Relief Vent for Atm. Tank

[RJB32482]

Hey Guys,
We are looking at closing off manways on a process tank that we blow slurry into and pump out of. The tank is maxed out at a liquid level of 60% capacity. This tank does have a seperate 8" Vent. The tank drawings has no MAWP for it (just states ATM.) For open vent capacity correlations, there is always a differential pressure term, but there can't be one here since I don't have a MAWP. If a MAWP was 1 psi, then I would know I would need x capacity at 1 psi pressure drop to know that I am safe. How would I determine the capacity of the vent if I don't have a difference in pressure (MAWP of vessel assumed to be 0 psig)

Thanks.

 

[Latexman]

A vessel designer can rate the vessel and tell you what the MAWP and MAWVacuum are.

Good luck,
Latexman

 

[RJB32482]

So is there anyway to see if the vent size is big enough to make the MAWP 0 psig and MAWV 0 psig (no pressure build or vacuum build in vessel). Or is it absolutely necessary to get the vessel approved for a specific MAWP and MAWV. Some people on the project believe we can do that (no vessel engineering needed).

 

[Latexman]

Look at your company or project rules and see who is accountable for sizing the vent. If that person is qualified and will go on record to provide sizing calculations that will pass an independent review by a qualified relief reviewer (or whatever the relief sizing work process is in your company), there should be no immediate issue to the project. These documents will become a *permanent* record in the plant's equipment files, so there is a chance this could become an issue in the future for the relief device sizer and reviewer should there be an incident.

I'm a qualified relief device sizer and reviewer in my company who has attended Root Cause Investigations on devices I sized or reviewed, and I'd make the advice of a vessel engineer part of the permanent relief device record, but that's just me.

Good luck,
Latexman

 

[djack77494]

I've run into this type of situation more than once, and, after much trying to avoid this conclusion, I am convinced that there is no minimum pressure or vacuum that an atmospheric tank is designed to. Therefore you cannot size an open vent line because to do so you MUST know the differential pressure. And you do not know it.

So I believe your only option is to have a qualified vessel designer rate your tank for its pressure and vacuum handling capability and then you can complete your vent sizing. Probably not the answer you wanted to hear - it certainly wasn't for me either, but I know of no other.
Doug