An aircraft maintenance facility had been using two BSD 50 hp rotary screw compressors to run their shop air operations overhauling a wide variety of commercial and military aircraft. They added a new testing application that used high volumes of air for short durations to simulate flight conditions. For this they purchased a complete clean dry air system with two DSD 150 hp units, refrigerated dryer, filters, 3800 gallon tank, etc. (with future plans for a third unit). This system included a Sigma Air Manager (SAM) for control and remote monitoring.
They intended to operate these systems separately most of the time, but they wanted to connect them so that the larger system could augment the smaller existing system (when possible) to reduce the operation of the compressors in the BSD system and allow for the DSD compressors to supplement the shop air. There was differing air quality between the two stations, however, so it was important that air from the cleaner of the two systems (the testing application) wasn’t contaminated with air from the system with lower air quality requirements (the shop air).
Our initial solution was to connect the two piping networks and link all four compressors to the SAM and run all at the same pressure. The challenge was that the system dedicated to the testing application required a higher operating pressure (118 psig) than the system dedicated to the shop air (100 psig), and the customer did not want to raise pressure in the shop air system. Additionally, the testing application consumed a large volume of air in a short period of time, and the customer did not want too much air bled from the shop air application when testing was taking place. In short, they wanted the testing application system to feed the shop air system occasionally, but they did not want the testing application to steal air from the shop system during high volume test periods. The customer wanted to put a check valve in between the systems to address protecting the air quality requirements in each system, but this proposed solution lacked the ability to protect pressure requirements for the shop air while the testing application was in operation.
While this system has some unique aspects, the basic idea is not unique. It is not uncommon for an operator to connect two separate systems in the same plant for backup air supply.
In some instances a customer does not have a centralized compressed air station, but instead has multiple systems within the plant (usually to protect specific applications). Often, users connect systems with a manual valve and open it as needed, but this is obviously inconvenient. Automating this function will avoid interruptions to operations.
In this particular case it was accomplished with a Kaeser DHS air main charging valve. Typically these valves are installed to protect air quality in a compressed air system – sensing air pressure upstream of the valve and then closing if upstream pressure dips below the set threshold. This logic also works well for this application when the valve is installed with its pressure sensor located on the side of the valve common to the “protected” system. In this customer’s case, that meant installing the DHS valve with its pressure sensor on the same side as the BSD/shop air system to ensure that pressure in the shop air system would not dip below 100 psig. The DHS had a setpoint of 105 psig – meaning that if pressure in the shop air system dipped below 105 psig, the DHS valve would close and isolate the two systems – effectively preventing pressure from falling too low. Once pressure rose back above the 105 psig setpoint in the shop air system, the DHS valve would reopen and both systems would again be common – allowing air from the DSD/testing system to supplement the shop. Note that the check valve was still recommended to prevent lower quality air from the shop from contaminating the higher quality air from the testing application. Despite the fact that the testing application runs at a higher pressure so air should not flow into that system, the check valve ensures that there is no contamination. See figure below for a visual representation.