Selecting the Right Flowmeter—Part 1

By Corte Swearingen
Reprinted from the July 1999 edition of Chemical Engineering magazine
("Choosing the Best Flowmeter")

Table 1: A Comparison of Flowmeter Options    Variable-Area Flowmeters    
Table 2: The Effect of Pressure Deviations on a Variable-Area Flowmeter
   Mass Flowmeters    Coriolis Flowmeters    
Differential-Pressure Meters    Turbine Meters    Oval-Gear Flowmeters    References

With the many flowmeters available today, choosing the most appropriate one for a given application can be difficult. This article discusses six popular flowmeter technologies, in terms of the major advantages and disadvantages of each type, describes some unique designs, and gives several application examples.

Dozens of flowmeter technologies are available. This article covers six flowmeter designs—variable-area, mass, Coriolis, differential-pressure, turbine, and oval-gear. Table 1 compares the various technologies.

Table 1
A Comparison of Flowmeter Options

Attribute Variable-area Coriolis Gas
mass-flow
Differential-
Pressure
Turbine Oval Gear
Clean gases yes yes yes yes yes
Clean Liquids yes yes yes yes yes
Viscous Liquids yes (special calibration) yes no yes (special calibration) yes, >10 centistokes (cst)
Corrosive Liquids yes yes no yes yes
Accuracy, ± 2-4% full scale 0.05-0.15% of reading 1.5% full scale 2-3% full-scale 0.25-1% of reading 0.1-0.5% of reading
Repeatability, ± 0.25% full scale 0.05-0.10% of reading 0.5% full scale 1% full-scale 0.1% of reading 0.1% of reading
Max pressure, psi 200 and up 900 and up 500 and up 100 5,000 and up 4,000 and up
Max temp., °F 250 and up 250 and up 150 and up 122 300 and up 175 and up
Pressure drop medium low low medium medium medium
Turndown ratio 10:1 100:1 50:1 20:1 10:1 25:1
Average cost* $200-600 $2,500-5,000 $600-1,000 $500-800 $600-1,000 $600-1,200
*Cost values can vary quite a bit depending on process temperature and pressures, accuracy required, and approvals needed.

Go to Top



Variable-Area Flowmeters

Figure 1
The plastic or glass tube of the variable-area flowmeter lets the user visually inspect the float, whose position in the tapered tub is proportional to the volumetric flowrate.
Design overview: The variable-area flowmeter (Figure 1) is one of the oldest technologies available and arguably the most well-known. It is constructed of a tapered tube (usually plastic or glass) and a metal or glass float. The volumetric flowrate through the tapered tube is proportional to the displacement of the float.

Fluid moving through the tube form bottom to top causes a pressure drop across the float, which produces an upward force that causes the float to move up the tube. As this happens, the cross-sectional area between the tube walls and the float (the annulus) increases (hence the term variable-area).

Because the variable-area flowmeter relies on gravity, it must be installed vertically (with the flowtube perpendicular to the floor). Some variable-area meters overcome this slight inconvenience by spring loading the float withing the tube (Figure 2). Such a design can simplify installation and add operator flexibility, especially when the meter must be installed in a tight physical space and a vertical installation is not possible.

Two types of variable-area flowmeters are generally available: direct-reading and correlated. The direct-reading meter allows the user to read the liquid or gas flowrate in engineering units (i.e., gal/min and L/min) printed directly on the tube, by aligning the top of the float with the tick mark on the flowtube.

The advantage of a direct-reading flowmeter is that the flowrate is literally read directly off the flowtube. Correlated meters, on the other hand, have a unitless scale (typically tick marks from 0 to 65, or 0 to 150), and come with a separate data sheet that correlates the scale reading on the flowtube to the flowrate in a particular engineering unit. The correlation sheets usually give 25 or so data points along the scale of the flowtube, allowing the user to determine the actual flowrate in gal/min, L/min, or whatever engineering unit is needed.

