When it comes to choosing a flowmetering device, some users assume “one size fits all.” In reality, there are significant differences between flowmeter types, and each design has its unique “pros and cons.” The basis of proper meter selection is a general awareness of flow measurement science — and a clear understanding of your specific application requirements.
Flowmeter users should also consider intangible factors such as familiarity of instrumentation personnel, their experience with calibration and maintenance, spare parts availability, mean time between failures, at the particular installation site.
With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid’s velocity or the change in kinetic energy. Velocity depends on the pressure differential forcing the liquid through a pipe or conduit. Because the pipe’s cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate. The basic relationship for determining the liquid’s flow rate in such cases is:
Q = V x A where Q = liquid flow through the pipe V = average velocity of the flow A = cross-sectional area of the pipe
Other possible factors affecting liquid flow rate include the liquid’s viscosity and density, and the friction of the liquid in contact with the pipe.
Volumetric flow rate is the volume of fluid passing through a given volume per unit time. Mass flow rate is the movement of mass per time. Its unit is mass divided by time - kilogram per second in SI units. Mass flow rate can be calculated from the density of the liquid (or gas), its velocity, and the cross sectional area of flow.
End users contemplating a flowmeter purchase should take the time to study the characteristics of respective measurement technologies, and analyze their advantages/disadvantages for different environments. Common industrial flowmeter designs include:
Differential Pressure – Operates by measuring the pressure differential across the meter and extracting the square root. These meters have a primary element which causes a change in kinetic energy, creating differential pressure in the pipe; and a secondary element measuring the differential pressure and providing a signal or read-out converted to the actual flow value.
Electromagnetic – Employs Faraday’s law of electromagnetic induction, which states voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor, and energized coils outside the flow tube create the magnetic field. The amount of voltage produced is directly proportional to the flow rate.
Coriolis – Provides mass flow data by measuring fluid running through a bent tube, which is induced to vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tube will deform and an additional vibration component will be added to the oscillation. This causes a phase shift over areas of the tube, which can be measured with sensors.
Thermal Mass – Utilizes a heated sensing element isolated from the fluid flow path. The flow stream conducts heat from the sensing element, which is directly proportional to the mass flow rate. The meter’s electronics package includes the flow analyzer, temperature compensator, and a signal conditioner providing a linear output directly proportional to mass flow.
Vortex Shedding – Forms fluid vortices against the meter body introduced in the pipe. These vortices are produced from the downstream face of the meter in an oscillatory manner. The shedding is sensed using a thermistor, and the frequency of shedding is proportional to volumetric flow rate.
Turbine – Incorporates a freely suspended rotor turned by fluid flow through the meter body. Since the flow passage is fixed, the rotor’s rotational speed is a true representation of the volumetric flow rate. The rotation produces a train of electrical pulses, which are sensed by an external pickoff and then counted and totalized. The number of pulses counted for a given period of time is directly proportional to flow volume.
Positive Displacement – Separates liquid into specific increments, and the flow rate is an accumulation of these measured increments over time. As the fluid passes through the meter, a pulse is generated which represents a known volume of fluid.
Ultrasonic – Operates by comparing the time for an ultrasonic signal to travel with the flow (downstream) against the time for an ultrasonic signal to travel against the flow (upstream). The difference between these transit times is proportional to the flow, and the flowmeter converts this information to flow rate and total.
Meter Selection Factors
For end users, there are a number of specific issues to keep in mind when choosing a flow-metering instrument.
When it comes to measuring the flow of liquids and gases, not all flowmeters are created equal. Different flowmeters are designed for optimum performance in different fluids — and under different operating conditions. That’s why it is important to understand the limitations inherent to each style of instrument.
Measuring the flow of liquid and gases demands superior instrument performance. From basic water to fluids with extreme viscosities, high solid concentrations and low conductivity, these applications present a wide range of flow metering challenges.
Measuring the flow of dense, viscous fluids can be difficult for many flowmeters. Thick and coarse materials can clog or damage internal meter components — resulting in poor accuracy and frequent downtime and repair.
