By the
combined gas law the quantity (P*V)/T remains constant throughout a closed system in steady state. If we can also assume that the temperature of the system remains approximately constant over the duration of interest, then the quantity P*V remains approximately constant too.
P*V ≈ constant
This is a pretty powerful tool. We can use it to equate the energies of two different volumes of gas at two different pressures.
P1*V1 ≈ constant ≈ P2*V2
P1*V1 ≈ P2*V2
The quantity P*V will be in units of
energy. This number will in fact be a estimate of how much stored energy is contained in the pressurized system. (But it's not quite a measure of how much energy is available in the system to do useful work!)
Here's an example. First let's look at how much energy is contained by the storage tanks.
P1 = 120 psig storage pressure
V1 = two 16 in^3 tanks = 32 in^3
P1*V1 = (120 psig)(32 in^3) = 3840 in*lbf
Now, how many times can you use that energy to actuate an example cylinder?
P2 = 60 psig point-of-use pressure
V2 = (n actuations)(5/16 in bore x 18 in stroke) = n(.0767 in^2)(18 in) = n(1.38 in^3)
P2*V2 = n(60 psig)(1.38 in^3) = n(82.8 in*lbf)
P1*V1 = P2*V2
3840 in*lbf = n(82.8 in*lbf)
Solve for n...
n = 46 actuations