Flow measurement is a critical aspect of chemical processing. The effectiveness of operations depends upon accurate data on flow measurement, as does maintaining compliance with regulations. In addition, a greater emphasis on sustainability is driving more manufacturers to closely monitor the consumption of precious resources and the byproducts generated in the process. At some sites, another crucial concern is custody transfer, with increasing energy costs spurring the need for improved fiscal metering of high-value products.
So, in this article, we’ll look at the key criteria in flow meter selection and go over some pointers for picking the most appropriate device.
Let’s start by reviewing the most frequently used measurement technologies and their advantages and disadvantages:
Coriolis. These meters contain a vibrating tube in which a fluid flow causes changes in frequency, phase shift or amplitude. Circuitry in the devices then converts this signal into an output that’s strictly proportional to the actual mass flow rate — in contrast to thermal mass flow meters, which depend upon the physical properties of the fluid.
One of the most important features of a Coriolis flow meter (Figure 1) is its ability to directly measure fluid mass with a very high degree of accuracy over a wide range of temperatures. Its unobstructed open-flow design is suitable for viscous non-conductive fluids that are difficult to measure with other technologies. With no internal moving parts, a Coriolis meter requires a minimum amount of attention once installed. However, such devices sometimes are considered too sophisticated, expensive or unwieldy for certain applications.
Differential pressure (DP). These meters measure the pressure differential across the meter and extract the square root. They have a primary element that causes a change in kinetic energy, creating differential pressure in the pipe, and a secondary element that measures the differential pressure and provides a signal or read-out converted to the actual flow value.
DP meters are versatile instruments that employ a proven well-understood measuring technology not requiring moving parts in the flow stream. Viscosity changes don’t affect the devices greatly. However, they have a history of limited accuracy and turndown, as well as complex installation requirements.
Electromagnetic. Such meters employ Faraday’s law of electromagnetic induction, whereby voltage is induced when a conductor moves through a magnetic field. The liquid acts as the conductor, with energized coils outside the flow tube creating the magnetic field. The produced voltage is directly proportional to the flow rate.
An electromagnetic meter (Figure 2) will measure virtually any conductive fluid or slurry, including process water and wastewater. The devices provide low pressure drop, high accuracy, large turndown ratio and excellent repeatability. The meters have no moving parts or flow obstructions, and are relatively unaffected by viscosity, temperature and pressure when correctly specified. Electromagnetic meters tend to be heavy in larger sizes and may be prohibitively expensive for some purposes.
Thermal mass. These meters utilize a heated sensing element isolated from the fluid flow path. The flow stream conducts heat away from the sensing element, with the rate directly proportional to the mass flow rate. The meter’s electronics package provides a linear output directly proportional to mass flow.
Thermal mass meters have a relatively low purchase price. They are designed to work with clean gases of known heat capacity, as well as some low-pressure gases not dense enough for Coriolis meters to measure. The main disadvantage of thermal technology is low-to-medium accuracy, although suppliers have improved the capabilities of these meters in recent years.
Turbine. Such meters contain a freely suspended rotor whose vanes rotate at a rate proportional to flow velocity. A sensor/transmitter detects the rotational rate of the rotor; the faster the fluid moves, the more pulses that are generated. The transmitter processes the pulse signal to determine the flow of the fluid in either forward or reverse direction.
Turbine meters incorporate a time-tested measuring principle, and are known for high accuracy, wide turndown and repeatable measurements. They produce a high-resolution pulse-rate output signal proportional to fluid velocity and, hence, to volumetric flow rate. Turbine meters are limited to clean fluids only. Use of ceramic journal bearings largely has addressed bearing wear — a common concern with this type of device. As a mechanical meter, turbines require periodic recalibration and service.
Impeller. Frequently used in large-diameter water distribution systems, these devices insert a paddle wheel perpendicularly into a process stream. The number of rotations of the paddle wheel is directly proportional to the velocity of the process.
The advantages of impeller meters include direct volumetric flow measurement (often with visual indication), universal mounting, fast response with good repeatability, and relatively low cost. However, their performance suffers in applications with low fluid velocity; the meters also are sensitive to flow profile. They suit clean, low-viscosity media.
Ultrasonic. There are two types of ultrasonic meters: transit time and Doppler. Both designs will detect and measure bidirectional flow rates without invading the flow stream. They can handle all types of corrosive liquids as well as gases, and are insensitive to changes in temperature, viscosity, density or pressure. A clamp-on ultrasonic meter (Figure 3) is ideal for troubleshooting, diagnostics and leak detection.