For applications with dirty, viscous liquids, chemical processors traditionally have chosen orifice plates, venturi tubes, target meters, variable area meters, mag meters, and various mass meters. Noninvasive ultrasonic meters are also an option. For the most part, intrusive technologies such as turbine, PD and vortex meters should not be considered for these environments.
On the other end of the spectrum, pharmaceutical and biotechnology plants often have filtration equipment producing a wide range of water qualities, including USP, de-ionized (DI) water, and water-for-injection (WFI). These operations require very accurate flow measurements at various stages of the process.
Turbine meters have long been popular for obtaining precise flow readings in clean, known liquids such as ultra-pure water. They normally operate in temperature ranges up to 350°F (177°C), and can be equipped with ceramic bearings which, unlike tungsten carbide and epoxy-impregnated graphite bearings, can withstand the corrosive effects of ultra-clean water.
All flowmeter users should understand fluid viscosity. Kinematic viscosity is the ratio of the absolute viscosity to the specific gravity, usually expressed in centistokes (cst), where the resistance to flow is measured in square millimeters per second (mm2/s).
Fluid viscosity sometimes fluctuates during a given measurement period. Viscosity and density also vary due to a physical change in the mixture of the fluid. In most cases, however, these changes are due to variations in the operating temperature of the fluid itself.
Using today’s advanced flowmeter electronics, real-time volumetric flow measurements can be obtained by measuring the process operating temperature and addressing a temperature verses viscosity look up table to determine the operating kinematic viscosity. The frequency is measured directly from the flowmeter and divided by the kinematic viscosity. This ratio of frequency and viscosity is used to determine the correct K-Factor for the specific operating temperature and viscosity.
In high-pressure flow applications, the “water hammer” effect can severely damage conventional flow measurement devices. Water hammer (or, more generally, fluid hammer) is a pressure surge or wave caused by the kinetic energy of a fluid in motion when it is forced to immediately stop or change direction. For example, if a valve is closed suddenly at an end of a pipeline system, a water hammer wave propagates in the pipe.
Ideally, an industrial flowmeter, whether it employs mechanical or electromagnetic principles, should respond instantly and consistently to changes in water velocity. Meters, however, are not perfect instruments and may not accurately register velocity in all measurement conditions encountered. Turbulent or pulsating flows can cause registration errors in meters.
In recent years, instrument suppliers have developed new PD meter configurations for pulsating flow streams encountered in batch applications. These meters, employing a bearingless design and non-intrusive sensors, can measure liquid flow where process pressure is above 1,000 psig and operating temperatures reach 400° F (204° C).
In situations where flowmeters are used to give an indication of the rate at which a liquid or gas is moving through a pipeline, high accuracy is not crucial. But for batching, sampling and dispensing applications, flowmeter accuracy can be the deciding factor between optimum quality and wasted product.
Typically, flowmeter accuracy is specified in percentage of actual reading (AR), in percentage of calibrated span (CS), or in percentage of full-scale (FS) units. Accuracy requirements are normally stated at minimum, normal, and maximum flow rates. Unless you know these requirements, your flowmeter’s performance may not be acceptable over its full range.
In applications where products are sold or purchased on the basis of a meter reading, absolute accuracy is critical. Otherwise, repeatability may be more important than absolute accuracy. Users should establish separately the accuracy and repeatability requirements of each application, and state both in their specifications.
Response time is another key flowmeter performance criteria. In the case of pharmaceutical dispensing, for example, the meter’s speed of response is critical to operational quality and consistency. Other applications are less concerned with response time, and are focused on accurate mass or volumetric measurements only.
Also, be sure to consider pressure drop when specifying a flow measurement instrument, especially for chemical and food plants with high viscosity fluids. Pressure drop is the decrease in pressure from one point in a pipe to another point downstream, and usually results from the friction of the fluid against the pipe. High flow rates in small pipes give large pressure drop. Low flow rates in large pipes give low pressure drop. In order to achieve a lower pressure drop, some users end up choosing a larger meter than originally planned — significantly increasing their costs.
Continuous or Total Flow Rate?
