NPT Fitting Pressure Limits 1

[tinnustechnology]

I have a simple pressure vessel, and am looking for guidance on calculating the safe working pressure. The vessel consists of a 3/4" schedule 80 red brass pipe with a red brass cap on one end and a mechanical pressure gauge on the other end. Fittings are NPT with Loctite 565 sealant. I plan to put dry ice in it and cap it, raising the pressure to about 1000psi of Co2. I get an SF of around 8 for the hoop stress. Are there any other bursting/safety issues that I should consider? If so, what is the proper methodology for rating the pressure of this design?

 

[SnTMan]

Per the fitting rating

 

[racookpe1978]

Is this an existing tank (PV) or a theoretical (future) design?

I have strong reservations about pressuring an unknown tank with gas (rather than water) since the reserve energy of high pressure gas is significantly more dangerous than water. If the tank bursts, then the excess pressure is quickly relieved. Not so with gas.

Frozen CO2 pellets to take up volume? What about the tank and piping strength at that reduced temps?

 

[tinnustechnology]

Thanks for the feedback guys. Following the fitting ratings seem easy enough, as I have found some NPVF fittings that are rated for >1000 psi.

racookpe1978 - this is an existing design. Just an 8 inch nipple, cap, and gauge. As noted I need to upgrade the fittings to a higher rating and then should be good to go. I just have not had any previous experience with very high pressures and it was making me a little nervous. Looks like it is as simple as specifying a purchasing the proper components.

 

[Compositepro]

You really should have a safety relief valve or rupture disc on it or else handle it as a potential pipe bomb.

 

[racookpe1978]

Er, uhm, ahhhh. If you have no safety relief valve or blowout plug/feature within the isolation boundaries of the presure vessel and pressure test, it IS a REAL pipe bomb, not a "potential" pipe bomb.

It just hasn't hasn't killed anybody yet.

 

[Duwe6]

Since you are designing outside of normal Sect VIII rules, there are actually rules to do this. Build 2 or 3 identical items and 'blow them up', prefferably in a blast pit. Divide the average rupture pressure by between 4 and 6, and -- voila MAWP

Anything else is just guessing with a REAL bomb.

 

[CRG]

Without knowing exactly what you are using for material, I really question your SF of 8. With an assumption that you are talking about ASTM B43, B31.3 piping code gives a allowable design stress of 8ksi up to 300 degF. Assuming no mill tolerance or corrosion allowance, it looks like you have an allowable pressure of around 800 psi.

When you calculated your SF, did you deduct for the thread depth, mill tolerance, or corrosion allowance?

 

[CRG]

Oops, I looked on the wrong line of my spread sheet for threaded pipe. The number above in my last post was for sch 80 2" pipe. The question was for 3/4" threaded pipe. So, assuming no mill tolerance or corrosion allowance, it looks like you have an allowable B31.3 pressure of around 1,177 psi for sch 80 3/4" ASTM B43 pipe.

 

[ricklts]

crg,

stand behind the fat guy. experience with the type of pipe/fittings you have described has shown a vast tolerance in the threading. Have you actually measured the threads on the end caps and mating pipe ends? If they were roll cut, work hardening could take place and increase brittleness in that area.

good luck

 

[MJCronin]

tinnus,

Is there something that prevents you from pressure testing your proposed vessel ?

I would pressure test to 1300 to 1500 psig or...

If you have enough $$ break a few of them, and, as suggested above, divide the average failure pressure by a generous factor, write a report and proceed with confidence.

Testing trumphs analysis.....

 

[vpl]

Tinus hasn't been back to the site since August 19

Patricia Lougheed

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Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[Duwe6]

Guess he couldn't stand our answers, excellent though they were.

 

[rmw]

Maybe he/she did the unthinkable (didn't stand behind the fat guy).

rmw

 

[vpl]

Maybe he was the fat guy ...

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[GenB]

tinnous, what's the operating temperature?
if ambient temp your 3/4in pipe and fittings(pipe not vessel) will withstand
3000 to 5000 psi.

 

[gr2vessels]

The eng tips doesn't seem to promote democracy;- we should be able to vote out the reckless posters and the profiteering ones who only take advice, but never contribute to the forum. Some people need extra help with their education, they won't even be bothered to thank for the help, since we are here for their benefit alone.
Cheers,
gr2vessels

 

[Duwe6]

"Vote 'em off the islan . . , er Forum"

Actually, not an entirely bad idea. Madam Moderator -- thoughts?