PV-BREATHER VALVES SETTING OVER API 650 Floating Roof Tank

[Epicuro]

I have an API 650 floating roof tank where gasoline is stored . Vapor pressure is 70 kPa.
Tank is ATM designed.
PV- breather valves are set @ 3,55mbarg/-2,55 mbarg.

3,55 mbar = 0,355kPa

We got an observation of "excessive" losses so we where requested to study a solution to increase set point for Overpressure,
let´s say ideally to reach 70kPa
so to move the set point higher from the current 3,55mbar (0,355kPa) to , ideally, 705mbar (= "slightly" above 700mbar ).

I am a bit confused because, doing so, we would be able to
(*)-pressurize the rim till 1718,25 mbar (1718 = 1013,25+705 ) making the tank a pressure vessel and no more an atmospheric vessel!

(**)I also believe that , in such situation, there is , keeping @ 3,55mbarg , an absolute pressure below 1016,80 mbarg
( atm +set point = 1013,25 + 3,55 = 1016,80 mbarg) or 101,68 Kpa of which 70 kPa is the partial pressure exerted by gasoline vapor

Are my statements (*) and (**) correct?
I would not increase the set points even because PV valves are not meant as devices to reduce emissions/losses.

thank you

 

[IFRs]

I don't think the PV will help your losses. The floating roof leaks, perhaps badly. As the tank sits idle, the vapor space above the floating roof will become saturated (after going through the explosive range). When the tank is filled and the liquid level rises, those highly concentrated, odorous (and potentially dangerous) vapors will be pushed out of the tank no matter what the PV setting is for an API 650 atmospheric tank. At the most, the PV will reduce or eliminate breathing losses from diurnal temperature changes. If the tank sits idle for a long time period during which the day/night temperature fluctuations cause enough of an increase in the pressure inside the tank to vent through the PV vent, setting a higher set point could help. This may be an unlikely ocurrance though.

I think there may be a misunderstanding of the gasoline's vapor pressure. This is a partial vapor pressure and the vapor space under the floating roof is at atmospheric pressure - no more or less. There is no pressure or vacuum under the floating roof. There is no pressure or vacuum above the floating due to the vapor pressure of the gasoline in that space. The only pressure or vacuum above the floating roof is due to thermal changes or filling/emptying activities because the tank roof is closed and the fixed roof vents have some set pressure. The vent set pressure should not exceed the weight of the roof plates, or the limits determined by API 650. 70kPa far exceeds the API 650 tank's design capacity. Don't do that!!!