5 replies to this topic

Dear Professionals,

I have a large storage tank (125,000 BBLS) which I need to convert to a Nitrogen blanketed tank.

I have calculated the maximum in-breathing (API 2000 Thermal + maximum pump out & assuming new Nitrogen failure) and maximum out-breathing (API 2000 Thermal + Max. Fill + new Nitrogen), so I have a maximum volumetric flowrate in and out (I assume I can assume Nitrogen to get a mass flowrate too).

The tank has 2 off 8" free vents (atmospheric vents).

I need to calculate the pressure in the tank (+ve and -ve) at the extremes of in-breathing and out-breathing to ensure that they are within the tanks tolerances.

I tried to do it by assuming a single orifice (11.32") and using K.A2.root (2.Rho.(p1 - p2)), but the result did not tally with manually iterating in an orifice sizing program to get the required flow rate.

How would anyone else approach this please?

[pleckner]

It is going to vary depending on the type of conservation vent you use. I suggest that you decide on the conservation vent you will eventually purchase, go to that vendor's website and look at the pressure curves they give for that conservation vent.

If you decide on ProtectoSeal, they have a great sizing program that correlates the pressure in the tank to the amount of venting.

[proinwv]

If I understand what you said, you have "free vents". Do you mean vents that are always open to the atmosphere? Then you have a constant flow of N2 out to the atmosphere along with the portion of the mixture that is product vapor?

[djack77494]

CJ,
You need to be careful about what you are doing here. I did not see any mention of the pressure that the tank can withstand. Do you know what that is?

If you want to maintain a nitrogen blanket on the tank, you should replace your free vents with conservation vents (i.e. valves). This would increase the operating pressure of the tank during normal operation (the blanket gas pressure instead of atmospheric) and during filling because you have an additional restriction in the vent line (the conservation vent valves). You must analyze the ability of your tank to withstand this additional pressure (and vacuum during drawdown operations). You may find that you need more or larger vents, or that it will not be possible to effect the proposed change.

Doug

Thanks for all the responses,

I will have a look on the website mentioned also.

Just to clarify a couple of points (which I omitted for clarity, but you have all picked up on!)

The tank is being converted only temporarily, for a change in duty whilst another blanketed tank is taken out of service for maintenance.

The tank will be free vented to atmosphere for that short duration as the preferred vendor for P&V valves (conservation valves) currently has lead times in excess of the maintenance outage. Once the other tank is repair this tank will be returned to atmospheric duty.

I do know the maximum design pressure of the tank, which is why I am trying to calculate the dP across the vent nozzles for the minimum flow I have calculated for the Daily Breathing Volume and for the maximum flow I have calculated for the Nitrogen supply at full bore flow.

Thanks for your help and suggestions.

[proinwv]

If your "vent nozzles" are a commercial vent, then the manufacturer should have flow curves to answer your question.