The advantage of the correlated meter is that the same flowmeter can be used for various gases and liquids (whose flow is represented by different units) by selecting the appropriate correlation sheets, where additional direct-reading meters would be required for different fluid applications. Similarly, if pressure or temperature parameters change for a given application, the user would simply use a different correlation sheet to reflect these new parameters. By comparison, for a direct-reading meter, a change in operating parameters will compromise the meter's accuracy, forcing it to be returned to the factory for recalibration. In general, the average accuracy of a variable-area flowmeter is ±2-4% of fullscale flow.
Figure 2
This variable-area meter with a spring-loaded float can be installed at any angle. This accommodation is not available for traditional variable-area flowmeters, whose operation relies on gravity.

Advantages: The major advantage of the variable-area flowmeter is its relative low cost and ease of installation. Because of its simplicity of design, the variable-area meter is virtually maintenance-free and, hence, tends to have a long operating life.

Another advantage is its flexibility in handling a wide range of chemicals. Today, all-PTFE meters are available to resist corrosive damage by aggressive chemicals. The advantage of a PTFE flowmeter with a built-in valve is that you can not only monitor the fluid flowrate, but you can control it, as well, by opening and closing the valve. If the application requires an all-PTFE meter, chances are the fluid is pretty corrosive, and many users would like the option of controlling the flowrate by simply turning a valve that is built into the flowmeter itself.

Disadvantages: One potential disadvantage of a variable-area flowmeter occurs when the fluid temperature and pressure deviate from the calibration temperature and pressure. Because temperature and pressure variations will cause a gas to expand and contract, thereby changing density and viscosity, the calibration of a particular variable-area flowmeter will no longer be valid as these conditions fluctuate. Manufacturers typically calibrate their gas flowmeters to a standard temperature and pressure (usually 70°F with the flowmeter outlet open to the atmosphere, i.e., with no backpressure).

During operation, the flowmeter accuracy can quickly degrade once the temperatures and pressures start fluctuating from the standard calibration temperature and pressure. Meters used for water tend to show less variability, since water viscosity and density changes very little with normal temperature and pressure fluctuations. While there is a way to correlate the flow from actual operating conditions back to the calibration conditions, the conventional formulas used are very simplified, and don't take into account the effect of viscosity, which can cause large errors.

Table 2
The Effect of Pressure Deviations on a Variable-Area Flowmeter

Maximum flowrate, L/min Fluid temperature, °F Outlet pressure, psi
Fluid type: Air
2.23 70 0
1.65 70 15
1.30 70 35
2.26 90 0
2.28 110 0
2.32 150 0
Fluid type: water
4.82 70 0
4.82 70 15
4.82 70 35
4.86 90 0
4.89 110 0
4.95 150 0

As Table 2 shows, the effect of pressure deviations can be quite significant. This table was created using data from a variable-area flowmeter that was calibrated for air at 70°F and with the outlet of the flowmeter vented to the open atmosphere (i.e. , 0 psi of outlet pressure).

The flowmeter was calibrated to read a maximum of 2.23 L/min at this temperature and pressure. When the outlet pressure increases as all other parameters remain constant, the flowrate drops off. This pressure change affects the viscosity and density of the gas and will cause the actual flowrate to deviate from the theoretical, calibrated flowrate. This relationship is extremely important to be aware of, and underscores the difficulty in measuring gas flow. Also note that even though gas flowrate changes with a change in gas temperature (with all other parameters remaining constant), this effect is much less significant with air than with other gases.

Table 2 shows this same variation with a meter calibrated for water at 9 psi venting pressure and a temperature of 70°F. Here, one can assume water to be incompressible. As shown, there is no direct effect on water flow with a change in back-pressure. The temp-erature change is not that significant either. But, for various fluids, a change in temperature could change the viscosity enough to degrade the accuracy below acceptable limits.

