Velocity type flowmeters generally tend to follow a linear relationship with respect to the volume flow rate. Unlike differential pressure type flowmeters, there is no square-root relationship in these instruments. Hence, their rangeability is much better as compared to other flowmeters. Furthermore, they prove to be less sensitive to changes in viscosity when used at Reynolds numbers (Re) more than 10,000. Nearly all velocity-type flowmeter housings are outfitted with flanges or fittings. This arrangement enables them to be joined directly into pipelines. Major types of Velocity flowmeters include turbine, vortex shedding, electromagnetic, and sonic designs.
A turbine flowmeter unit is constructed of a multiple-bladed rotor installed with a pipe in a perpendicular direction to the fluid flow. As the liquid flows through the blades, the rotor rotates. The rotational speed of the rotor is correlated to the flow rate of the liquid. The speed of the rotor can be sensed by various mechanisms such as magnetic pick-up, photoelectric cell, or gears. A tachometer can also be attached to the turbine for measuring its rotational speed which in turn helps in determining the liquid flow rate. After proper installation, turbine meters offer good accuracy, mainly with low-viscosity liquids. Hence, turbine meters are widely employed in applications where accurate measurements are required. When a turbine flowmeter is calibrated and operated for a single fluid, constant conditions, its turndown ratio can go beyond 100:1. Moreover, accuracy of a turbine flowmeter can be as good as +/-0.1%.
The major problem associated with the usage of turbine meters is bearing wear. To prevent this problem, a “bearingless” turbine meter design has been introduced recently.
Various turbine flowmeter designs have been manufactured. However, they all operate on the same basic principle which says “If a fluid moves through a pipe and acts on the vanes of a turbine, the turbine will start to spin and rotate. The rate of spin is measured to calculate the flow”.1
A Turbine meter is shown in the figure below:
Vortex flowmeters are also referred to as vortex shedding flowmeters or oscillatory flowmeters. These types of flowmeters are used to measure the vibrations of the downstream vortexes caused by an obstruction in the flowing stream. Each obstruction has a vital liquid flow speed at which vortex shedding takes place. This vortex shedding occurs at the instant when alternating low pressure zones gets created in the downstream. These sporadic pressure zones enable the barrier to move towards the low pressure zone. By means of sensors gauging the vortices the flow rate can be easily detected. Hence, major components of a vortex flowmeter include
- a bluff body strut-installed across the flowmeter bore
- a sensor to indicate the presence of the vortex and to produce an electrical impulse
- a signal amplification and conditioning transmitter which gives an output proportional to the flow rate
A Vortex flowmeter is shown in the figure below:
- Vortex meters are equally appropriate for flow rate or flow totalization measurements.
- Use of vortex meters is usually not preferred for slurries or high viscosity liquids. Also their usage is not suggested for batching or other intermittent flow applications.
- Since there is rise in viscosity with the drop in Reynolds number, vortex flowmeter rangeability degrades as and when the viscosity increases. The maximum viscosity limit, as a function of permissible accuracy and rangeability, is found to be somewhere between 8 and 30 centipoises.
- In case of gas and steam services, one can gain rangeability better than 20:1 whereas in low-viscosity liquid applications, rangeability offered by a properly sized vortex meter is over 10:1.
- With Reynolds numbers more than 30,000, inaccuracy of majority of vortex flowmeters is 0.5-1% of rate.
- Vortex metering error increases with the decreasing Reynolds number.
- Vortex flowmeters are available in typical flange sizes ranging from 1/2 in. to 12 in.
- Wafer body vortex flowmeters i.e. flangeless flowmeters are inexpensive as compared to flanged meters. However, flanged meters are considered ideal for applications where the process fluid is perilous or is at a high temperature.
- Nowadays, nearly all vortex meters make use of piezoelectric or capacitance-type sensors to determine the pressure oscillation around the bluff body.
Vortex Shedding Frequency
The vortex shedding frequency is the vibrating frequency of the vortex shedding. It is the frequency which is directly proportional to the velocity of the liquid flowing in the pipe, and hence to volumetric flow rate. It is independent of fluid properties such as density, viscosity, conductivity, etc., except that the flow must be turbulent for vortex shedding to occur. The basic relationship between vortex shedding frequency and fluid velocity is given below:
St = f(d/V)
In the above equation, St represents the Strouhal number which is a typical dimensionless calibration factor used to differentiate a variety of bluff bodies.
Other parameters are F = vortex shedding frequency
d = width of the bluff body
V = average fluid velocity.
The Strouhal number is generally defined as the ratio of the interval between vortex shedding (l) and width of the bluff body (d). If the Strouhal number of two different bluff bodies is equal, then they will work in the same manner.