Flow measuring instruments and their technologies

There are several different flow measuring instruments available, each with its own technology and application.

Differential pressure flowmeters correlate flow to the measured pressure differential across the in-line sensing elements listed here:

• Flow nozzle
• Orifice plates – concentric orifice, eccentric orifice, conditioning orifice
• Pitot tube
• Averaging pitot tube, annubar
• Variable area flowmeter, rotameter
• V-cone flow element
• Venturi tube
• Wedge flow element (slurries, heavy liquids)

Differential pressure transmitters have high and low pressure ports connected to a valve manifold so that impulse tubing is connected to any of the flow sensing elements shown here. Valve manifolds are available in three-valve and five-valve configurations and are suitable for most applications, providing an ideal low-cost solution for connecting a transmitter to its impulse tubing. The three-valve configuration provides span and zero calibration, with additional ports available to connect test instruments. The five-valve configuration provides the additional ports for draining or purging of the impulse tubing. The impulse tubing size is usually ½ inch. Sometimes impulse tubing is heat traced if the fluid freezes inside them.

In situations where the process fluid needs to be isolated from the sensor due to extreme temperatures, corrosiveness or sanitary reasons, differential pressure transmitters with valve manifolds and impulse tubing are replaced with differential pressure transmitters with diaphragm seals and capillary tubing. The diaphragm seals connect directly to the process. The transmitter, diaphragm seals and capillary tubing contain a fill fluid, the type of which is dependent on the application. There are several fill fluids to choose from, listed in the instrument manufacturers’ specifications. Food-grade fill fluids are used in food and pharmaceutical industries; other fill fluids are used in chemical industries.

Mass flowmeters correlate the volumetric flow and density of the fluid through the sensing elements listed below. Gases are most often measured by thermal mass flowmeters, whereas liquids are measured by Coriolis mass flowmeters but measure gases also. Solids are measured by impact flowmeters of several types.

• Coriolis mass flowmeter correlates mass flow to the fluid’s inertia sensed when induced by vibration within the meter tubes. It is excellent for custody transfer measurements of fluids.
• Thermal mass flowmeter correlates mass flow to the quantity of energy transferred from the heated probe to the flowing fluid.
• Bulk solids measuring types
  • Weigh belt/feeder uses a combination of the continuous weighing system load cells and speed sensing of the belt or feeder to determine a mass flow rate.
  • Coriolis correlates mass flow rate to torque measured by a load cell on a motor-driven paddle wheel, rotating at a constant rate while solid particles are flowing onto an area slightly offset from the hub.
  • Impact plate and deflection chute correlates mass flow of falling bulk solids to the reactive force imparted to an impact plate or deflection chute, as measured by installed load cells.
  • Radiometric nuclear source correlates mass flow to the quantity of radiation absorbed by a receiver mounted opposite of a nuclear source.

Open channel flowmeters correlate flows that are open to the atmosphere to the measured levels and measured velocities of the fluid through flumes or weirs, by restricting the flow.

• Parshall flume makes the channel narrower.
• Weir forces the fluid over a plate in a channel or box.

Velocity flowmeters correlate flow to the measured flow velocities and other properties of the flowmeters sensing elements listed below.

• Magnetic flowmeter correlates flow to the voltage measured when induced by fluid passing through a generated magnetic field.
• Vortex flowmeter correlates flow to the frequency of vortices created within the flow element. It is excellent for steam, clean liquids and clean vapors measurements.
• Swirl flowmeter is very similar to the vortex shedding flow meter, but it has a swirl element at the inlet and a de-swirl element at the outlet.
• Turbine flowmeter correlates flow to the speed of the internal rotating turbine or gears, respectively. It is excellent for custody transfer measurements of fluids.
• Both the Doppler ultrasonic flowmeter and the transit time ultrasonic flowmeter correlate flow to ultrasound either time of travel or reflected from the transmitter, respectively, but ultrasound must penetrate or reflect to work. There are two types: in-line, and clamp-on.

Other Types of Flowmeters

• Positive displacement flowmeters correlate flow to the measuring of specific volumes and the speed of internal rotating turbine or gears, respectively. It is excellent for custody transfer measurements of fluids.
• Target flowmeter correlates the deflection of a plate mechanism in the line by the fluid flow, which in turn generates an analog signal.

Flow switches correlate flows above or below a setpoint for the switch types listed below. Switches should be selected so that the normal process variable actuation setpoint lies between 40 and 60 percent of the maximum full scale range of the switch. These switches have contact closures or openings based on your failsafe operation. High flow switches use the normally closed contact, and it opens on high flow above the setpoint. For flows below that setpoint, the contact closes. Low flow switches use the normally open contact, and it opens on low flow below the setpoint. For flows above that setpoint, the contact closes. Confusing? Maybe not. Think of it as if you lose power or a wire breaks you want the circuit to open as if it is in the failsafe state. High flow opens and low flow opens.

Types of flow switches include:

• Paddle
• Vane
• Rotor
• Thermal
• Differential pressure

Mechanical flow indicators correlate flow to the visual presence of the fluid through the in-line sensing elements listed below.

• Rotameters
• Vane
• Piston
• Turbine
• Sight flow indicators/flow glasses

Flow measuring instruments are widely used throughout industry. Understanding the different types of flow measurement instrumentation is critical in industrial applications. Next time, we will look at the steps involved in specifying flow measuring instruments.

Matrix Technologies is one of the largest independent process design, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our manufacturing operations management capabilities and manufacturing process control solutions, contact Bob Kurczewski, Technical Consultant, Process & Electrical Design Department.

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