Compressed air thermal mass flow meters are great tools to monitor and manage your energy consumption. They can detect leakages much faster than a pressure sensor, they can be used for cost allocation and they can be used for efficiency monitoring and maintenance management. However, it is important to know how to install them correctly. In this FAQ we will give you some expert tips on how to get the most out of your flow meter.
1. Select the right type flow meter for your application
Is the air wet or dry? Is an In-line or probe style best for your application? Select the right technology and type to match your situation. This can be done by using our online tool VPCalculator, or by checking with the tables in our brochures. Good practise is to plan for the normal (average) flow to be 50% to 75% of the meter’s full range. We do not recommend using a flow meter at < 10% of its full flow range. Whenever possible, select a flow meter that will operate near its midrange specification.
2. Make sure the air quality matches the flow meter
Regardless of type, any flow meter is sensitive to pollutants in the air. For example, sticky particles can adhere to a thermal mass flow meter and small abrasive particles can wear away the sensor surface over time. We recommend placing thermal mass flow meters in dry air downstream of all air treatment filters to prevent sensor degradation. Also, for differential pressure flow meters a combination of too much water and dirt can clogg the sensor.
3. Create enough straight pipe length
A symmetric, fully developed turbulent flow profile is what is needed for accurate flow measurement. Check the piping table, which you can find in the flow meter’s user manual, for the minimum recommended pipe length. Make sure to install the meter at the recommended distance from bends, valves and other objects that can distort the flow profile. Also, be aware that the flow profile can change when velocity increases. As a rule of thumb, most manufacturers recommend a minimum of “20 times the pipe diameter” downstream of a 90-degree bend. There are manufacturers that claim a shorter pipe length is adequate for their flow meter technology. But you cannot change the laws of physics and that is why we advise to use the longest pipe length that is available before and after the flow meter.
4. Beware of temperature sweeps
If temperature changes rapidly, this can affect the measurement accuracy of the flow meter and extreme temperatures can shorten its service lifetime as well. Large temperature variations, which can be found downstream of malfunctioning desiccant dryers, should be avoided. It is also good practice to check the maximum temperature rating of your flow meter to make sure worst- case conditions, at the location where the meter is installed, will not overheat and damage the flow meter.
5. Make sure the compressed air flow meter is 3-in-1
A thermal mass flow meter only measures mass flow. Without pressure and temperature, this does not give a clue about what is really going on. So, stop wasting your time looking only at flow, but get the complete picture. All VPFlowScope products, both thermal and differential pressure, measure flow, pressure and temperature.
6. Be aware of reverse flow conditions
Flow in compressed air pipes can reverse direction unexpectedly and only a bi-directional flow meter can reveal this. This is why all VPInstruments’ flow meters are equipped with a bi-directional sensor. Reverse flow can reveal issues with ring networks, multiple compressor rooms, non-return valves, leakage inside compressors, drains, filters and dryers. Measuring flow in both directions is crucial information to the understanding of your compressed air system.
7. Understand the effect of a receiver
Measuring flow and pressure before or after the receiver tank makes a big difference! If you have multiple receivers, the situation becomes even more complex. If you do not take the receiver into account, you can draw the wrong conclusions about compressor capacity or equipment capacity. So, consider how the tank affects your system when measuring near receiver tanks and be aware that a long pipe acts as a receiver as well.
A flow meter for every situation
Your production process consists of many different steps, as previously explained. This calls for different types of flow meters. Fortunately, VPInstruments offers flow meters for every part of the compressed air system. From supply side to demand side. These flow meters are all part of the same “product family”, so the way to use them is mostly the same. They can each be used in different situations, however.
VPFlowScope DP is especially designed for wet and dirty compressed air flow measurement. When there is no need for such precautions, the VPFlowScope M is a great choice. This flow meter is suited to measure dry compressed air and technical gass (after a drier). Our In-line flow meters, meanwhile, are built for small pipes up to 2”.
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