NPTF fitting/port question 2

[kvweid12]

I have an application where we currently thread a steel NPT plug into an aluminum housing. I am looking to change both parts from NPT to NPTF to avoid having to apply Teflon tape (this is a production part and would save assembly time). From the other research I have done, the NPTF thread should create a good seal without the Teflon tape, as the parts are designed to deform together. My worry is that while the aluminum part will deform correctly, the steel won’t. That would still leave one spiral leak path and we will end up having to use the Teflon tape anyway. Also, I have seen comments that NPTF should still use the Teflon tape to avoid galling at installation. I was hoping to be able to use a spray-on lubricant to avoid the galling. Does anyone have experience with either of these issues? Thanks!

 

[faqs]

I am a fan of NPT ports (in an industry that villifies them) but in my opinion NPTF is not a solution. Both NPT and NPTF need Teflon tape.

 

[hydtools]

In all my years using NPT and NPTF I have alway used tape or sealant. If you are changing ports change to straight thread o-ring ports. No tape, no sealant. The o-ring does the job.

Ted

 

[kvweid12]

I have to have both NPT and BSPP available for customers; it just varies per order what gets plugged and what gets used...

 

[MikeHalloran]

Agreed, the 'F' in NPTF is for 'fantasy'.

If you used o-ring ports, you could use different adapters for each order, and eliminate the extra holes and plugs.



Mike Halloran
Pembroke Pines, FL, USA

 

[1gibson]

If you're just worried about assembly time, use pipe dope instead of teflon tape.

 

[tr1ntx]

Don't you always want something (tape, anti-seize, pipe dope, etc.) between steel and aluminum?

Maybe that is an "old wives' tale" or "industrial folklore" I've picked up somewhere along the way.

 

[moltenmetal]

Your choices are straight thread with o-ring or NPT with sealant. Your sealant choices are many. Dry fitting to seal between these materials is not an option if removability later is also required.

 

[kacarrol]

I agree with tr1ntx, I was always under the impression that mixing steel and aluminum in a threaded coupling is a bad idea. Isn't aluminum used as a sacrificial annode for some steel structures?

 

[hydtools]

Nothing wrong with mixing steel and aluminum in a threaded connection. Have used steel plugs in aluminum housings for years with no problem.

Ted

 

[bcd]

Mike hit the nail on the head. "F" is for fantasy. Great in theory, but plan on using some sort of thread sealant. The tools and equipment used to cut the threads are not always perfect so you will have leaks if you don't apply some sort of sealant.

 

[moltenmetal]

Whether or not mixing aluminum and steel in a threaded joint is a problem depends greatly on your service.

Using threaded joints without a gasket/o-ring or sealant is a different matter and is usually a bad idea.

 

[hydtools]

Up to 5000 psi hydraulic components, indoor and outdoor service. Always used sealant.

Ted

 

[1gibson]

Steel spark plugs in aluminum heads

 

[moltenmetal]

Spark plugs are straight thread with either a gasket or a conical seal face. Yeah, you can call that a "threaded" joint, but it's about as similar to an NPT joint as a flange is to a union.

In a service with any significant amount of water INSIDE the system, or where the system is immersed externally, I wouldn't use a steel plug in an aluminum component! So yes, the success of this pairing of materials DOES depend on service. And without a gasket or sealant in an NPT joint of any design- forget about it!

 

[rfmatola]

I used to insist on NPT for all pipe fittings. Now that I have been forced to use others like Metric plugs, BSP and EO and JIC fittings, NPT has got to be the worst. Having to use teflon tape to get a seal is such a waste of time and money. Try Metric pipe plugs for your application.

 

[johnchrc]

Have you considered a #2 Parker Plug 2 HP5ON-SS. Use std SAE port. Very common, many sizes and materials, off the shelf. Hollow hex plug. See Pro/E part file.

Link:

http://www.parker.com/portal/site/P...gnextdiv=687620&vgnextpartno=2 HP5ON-SS&Wtky=

 

[JLSeagull]

In lieu of tape, some use a liquid thread sealant paste. Some include Teflon; others are Teflon free. Teflon such as PTFE is listed on some paste suitable for oxygen based applications etc.

I like PTFE based seal tape and prevent any from hanging over the edge. One tape issue is leaving fragments that can blow down the tubing etc.

Some thread seals are listed and colored for different applications. Potable water should be different than other water or gas applicatons to accommodate plumbing inspection. The PTFE temperature limit below about 300 degree F would not suit some other applications. Some sealants permit easy removal and others are nearly permanant. Some thread paste is like pipe dope; others not.

I don't remember using a spray by intention although even WD-40 etc. could happen in a pinch. Use good judgement along with supplier documentation to obtain a good fit.