# Check valve in compressed air pipe network?

I am working on a building that would like to used compressed air for some pneumatic actuators. They would like to bring the air in from an adjacent building that has more than enough capacity, but the air quality coming over is not sufficient and I need to add a dryer and filters to bring it to the correct quality.

Depending on the usage in the other building, I could feasibly see air that has run through the dryer and filters in the new building being effectively siphoned back to the other building. Is there a reason that I could/should not install an inline check valve where the piping comes into the new building right before the filters and dryer? Check valves are used extensively on the compressors and receivers, but a brief look online didn't turn up anywhere where people were using them as I propose.

The problem comes down to pressure. If your supply building (building 1), which has capacity, is far enough away, the pressure at the end of that line may be significantly lower than the line in new building, (building 2), especially before the filters. If this is the case, the new line won't have enough force to open the check valve against the old line, and none of the new air will get into the new line.

As such, a careful consideration and monitoring of the pressures should be done before beginning. Monitor the use of building 2 during slow times, during peak usage, and then model if for the new higher usage. Ideally use 3-4 points, knowing the flow rates and pressures for each. Plot them on a curve in excel, using a quadratic regression, and figure out where the pressure will be at the new flow rate at the new use rate.

You should note that the compressor's pressure should drop on that curve. Then measure building 1's usage as well, ideally at similar flow rates and pressures if possible. Finally, use some basic fluid dynamics losses to find the pressure loss after routing from so far away.

If $Q_{Overcapacity}$, is the new flow rate, $P(Q_{Overcapacity})$ is the pressure at the new flow rate, and $P(0)$ is the pressure at no flow rate, the system will only provide new air if the following equation holds true:

$$P_1(0) > P_2(Q_{Overcapacity})$$

If this equation does hold true, then the actual flow rate of the new line can be determined by when the following equation is true:

$$P_1(Q_{Makeup}) = P_2(Q_{Remainder}) + P_{losses}(Q_{Makeup})$$

In this scenario, $Q_{Makeup}$ is the flow rate of the makeup line, $Q_{Remainder}$ is the remainder to meet the complete flow demand. If this pressure is to low for the equipment to operate, then the entire system needs to be scrapped and a higher pressure source is required anyways.

If you know that the check valve won't contribute too much frictional loss and that mixing the air from building 2 (higher quality air building) with air in building 1 (supply building) cannot happen, a check valve will be fine. The only other consideration I would have is to give a way to vent the air in building 2 because if there is some air stuck between the check valve and use point, there is no way to depressurize it safely for any future work. Usually the filter bank can be opened to atmosphere and you can bleed the pressure that way.