Ultrasonic meters have no moving or wetted parts, suffer no pressure loss, offer a large turndown ratio, and provide maintenance-free operation — important advantages over conventional mechanical meters. However, the precision of these meters becomes much less dependable at low flow rates. Unknown internal piping variables can shift the flow signal and create inaccuracies.
Variable area. These meters are inferential measurement devices consisting of two main components: a tapered metering tube and a float that rides within the tube. The float position — a balance of upward flow and float weight — is a linear function of flow rate. Operators can take direct readings based on the float position within transparent glass and plastic tubes.
Simple, inexpensive and reliable, variable area meters are appropriate for many applications. However, they must be calibrated for viscous liquids and compressed gases. Furthermore, they offer limited turndown and relatively low accuracy.
Vortex. Such meters make use of a principle called the von Kármán effect, whereby flow will alternately generate vortices when passing by a bluff body (a piece of material with a broad flat front that extends vertically into the flow stream). Flow velocity is proportional to the frequency of the vortices. Flow rate is calculated by multiplying the area of the pipe by the velocity of the flow.
Vortex meters have no moving parts that are subject to wear and, thus, don’t require regular maintenance. However, they only can measure clean liquids. The devices are particularly well suited for measuring gas emissions produced by wastewater. Vortex meters may introduce pressure drop due to their obstructions in the flow path.
Oval gear. These meters utilize a positive displacement (PD) meter design, whereby fluid enters the inlet port and then passes through the metering chamber before exiting through the outlet port. Inside the chamber, the fluid forces the internal gears to rotate. Each rotation of the gears displaces a specific volume of fluid. As the gears rotate, a magnet on each end of the gear passes a reed switch, which sends pulses that a microprocessor translates into a flow rate shown on an LED display.
The latest breed of oval gear meters directly measures actual volume. It features a wide flow range, minimal pressure drop and extended viscosity range. This design offers easy installation and high accuracy, and measures high-temperature, viscous and caustic liquids with simple calibration.
Nutating disc. Most commonly used in water metering applications, these devices have a disc attached to a sphere mounted inside a spherical chamber. As fluid flows through the chamber, the disc and sphere unit wobble or “nutate.” This effect causes a pin, mounted on the sphere perpendicular to the disc, to rock. Each revolution of the pin indicates a fixed volume of liquid has passed. Meters with aluminum or bronze discs can monitor hot oil and chemicals.
Nutating disc meters have a reputation for high accuracy and repeatability. However, viscosities below their designated threshold adversely affect performance.
Key Selection Factors
In a typical plant, fluid characteristics (number of phases, viscosity, turbidity, etc.), flow profile (laminar, transitional or turbulent), flow range and accuracy requirements all are important considerations in determining the right flow meter for a particular measurement task. Additional considerations such as mechanical restrictions and output-connectivity options may impact the choice.
For process operations, the key factors in meter selection include:
Process media. Whether the fluid is liquid, gas or multiphase influences the suitability of a particular device for the service. Different flow meters are designed to operate best with certain fluids and under specific operating conditions. That’s why understanding the limitations inherent to each style of instrument is important.
Measurement type. Do you need a mass or volumetric flow measurement? While volumetric readings are convertible into mass measurements given an accurate density, volumetric measuring devices like variable-area and turbine meters can’t distinguish density-altering temperature or pressure changes. So, mass flow measurement would require additional sensors for these parameters and a flow computer to compensate for the variations in these process conditions. Thermal mass flow meters are virtually insensitive to variations in temperature or pressure.
Flow rate information. A crucial aspect of selection is determining if continuous or totalized flow rate data are needed. A typical continuous flow measurement system consists of a primary flow device, flow sensor, transmitter, flow recorder and totalizer.
Desired accuracy. The difference between on- and off-specification product often depends upon flow meter accuracy. This is specified in percentage of actual reading, calibrated span or full scale. It normally is stated at minimum, normal and maximum flow rates. You must clearly understand these requirements to get a meter with acceptable performance over its full range.
Application environment. A flow meter can face widely varying conditions in a plant. So, you must decide whether the low or high flow range is most important for a metering application; this information will help in sizing the correct instrument for the job. Pressure and temperature conditions are equally important process parameters. You also should consider pressure drop in flow measurement devices, especially with high-viscosity fluids. In addition, viscosity and density may fluctuate due to a physical or temperature change in the process fluid.
