Plants can cut their costs by only using the highest accuracy flow instruments in areas where they are required, while using less accurate devices for non-critical functions. In general, the accuracy and cost of a device are proportional.
For critical functions, such as addition of ingredients in a pharmaceutical formulation, an extremely accurate device is important. However, not all process applications require such accuracy. For example, less accurate sensors usually are fine for monitoring the flow in cooling water lines and pneumatic filtration and lubrication systems, as well as for ensuring that pumps don't run dry.
Several types of sensor technologies commonly handle non-critical functions.
MECHANICAL FLOW SENSORS
These produce a signal based on a physical part contacting the media being monitored. Though they generally are less expensive than their electronic counterparts, their mechanical parts may wear out more quickly and require more frequent replacement.
Paddle wheel sensors insert in the flow a paddle that turns at the same rate as fluid moving past it. Generally, these sensors provide a linear analog signal proportional to the flow rate or a frequency output. The devices are quite simple, accurate and relatively inexpensive. However, they have a defined lifespan. When the device fails — essentially breaking apart in the flow — parts of the paddle can travel downstream and end up in an expensive piece of machinery or an end product. Media that contains particle contaminants could slow or stop the paddle from spinning, producing a false reading or damaging the meter altogether. Also, pressure drop across the meter is higher than for a non-insertion instrument.
Ideal applications include services with fluids with relatively stable flows that don't contain contaminants.
Variable area sensors insert a tube with a float inside a vertical run of pipe. The float's position corresponds to a specific flow rate, generally with 2%–6% accuracy. The tube actually is tapered, wider at the top for larger flow rates and narrower at the bottom for lower ones. The devices are simple to install and use, and very affordable. However, they have mounting limitations and must disturb the flow to obtain a measurement. Moreover, these sensors typically aren't automated — and therefore only are useful when someone is present to read the sensor to determine a course of action based on the flow rate. Additionally, a user must know how to properly read the float; the method varies by manufacturer.
These sensors suit services with stable flows and where an operator regularly takes readings.
ELECTRONIC FLOW SENSORS
Such devices use internal electronic components to produce a signal. They disrupt the media being sensed less than mechanical sensors and can provide more accurate measurements but are more expensive.
Vortex meters restrict the flow using an invasive vane and count the frequency of eddy currents produced as a result of the disturbance. The units require turbulent flow for accurate reading; therefore they aren't appropriate for low flow situations, batching or irregular flow applications where measuring the initial and final flows is important in the overall measurement. Vortex meters have a shutoff point — their electronics force a zero reading at low flow rates.
They suit applications with clean fluids and stable flows.
Calorimetric sensors use two temperature-sensing elements insulated from each other. One of the elements is in line with a heating element. The heating element maintains a constant temperature in the area between both temperature sensors when there's no flow. Flow creates a temperature difference between the two elements that is proportional to the flow rate — the lower the thermal conductivity of the fluid, the faster the fluid must flow to be detected.
Calorimetric monitors contain no moving parts. They work well with contaminated media because the inline units don't disrupt the flow and probe-style devices must be inserted only a few millimeters into the flow to get a signal. They are very repeatable and ideal for flow/no-flow and set-point applications, but rarely are considered accurate enough to be classified as a meter. If fluid temperature rises quickly, the calorimetric monitor could read this as a change in flow rather than temperature.
Ideal applications include flow/no-flow monitoring for pumps, ventilation systems and dosing operations.
Magnetic inductive sensors contain a solenoid coil, measuring tube and electrodes. They take advantage of Faraday's law of induction. The solenoid coil produces a magnetic field and the charged particles entering the magnetic field are driven to the outside of the tube wall. This creates a voltage, proportional to flow rate, that is measured by the electrodes.
Many such meters can totalize flow. And, unlike paddle wheel and vortex meters, which don't work reliably with highly viscous fluids, viscosity generally has no effect on magnetic inductive units. In addition, suspended solids and debris in the fluid don't affect them. However, the fluid must be electrically conductive, which rules out use with hydrocarbons.
Magnetic inductive meters are ideal for fluids containing particles and debris and for flow conditions requiring totalizing.
ONLY PAY FOR WHAT YOU NEED
I've highspotted devices most commonly used for non-critical flow detection, but numerous other technologies can handle process applications. The accuracy required, media, and environmental conditions will dictate which sensor best suits a particular application.
Remember, when dealing with non-critical flows, consider less accurate devices with fewer features. They can provide suitable performance at lower cost.
RICHARD TALLANT is a product manager for Turck, Minneapolis, Minn. E-mail him at Richard.Tallant@turck.com.