ALL ENCOMPASSING FLOW METERS - Vortex Flow Meter
Vortex Flow Meter works on the principle of Karman Vortex Street. Any medium passing through the pipeline flows around the bluff body and sheds a series of alternating vortices on each side of the body. This phenomenon is known as Vortex Shedding. These vortices shed downstream of the bluff body and dissipate as they flow further. This pattern of vortices is called a Karman vortex street.
A vortex flow meter primarily consists of a bluff body, a sensor assembly and a transmitter. A bluff body or a shedder is nothing but a non-streamlined object or a barrier placed perpendicular to the axis of the pipeline, around which the medium flows. The frequency of the vortices, i.e. the number of vortices shed per second, is directly proportional to the velocity of the medium. This vortex shedding frequency is used to calculate the mass flow. The sensor assembly records the pressure and velocity oscillations generated on each side of the bluff body by the vortices and generates a digital linear output signal. The vortex shedding frequency is calculated using the following formula:
d
Where:
f = Frequency of vortex shedding
St = Strouhal number
V = Flow velocity
d = Width of the bluff body
Strouhal Number, St
The Strouhal number in the above formula is also known as “reduced frequency”. It is a dimensionless parameter which is a measure of the vortex shedding frequency and the velocity of the flow medium. It is calculated using the formula:
U
Where:
f = Frequency of vortex shedding
d = Width of the bluff body
U = Velocity of the flow medium
The Strouhal number is a function of the Reynold’s number. Reynold’s number is also a dimensionless parameter which is used to determine how the flow pattern of different fluids will change. The Strouhal number should remain constant when the Reynold’s number ranges from 2×104 to 7×106.
Calculation of volume flow rate
When the vortex shedding frequency is known, the volumetric flow rate can be calculated using the formula:
q = f*d/k
Where:
q = Volumetric flow rate
f = Vortex shedding frequency
k = k factor, which is a ratio of the pulses transmitted to the unit volume
Depending on the mounting style, the types of vortex flow meters include:
1. Insertion flow meter
Each insertion type of vortex flow meter consists of a built-in bluff body and three sensors, namely one sensor to record the vortex shedding frequency, an RTD temperature sensor, and a solid-state pressure sensor. These components are located in close proximity to each other in a short tube and together they form an assembly called an insertion sensor head. The sensor probe is inserted carefully into a hole made into the pipeline using flanged or male NPT process connections.
This flow meter is also provided an LCD display and keypad to program and monitor the system settings. The insertion flow meter used in pipes which are 2 inches or larger. Since it is a retractable meter, there is no need to shut down the system to take the required measurements.
2. Inline flow meter
An inline flow meter is similar to the insertion flow meter in terms of its components. It also has a bluff body and three sensors to register the vortex shedding frequency, temperature, and pressure. It differs from an insertion flow meter in that it replaces a part of the pipeline and not inserted through it.
Many flow meters measure the velocity, temperature, and pressure of a medium at different locations of a system without offering adequate means to compensate for the difference in process conditions. An inline flow meter, however, measures these variables at a single point.
An inline flow meter in turn has two main types depending on the end connection:
