Tank Vent Orifice Plate

[mal78ibu]

Need some thoughts as to go about the calculations.

I have a tank of NH3 padded with N2 to 140 psig that I want to vent down to a recovery system, but I want to restrict the flow to 50 scfm during the vent down. I am planning on using 1" pipe but will need an orifice to restrict the flow so that I don't overcome my nh3 recovery system.

Thoughts?

 

[TD2K]

To what pressure do you want to vent it down to? The tank may be padded with N2 but depending on the storage temperature, a significant portion of the vapor will be ammonia. If you just open the blowdown line to your recovery system and flow through the orifice, the ammonia will keep flashing off from the bulk liquid as the tank depressures. If you could just keep venting ammonia till the tank pressure is down to atmospheric pressure, the remaining liquid will be pretty cold (about -28C at 1 atmosphere absolute pressure).

Sizing the orifice is relatively straight foward. Assuming the orifice is located near the ammonia storage tank, I'd use a flow rate of 50 scfm and 140 psig inlet pressure as the starting point. You'll have to determine a MW for the vapor (mixture of ammonia and nitrogen). Then, you'll need to see what pressure drop you get through the 1" piping downstream of the orifice over to the ammonia recovery skid and use that as the outlet pressure on your orifice. The calculated downstream pressure may not be significant in terms of the sizing the orifice if you have choked flow across the orifice but if the backpressure puts you into non-chocked flow, you'll need a larger orifice (assuming chocked flow in sizing the orifice will give you the smallest orifice).

 

[BenThayer]

as the pressure falls off, the flow will fall off substantially. you might consider a globe valve so you can slowly open it further.

otherwise, add a pressure regulator and drop the pressure to something rather low like 20 psig and then size the orifice for that inlet pressure. then the regulator can "control" the flow or at least keep the pressure to the orifice constant for a constant feed rate to the recovery system.

i too am a bit surprised that you have nitrogen pad pressure on an ammonia system at 140 psig. if there is liquid present at 80°F, that would give ~ 140 psig.

 

[dcasto]

you don't have a tank of NH3 padded with , you have an equalibrium mixture of NH3 and N2. At 140 psig pure NH3 liquid with a vapor phase (saturated liquid) is at exactly 75F.

With added N2, you do not know exactly what is in the gas and in the liquid phase. The gas phase could contain 20 of 30% NH3, as was stated above. As you vent off the gases, the NH3 will boil and the temperture will drop like a rock in the tank and that is an indication of how fast the tank is venting and when you gotten a large amount of the N2 out.