- Published on Monday, 30 September 2013 09:06
- Written by David W. Spitzer
There are many types of flow devices used to measure liquids and gases in closed pipes and open channels. Knowing which flowmeter is appropriate for which application requires knowing the difference between the many choices. This column seeks to give readers a basic understanding of the different types of flowmeters and what to consider when selecting them.
Positive displacement meters are used for both liquids and gases, often with high accuracy. They come in a variety of types, such as oval gear, reciprocating piston, nutating disc and rotary piston. They repeatedly entrap fluid, emulating a bucket—one bucket in, one bucket out. This type is the only flowmeter technology that measures the actual volume of the fluid. Positive displacement flowmeters can accurately measure highly viscous materials up to hundreds of thousands of centipoise.
Differential pressure flow measurement is still one of the widest used technologies in the process industries. The method is relatively inexpensive and utilizes differential pressure transmitters that the plant often uses for other process measurements (such as pressure and level). These flowmeters operate by sensing the differential pressure across an orifice (or other restriction) where the flow rate is proportional to the square root of the differential pressure produced.
There are a wide variety of types and styles of mechanical flowmeters with moving parts that measure velocity such as turbine, paddlewheel, impeller and propeller flowmeters. The speed of their rotor is proportional to the velocity of the flowing fluid. These flowmeters can be quite accurate (better than 1% of reading) or only approximate (such as 5% of span). They can be spool piece style, barstock style or insertion devices that can range from inexpensive to medium priced. Models with high precision and special materials can be relatively expensive.
Full-bore spool piece magnetic flowmeters (Figure 1) can accurately measure the average velocity of electrically conductive fluids over a wide range of flows and flowmeter sizes. Other flowmeters that rival the installed accuracy of a magnetic flowmeter include turbine flowmeters, correlation-type ultrasonic flowmeters with calibrated spool pieces and Coriolis mass flowmeters.
Ultrasonic flowmeters operate by either measuring the transit time between two sensors or the Doppler shift caused by the movement (velocity) of the solids-bearing liquid. Transit time ultrasonic flowmeters may have clamp-on, wetted or inline sensors whereas Doppler ultrasonic flowmeters usually are supplied with clamp-on sensors. Ultrasonic flow meters are perhaps the most misapplied flow meters around. When they were first introduced, they were supposed to replace all other flow meters. But as with many tools, they settled into niches over time.
Thermal dispersion flowmeters are used for flow measurements in low-flow applications (Figure 2). This technology is commonly applied to the mass flow measurement of gases by measuring the thermal dispersion caused by a gas flowing past a heated temperature sensor. Thermal insertion flowmeters can be used to measure flows in large pipes.
Fluidic meters operate by forcing the fluid to continuously oscillate in a repeatable and detectable fashion. Fluidic flowmeters have no moving parts and use the fluid itself to generate oscillations. The most common types of fluidic flowmeters include vortex shedding, swirl and Coanda effect flowmeters. A traditional vortex shedding flowmeter uses a bluff body (shedder bar) to produce repeatable oscillations (Figure 3). Swirl flowmeters use a stationary “rotor-like” device to twist the flow in conjunction with a restriction to contain the oscillations and make them repeat. Coanda effect flowmeters use hydraulic feedback to force the fluid to oscillate in proportion to its velocity.
Coriolis mass flowmeters (Figure 4) use the properties of mass to measure the mass flow of liquids and gases. These meters can also measure density and allow determination of the volumetric flow. Most Coriolis mass flowmeters have an internal temperature measurement that approximates the temperature of the flowing fluid.
Open channel flowmeters operate by measuring the head height (level) within or upstream of an open channel flow restriction. The level can be measured with various types of sensors, such as capacitance, float, ultrasonic and radar level sensors. In open channel installations, the flowmeter primary is the flow restriction (such as a flume or weir) and the secondary instrument is the level sensor used to automate the flowmeter reading. It should be noted that even a manually-read yardstick could be used to measure the level.
SELECTING AN APPROPRIATE FLOWMETER
When faced with so many choices, what goes into finding an appropriate flowmeter for the application?
The first cuts should be clear—liquid, solid, gases. Each of these categories eliminates many types of flowmeters. Then further cuts can be made. For example, if the flow is liquid, the second cut might be whether the pipe is full or not.
Beyond these distinctions, there are other considerations, such as price. When it comes to price, choices often are based on whether the price tag is under or over about $1,000, an amount that often divides “inexpensive” and “expensive” flow elements.
Additional considerations might justify a second look at the price tag. For example, it is hard to find a 4-inch (100 millimeter) flowmeter with 1% of flow rate accuracy for under $1,000. If better accuracy is a requirement, the price tag may be well over $1,000.
One can then consider the constraints of the application. Does the application require high accuracy? If not, relaxed accuracy requirements might allow the use of a less expensive flowmeter. Good accuracy is often desirable when feeding high value fluids and batching, or when product quality is dependent upon the correct fluid flow. However, the best flowmeter accuracy is usually reserved for custody transfer applications where the flowmeter determines the amount billed for material that is bought or sold.
High temperature, high pressure and a wide variety of flow needs can also justify looking at the price tag. For example, if 10:1 turndown is required, several relatively inexpensive meters may perform well. However, the available choices diminish rapidly if 100:1 or 300:1 turndown is required.
Chemical compatibility and abrasion resistance are other considerations that can make the flowmeter choice more expensive. For example, there are a few flowmeters available larger than 1-inch in size that are highly corrosion resistant—and most are expensive.
Finally, be sure to consider the useful life expectancy of the flowmeter so the flowmeter is appropriate to avoid over-engineering or under-engineering the application.
David W. Spitzer is a principal in Spitzer and Boyes, LLC, which offers engineering, seminars, strategic, marketing and distribution consulting and expert witness services for manufacturing and automation companies. Spitzer has written multiple books and more than 250 articles about flow measurement, instrumentation and process control. Reach him at 845.623.1830 or www.spitzerandboyes.com.