While many plants operate a single centralized compressed air station supplying all points of use, it is not uncommon to find some with multiple systems within the plant. In very large plants with multiple buildings, this may simply be the more economical way to go- especially in colder climates which would necessitate desiccant dryers and possibly heated piping that would not be needed everywhere in the plant. Here we will address another case, in which separate air systems are employed to ensure specific pressure and air quality to specific applications. But even in these cases, users often choose to link otherwise independent systems for backup purposes. Often, users connect systems with a manual valve and open it as needed, but this is obviously inconvenient and cannot be well regulated. Automating this function will avoid interruptions to operations. In the case below, we explore how one customer approached this. The twist: the two systems operated at different pressures and air quality levels.
An aircraft maintenance facility had been using two BSD 50 hp rotary screw compressors for main shop air to run their operations overhauling a wide variety of commercial and military aircraft. Then they added a new testing system 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 at times the BSD system ran short of air and needed additional volume. The customer wanted to connect the two systems together so that the larger system could augment the smaller system when possible – feeding air from the DSD compressors in the testing application to the overworked BSD compressors in the shop air system.
The testing air system had higher air quality and it was important to prevent the lower quality shop air from contaminating the testing system air. Essentially, the customer wanted air to flow from the testing application to the shop air application when necessary, but under no circumstances could air from the shop location enter the testing application system.
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 (it would have wasted considerable energy). Additionally, the testing application consumed a large volume of air in a short period of time, and the customer did not want any 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.
This particular case called for a somewhat creative solution, and was accomplished with a Kaeser DHS air main charging valve. Typically these valves are installed to ensure air quality in a compressed air system – sensing air pressure upstream of the valve and closing it if upstream pressure dips below the set threshold (which would cause higher velocity flows that would prevent proper contact time in dryers, for example).
This logic also works well for this application when the valve is installed with its pressure sensor located on the “protected” side of the valve. In this customer’s case, that meant installing the DHS valve with its pressure sensor on the shop air side to ensure that pressure in the shop air system would not ever dip below 100 psig, and would never be impacted when the testing application was running in the other system. Most of the time, the test system is allowed to supply air to the shop, but during testing (a high flow demand event), it is possible for the test system pressure to drop below the shop air pressure. At these times, the DHS engages to prevent loss of pressure to shop air users AND to ensure that lower quality shop air does not contaminate the more sensitive testing instruments.
The DHS had a setpoint of 105 psig – so that if pressure in the test system dipped below 105 psig, the DHS valve would close and isolate the two systems – effectively preventing pressure from falling too low in the shop. 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 again.
Note that a check valve was recommended to prevent lower quality air from the shop from contaminating the higher quality air from the testing application. While the testing application generally runs at a higher pressure than the shop, it may occasionally drop below during high demand events so the check valve ensures that there is no contamination on the testing side. See figure below for a visual representation.
This application was an interesting challenge with somewhat unusual requirements and restrictions. The initial solution relying on the check valve pointed us in the right direction but needed some refinement. Getting a full understanding of system dynamics and then exploring different possibilities for control were the keys to success.