More Details or Order Online:

Gilmont Unshielded Variable Area Flowmeters

Gilmont Shielded Variable Area Flowmeters

Gilmont Shielded Variable Area Flowmeters without Valve

The bottom line is that the user must be aware of any variation between calibration conditions and operating conditions for gas flows, and must correct the reading according to the manufacturer's recommendations. Some users have the manufacturer calibrate the meter to existing conditions, but this presumes that operating conditions will remain the same—which they rarely do.

The effect of viscosity changes is another potential disadvantage of the variable-area meter when measuring liquids. When a viscous liquid makes its way through a variable-area flowmeter, drag layers of fluid will build up on the float. this will cause a slower-moving viscous liquid to yield the same buoyant force as a faster-moving fluid of lower viscosity. The larger the viscosity, the higher the error. The general rule of thumb is as follows—unless the meter has been specifically calibrated for a higher-viscosity liquid, only water-like liquids should be run through a variable-area flowmeter.

Sometimes, for liquids that are slightly thicker than water, a manufacturer-supplied correction factor can be used without the need to recalibrate the whole meter. As always, check with the manufacturer if you plan on deviating from its calibration fluid and calibration conditions. For a more-detailed discussion of the proper correction equations to apply to variable-area flowmeters in both water and gas service when they deviate from standard conditions, consult Refs. 9 and 10.

Applications:
Variable-area flowmeters are well suited for a wide variety of liquid and gas applications, including the following:

  • Measuring water and gas flow in plants or labs
  • Monitoring chemical lines
  • Purging instrument air lines (i.e., lines that use a valved meter)
  • Monitoring filtration loading
  • Monitoring flow in material-blending applications (i.e., lines that use a valved meter)
  • Monitoring hydraulic oils (although this may require special calibration)
  • Monitor makeup water for food & beverage plants

Go to Top



Mass Flowmeters

Figure 3
Because the mass flowmeter measures mass flow rather than volumetric flow, this popular device is relatively undaunted by fluctuations in line pressures and temperatures, especially compared with a variable-area flowmeter. The unit shown provides an integral digital display, as well as a built-in control valve.
Design Overview:Mass flowmeters are one of the most popular gas-measurement technologies in use today (Figure 3). Most thermal mass flowmeters for gases are based on the following design principles, which are shown in Figure 4. a gas stream moves into the flowmeter chamber and is immediately split into two distinct flow paths. Most of the gas will go through a bypass tube, but a fraction of it goes through a special capillary sensor tube, which contains two temperature coils.

Heat flux is introduced at two sections of the capillary tube by means of these two wound coils. When gas flows through the device, it carries heat from the coils upstream to the coils downstream. The resulting temperature differerential creates a proportional resistance change in the sensor windings.

Special circuits, known as Wheatstone bridges, are used to monitor the instantaneous resistance of each of the sensor windings. The resistance change, created by the temperature differential, is amplified and calibrated to give a digital readout of the flow.

As shown in Figure 3, the mass flowmeter is available with a built-in valve for flow-control applications. This allows for external control and the programming of a setpoint for a critical flowpoint. Most mass flowmeters also have an analog or digital output signal to record the flowrate. The average mass flowmeter has an accuracy of 1.5-2% of fullscale flow.

Figure 4
Inside a mass flowmeter, the gas is split. Most goes through a bypass tube, while a fration goes through a sensor tube containing two temperature coils. Heat flux is introduced at two sections of the sensor tube by means of two wound coils. As gas flows through the device, it carries heat from the upstream, to the downstream, coils. The temperature differential, generates a proportional change in the resistance of the sensor windings. Special circuits monitor the resistance change, which is proportional to mass flow, and calibrate it to give a digital readout of the flow.

Advantages: The main advantage of a mass flowmeter for gas streams is its ability (within limitations) to "ignore" fluctuating and changing line temperatures and pressures. As mentioned above for variable-area flowmeters, fluctuating temperatures and pressures will cause gas density to change, yielding significant flow errors. Because of the inherent design of the mass flowmeter, this problem is much less significant than that found in variable-area flowmeters. Mass flowmeters measure the mass or molecular flow, as opposed to the volumetric flow. One can think of the mass flowrate as the volumetric flowrate normalized to a specific temperature and pressure.

