Overpressure protection on a 5 psig rated vessel

[PointComfort]

I have an blowoff tank rated 5 psig that has a vapor relief system on top (counterweight valve set at 3.5 psig) to release vapor to atmosphere. When we hydraulically overfill the tank (normal operation level is 50%), slurry is sprayed to the atmosphere through the vapor relief valve. Obviously, when this happens, the vessel gets overpressurized (above the 5 + 10% psig). We want to avoid that.

Knowing that the vessel is rated 5 psig (i.e. below the 15 psig) Would ASME allow us to install an instrumentation-only system to protect the vessel?

I planned to install simply an overflow (with water seal), but this gets quickly expensive. The all-instrumentation solution would be far cheaper, provided that ASME allows us to do it.

Keep in mind that even though the vessel is rated 5 psig, we happened to overpressurized it to up to 20 psig...scary ...

Any background would help

Thanks

 

[JStephen]

ASME doesn't police vessels, and the ASME B&PV Code exempts vessels under 15 PSI, so that's not really an issue.

If the product is flammable, you may want to check into the requirements of NFPA 30, Int'l Fire Code, or other building codes that might specifically require venting on tanks or vessels. You might also want to see if OSHA addresses anything like this.

It would seem to me that regardless of the instrumentation, you'd want some means of avoiding or dealing with overpressure than just the controls.

 

[arto]

Watch out for when somebody hooks it up to city pressure

 

[PointComfort]

Thank you JStephen,

I thought that even though the vessel itself is rated 5 psig, if there is a possibility to build up pressure in that vessel above 15 psig, then ASME kicks in.

In my case, I have a pump feeding the vessel (dead head: 120 psig), so I literraly can blow up the vessel in seconds.

ASME BPV Section 8 Division 1 states:

"UG-125 “Pressure reducing valves and similar mechanical or electrical control instruments, are not considered as sufficiently positive in action to prevent excess pressures from being developed.”

So if I get you, my vessel isn't governed by ASME (rated <15 psig) and therefore I can protect it with instrumentation only?

 

[jte]

Point-

I think the resolution to your question lies in a reasonable HAZOP in which a process engineer evaluates what pressure the vessel might reasonably be exposed to. If it is normally operated at less than 14 psig but a single operator opening a wrong valve kicks it up to 120 psig, then that most likely will kick the design pressure to 120 psig (plus appropriate margin as needed). With the design pressure at 120 psig the vessel will fall into the scope of ASME VIII Div. 1 and thus require a pressure relief device sized relatively small and set at ~120 psig. Alternatively, a flow evaluation could be made to determine what size the relief device needs to be to keep the possible pressure from a single contingency form rising above a lower design pressure / MAWP.

You already have proof that the vessel can reasonably expect to be exposed to 20 psig at least on rare occasions. That counts as a reasonable contingency. So it seems to me that this vessel needs to be stamped or the overall system modified to keep a reasonable scenario from bringing the pressure to 15 psig.

jt

 

[psafety]

1.- Not within the Scope of ASME.

2.- Slurry/liquid carryover via the vapor vent, the venting velocity is too great, and it sounds as if the physical size of the vent is too small if the pressure climbs to 20-psig.

3.- The vent relief area needs to be re-evaluated.