One step in flowmeter selection is determining whether flow rate data should be continuous or totalized, and whether this information is needed locally or remotely. If remotely, the user must decide whether the transmission should be analog, digital, or shared. The choice of a digital communications protocol such as HART and fieldbus adds another element to this decision. If shared, make sure the required (minimum) data-update frequency is taken into accounts.
In large chemical processing plants, flow readings are normally supplied to a Programmable Logic Controller (PLC) or Distributed Control System (DCS) for use in overall process control and optimization strategies.
In terms of flow, at room temperature and low pressures, volumetric and mass flow rates will be nearly identical. However, these rates can vary drastically with changes in temperature and/or pressure, because the temperature and pressure of the gas directly affects the volume.
When measuring the flow of compressible materials, volumetric flow is not very meaningful unless density (and sometimes also viscosity) is constant. When the velocity (volumetric flow) of incompressible liquids is measured, the presence of suspended bubbles will cause error; therefore, air and gas must be removed before the fluid reaches the meter.
From acids and juices, to argon, chlorine, hydrogen, ammonia and even phosgene, corrosive process media can mandate using a flowmeter with specialized materials of construction. In addition to 316 stainless steel and Hastelloy, standard flowmeter wetted parts are manufactured from Tantalum, Monel, Nickel, Titanium, Carbon Steel, and Zirconium.
Flowmeter manufacturers have invested considerable time and resources to develop meter designs utilizing thermoplastic materials able to handle corrosive liquids and gases. For example, all-plastic PD meters are ideal for aggressive liquid flow applications, including acids, caustics, specialty chemicals, and DI water.
Ultrasonic flowmeters have no moving or wetted parts, suffer no pressure loss, and provide maintenance-free operation— important advantages over conventional mechanical meters such as vortex meters, and also, in many cases, coriolis mass meters. Clamp-on ultrasonic meters are mounted completely external to the pipe wall, so they are not affected by corrosive or erosive liquids, and are not damaged by solids, gases or particles in the process liquid.
Pipe Size, Configuration
When evaluating a flowmeter installation, users should know the flow direction (avoid downward flow in liquid applications), pipe size, pipe material, flange-pressure rating, accessibility, up or downstream turns, valves, regulators, and available straight-pipe run lengths in their facility.
The amount of available installation space may dictate your choice of flowmeter technology. Where there is limited room, larger instrument designs like Coriolis meters may not be feasible. Vortex, turbine and PD meters, as well as a handful of other technologies, are a wiser choice under these circumstances.
Nearly all flow instruments must be installed with a significant run of straight pipe before and after the location of the meter. Elbows, reducers, chemical injection ports, filters, screens and valves can cause radial, tangential and axial swirling effects within the pipe. In combination, these changes can rapidly distort the velocity profile, degrading the flowmeter’s accuracy and repeatability.
Many flow measurement instruments require straightening vanes or straight upstream piping to eliminate distorted patterns and swirls. This involves the installation of additional diameters of straight pipe run before the flow straightener, and between the straightener and the flowmeter itself.
In the pharmaceutical, food & beverage, and biotech industries, contamination- free processing is critical. The integrity of the sanitary manufacturing application is essential for full compliance to the validation process.
The potential for contamination increases with the introduction of peripheral components, such as flow, temperature and pressure measuring instrumentation required to ensure process parameters remain within acceptable limits. As a result, these inline devices must themselves meet standards set by governing agencies to ensure there are no weak links in the sanitary chain.
Sanitary standards for flowmeters and other measurement devices address issues such as process connections, materials of construction, surface finishes, electronic instrument housings, autoclave sterilization, and diaphragm seals.
As a rule of thumb, flowmeters with few or no moving parts require less attention than more complex instruments. Meters incorporating multiple moving parts can malfunction due to dirt, grit and grime present in the process fluid. Meters with impulse lines can also plug or corrode, and units with flow dividers and pipe bends can suffer from abrasive media wear and blockages.
Moving parts are a potential source of problems, not only for the obvious reasons of wear, lubrication and sensitivity to coating, but also because they require clearance spaces which sometimes introduce “slippage” into the flow being measured. Even with well-maintained and calibrated meters, this unmeasured flow varies with changes in fluid viscosity and temperature. Temperature changes also change the meters internal dimensions and require compensation.