Create enough straight pipe length
Having a flowmeter is one thing, but installing one is another. It’s very important to select the right installation point for your flowmeter. It’s important to know what to take into account and what needs to be avoided at all costs. The installation point is crucial for correct measurements!
Why the piping table?
Gas flow in pipes follows certain rules, which must be observed for optimal measurement results. A symmetric, fully developed turbulent flow profile is what is needed for accurate flow measurement. Bends, valves and other objects will distort the flow profile, so it is important to install the flow meter with sufficient distance from the distortion.
As a rule of thumb, most manufacturers recommend a minimum of “20 times the pipe diameter” downstream of a 90-degree bend. For some exceptions the upstream length needs to be longer, or it can be shorter. A few common examples in compressed air systems, we have included in our piping table below. The table shows the minimum amount of length, upstream and downstream, depending on the installation.
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.
If possible, you can always choose a longer upstream length, as these are minimum values. The up- and downstream lengths are used industry wide as guidelines, but will never be a guarantee for obtaining the “true value”.
Piping table
The piping table provides a guideline for proper distances between upstream or downstream objects and the flowmeter. The upstream length is the length between the last non-straight object and the flowmeter. If the upstream length is straight, and the distortion is downstream of the flowmeter, you can use the column “downstream length” as a guideline. In very complex situations, with multiple up- and downstream objects, you should consider another location.
This table is a practical guideline and is not exact science. Practical situations can have multiple sources of distortion, therefor VPInstruments does not take any responsibility for correctness.
