Testing with the Airometrix LP Flow Meter is simple, quick, and repeatable. The ideal meter tie-in location is after the aftercooler and moisture separator and as close to the output of the compressor as practical. This can be on a bung in a receiver, at a drop leg or tap in the compressor room, or at a service port where a portable compressor is typically connected. The main requirement is that the compressor is capable of being isolated from the system with the full flow of the compressor going to the LP Meter. Note the installation diagram below:
Testing the Compressor Performance
Once the meter has been plumbed in and the compressor has been isolated from the rest of the system, the compressor is started and the different orifices on the meter are opened and closed until the compressor can just barely keep up at system pressure. Add the valves which are open and this is the Free Air Delivered (FAD) of the compressor at the test pressure. In order to build a performance curve for the compressor, flow tests are performed at various pressures to determine the compressor output over a span of loads. This is accomplished by opening or closing more or less orifices and noting the changes in pressure with flow. It is recommended to take simultaneous true power readings (kW) on the compressor at each load point to get a full picture of compressor performance (flow, pressure, and power).
The performance data obtained from the test can be used to diagnose control function and other hard to measure performance information. It can also be used to chart inlet valve control, and accurately determine compressor power and flow turndown.
Testing for Leak Volume
Additional testing can be performed to determine system leak volume and consumption of equipment and processes. To test a system for leaks, the system is brought on-line without any equipment operating (usually performed on lunch breaks, off shifts, maintenance shifts, or annual shutdown), and a second flow test is performed on the compressor at the previously tested system pressure. The results for the second test with the system pressurized will be a lower air flow through the meter, the difference in the two tests is the leak volume in the system. For instance, if an isolated compressor is tested at 100psig at 1000cfm, and then retested at 100psig with the system pressurized and a flow of 600cfm through the meter, then the leak volume in the plant is 400cfm. If areas of the plant can be isolated, a leak test for each section can be administered and effort focused in the sections with the greatest volume of leaks.
Consumption Testing
Consumption tests can be performed using the same approach as the leak test which is to test before and after equipment is turned on and off. The difference in flow readings between the on and off conditions is the consumption of that piece of equipment. The same procedure can be used to test building consumption, process lines, or any other end use which can be isolated. A similar procedure can be used to find overall system consumption plus leaks when the plant is fully operational by testing each compressor off line one at time to determine the flows at full load. With the plant running, the compressors are brought to full load by opening orifices on the LP meter and the flow through the meter is subtracted from the known maximum capacity of all the on-line compressors. This is the consumption plus leaks for the entire system. If the leak volume has already been determined, the leak volume can be subtracted as well and the remainder is the consumption of the plant equipment.
In most facilities, only one meter is required to test all of the compressors for the system. The meter can be quickly disconnected from one location and moved to test another compressor as long as the meter is sized large enough to accept the compressor flow. Actual testing time is limited by how fast the compressor can react to a load change. Typical test time for positive displacement compressors is 10 minutes or less, and one hour or less for dynamic machines.