A more intuitive way to understand mass versus volumetric measurement is to imagine a gas-filled ballon. Although the volume of the balloon may be altered by squeezing it (changing the gas pressure), or by taking the balloon into a hot or cold environment (changing the gas temperature), the mass of the gas contained inside the balloon remains constant. So it is with mass flow as opposed to volumetric flow.

A variable-area flowmeter measures volumetric flow. The flowrate on the flowtube reflects the volume of gas passing from the inlet to the outlet. This volume can change when gas temperatures and pressures change. Because a mass flowmeter is measuring the actual mass of gas passing form inlet to outlet, there is very little dependence on fluctuating temperatures and pressures. If you were piping an expensive gas, you would certainly want to keep track of the amount of gas used based on mass, not volumetric, flow.

More Details or Order Online:

Aluminum Thermal Gas Mass Flowmeters

316SS Thermal Gas Mass Flowmeters

Makers of mass flowmeters measure their products' ability to withstand changing pressures and temperatures by giving coefficients that state the deviation of accuracy per degree or psi change. For example, typical coefficient values are 0.10% error per degree C, and 0.02% error per psi. This means that each degree or psi change away from the meter's calibration conditions will degrade the accuracy by these coefficient amounts. So, although there is a dependence on pressure and temperature for a mass meter, its is very small, if not negligible. This is the biggest advantage of a mass flowmeter. Another is that there are no moving parts to wear out.

Disadvantages: Aside from the fact that the gas going through the mass flowmeter should be dry and free from particulate matter, there are no major disadvantage to the mass flow technology. Mass flowmeters must be calibrated for a given gas or gas blend.

Applications:
Applications for mass flowmeters are diverse, but here are some typical uses:

  • Monitoring and controlling air flow during gas chromatography
  • Monitoring CO2 for food packaging
  • Gas delivery and control for fermenters and bioreactors
  • Leak testing
  • Hydrogen flow monitoring (e.g., in the utility industry)
  • Control of methane or argon to gas burners
  • Blending of air into dairy products
  • Regulating CO2 injected into bottles during beverage production
  • Nitrogen delivery and control for tank blanketing

Go to Top



Coriolis Flowmeters

Design Overview: The Coriolis flowmeter is named for the Coriolis effect, an inertial force discovered by 19th-century mathematician Gustave-Gaspard Coriolis. as a result of the Coriolis force, the acceleration of any body moving at a constant speed with respect to the Earth's surface will be deflected to the right (clockwise) in the northern hemisphere, and to the left (counter-clockwise) in the southern hemisphere.

The basic design of the Coriolis meter makes use of this Coriolis force by subjecting a set of curved measuring tubes to rotary oscillations about an axis. This oscillation is normally driven by two electromagnetic coils, which also physically couple the two curved measuring tubes. As a particular fluid flows through the tubes, it will move through points of high rotational velocity, to points of lower rotational velocity.

Figure 5a (left). In a coriolis flowmeter, the Coriolis force FCor, pushes out toward the z-axis as the fluid moves up through the tube. this force develops as the tube rotates at a rate of W around the x-axis, and causes the tube to distort out of the x-y plane
Figure 5b (right). As an example of a single-tube Coriolis flowmeter, this figure shows the fluid forces that generate the twisting motion of the flow tube
Upon approaching the tube plane in which the rotational axis is located, the rotational motion of the fluid element is decelerated at a uniform rate, until it finally reaches zero in the plane of the rotational axis. As the fluid element flows away form the rotational axis plane, toward points with higher rotational velocity, it is uniformly accelerated to increasingly higher rotational velocities. This produces a force (the Coriolis force) that causes a twisting motion withing the sensor tubes (Figure 5a).

