Flow Is Everything

If you work with compressed or hydraulic air systems, one area of particular concern revolves around the fact that, well, air is invisible, right? This means that without exact testing measures, you really are unable to determine if there is a leak in this critical system. Such injections of error into your plant's operations could cost you millions of dollars each year. There are a number of devices that may be used to assist in air and gas flow measurement and control.

Meters help determine if a leak is occurring. They can also measure compressor performance. That is a great benefit as your operators and maintenance staff will note measurement warnings of system failures and can then repair or adjust compressors that have decided to take a vacation from proper operations.

With airflow demands, a simpler air flow meter is better in the case of a mechanical meter. No batteries are required for operation; there's no software to code or update; the volume of air that escapes from a precision machine orifice can be determined with a high degree of accuracy. As that indicator never alters, over the years the same result can be expected. Any measurable drop or increase to the predetermined rate is a sign of trouble.

With magnetic flow meters, accuracy is the main attraction. Used mostly for liquid velocity measurement, these are versatile instruments that allow for the volumetric flow to be inferred. Plus, accurate measurements and is simple to alter, even after the unit has been installed in your plant. Faraday's Law of electromagnetic induction is the key to a magnetic flow meter. When a conductive liquid moves through a magnetic filed, electrodes that are located on the wall of the flow tube itself generate a voltage signal. More voltage is generated when the fluid is fast moving, with the voltage indicator being in direct proportion to the flowing liquid's movement. The voltage signal is then processed by a transmitter, which determines the flow of the liquid.

Different Strokes

 There are several types of flow meters one can consider for plant operations. They range from orifices to venturies, nozzles and rotameters, pitot tubes, calorimetrics, turbine, vortex, electromagnetic, Doppler, ultrasonic, thermal and coriolis. Some you may have heard of, others not ¾ the most common principles for metering the flow of fluids include differential pressure, open channel, positive displacement, mass and velocity.

Differential pressure flow meters include Flow Nozzles, Orifice Plates, Venturi Tubes and Variable Area Rotameters. The Bernoullis Equation is used to determine the flow by measuring the pressure drop over obstructions inserted into the flow itself.

Flow nozzles are most often used in industrial applications to measure air and gas flows, is available in many materials, is relatively inexpensive to install and may be used in a broad range of applications.

The calorimetric flow meter is used for fluid flow measurement. The operation is based on two temperature sensors that are in close contact with the fluid. They are thermally insulated, one from the other. The flowrate itself is monitored through one of the two sensors constantly being heated. The cooling effect of the fluid on that sensor results in the analysis. As there is a constant temperature difference between the two sensors, heat energy is drawn from the heated sensor and the temperature difference between the two sensors becomes reduced when the fluid flow increases. The reduction is therefore proportional to the fluid's flow rate.

The Coriolis flow meter uses the Coriolis effect to measure the amount of mass that moves through the element. A U-shaped tube is used that vibrates in an angular harmonic oscillation. The tubes deform and an additional vibration is added to the oscillation. These causes a phase shift on some places of the tubes and sensors then make the measurements. These flow meters can also be used to measure fluid density and, due to their direct mass measurement capabilities, sets them apart from other technologies.

With an ultrasonic Doppler flow meter, if a fluid moves toward a transducer, the returning signal's frequency increases, while the opposite is true when the fluid moves away from the transducer. The frequency difference is then used to calculate the fluid flow speed.

Open channel flow meters measure the height of the liquid as it passes over a flume or weir obstruction placed in the channel.

When using an orifice plate, the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe is measured ¾ the plate that obstructs the flow uses a precisely measured impediment that narrows the pipe and forces the constriction of the flowing fluid.

Most often used are pitot tubes, especially within air applications such as HVAC and ventilation systems. The flow velocity is measured through the conversion of the flow's kinetic energy into potential energy. However, the pitot tube is only suitable for point measuring.

Glass or plastic tubes comprise the rotameter. At the top of the tube is the larger end within which a metering float is free to move about. The float rises in the tube as the upward pressure differential and buoyancy of the fluid overcome gravity's effects. The flow rate is indicated by the height of the float, and calibration and graduation of the tube results in designation of flow units. 

With a velocity flow meter, the speed in one or more points in the flow is calculated.

Within the vortex flow meter, there is an obstruction placed in the downstream flow. Vortex shedding occurs at a critical fluid speed and the alternating low-pressure zones cause that obstruction to travel towards the low-pressure zone where sensors then gauge the strength of the vortices and the flow is then measured.

 Applying Knowledge

 To ensure your plant's use of the appropriate technology, make certain all of the relevant choices are studied and tested before application. Sometimes the least expensive route is proper and, other times, the higher end flow meter is the best solution. A knowledgeable plant manager is one who can ensure the best tools are applied to the demanding projects at hand.

 Written by Hartley G. Lesser for Fox Thermal Instruments

Bio

Founded in 1994, Fox Thermal Instruments provides a broad range of reliable, accurate and full-featured flow meters and flow and level switches to industrial control OEMs and end-user customers. The Fox management group has decades of experience in thermal flow design, application and manufacturing techniques.