External Floating Roof Tank safety devices

[NickVil]

Hi,
Is it recommendable to install a pressure-vacuum valve on an external floating roof tank? We have a gasoline service tank and we need to know about if is neccesary to install that PV valve.

Regards

 

[IFRs]

The only PV valve normally needed on an external floating roof tank is one that protects the floating roof when it is subjected to pressure or vacuum when product is pumpled into or out of the tank when the floating roof is supported not by liquid but by it's legs. The only other reason for a PV vent I can think of might be in case you expect to introduce large amounts of gas from pigging or blending operations and you are concerned that the gas would upset the floating roof before it was absorbed back into the liquid.

 

[NickVil]

Thanks IFRs for your comments.
The case I was considering for that tank is the possibility of the roof clogging when product is pumped into or out of the tank.
The API 2000 recommend PV valves on atmospheric tanks with petroleum products with flash point below 37.8ªC (100ªF). This standard doesn't specify safety devices according with the type of roof.

Bestr Regards

 

[0707]

NickVil

Point C.3.9 VENTS of Appendix C on Welded Steel Tanks for Oil Storage API STANDARD 650 states:

“Suitable vents shall be provided to prevent overstressing of the roof deck or seal membrane. The purchaser should specify liquid withdrawal rates so that the fabricator may size the vacuum vents.

Vents, bleeder valves, or other suitable means shall be adequate to evacuate air and gases from underneath the roof during initial filling”.

Maybe this answers your question

Regards

Luis Marques

 

[NickVil]

Ok Luis,
You're right the API 650 section that you've mentioned I think it means to put vents under the floating roof.
What about the NFPA 30 ? In point 4.2.5.2.2 says: "For vertical tanks, the emergency relief venting construction shall be permitted to be a floating roof...."

Regards

 

[IFRs]

You said this was an external floating roof tank. You can't possibly overpressure the tank - it has no fixed roof. By the roof "clogging" do you mean that it may refuse to go up for some reason?

 

[NickVil]

That's right IFRs. The existent tank is an external floating roof. When i said "the roof "clogging"", I was trying to said that for some reason, mechanical reason, the roof will not go up or not go down with the fluid pumping into or out the tank. So, with that scenario, if the fluid is pumping out and the roof is not going down because is plugged or stuck, we would need some safety device that allows keep the tank with atmospheric pressure.

Best Regards

 

[IFRs]

In my 28+ years of floating roof experience, I have not come across this concern before. That does not make it an invalid concern, but one that bears further review. The floating roof should have some P/V vents on it, not to mention penetrations for pipes, emergency drains, possibly a sump, legs, instruments, etc. If the roof got hung up and would not move and the liquid were drawn down further, the vacuum under the floating roof would either be relieved through those openings or the floating roof would deform greatly and expose areas around the seal that would serve as vacuum relief. The floating roof is designed to be supported on its legs, which are numerous. If it were supported on a few points separate on the tank shell, it would crumple. The seals are not strong enough to support the floating roof - they would just get torn off. The floating roof is smaller than that tank in all directions - even when tilted it is hard for it to get hung up, especially since you have no columns. A floating suction or rolling ladder is also unlikely to be strong enough to jam and hold the floatiing roof up. I guess I can't envision how your EFR could hang up and survive pump-out to act as a diaphragm and allow high vacuum to form underneath it. Perhaps I am assuming too much, picturing a much different situation than you really have. Is this a very small or unusually shaped tank? Does the floating roof not have any penetrations? Is the seal some design I have not seen yet? I am open to a consult or visit, if it would help.