Flow conditioning

Flow conditioning regulates the way a gas or liquid moves through a pipe (the flow profile). This is particularly important for flow measurement. The flow profile is disturbed by system components such as bends and control valves. Virtually every flow meter is sensitive to the flow profile, as the accuracy deteriorates. Examples of flow meters where the flow profile is very decisive for the performance of the instrument are ultrasonic flow meters and pluggable thermal flow meters. The only flow meters that are not adversely affected by the flow profile are a Positive Displacement meter and the Coriolis mass flow meter (largely).

Illustration: a cross-flow flow profile after a turn.

Illustratie: een rondgaand profiel, ontstaan door 2 bochten achter elkaar.

A developed flow profile

Suppliers therefore list the minimum upstream and downstream lengths in their flowmeter specifications, usually expressed in pipe diameters. So if an instrument in a DN200 pipe needs 20D up- and 5D downstream, in practice this corresponds to 4000 mm and 1000 mm up- and downstream lengths. Those are the minimum values to not get into trouble with the expected measurement accuracy under standard conditions. But the flow profile is not guaranteed. Due to fluctuations in pressure and temperature, it is therefore wise to allow for more inlet and outlet lengths if there is room for them.

If a medium flows through a straight pipe, a "developed flow profile" is created after a certain length. The characteristic of a developed flow profile is that this flow profile no longer changes in the pipeline. It is a turbulent profile and, precisely because of this turbulence, the profile is symmetrical and uniform. A flow profile is influenced by bends, control valves, reducers and thermowells. And especially at low viscosities such as gases and thin fluids. A viscous oil will suffer much less from distortions.

Illustration: a laminar flow profile

Illustration: a turbulent flow profile.

Flow conditioners

To improve the performance of a flow meter, a flow conditioner can be installed. A flow conditioner influences the flow profile so that it is known and controlled. This can be a developed flow profile, or perhaps a flat flow profile. The result depends on which flow conditioner is used. Some flow conditioners reduce the swirl, others create heavy turbulence, flattening the flow profile. A random flow conditioner therefore does not give a guaranteed improvement of the measurement result, but a well-proportioned flow conditioner generally gives many advantages:

• In combination with a flow conditioner, the system is robust and highly insensitive to installation conditions.
• The flow conditioner allows the minimum straight inlet length to be reduced (for ultrasonic flow meters, the straight inlet length decreases from about 10D to about 4D) or increased accuracy at the specified straight inlet length.
• Even better performance with a calibration of the entire meter run. By compensating for Reynolds, the meter is completely independent of the fluid properties.

Well-known flow conditioners are the perforated plates, such as the Zanker and Gallagher plate. These hole plates are clamped between the flanges and have a relatively small influence on the swirl, but do ensure a more symmetrical profile. A tube bundle is a flow conditioner which consists of a bundle of tubes placed in the longitudinal direction of the pipe, which do counteract the swirl. However, they have relatively little influence on the symmetry of the flow profile. Another well-known flow conditioner is the Vortab. For larger pipes the contraction technique can be an interesting option. Rectangular ducts, which are often used around steam boilers and incinerators, can also be used to condition the flow.

Flow conditioners in calibration applications

The challenge with flow profiles and flow meters has two sides:

1. the sensitivity of the type of instrument (the measurement method)
2. how has the instrument been calibrated

So far only the first factor has been discussed. In calibration applications you want to have a predictable flow profile. Therefore a flow conditioner is often used. The flow meter is calibrated including the flow conditioner.

During calibration it is specified how much straight length is used. In fact, it specifies the flow profile that is being calibrated. Because if the accuracy of the measuring instrument is influenced by the flow profile, a statement about the accuracy should also include the flow profile conditions under which the accuracy was achieved.