Determining Air Flow.

[sam01]

Typically chemical bulk storage deliveries on a tanker truck use air to unload the contents of about 5000 gallons. The tanker trucks utilize a compressor which pressurizes the tank to 20 to 30 psi. The compressor puts out about 15 cfm. Once the tanker truck is toward the end of the unloading its contents, there is a 5000 gallon storage vessel of compressed air at 30 psi. The atmospheric storage tanks that accept the bulk chemical deliveries are made of fiberglass. The vent of the fiberglass tanks are connected to a desicant drier that is rated at about 80 cfm for pump out rate. How do I figure out what the air flow rate would be once the chemical liquid flows out of the tanker truck. My concern is that the air flow rate is at supersonic speed which would exeed the flow rate of the desicant drier. This could rupture the fiberglass storage tank by an increase in back pressure. Can someone tell me if the air flow rate is exceeded during this instance. Thanks.

 

[pmover]

Once the liquid empties, the piping system is now air instead of liquid. Thus, different flowrates.
Not knowing much about the piping system, here are some suggestions.
It seems that when the tank is empty or near empty, the sound of air rushing through the piping will be observed and the valve at the tanker truck can be closed. Thus stopping the sudden rush of air. The compressor can then be shutdown and opening a vent valve on the truck will vent to atmosphere without passing through the tank and drier. This assumes a vent and isolation valves are present on the tanker truck.
Unless, the situation requires the air in tanker truck to be passed through drier. Then, once the liquids have emptied the truck, shutdown the compressor and somehow control the air flow to the tank until the tanker truck is at atmospheric pressure.
Or, shutdown the compressor half way through the operation and allow the air in tanker truck to expand to lower pressures. This will lengthen the time required to empty the truck, but the task will get done. If necessary, intermittently operate the compressor to push out the liquid. I suppose the storage tank level is greater than the tanker truck.
Construct structural platform and feed the storage tank by earth's best friend - gravity.
Good luck.

 

[Haf]

Sam,

Although it is possible that the flow is choked (i.e., sonic), it is unlikely that the flow is supersonic, since achieving supersonic velocity generally requires the existence of a converging/diverging nozzle somewhere in the system. Although there is some dependence on the geometry of the delivery system, the air velocity can generally be determined by the ratio of the back pressure over the delivery (tank) pressure. Using the isentropic flow relations (look up in any fluids textbook; there are usually tables in the compressible flow section), you can determine the Mach number of the flow. If the ratio is less than 0.528, the flow is choked (i.e., the Mach number is 1). Since you know the pressure and temperature (presumably atmospheric?) in the storage vessel that delivers the air, you can calculate the temperature, static pressure, stagnation pressure, and density at the exit, again using isentropic relations. You can then calculate flow rate simply using the formula Q = rho*V*A. If the flow is choked, you can save some time by using Fliegner's formula to calculate flow rate. If you have trouble finding the formulas or tables, let me know, and I'll get them to you. Just wanted to avoid typing them out if I can.

Good luck,

Haf

 

[psafety]

Sam,
You are on the correct track. The tanker is now a stored volume of air and the resistance to the flow of air is the interconnecting piping. I have ran across the scenario many times while performing hazard analysis of such systems.
You'll have to determine the maximum rate of flow through your system at P1 being the tanker pressure and P2 the storage vessel. It can and does get ugly sometimes. Let me know if more info is needed.
Don Coffman

 

[sam01]

Thanks for everyone's help, sorry I didn't get back to this sooner. Haf can you please send me the formulas or tables for choke flow. Thanks again.

 

[Haf]

Sam,

If the flow is choked, you can use Fliegner's formula to determine flow rate. Here it is, simplified for air (i.e., gamma = 1.4, R = 287.0 J/(kg*K)).

(dm/dt)*T0^0.5/(P0*A*) = 0.04042 (kg/s*K^0.5)/(Pa*m^2)

where
dm/dt = mass flow rate in kg/s
T0 = stagnation (total) temperature in K
P0 = stagnation (total)pressure in Pa
A* = throat area in square meters

Remember, this formula only applies if the flow is choked (i.e., the storage pressure to back pressure ratio is less than 0.528). If the flow is not choked, a little more work is necessary to determine mass flow rate, but the problem is still very managable. Of course, choked flow is the highest mass flow rate possible through the system and therefore represents a worst-case scenario.

Haf