PVRV 3

[nanthakumar]

Hi

currently we have a tank containing very flammable liquid. This tank uses nitrogen via a PCV for blanketing purpose. This tank is also equipped with a PVRV.
If the tank pressures exceeds a certain value then the PVRV will be lifted and the gas will be vented to the emergency vent header. Likewise if the Nitrogen PCV fails then the tank will start pulling vacuum ,then air will be drawn into the tank because the vacuum side of the PVRV is not connected to anything except to the atmosphere.

Can we say that if air is pulled into the tank, the flammable gas is too rich (UEL) to ignite.
Can anyone share their thoughts on this type of arrangement?
IS there any requirement for flammable tank in terms of venting or pulling in vacuum during emergency?
Thanks

 

[saleda]

in industries that have to deal with highly flammable liquids first and most necesssry task is to to eliminate the ignition source. proper earthing for reducing static charge risk. and the vent of tank can have condenser with appropiate utility running to keep vapor below LEL.

 

[TD2K]

I don't think you can argue that the atmosphere in the tank is too rich under the scenario you have posed.

Whether the atmosphere in the tank is going to be too rich will depend on the UEL, the fluid's vapor pressure, how much air is drawn into the tank for the vapor volume that is there AND how well the two are mixed. Even if upon mixing you can show that you would be above the UEL, there would be a period of time where you could be in the flammable region.

That may just have to be a risk you take after looking at the reliability of your tank blanketing system. I'd look at what tools I have given Operations that there is a problem with the blanketing system. These could be low pressure alarms on the tank indicating that air is about to or has been drawn into the tank to avoid vacuum.

 

[Compositepro]

Auto gas tanks operate above UEL, as one example. If you have a N2 blanket someone has already decided on the safety strategy. It is a matter of risk benefit analysis. Oxygen reaction with the tank contents is another consideration in blanketing.

There is probably not much you can do to conserve N2 if the blanketing system was properly designed to begin with. An inert gas generator may be cheaper than N2. These burn fuel to produce low oxygen gas from air.

 

[nanthakumar]

Hi

looking at the scenario you guys mention, i think we are going to add a pressure transmitter to the tank to help us identify if the blanketing system is working or not.

Thanks again for the valuable feedback.