Mineral oil leaks

[bschwak]

[/indent]thread378-146020

Responding to the thread above. We use spa heaters in a recirculating system. The spa heaters are 1 1/2" NPT female threaded on both ends. We are plagued by oil leaks. It seems like I have tried every solution out there: Blue Monster tape(with and without dope), copper anti-seize, x-pando, and nothing seems to be holding up. I've replaced heaters and fittings many times. The x-pando seemed to do the trick, but just recently had a failure after a few months, and has begun leaking. I'm at a loss with how to proceed. At this point I'm wondering if our application is incorrect. Perhaps a tighter class of fit than NPT can provide? Any advice would be appreciated.

Thanks

 

[Stick.]

All my opinions are formed based on high pressure hydraulic systems (3,000 psi) and a lot of it comes from second hand lessons from older engineers, but in general, I just assume that NPT fittings will leak eventually. If you don't want leaks, switch to a more modern, leak free type of fitting or weld it. Hopefully someone with more relevant experience can suggest the right magic goo that will stop the leaks, but it wouldn't hurt to see if you can find heaters with better fittings on them.

 

[Heaviside1925]

OP,
I agree with Stick. If you can't handle leaks, don't use NPT. When I hear heater, I think of cyclic heat cycles and cyclic thermal expansion and contraction, think of how a rigid threaded connection is going to respond to that. As suggested, weld it if possible but if you need to uncouple it, then consider a gasketed fitting. Might be able to give you a more focused answer if you provide materials of construction, temperatures and any other relevant information.

 

[EdStainless]

There are o-ring sealed hydraulic fittings that would work fine for this service, if you selected the correct o-rings.
Pipe threads are tapered and truncated which assures a leak path through them.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Compositepro]

NPT has been around for well over 100 years, and is very reliable when made and installed correctly. It is a disservice to say that they can be expected to leak. These days few technicians really know how to identify an improperly formed thread or how to seal and tighten NPT. I suppose that might be used as a justification to not use NPT. When I started working in the early 80's, the chemical plant had a crew of very experienced pipefitters, and thread leaks were not a problem. I was amazed at how much torque they used. Without proper training and experience, any coupling method will be unreliable.

Threads will leak without sealant, by design. Using too much sealant is more likely to produce leaks than using too little. The threads must have metal to metal contact between the flanks of the threads in order to carry pipe loads. The sealant only seals the spiral gap at the thread tip and valleys. Excess sealant will extrude out of the joint during tightening, and too much will require very high torque to get metal to metal contact. Pipe tape has to be wrapped in a spiral opposite the thread helix so that it does not strip-off when tightening, which cannot be seen. Without metal to metal thread contact, pipe loads pass through the "plastic" sealant, and the sealant will flow under pipe loads and leak eventually. When I first started doing my own pipefitting as a DIY leaks were not uncommon. That did not cause me to declare that NPT was unreliable. But understanding how to do it right was not a "no-brainer".

It is not a good idea to use spa heaters to heat oil. Heaters designed to heat water have a much higher wattage per square centimeter than is suitable for oil. This results in high surface temperatures that will cause coking and decomposition of the oil.

 

[LittleInch]

Are those heaters metal or plastic?

Whats the pressure rating?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bschwak]

So the heater specs are as follows:
VULCAN Spa and Hot Tub Heater: 1.5kW/6.0kW, 120/240V AC, 1-phase, 5/15 gpm, Flow Through (from Grainger). Female threaded on both ends. Low pressure, temperatures generally in the range of 50-80C. We pump a food grade mineral oil through the system.

 

[LittleInch]

Designed for very low pressure water then (you don't give a pressure rating or what pressure you're operating at), not higher pressure thin, slippery mineral oil.

It's therefore unsurprising it leaks.

You need something where you have a resilient seal / O ring / gasket / metal to metal cone where torque can increase sealing capability. A 1 1/2" Female NPT won't do that IMHO. So you're back to getting a heater with different end connections. Or weld it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bschwak]

Thank everyone, I think we need to look into a redesign of our heating systems

 

[Heaviside1925]

If you don't want to throw the baby out with the bath water, consider Compositepro's comments. If your set-up is performing in an acceptable manner except for the leaks, it might be worth taking the heater and piping to a local machine shop to have a machinist inspect the threads for tolerance and damage. They may be able to provide a repair and/or another solution as a path forward.

 

[LittleInch]

What are you heating though and at what pressure?

Those spa heaters etc don't look up to operating at 80C and anything more than a couple of metres of water will stress them out leading to your leaks.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bschwak]

LiitleInch,
We use these in a variety of portable vessels that have an oil jacket. I have no idea of what our operating pressure is, I doubt we've ever measured it. The material being heated is generally a petroleum and/or wax based cosmetic product. These systems are homemade in most cases. We're also running taco (I believe residential) pumps to move the oil through the heater.
You're right, 80C is probably pushing their abilities, but we do commonly run between 65 and 70C.

 

[Compositepro]

Perhaps you should consider buying a number of these inexpensive wax melters.

https://m.vevor.com/wax-melter-c_12...lk-production-business-or-home-p_010251301490

 

[bschwak]

Compositepro,
Our vessels are all different sizes, ranging between 100 and 500 liters. They also commonly require agitation with either a propeller or scraper mixer. I don't believe a wax melter would be an effective application. Some of our vessesls use "dry" heat, and obviously we don't have this issue with that particular type.

 

[LittleInch]

So is this mineral oil system sealed or "open", i.e. does it run back into a tank which has a vent to the open air?

If it sealed you could easily be getting some pretty high pressures though a lot will be taken out if you're using flexible pipe or jackets.

"homemade" systems at 80C eh? Better not have any of them burst or spray anyone with liquid at that temp or you will get life changing injuries and a very large bill.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bschwak]

Little inch,
Not sealed systems, they all have an elevated vent.

 

[TugboatEng]

If you must use NPT I STRONGLY recommend Loctite 577 for most applications.

Proper bracketing of the pipe to prevent movement is critical.

 

[MJCronin]

As a last resort, perhaps seal welding the NPT fittings ?... (the system must be drained and inerted)

Can you provide a PID ??

Perhaps seal welded NPT unions instead of flanges for joints that require break-dowm ?

It is an acceotable sealing solution as per ASME B31.1 and B31.3

Can anybody on the interwebs provide a proven "piping line spec" for mineral oil ?

OK,.... Who agrees and who thinks I am an old idiot ?

Cheers !!!

MJCronin
Sr. Process Engineer

 

[r6155]

I suggest, before installation, to implement a quality control system: a hydraulic testing device of each fitting, say at 10 bar.

Regards