The accuracy of most flow meters depends on the flow profile of the substance in the piping system. Upstream disturbances have been observed to have the greatest impact on the flow profile of the flow stream which in turn affects flow meter accuracy. The desired flow profile can be achieved in a typical installation without flow conditioning using 25 to 40 pipe diameters of straight run piping before the flow element and about 4 or 5 pipe diameters downstream of the element. These requirements vary quite considerably according to the upstream (and downstream) disturbances and the beta ratio. In most practical application of flow measurement, it is not always possible to provide sufficient straight run to secure a “fully developed flow profile. What practical solution then exists if sufficient straight run pipe is not achievable? The engineering solution is always some form of flow conditioning using devices called flow conditioners.
Recommended Minimum Straight Run Pipe Lengths without Flow Conditioner
Where sufficient straight run pipe lengths can be provided, a flow conditioner is
not needed. In this scenario, the commonly used standards for flow measurement in differential pressure devices - ASME MFC-3M, AGA 3 (Report Number 3), ISO 5167 (1991) – specifies minimum pipe lengths upstream and downstream of the flow meter to be used in custody transfer flow measurement systems. These recommendations are tabulated below:
Recommended Minimum Straight Run Pipe Lengths without Flow Conditioner
Where sufficient straight run pipe lengths can be provided, a flow conditioner is
not needed. In this scenario, the commonly used standards for flow measurement in differential pressure devices - ASME MFC-3M, AGA 3 (Report Number 3), ISO 5167 (1991) – specifies minimum pipe lengths upstream and downstream of the flow meter to be used in custody transfer flow measurement systems. These recommendations are tabulated below:
| Standards | Recommendations | |
|---|---|---|
| Upstream Piping | Downstream Piping | |
| ASME MFC - 3M | 54D | 5D |
| AGA 3 | 95D | 4.2D |
| ISO 5167 | 60D | 7D |
What is Flow Conditioning?
The objective of flow conditioning is to create a known and repeatable velocity profile of the substance being measured in order to improve the accuracy of the flow metering process. This is especially critical for compressible fluids like gas when flow is being measured by differential pressure devices that have a restriction in their flow path.
Anytime there is a change in the size or direction of a pipe in which a fluid flows, or a disturbance in the flow path, the flow profile is distorted and the accuracy of the flow measurement setup is impacted. To create the desired flow profile, the flow stream is conditioned with a flow conditioner.
What is a Flow Conditioner?
A flow conditioner is a device which consists of straightening vanes or pipes running parallel to one another that helps to improve the flow profile of a given fluid stream after passing through an upstream disturbance or obstruction. Typically, the effect of most flow disturbances can be overcome through the use of sufficient straight pipe length, upstream of the meter, however this is not always practical. In such impractical scenarios, flow conditioners are used to shorten the upstream pipe runs requirement. Flow conditioners or straighteners are effective in eliminating swirl and helping to restore grossly distorted flow profiles to guarantee accurate flow measurement. They are usually installed upstream of the flow meter.
Examples of typical flow conditioners in use are pictured below:
Folded vane and fin type straightening vanes are normally used on gas flow measurement installations whilst the tubular type is normally used on steam or liquid flow measurement installations. It is usually recommended that vanes be installed only in extreme cases after all other options have been carefully considered
