1. Working Principle of Vortex Flow Meters:
The principle of the vortex flow meter is to place a bluff body in the flow meter pipe. When fluid flows past the bluff body, due to the flow separation at the bluff body’s surface, two alternating vortex streets are generated downstream, known as Karman vortex streets. The vortexes on both sides rotate in opposite directions. Theoretically, Karman proved that when h/L = 0.281 (where h is the width between the two vortex streets, and L is the distance between two adjacent vortexes), the vortex street is stable.
The vortex flow meter is a fluid oscillation-type flow meter based on the Karman vortex principle. When a non-streamlined, symmetric object (called a vortex shedding body in the vortex flow sensor) is placed in a flowing fluid, two regular vortex streets, known as Karman vortex streets, will be generated downstream on both sides. The vortex frequency is proportional to the flow velocity:
F=St⋅u/d
where:
- F is the vortex frequency,
- d is the width of the vortex shedding body,
- u is the flow velocity,
- St is the Strouhal number.
The value of St is related to the width of the vortex shedding body (d) and the Reynolds number (Re). When Re < 2 × 10^4, St is variable. When Re is in the range of 2 × 10^4 to 7 × 10^6, St remains constant, which defines the primary measuring range of the flow meter. The Reynolds number (Re) is a dimensionless number representing the ratio of inertial forces to viscous forces in the fluid flow.
The formula shows that when d and St are constant, the vortex frequency F is proportional to the average flow velocity u. By measuring the vortex frequency, the fluid flow rate can be obtained, which is the fundamental principle behind the vortex flow meter.
2. Features of Vortex Flow Meters:
- Vortex flow meters can be used in any situation where vortex streets can be formed, not only in closed pipelines but also in open channels.
- The application range is broad; it can measure gases, liquids, and steam.
- Vortex flow meters have no moving mechanical parts, requiring minimal maintenance. The instrument constant is stable. Compared to orifice flow meters, vortex flow meters have a broader measuring range, lower pressure loss, higher accuracy, and do not require pressure guiding pipes, making installation and maintenance easier.
- However, many environmental parameters related to vortex flow meters are often overlooked in practice, which may affect their performance.
- Vortex flow meters have a large measuring range, typically 10:1.
- Mechanical vibration should be avoided during use, especially lateral vibration of the pipeline.
- The medium temperature can significantly affect the performance of vortex flow meters.
3. Common Faults in Vortex Flow Meters:
Long-term inaccurate readings: This could be due to the improper diameter selection of the vortex sensor or changes in the process conditions, leading to a larger-than-required specification being selected. Reducing the selected diameter as much as possible can improve measurement accuracy.
No indication at all: This could be due to insufficient length of the straight pipe section before the sensor, affecting measurement accuracy.
Large fluctuations in readings, making it difficult to obtain measurements: This could be caused by incorrect parameter settings, resulting in errors in the display of the secondary instrument.
Indications not returning to zero: This may be due to improper calibration or installation issues.
No indication at low flow rates: This can occur when the selected diameter of the vortex meter is too large for the actual flow rate, making it difficult to measure low flows accurately.
Inaccurate indications at low flow rates, but acceptable at high flow rates: This could be due to the same issue of selecting a meter with too large of a diameter for the given process conditions.
Slow response to flow changes: This could be caused by issues with the display head or improper mounting.
Inconsistent K-factor: If the K-factor (a constant related to the sensor) differs across various sources, it may cause measurement inaccuracies.
Other Causes:
- Installation issues: Such as insufficient straight pipe sections or incorrect wiring.
- Secondary instrument failures: Problems like broken circuit boards or faulty display units.
- Environmental issues: High humidity or poor conditions where sensors are installed can also lead to issues, which can be resolved by using separate type flow meters.
By addressing these potential issues through adjustments and maintenance, the proper functioning of vortex flow meters can be ensured.