Universal Vortex Flowmeters

Selection of the best flow meter for your specific application is a critical step, one that will affect the quality of your process for years. Choosing the wrong meter can lead to inaccurate readings, high maintenance costs, and expensive downtime.  The following section is designed to explain the technology of vortex flow meters and the specifications of the Clark Solutions vortex product line.  Our goal is to ensure that the vortex meter you select meets the requirements of your specific application.

HOW VORTEX FLOW METERS WORK
The operation of the Universal vortex flow meter is based on the vortex shedding principle. As fluid moves around a body, vortices (eddies) are formed and move downstream. They form alternately, from one side to the other, causing pressure fluctuations. These are sensed by a piezoelectric crystal in the sensor tube, and are converted to a 4-20 mA or pulse signal. The frequency of the vortices is directly proportional to the flow. This results in extremely accurate and repeatable measurements with no troublesome moving parts.

MATERIAL SELECTION
When choosing the best pipe material for your process, it is necessary to review the fluid to be transported, its concentration, the minimum and maximum operating temperatures, and the pressures to be accommodated. Choosing a flow meter is a similar process, but it is necessary to review a few additional considerations, such as fluid viscosity, suspended particles, density of the fluid and,  most importantly, expected flow range. One advantage of utilizing a Universal vortex flow meter is that there are no gaskets or elastomers in the meter. Therefore, you only need to be concerned with the thermoplastic material used for the body construction. In a thermoplastic piping system, the material chosen for the flow meter should match that of the pipe if at all possible. If you are  planning to install a meter in a metal pipe system, you must consider three operating conditions: temperature, media, and pressure. Chemical resistance data is available on request from Clark.

When choosing a flow meter, it is necessary to verify with the supplier that the unit selected is suited for your specific flow range needs. Most manufacturers state flow range capabilities by publishing maximum allowed flow rates. Then they provide a turndown ratio to determine minimum flow rate. To use the turndown ratio, simply divide the maximum rate by the ratio to determine the minimum rate.

Universal vortex flow meters offered by Clark have a 12:1 turndown ratio (exceptions: V1 1/4" unit and V4 1/2" unit; 8:1). Bulletins on models V1, V2, V3, and V4 indicate their minimum and maximum flows.

LINE FLUIDS
Many factors may affect the capability of a meter to accurately measure the flow of specific fluids. Different solutions have varying effects on meters. For instance, heavy particle suspension will wear down internal parts on some meters or cause sensing inaccuracies for non-obtrusive metering systems. For vortex flow meters, high viscosities tend to dampen the formation of vortices and reduce the effective range. Particles and internal bubbles do not usually affect vortex meters. PVDF models work very well in slurry services. However, slurries containing grit will wear down the bluff body, although it can withstand a 5% reduction before accuracy is affected. Also, long fibers will catch and build up on the bluff, decreasing accuracy. Standard factory calibration is for 32 SSU (1 CST) viscosity liquid. Viscosity above 1 CST will raise the minimum readable flow rate, reducing rangeability. The effect is linear to viscosity. No adjustments are required for specific gravities up to 2.0. Liquids with high specific gravities will adversely affect the permissible amount and duration of overrange flow. The following chart indicates the reduction of range based on viscosity. 

Depending on your application, accuracy and repeatability may be critical. Accuracy is measured as a percentage by which the meter reading could vary from the actual flow. Repeatability is the percentage by which the meter may vary for a specific flow rate from reading to reading. In other words, if you are operating at a flow rate of 50 gpm and the rate increases to 75 gpm and then returns to 50 gpm, repeatability indicates the percentage within which you will now read the 50 gpm flow rate versus the original reading. Accuracy is normally published by the manufacturer in two formats: accuracy of full scale, or accuracy of rate. Accuracy of full scale is a percentage of the maximum flow rate, no matter what the actual flow. Accuracy of rate is a percent of the actual flow rate of the fluid the meter is currently reading.

When choosing a meter for an application where accuracy and repeatability are critical, it is necessary to use a meter that is wet calibrated by the manufacturer, and supplied with documentation of that calibration. A meter that is not individually calibrated cannot be relied on as truly accurate to its specification. Production tolerance differences can and will affect a meter’s accuracy. For this reason, meters must be wet calibrated to ensure specified accuracy and functionality for the user. The small size of the electronics used in Clark Universal vortex flow meters keeps the size of the meters very compact. The electronics are factory calibrated, but can be recalibrated and re-spanned in the field if desired. The electronics package is easily removed and replaced by disconnecting one wire only, making repair a very easy process. An NIST Traceable Calibration Certificate is available on request.

CONTINUED ON PAGE 2