If v is the velocity of the fluid in the measuring tube, m/s, w the instantaneous angular speed of rotation, radians/s, and m the mass of liquid in the tube section, kg, then the following applies to the Coriolis force, kg(m/s) (Note that if the flow is low, you may be using different units to represent smaller forces):

  FCor = -2m(w x v)      

The design of the Coriolis flowmeter takes advantage of this force in the following manner. First, the electromagnetic drivers initiate a vibration or oscillation in the sensor tube. This oscillation occurs even when there is no fluid moving in the meter.

The amplitude and frequency of this oscillation varies from manufacturer to manufacturer, but in general, the amplitude is about 3 millimeters, and the frequency is roughly 75-100 cycles/s. As the fluid element passes through the sensor tubes, the Coriolis forces come into play. The Coriolis forces cause a twisting, or distortion, in the measuring tube, which causes a vibrational phase difference between the two tubes.

Some designs use only one sensor tube (figure 5b). In this case, the distortion caused by the Coriolis force in the tube is compared to the tube at "no flow" conditions. In both cases, however, a correlation to the mass flowrate is achieved, because the measured phase difference or distortion is directly proportional to the mass flowrate of the fluid. Meanwhile, temperature-compensation techniques nullify the temperature dependence of the tube oscillations, creating a high-accuracy correlation to mass flow.

Advantages: The biggest advantage of the Coriolis design is that it measures mass flow instead of volumetric flow. Because mass is unaffected by changes in pressure, temperature, viscosity and density, reasonable fluctuations of these parameters in the fluid line have no affect on the accuracy of the meter, which can approach 0.05% of mass flow.

Coriolis meters can also determine fluid density by comparing the resonant frequency of the fluid being measured with that of water. Knowing density, the software can then convert mass to volume or percent solids.

Since there are no obstructions in the fluid path, Coriolis meters have inherently low pressure drop for low-viscosity liquids. Turndown ratios (the ratio of maximum to minimum flow) of 100:1 are not uncommon. In addition, the lifetime and reliability of the Coriolis meter are high as the flow path is free of moving parts and seals. And, if installed properly, vertically installed Coriolis meters are self draining, so they will not hold fluid when the line is down. A variety of wetted parts, communications outputs and connections are available.

Disadvantages: Because of their high accuracy and reliability, Corilois meters tend to be relatively expensive. This is not necessarily a disadvantage, however, if one looks at the relatively low cost of installation and ownership over time (Table 1). Because of their accuracy, Coriolis meters can help increase operating efficiency and save on production costs.

The main limitation of the Coriolis meter is that pressure drop can become large as fluid viscosity increases. For viscous products, check with the manufacturer to make sure the pressure drop at you max flowrate is acceptable and within your design parameters.

Applications:
Coriolis flowmeters are suitable for:

  • General-purpose gas or liquid flow
  • Custody transfer
  • Monitoring concentration and solids content
  • Blending ingredients and additives
  • Conducting a primary check on secondary flowmeters
  • Metering natural-gas consumption
  • Monitoring such fluids as syrups, oils, suspensions and pharmaceuticals

Go to Top



Differential-Pressure Meters

Design overview: While many different types of differential-pressure flowmeters are available, this discussion will focus on one type. The technology discussed here involves the measurement of a pressure differential across a stack of laminar flow plates (Figure 6). During operation, a pressuredrop is created as fluid enters through the meter's inlet. The fluid is forced to form thin laminar streams, which flow in parallel paths between the internal plates separated by spacers.
Figure 6
Using a differential-pressure flowmeter, a pressure drop is created as fluid enters the inlet. The fluid is forced to form thin laminar streams, which flow in along parallel plates. The pressure differential created by fluid drag from one end of the laminar flow plates to the other is linear and proportional to the flowrate of the liquid or the gas.

The pressure differential created by the fluid drag is measured by a differential-pressure sensor connected to the top of the cavity plate. The differential pressure from one end of the laminar flow plates to the other end is linear and proportional to the flowrate of the liquid or gas.

