Volume Bottle for pneumatic actuator

[markstg]

I would like to calculate the volume bottle size required to close a fail open pneumatically operated on/off valve 3 times, when the Instrument Air Supply header has failed. My actuator size is 10,500 cubic inches. The volume bottle would have 100psig air and the actuator requires 60 psig air to operate. Does anyone know how to calculate the required volume bottle size?

Thanks in advance.

 

[israelkk]

Pressure * Volume = constant (at constant temperature)

Assuming bottle volume = V1

V1*100 psi = (V1 + 3 * 10500)*60 psi

Giving

V1 = 47250 cubic inch

To be on the safe side take larger bottle to acomodate for leakage.

 

[JimCasey]

Automax offers "Autosize" software, and one of the features is failsafe tank sizing. I plugged in the parameters you gave and got the minimum tank size to be 422.8 gal. That's a little more than twice what IsraelKK calculated. Buy a 500 gallon tank.

 

[markstg]

Thanks for your help. The PV=constant method undersizes the volume tank as JimC noted. Thus the PV=constant is not a good model for this. Using an adiabatic process model, PV^k=constant where k is the specific heat ratio (for air = 1.4) gives the correct result, and matches what JimC has found.

Thanks again.

 

[israelkk]

markstg

Note that I specifically stated "at constant temperature". I just wanted to give you an idea to the way to solve the one variable equation to calculate the volume and you did it.