leaking NPT joint on a rotary pump

[tjagiello]

Hi guys,

I've got a small stainless steel rotary pump which has 3/8" NPT female threads and I'm trying to mount some 3/8" NPT male - 3/8" BSPP adapters (SS as well) on it. I've tried teflon tape but no matter what I do I can't remove a leak from one of the adapters. I've tried various torques, various amounts of teflon tape and still one adapter leaks from the NPT thread. The manual for the pump says that use of pipe dope should be avoided, but I'm seriously considering using Loctite 542 on those threads as I'm sick of disassembling the connections to re-do the joints.

Oh and yes I agree that PTFE tape shouldn't be used on pumps, but the manual says to use that instead of pipe dope. Not sure why to be honest.

Anyone has any ideas?

Regards,
Tom

 

[rneill]

I think you probably meant Loctite PST 592 as I'm not aware of a 542 product. The PST 592 would be a good choice as it is robust and recommended for vibrating services. It will take about 3 hours to set with stainless steel fittings (only 30 min with carbon steel). Supposed to be good to 10,000 PSI and 204 C.

 

[tjagiello]

Hi,

might be a different number in the US/UK? This is the stuff I was talking about:

http://www.loctite.sg/sea/content_data/93786_542EN.pdf

I've removed the pump from the motor (it's a magnetically coupled pump) and it turns out there's a kink in the NPT thread on the pump. It rips the teflon tape when it's being screwed in and that's why it leaks, everytime in the same place. I guess a sealant would take care of it, but:

- the 542 product is not so great for SS, only 35% strength compared to normal steel.

- I'm worried about the mess it leaves when you unscrew the fitting. Just had one fitting previously sealed with 542 loosened up and it was quite hard to clean out the female thread on that joint (before resealing). I guess it would be even harder on the pump as the hardened bits can fall inside and damage the pump.

Just as a side note the system will run at around 9bar, 20*C, so perhaps I don't really need too much strength on the joint?

Regards,
Tom

 

[iainuts]

Can you run a tap in and clean up the threads? I've used pipe threads on pumps up to 10,000 psi and down to -360 F and they work great with just Teflon tape.

 

[tjagiello]

Hi,

nope no way to correct that as I haven't got the tools. Plus it would probably be quite tricky without getting anything inside the pump.

Regards,
Tom

 

[Latexman]

Are you sure that joint is actually NPT? Carry the parts to a local shop that has the right tools, or calling them in may be a wise decision at this point.

Good luck,
Latexman

 

[tjagiello]

Hi Latexman,

yup definitely NPT, it even says so in the manual. I'm still not sure why the manufacturer of this pump (Fluid-o-tech) doesn't recommend liquid sealants.

Regards,
Tom

 

[hydtools]

Perhaps there is a small crack in the pump port and it opens when the fitting is tightened. There is no chance that tape will seal the crack. Inspect the port closely when the fitting is installed.

Ted

 

[unclesyd]

We have hundreds of small SS pumps of all descriptions and all pumps with female pipe connections we will normally chase the threads and install a nipple or bushing to where our pipe makeup is not on the pump proper.

Our pipe sealant of choice for the pump side is X-pando sealant/dope. Something to keep in mind X-pando will make a joint a little hard to break but it can be done. This is a plus in most of our pump systems since we will try other sealants on the make and break joints.

http://www.xpando.com/pjcfaq.html

 

[tjagiello]

Hi guys,

yes there is a small kink/crank in the top part of the thread which is what causes the tape rip and leaks. I guess there's no other way than liquid sealant, although I will wait and see what the company technician says.

Regards,
Tom