Selection criteria for flow instrument

Low-Pressure Drop:

A device that has a low-pressure drop presents less restriction to flow and also has less friction. Friction generates heat, which is to be avoided. Erosion (due to cavitation and flashing) is more likely in high-pressure drop applications.

Less Unrecoverable Pressure Drop:

If there are applications that require sufficient pressure downstream of the measuring and control devices, then the pressure drops across these devices need to be taken into account to determine a suitable head pressure. If the pressure drops are significant, then it may require higher pressures. Equipment of higher pressure ratings (and higher cost) are then required.

High-Velocity Applications:

It is possible in high-velocity applications to increase the diameter of the section which gives the same quantity of flow but at a reduced velocity. In these applications, because of the expanding and reducing sections, suitable straight pipe runs need to be arranged for suitable laminar flow.

Operate in Higher Turbulence

Devices that can operate with a higher level of turbulence are typically suited to applications where there are limited sections of straight length pipe.

Fluids Containing Suspended Solids:

These devices are not prone to mechanical damage due to the solids in suspension, and can also account for the density variations.

Require Less Straight Pipe Up and Downstream:

This is generally a requirement applied to equipment that can accommodate a higher level of turbulence. However, the device may contain straightening vanes which assist in providing laminar flow.

Price does not Increase Dramatically with Size:

This consideration applies when selecting suitable equipment, and selecting a larger instrument sized for a higher range of operation.

Good Rangeability:

In cases where the process has considerable variations (in flow for example), and accuracy is important across the entire range of operation, the selecting of equipment with good rangeability is vital.

Suitable for Very Low Flow Rates:

Very low flow rates provide very little energy (or force) and as such can be a problem with many flow devices. Detection of low flow rates requires particular consideration.

Unaffected by Viscosity

The viscosity generally changes with temperature, and even though the equipment may be rated for the range of temperature, problems may occur with the fluidity of the process material.

No Obstructions

This primarily means no pressure loss. It is also a useful criterion when avoiding equipment that requires maintenance due to wear, or when using abrasive process fluids.

Installed on Existing Installations

This can reduce installation costs, but more importantly can avoid the requirement of having the plant shutdown for the purpose or duration of the installation.

Suitable for Large Diameter Pipes

Various technologies do have limitations on pipe diameter, or the cost increases rapidly as the diameter increases.