Vortex flow meter is based on VON KARMAN VORTEX operating principle to measure volumetric flow for high rangeability measurement requirement.
Vortex Flow Meter
A fluid passing a fixed solid obstruction ( diametrically across a pipe generates a succession of vortices. The vortices are generated alternately on opposite flanks of the shedder bar. They then separate from the flow element and progress downstream. The separation of the vortices from the flow element is called ‘vortex shedding’. The shedding frequency and hence of the volumetric flow rate.
Various methods of sensing the frequency generated by the vortex formation have been developed, such as differential pressure, thermistor, piezo electrical and variable capacitance sensors. The electronics of the meter can be an integral part of the meter or mounted remotely (for instance for high temperature applications or to allow easy access to the electronics).
Although mass vortex flow meters are being developed, the present generation of vortex flow meters only produce a signal proportional to the volumetric flow rate.
Vortex flow meters are characterised by wide turn downs and can be used on a wide range of process fluids, i.e. liquids, gas and steam. Vortex meters are uni-directional, velocity profile dependent, volume flow meters.
Vortices only occur from a certain fluid velocity onwards, consequently they have an elevated zero the refereed to as cut-off point. The minimum measured flow is limited by any one of the following factors:
- a low Reynolds number, causing the vortex shedding phenomenon to cease;
- minimum fluid velocity;
- too low a signal/noise ratio (sensors cannot make a distinction between signal frequency and background noise).
To ensure proper operation of the meter at minimum flow conditions (e.g. during start-up), it shall be sized for those minimum conditions, which usually result in a meter that is one or two sizes smaller than the line size (e.g. a DN 80 or DN 50 vortex meter in a DN 100 pipe). If two sizes of vortex flow meters are both able to cover the minimum and maximum flow rate, the smaller size meter should be selected.
1 Many operational problems associated with vortex meters are related to oversizing. Oversizing increases the cut-off point, which might make the meter unsuitable for proper control.
The maximum flow to be measured should not be less than 35% of the maximum measurable flow rate (the capacity) of the chosen vortex meter and the settings for low flow switch functions should amply exceed the cut-off point.
2 Selection of the lowest possible cut-off point is particularly critical for meters used in closed control loops, as vortex shedding introduces instability (hunting) at the cut-off point. If operation in
the cut-off region cannot be prevented,the use of flow rate estimator on the basis of control valve position could be considered. Such an estimator requires Principal’s approval.
3 The cut-off phenomenon and the fact that the meter does not make a distinction between forward and backflow makes the vortex meter less suitable for backflow detection.
Vortex meters will only function properly under truly single-phase fluid conditions. In liquid applications, the pressure profile across the vortex meter shall not result in cavitation under any operating condition. Cavitation will cause signal drop-out and might damage the meter and downstream piping.
Vortex meters are susceptible to oscillating flows and mechanical vibration. If the frequencies enter the vortex frequency range, major systematic or random errors are introduced.
The swirl flow meter is a special type of vortex flow meter. An additional swirl is added to the incoming process fluid via a set of helical blades installed in the inlet of the meter body. The advantage of this type of vortex flow meter is its relative insensitivity to the flow pattern of the incoming fluid, which means the straight length requirements are less stringent than those for standard vortex flow meters. Another advantage is its lower allowable Reynolds Number.
Selection of vortex flow meters
The following shall be considered in vortex flow meter selection:
Turndown requirements, taking into account all normal and abnormal operating conditions.
The Reynolds number shall be at least 20,000, but preferably above 40,000 under any normal or abnormal process condition. Furthermore, the minimum fluid velocity, specified by the Manufacturer, shall be met under any normal or abnormal process condition that requires a reliable measurement. The minimum flows to be measured shall amply exceed the minimum measurable flow of the selected meter, specified by the Manufacturer, in order to achieve reliable measurement. In selecting a vortex flow meter, the meter turn-down shall be balanced against the permanent pressure drop.
Vortex meters in liquid service should be selected so that cavitation does not occur during any normal or abnormal process condition.
Vortex meters should not be located downstream of positive displacement pumps/compressors without suction and discharge dampers.
Vortex flow meters should not be used in wet gas or wet steam applications or in any other two-phase application (e.g. liquids with gas bubbles or foam, flashing liquids, half-full lines).
Vortex flow meters should not be considered for very viscous, waxy or erosive services.
Vortex flow meters shall be provided with process flanges.
Non-wetted sensors are preferred. For critical applications, the sensors shall be replaceable during operation.
Note : Should take note of the largest available vortex meter sizing to match large piping line sizes e.g. greater than 12” Installation requirements
For general installation requirements,.
- In general, the orientation (horizontal or vertical) of the vortex flow meter is not critical. If mounted in horizontal pipes, it is recommended to mount vortex meters with the shedder bars in the horizontal plane in order to reduce the build-up from debris and other hard deposits.
For gas measurements in horizontal lines, the meter should preferably not be located at the lowest point in order to avoid condensate impacting on the measuring element.
For vortex meters in a vertical line, the flow shall be upwards.
- In liquid services, the horizontal or vertical flow line must remain full under all operating conditions.
NOTES: 1. Where horizontal positioning of a shedder bar in horizontal lines causes interference with adjacent piping, the shedder bar may be mounted at a 45 degrees angle.
2. For specific meter designs, the Manufacturer’s above recommendations.
3. If pressure measurement is required for density compensation, the reference pressure at which the Manufacturer has determined the meter is relevant. It is often assumed that this is the absolute pressure at the shedder bar.
NOTE: The pressure at the shedder bar can be calculated from the measured upstream pressure and Manufacturer’s data on permanent pressure loss and vortex meter.
Straight length requirements shall be as specified by the Manufacturer.
Upstream pipe or flange transitions shall be smooth and flush with the pipe wall, i.e. free from welding roughness and burrs. Otherwise, vortices may be created which may adversely affect the performance of the flow meter.
For services where condensable gases during abnormal operation will convert into liquid, draining facilities have to be provided to prevent damage of the shedder bar by liquid slugs at plant (re)start.
Install the vortex flow meter only after flushing or air blowing activities have been completed.
A welded vortex flow meter body may be considered depending on the application and make. The use of welded vortex flow meters is subject to the approval of the Principal.
For critical services, online removable sensor is preferred to enable online maintenance.