vapor blow through for tank filling

[jrwchem]

Hello,

We have a storage tank that will be filled via truck deliveries. The truck will be pressurized to 20 psig with N2 to offload the inventory into the tank. I would prefer to use a pump, but the client prefers N2 offloading.

A pressure regulator is designed to provide a maximum N2 flow of say X lb/hr.

Per API 2000, it is wise to consider vapor blow through for vent sizing.

My initial thought is that the tank relief/vent must be sized to handle X lb/hr, since this is the maximum flow the upstream PRV can handle.

However, it has been suggested that a transient case be considered, in which we imagine the truck is empty and pressureized to 20 psig, and the inital flow of N2 from the truck to the tank is considered. It turns out you can get quite an inital "woosh" of flow, which results in original vent being quite undersized. This would be a temporary situation, which would only appear to happen if the truck is filled with N2, blocked in, and then the valve is opened suddenly.

Any thought or comments on the credibility of this case? Any suggestions?
thanks.

Jonathan

 

[BenThayer]

decide if you think it is a credible case and then size accordingly.

fwiw, i think the case sounds feasible. but then ask how big is the tank and what volume can be compressed in it.

and why do you need 20 psig?

and what happens when the truck goes empty and the gas at what ever pressure suddenly flows in directly?

transients are when and where many events happen because no one considered what sequence the operators will be doing to actually operate the equipment.

 

[wrm]

Check out this incident report. It has a lot of details describing the situation you describe

http://ncsp.tamu.edu/reports/EPA/surpass.pdf

 

[dcasto]

I'd size for the failure of the N2 regulator. If the regulator fails, then you could get 20 times X lb/hr. Then You'd think again about the size of the regulator and make sure it is not to large.

 

[shahyar]

Your thought is correct.
You need to size vent over the tank based on "last woosh of gas".
However, pressure is not 20 psig necessatily. The pressure in the truck vessel is equal to vertival coloumn of your liquid, roughly equal to tank height (so, it is always less than 20 psig).
Hope this help

 

[CMA010]

Perhaps stating the obvious but don't forget that the vent will have to be sized for the N2 case plus the normal venting requirements.

 

[jrwchem]

Thank you everybody for your comments. I have given some further thought to my original question, and feel comfertable at this point. I will share my conclusions:

1) The psv SHOULD be sized for the "woosh" of nitrogen based on the worst case tank pressure (generally ~ 30 psig - assume truck PSV set at 30 psig), and worst case tank pressure (0 psig). While this woosh might be a transient (i.e the pressue regulator ultimalty wont be able to keep up, and hence the tank pressure will eventually drop) it will still occur initially and the psv must be able to accomodate the woosh.

2) The vapor blow through can easily dominate the other venting requirements (thermal and outbreathing due to liquid influx)

3) Part of the venting capability was in the form of a 24" manway/emergency vent. I was hesitant to use N2 offloading since this vent does not vent to a safe location. The operators were hesitant as well.

In the end, we went with using a pump. Who knew?

Thanks anyways.
Jonathan