occured on the "wet" system. This can generate very high internal hydraulic pressures in parts of the piping. I have seen this result in leakage from pipe thread joints after the thaw.

 

[Nick2003]

There are fittings and there are fittings !!!
Ensure you have used the correct material as well, Galvanic corrosion may be a long term issue. Why not try a better quality fitting such as Parker or even better Swagelok. I have seen the latter last even the most testing of applications.
Also make sure you use a good quality sealant.

 

[arto]

A few thoughts:
How about porosity in the fittings in the thread region?

How about gage the thread using the "number of turns" method? [Go in tight, then count how many until the gage falls out. - makes sure the pipe bottoms out on the taper, not on the end - that way there's less of a gap for the goop to seal]. Some mass-produced valves/fittings have the proper thread form & taper, but the diameters are bigger than NPT standard. [Like the last thread was cut off the tap]

Also check tooling "lines" on the fitting where the tapped stopped [if a collapsable tap was used in production] for possible leakage paths.

Use pipe dope [paste]instead of teflon tape? seals better/no "stringys" to get stuck in equipment downstream.

 

[qaqc1]

The threads are varying from sizes 1/2" to 2". There were not any problems of pressure surges or freezing. We are trying to check out if the correct sealnt was used. The threads gauge correctly & profile is also OK. What beats me is what is the reason for leak to start over a period of time.

Thanks for all the suggestion

 

[TBP]

Nick2003 - It would need to be a very special application for sprinkler lines to use Swaglok or Parker fittings. The cost to do something like a warehouse would be astronomical. Nobody would pay for it.

Arto - I have had FAR more problems with leaking joints when using pipe dope, as opposed to Teflon tape. If it's put on correctly, there will be no strings.

It's my observation that the overall quality of both pipe and fittings has been on a steady slide for 25 years, especially the lower end material. 150# malleable fittings are awful, as are A53F pipe & nipples. I also suspect that there has been some changes in the ingredients of pipe dopes. Ingredients that were expensive and/or quietly determined to be toxic have been substituted or eliminated? For whatever reason, things don't work nearly as well as they once did. Correctly doped factory nipples, properly tightened into brand name fittings should NOT leak. Yet a significant number do, especially on a particular steam system where we are not permitted to use Teflon of any sort, even in pipe dope. It's absolutely maddening. There is not normally a problem if Teflon tape is used, but tape may very well not be permitted in fire protection sprinkler system applications.

 

[armanto]

Hello

My suggestion is to use NPTF THREAD for the fitting.

NPTF THREAD is used for sealing application and required more precision manufacturing using, L1, L2 and L3 gauges.

Oil field used this type of thread for flange/ flens, well head up to 5000 psig.

Please open your machinary hand book, it is over there.

Good luck

Armanto

Alpha

 

[bcd]

Hi qaqc1,

I have seen vibration loosen an NPT fitting on many occasions. NPT threads, if made correctly, contact at the pitch diameter when fully tightened. Excess sealing compound or PTFE tape gets pushed to the cavity at the tips of the threads. The sealent that congregates at the tips of the threads seal the spiral-shaped leak path. It is not uncommon for someone to put too much PTFE tape on the threads such that when fully tightened, there is not metal-ot-metal contact at the pitch diamter. The assembler will feel resistance when tightening, but their is still a small amount of PTFE between the thread flanks. This type of connection is highly susceptible to loosening and leaking if subjected to a high side load or vibration.

Second possiblitiy is incomplete threads in the wrench make-up region of the threads (L3). Usually caused by a worn tap. The ANSI spec. for NPT threads (B 1.20.1) only requires a check of the handtight region (L1). As stated earlier, the dryseal type thread, NPTF per B 1.20.3, is a better connection because L3 muct be checked per the specification. There is a possibility that the threads bottomed-out at the end of the L3 zone, again preventing contact between the threads at the pitch diameter. A quick check is to fit the parts and count the number of turns in the handtight and wrench tight regions. You can catch gross machining errors with this check.

bcd

 

[corrosionman]

You say dissimilar metal corrosion is not the problem.Almost certainly the different components are different types of material. Do a simple test as follows:- put the two materials in water and check if any potential between them. Possibly it will be only 0.1 volt but over 6 months that will do lots of damage.

 

[qaqc1]

Thank you everybody, some of the responses were very good.

 

[EdDanzer]

Your problem is in the female thread, when the tap stops it leaves a line full length of the thread a few thousands deep at every flute. In medium pressure hydraulics (1000 psi) we tried all types of thread sealants, and tape. The best over time has been black Permatex pipe joint compound, small bottles are PN 51H. The expansion differences in the two materials will force Teflon tape out if the joint as well as hardening types, and some materials are to soft and flow out with time from gravity.

 

[earnest]

corrosionman's suggestion is v. good.
I had a similar problem with a stainless NPT fitting in Aluminum - medium was steam/water - sealant was locktite PST. As built, nothing leaked, but after long term testing leaks often developed. What we noticed was, if there was galling during assembly - chance of later leak was increased. Galling was evident by tearing of threads during disassembly. I recall brass fittings performing better.