air start motor attached to pressure vessel

[mechie17]

I am working with a system that starts with a tank of pressurized air, goes through some piping, a pressure reg valve, some more piping, a solenoid valve and then a turbine style air start motor. I am trying to analyze the system for a 20 second run time (the tank is pressurized to some standard, a solenoid is opened at t=0 and the valve stays open for 20 sec.) I need to know the pressure and flow at the turbine and also volume of air left in the tank after the 20 seconds. Since each component has its own restrictions on volumetric flow what would be a good method to do this? Also, since the head loss of the components is based on the flow rate, how do I include that in the calculation since it keeps changing? Feels like some sort of differential equation, but I'm not sure how to begin.

 

[dcasto]

No differntial EQ. The pressure reulator will compensate for all the piping losses, thats why its there. Now it compensate for installing a 1" line to the starter, That line must be able to handle the flowrate of the starter with just a few psi drop. Likewise the regulator can not compensate for not having enough supply air.

The starter has a consumption rate in CFM, so CFM times minutes is how much air you will have to remove from the tank. Typically the starter is rated in SCFM so assume that calculation reveals that you nee 500 CFM at 90 psig. If the tanks holds 1000 SCF at 200 psi it holds about 500 SCF at 100, so the tank can supply the 500 SCF at a pressure ABOVE the 90 psig the starter needs. Your regulator must be able to pass the listed flowrate with a 10 psi DP. Finished.

 

[mechie17]

The problem is that the consumption rate is based on the pressure the ASM sees. The reason this is a problem is 1) there are actually two ASMs connected but one turns off after 4 seconds then starts again at 10, then turns off again at 14 and 2) because the pressure regulator needs to be set higher to accomodate the flow of the two ASMs there will be a point where the pressure coming from the tank will be below set. Then the pressure seen at the ASM will be variable.
I'm sorry I did not mention these before, I guess I oversimplified.

 

[EdStainless]

well I think that DC is still right. You only need to consider everything after the regulator, until you drop the tank pressure far enough to no longer support the regulator outlet pressure.
If the pressure at the ASMs is fluxuating, then the system is undersized. Even in this cast teh ASM should only see two possible pressures. The pressure with one running or the pressure with two running. So you only have two flow states to address.

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