PV Vent for Floating Roof

[Chemtech2]

Does Pressure-Vacuum vent sizing depend on the inbreathing / Outbreathing sizing or evaporative losses. Since the roof floats on the liquid under normal circumstances, how do we size for initial filling and emptying operations.

Thx. in advance

 

[sshep]

The floating roof needs an vacuum/pressure vent independent of the tank (i.e. internal floating roof). This breather is only required when the roof is standing on its legs and not in contact with the liquid. My experience has been that this is typically a nozzle on the roof sealed by a plate. A guided shaft longer than the legs extends below the plate to lift it so that the pressures above and below the roof can equalize during filling and emptying. There may be other options. In any event, the vendor supplying the floating roof should propose and design some sort of venting detail as it is a neccessary provision.

 

[Chemtech2]

sshep ,
Thanx for your input. The floating roof has 2 venting devices. The one that you mention is generally referred as Vacuum breaker and which is helpful while filling ( when the roof is still on its leg). There is this PV vent as well. I am not very confident if it is the RIM vent or a venting device for inbreating and outbreathing to prevent evaporative losses.

SD

 

[rmw]

Did your tank have the floater added to it. Was it a tank without a floating roof first, already with a PV vent?? And, maybe, when the floater was added, nobody ever changed the PV vent to a vented opening.

One of the ways I identify tanks with floaters while driving by a plant or tank farm is to look at the vents. If there are permanent vent openings around the perifery of the top of the tank, I can be fairly well assured it has a floating roof inside. If it has a PV vent or vents, I do not suspect a floater.

Putting the discussion of the vacuum breaker in the floating roof for when the liquid level is below the height of the lets, lets look at the requirements of a PV vent on a tank.

The PV vent has to be sized for the charge and discharge rates of the tank, however that is accomplished, pump(s) gravity, however. The vent sizing in each direction (pressure/vacuum) is determined by how rapidly the tank is filled or vacated. In the case of a tank without a floater, the relieving capacity has to be the sum of the pump in rate, as well as the maximum evaporation rate.

This should not apply in the case of a floater, unless it does not have good seals, or there is a high evaporation rate of whatever is "wiped" along the walls as the roof lowers.

A PV vent would not know if the tank had a floater or not, just that a certain amount of air/vapor was being displaced by the changing level.

One place you can get some information and even order a vent sizing program is

http://www.protectoseal.com

If your tank was, in fact, converted, and you have a PV on a tank that now has a floater, and the PV was properly sized for the liquid before, and nothing in that regard has changed, you are probably OK, since you no longer have the evaporation rate to contend with. However, having a PV on a tank with a floater is suspicious to me, so do not take our word for it, get a sizing program, and check it out.

rmw

 

[tkequipments]

As far as I understood, the P/V is installed on your floating roof. As you said, it could either be a rim vent or a breather. But I don't think it is a rim vent as rim vent are only used for pantograph type peripheral seals, and rim vent are pressure only vents. Thus I would suggest it is a breather.
In that case, it should be sized according to API 2000, taking as basic data a tank capacity corresponding to your tank with a height equal to the height of your floating roof when resting on its legs (usually 1,80 m).

http://www.larco.fr

 

[rmw]

Sandeepdalvi,

There are a couple of other threads current right now that deal with the question you asked.

Go to:

124-65528
794-82669

Some mighty interesting reading there regarding what you want to know.

rmw