The importance of bi-directional compressed air flow measurement

Bi-directional flow occurs more often than you would expect in compressed air systems. In this article you will find a few examples of bi-directional flow situations. In these cases, a bi-directional flow meter is the only way to measure correctly and to know what is going on in your compressed air system.

Example case: Ring network with two compressor rooms

A large electronic component manufacturer has a compressed air system that consists of a ring network with air supplied by two compressor rooms (see picture). In compressor room B two flow meters were installed after the receiver tank, due to space restrictions. At the beginning, these were uni-directional flow meters. In other words, they could not see the difference between forward and backward flow.  

Receiver tank acts as big balloon

After a couple of days strange measurements were observed. The flow meters, manufactured by VP Instruments, were showing considerable measurement values when the compressor in room B was turned off. Measurements with VPFlowScope bi-directional flow meters showed that the large receiver tank is filled by the compressors from the other compressor room when the local air demand in the ring network is low. Moreover, it delivers air to the network when the local demand is high. You can see the receiver tank as a big balloon interacting with the network.

 “Merging traffic” situation

Furthermore, when the compressor in room B is running loaded, compressed air is fed into the ring network. In the T-junction, the flow merges with existing “traffic” based on how the consumption in the ring mains is balanced. If the consumption is well balanced, it might be perfectly symmetric (50% left, 50% right), but when the consumption on one side is much higher, the air flow will distribute differently. The air coming from the compressor merges with the air which was already flowing through the ring network. This can result in a high or low reading, depending on the actual demand and the resulting flow direction.

Other bi-directional flow examples

Bi-directional flow meters can reveal important issues in many compressed air systems. Other examples where you can reveal issues:

• Leaking non-return valves just after the compressor, or leaking condensate drains. The bi-directional flow meter will tell you when it happens.
• Overseen branches
• Systems with multiple compressor rooms
• Decentralized receivers can also act as a local buffer
• Oscillations in the pipe network can be seen as consumption instead of near zero flow

Conclusions

Uni-directional flow meters will show a positive flow when reverse flow occurs. This can lead to wrong conclusions about the system behavior. Discover your actual consumption and avoid such misreading with bi-directional flow meters, like the VPFlowScope. It is recommended in ring networks to always use bi-directional flow meters. In case of doubt, just use a bi-directional flow meter. Costs for this additional feature in your flow meter is insignificant compared to drawing the wrong conclusions.

Read the entire article with more examples and details here.