Minimum Factor of Safety (FOS) for small diam pressure lines?

[submariner1]

Hello,
I'm trying to determine the validity of an imposed requirement for minimum FOS of 4.0 for a short length (~18&quot of 1/8" od pressure line. The pressure line routes 10000 MEOP Helium gas to downstream components. The pressure vessel for the gas is designed with FOS of 2.0. The imposed FOS 4.0 requirement for pressure lines/components comes from MIL-STD-1522.

Serious cost, schedule implications involved with developing a pressure line/fitting assembly that meets the 4.0 FOS, due to bend radii vs. maintaining inside diam. of tubing. To avoid weight penalty and keep wall thicknesses reasonable to permit bend, pressure line redesign will likely result in use of a stronger (costlier) material, and application-specific manufacturing technologies (again, more cost).

Given the vessel reqmt of FOS 2.0, can someone attempt to explain the benefit of mandating a downstream pressure line/component FOS of 4? Seems to me the greater personnel hazard will be caused by vessel burst. Additional items: The system will not be transported pressurized. Personnel exposure to the pressure line system during pre-operation evolutions, after vessel pressurization, will be controlled/minimized. During such exposure, personnel protective equipment will be mandated.

Any help is appreciated,
Jim

 

[BillBirch]

Jim,
I think it is not unreasonable for the piping system to have a higher FOS than the vessel given the possibility of mechanical damage to the piping. I would consider looking at the Sno-Trik range of tube fittings. This range easily can handle 10000 psi at the size you require with the following limitations: -
1) Compression elbows must be used ( cannot bend tube).
2) All tube ends must be bevelled at fittings.

Hope this helps,
Bill Birch

 

[MJCronin]

submariner,

You might want to look at the rules, under which the allowable stresses for ASME B31.1/B31.3 piping systems were developed.

Among other criteria, a FOS of 4 was used on the ultimate tensile strength for the pipe material at design temperature. Perhaps this philosophy was used in the design rules for your system.

Your reasoning about the vessel being the "weakest link" in an overpressure condition is correct.

However,you should also realize that the FOS rules for B31.1 "large-bore" piping systems were meant to make up for other uncontrollable factors ( weld defects, coldspring at installation....etc. etc) These factors, I assume, are not present or relevent in your 1/8" tubing system...


Hope that this helps !!


MJC

 

[submariner1]

Thank you both for answering... due to the application, we are looking at single stock/piece, vice use of fittings. Reliability hedge against leakage over time (static pressure state for greater than 15 years). We have decided (for the meanwhile) to go with the FOS 4 design. The impacts have been deemed reasonable by the customer, given some concerns about being viewed as 'non-compliant'. I will continue to look into alternatives/arguments against. Again, thanks.

Jim