Threaded connections for Automatic air vent and Pressure Relief valves for Jet fuel service

[vjr0512]

I have the filter water separator and the Filter monitor vessels for the Jet fuel service and they have the threaded connections at the top of these vessels for air vent and the Pressure relief valves. Our client is having a concern that for the jet fuel service we need to avoid the threaded connections and also while removing the air vent valve for any maintenance and fixing back the threads may get damaged and insisting to give flanged connections.

Vendor refuses to comply with this as this is their standard design and cannot change it. I have also checked all other suppliers of similar aviation fuel service filters and monitors and there also it is threaded connection only.

The vendor is not able to give any technical reason why it should be threaded connection and my query is why all the vendors have such air vent valves in threaded connection?

Thanks to reason it out and reply

 

[moltenmetal]

Add a threaded flange to the relief valve, which is the only part you'll be disassembling frequently. Add a threaded nipple and flange to the vessel at the connection point. What's the big problem?

Avoiding threaded connections is one thing, and eliminating them entirely is quite another.

NPT threaded joints at sizes 2" and smaller, when used within their reasonable limits of temperature/pressure and when used with the right thread sealant, are not unsafe. I would have no problem using threaded connections on small connections at moderate pressure and ambient temperature in a chemical plant. Whether there is an absolute prohibition against using them in jetfuel service in accordance with some fuels regulation in your region is totally unknown to me.

That other vendors also use threads should tell you something. It tells me that this is NORMAL in that particular industry and that your client's absolute prohibition of threaded connections is not normal.

If for some reason your client wants to eliminate the threaded connections entirely, they'll need to either not use the vendor's standard component, or to modify it after receiving it (if that's even possible) by welding and then hydrotesting the welds. If the part is aluminum, I wouldn't mess with welding personally. None of this is the vendor's problem, unless they promised you no threads before you ordered the part and then tried to give you less than what you ordered.

 

[vjr0512]

Thanks sir for your reply . Sometimes some may not go through the topics not interested to them. That is why i double posted. I will avoid it in future.

 

[Tmoose]

Are the threads straight ("parallel") or tapered pipe threads?
The less common "straight" pipe threads require an additional feature (o-ring) to seal.

As others said, are the housings aluminum, inviting easier thread damage?

When working with hydraulic systems (100s or 1000s of psi) over the past few years there seemed to be some tendency for US manufacturers to use tapered pipe threads on relatively small fittings ( under 1/2 inch NPT or so), and flanges on larger fittings. European suppliers seemed to adopt flanged, o-ring joints much more often. I never dug deep into the various ISO standards even though I expected to see something was spelled out there.

Gotta think a tapped hole is a lot quicker/easier/cheaper than a fabricated and welded flange.

Would your specification allow an "adapter" fitting as suggested by MoltenMetal? Although that would introduce 2 of the dreaded threaded connections.

 

[Cockroach]

Would it not be a safety issue? Usually this is where I get the resistance.

NPT forms are particularly weak in shear, any side load, wanted or unwanted, would snap the relief valve off at the stem. This would have a tremendous amount of safety concerns given your medium is a jet fuel. Often the NPT stem is supported against shear, the resulting geometry looking more flange like in nature. So I suspect buddy simply wants usage of a flange and be done with it.

The other reason is as mentioned by MoltenMetal. If you are to inspect the component often, then that means unfastening it from the pressure vessel. NPT are good for about two or three fits, then the interference threading starts to gall and is proned to pickup. You could over torque the piece during mating, again, safety issue about stripping threads. This is not saying that the technician is not competent which is usually the interpretation, just saying that a flange style assembly offers considerable benefits should something unintended happen.

I bring up these two points as I have often encountered them from third party inspectors in oilfield valve systems for mainline block and bleed ball and gate valves. NPT are typically points for discussion during design review meetings.

Hope this helps or at least clarifies some of your concerns.

Regards,
Cockroach

 

[vjr0512]

Thanks all of you sirs for the wonderful reply .