Vortex Flowmeters

Multivariable vortex flowmeter combines flow, temperature
and pressure measurement into a single compact instrument
Image courtesy Krohne

Vortex shedding flowmeters provide process operators with consistent fluid flow rate measurements across a wide range of applications. These flowmeters measure the volumetric flow rate of steam, gas, and low viscosity liquids, boasting both versatility and dependability when used in conjunction with process systems.

Vortex shedding refers to the phenomenon wherein flowing gas or liquid form vortices around a solid object placed in the flow path. The measurement technology returns an indication of the process fluid velocity, which can then be used to provide volumetric or mass flow data. Vortex technology is well suited for many applications involving cryogenic liquids, hydrocarbons, air, and industrial gases. Vortex flow measurement requires contact between portions of the measurement instrument and the process media, so these flowmeters are commonly fashioned from a range of corrosion resistant materials.

The process of measuring the flow involves both the flowmeter and the ability for other instrumentation to measure the vortices themselves in order to calculate velocity. Ultrasonic sensors have become popular tools for measuring vortices. Applications involving flow measurement of high viscosity fluids are not suited for vortex technology because extremely viscous fluids do not behave in the same manner as lower viscosity fluids when their flow path is obstructed. Splitting higher viscosity fluids into concordant vertices is extremely difficult due to the internal friction present in highly viscous liquids.

Additionally, in order to split these process liquids, the piping through which the process material flows must be straight, and any disturbance or vibration in the pipe may impact the measurement. A vortex flowmeter will be in a fixed installation. This stationary element, operating without electrodes, can be advantageous for flow measurement in chemical applications utilizing low viscosity fluids.

The vortex shedding flowmeter is widely used for the measurement of steam flow. The high pressure and elevated temperature of steam, along with the variation that exists in most steam systems, have little negative impact on the operation of a vortex flowmeter. Vortex shedding flowmeters are often volumetrically based in terms of measurement, but their output can be combined with other fluid information to calculate mass flow. A product variant commonly available will combine the vortex flow measurement with temperature and pressure compensation, delivering three process measurements from a single installed device.

Share your process and flow measurement challenges with instrumentation specialists, leveraging your own knowledge and experience with their product application expertise.

Measuring Differential Flow in Industrial Process Control

Measuring differential flow

The differential flow meter is the most common device for measuring fluid flow through pipes. Flow rates and pressure differential of fluids, such as gases vapors and liquids, are explored using the orifice plate flow meter in the video below.

The differential flow meter, whether Venturi tube, flow nozzle, or orifice plate style, is an in line instrument that is installed between two pipe flanges.

The orifice plate flow meter is comprised the circular metal disc with a specific hole diameter that reduces the fluid flow in the pipe. Pressure taps are added on each side at the orifice plate to measure the pressure differential.

According to the Laws of Conservation of Energy, the fluid entering the pipe must equal the mass leaving the pipe during the same period of time. The velocity of the fluid leaving the orifice is greater than the velocity of the fluid entering the orifice. Applying Bernoulli's Principle, the increased fluid velocity results in a decrease in pressure.

As the fluid flow rate increases through the pipe, back pressure on the incoming side increases due to the restriction of flow created by the orifice plate.

The pressure of the fluid at the downstream side at the orifice plate is less than the incoming side due to the accelerated flow.

With a known differential pressure and velocity of the fluid, the volume metric flow rate can be determined. The flow rate “Q”, of a fluid through an orifice plate increases in proportion to the square root the pressure difference on each side multiplied by the K factor. For example if the differential pressure increases by 14 PSI with the K factor of one, the flow rate is increased by 3.74.

Understanding Hot Tapping: Insertion Flowmeter Example

Hot tapping insertion probe.

The ability to remove an insertion flowmeter probe is important for service and calibration. In many situations, it is not desirable to shut down the process and drain the pipe. In these cases, a method for removal known as "hot tapping" is preferred. Hot tapping (also known as pressure tapping) is a technique where a connection is made to an existing pipe or pressure vessel without disturbing flow or having to empty the pipe or vessel. This allows a pipe or tank to be in operation while maintenance or modifications are being done.

This video demonstrates the steps (and precautions) to remove a Seametrics flowmeter insertion probe from a live process (hot tap).  Insertion type flowmeters use a ball valve as the shut-off device and as isolation from the process media in the pipe. The video outlines the sequence of loosening the lock nut, raising the probe, then shutting off the valve before removal.

Additionally, here is a good document with more specific information on the insertion flow meter / hot tapping process:

Flow Meter Hot Tap Instructions from Instrument Specialties, Inc.

For more information on any flowmeter installation, contact:

Instrument Specialties Inc.
3885 St. Johns Parkway
Sanford, FL 32771
phone 407.324.7800
fax 407.324.1104

www.isisales.com