Fluid characteristics. You must pick a meter compatible with the fluid and operating conditions. Many plants handle abrasive or corrosive fluids that may move under aerated, pulsating, swirling or reverse-flow conditions. Thick and coarse materials can clog or damage internal meter components — hindering accuracy and resulting in frequent downtime and repair.
Installation requirements. Planning a flow meter installation starts with knowing the line size, pipe direction, material of construction and flange-pressure rating. You also must identify possible complications due to equipment accessibility, valves, regulators and available straight-pipe-run lengths. Nearly all flow meters require a run of straight pipe before and after their mounting location. Where this isn’t possible, you can use a flow conditioner to isolate liquid flow disturbances from the flow meter while minimizing the pressure drop across the conditioner.
Power availability. Pneumatic instrumentation once was used in most applications in hazardous areas to obviate bringing in a power source that might cause an explosion. Today’s installations normally call for intrinsically safe instruments; these rely on safety barriers that limit current to eliminate any potential spark. Another option is to employ fiber optics. Turbine flow meters offer an advantage in environments where a power source isn’t available because they don’t require external power to provide a local rate/total indicator display for a field application; the devices instead rely on a battery-powered indicator. Solar-powered systems also may make sense in remote areas without power.
Necessary approvals. Plants must comply with relevant standards and regulations for the use of flow measurement equipment in hazardous locations. These include: FM Class 1 Division 1, Groups A, B, C and D; and FM Class 1, Zone 1 AEx d (ia) ia/IIC/T3-T6. Standards such as the Measuring Instruments Directive in the European Union apply to fiscal- and custody-transfer metering for liquids and gases. In terms of environmental emissions, industrial flow meters must meet the Electromagnetic Compatibility Standards EN55011:1992 and EN61326-1:1997.
Output/Indication. You must decide whether measurement data are needed locally or remotely. For sending data for remote indication, the transmission can be analog, digital or shared. The choice of a digital communications protocol such as HART, Foundation Fieldbus or Modbus also figures into this decision. In a large plant, flow readings typically go to an automation system for use for process control and optimization.
Other Important Issues
Higher accuracies and broader capabilities in flow meters cost more. So, determine what you actually need. Evaluate process conditions, including flow rates, pressure and temperature, and operating ranges to find a meter suited to the specific application. Sacrificing features for cost savings or opting for a lower-priced alternative that would be applied outside of its capabilities are false economies.
All meters are affected to some extent by the medium they are monitoring and the way they are installed. Consequently, their performance in real-world conditions often will differ from that in the reference conditions under which they were calibrated. For the lowest uncertainty of measurement, PD meters generally are the best option. Electromagnetic meters provide for the widest flow range while turbine meters usually are the best choice for the highest short-term repeatability. Despite their high initial cost, Coriolis meters are ideal for measuring particularly viscous substances and wherever readings of mass rather than volume are required.
You always should examine long-term ownership costs. A flow meter with a low purchase price may be very expensive to maintain. Alternatively, a meter with a high purchase price may need very little service. Lower purchase price doesn’t always represent better value.
Generally speaking, flow meters with few or no moving parts require less attention than more-complex instruments. Meters incorporating multiple moving parts can malfunction due to dirt, grit or grime present in the process fluid. In addition, impulse lines needed with some meters can plug or corrode, and units with flow dividers and pipe bends sometimes suffer from abrasive-media wear and blockages. Changes in temperature also affect the internal dimensions of the meter and require compensation.
The need to recalibrate a flow meter depends upon how well the instrument fits a particular application. If the application is critical, check meter accuracy at frequent intervals. Otherwise, recalibration may not be necessary for years for non-critical applications and ones where conditions don’t vary.
No matter the chosen flow-meter technology, overall system accuracy won’t exceed that of the equipment used to perform the meter calibration. The most-precise flow calibration systems on the market employ a PD design. This type of calibrator, directly traceable to the U.S. National Institute of Standards and Technology via water draw validation, provides total accuracy of at least 0.05%.
Selecting the right flow meter can significantly impact operational and business performance. So, educate yourself about basic flow-measurement techniques and available meter options, and don’t hesitate to consult with a knowledgeable instrumentation supplier in the early stages of a project. That will help you ensure a successful application once the equipment is installed.
BRIAN KETTNER is a product manager for Badger Meter, Milwaukee, Wis. E-mail him at email@example.com.