What makes this technology unique is the linear relationship between differential pressure, viscosity and flow, which is given by the following equation

   Q = K[P1-P2)/n2]      

where (units vary per approach):
Q = Volumetric flowrate
P1 = Static pressure at the inlet
P2 = Static pressure at the outlet
n = Viscosity of the fluid
K = Constant factor determined by the geometry of the restriction

This direct relationship between pressure, viscosity and flow allows the meter to switch easily among different gases without recalibration. This is normally accomplished by programming in the various gas viscosities and allowing the user to dial in the appropriate gas, via a set of switches.

Variances in temperature and pressure, which often cause errors in variable-area flowmeters, can be easily handled by adding a pressure sensor (separate form the differential-pressure sensor in the basic design) and a temperature sensor to the design, to constantly monitor fluctuations in stream pressure and temperature, and correct the flow readings to standard pressure and temperature (77°F and 1 atm). This is critical for gas flowmeters, which are very sensitive to these parameters. Typical accuracy for the design is ±2-3% fullscale.

Advantages: As with mass flowmeters, the differential-pressure meter has no moving parts to wear out. And, unlike with mass flowmeters, users of differential-pressure meters can measure different gases, such as air, hydrogen, ethane, methane, nitrous oxide, carbon dioxide, carbon monoxide, helium, oxygen, argon, propane and neon, by setting a switch on the unit, without the need for recalibration.

For control applications, these meters are available with a built-in proportioning valve for onboard or remote control of the flowrate. With a wide variety of flow ranges and models for both gases and liquids, the differential-pressure meter is one of the most versatile designs currently on the market.

Disadvantages: These meters are generally reserved for use with clean gases and liquids. particulates with diameters >20 to 30 micrometers could get caught between the plates.

Applications:
Viable applications include the following:

  • Chemical applications (ratio, metering, and additive control)
  • Pharmaceutical applications (liquid injection and batching)
  • Research and development, and laboratory applications (gas blending, injection and aeration)
  • Food and beverage applications (CO2 measurements, air drying, and process control)

Go to Top



Turbine Meters

Design Overview: Many designs exist for turbine flowmeters, but most are a variation on the same theme. As fluid flows through the meter, a turbine rotates at a speed that is proportional to the flowrate (Figure 7). Signal generators, usually located within the rotor itself, provide magnetic pulses that are electronically sensed through a pickup coil (the yellow pickup coil shown in Figure 7) and calibrated to read flow units. In some designs, an integral display may show both the flowrate and the total flow since power-up. Turbine meters are available for both gas and liquid flow.
Figure 7
This cutaway view of a turbine flowmeter shows the turbines and signal generators used to produce voltage pulses that are proportional to the flowrate.

Because of the rotating blades in a turbine meter, the output signal will be a sine wave voltage (V) of the form:

   V=KwsinNwt        

where:
K = The amplitude of one sine wave
w = The rotational velocity of the blades
N = The number of blades that pass the pickup in one full rotation
t = Time

Because the output signal is proportional to the rotational velocity of the turbines—which, in turn, is proportional to the liquid flow—the signal is easily scaled and calibrated to read flowrate and flow totalization. Turbine flow sensors generally have accuracies in the range of ±0.25-1% fullscale.

Advantages: The main advantages of the turbine meter are its high accuracy (±0.25% accuracy or better is not unusual) and repeatability, fast response rate (down to a few milliseconds), high pressure and temperature capabilities (i.e., up to 5,000 psi and 800°F with high-temperature pick coils), and compact rugged construction. Some manufacturer's have taken turbine meter design to the next level by incorporating advanced electronics that perform temperature compensation, signal conditioning and linearization, all within a few milliseconds. This advanced technology will allow the meter to automatically compensate for viscosity and density effects.

More Details or Order Online:

Turbine Meters with
4-20 mA Output

Turbine Meters with Battery-Powered Display

Disadvantage: The disadvantage of the turbine meter is that is relatively expensive and has rotating parts that could clog from larger suspended solids in the liquid stream. And, most turbine meters need a straight section of pipe upstream from the flowmeter in order to reduce turbulent flow. This may make installation a challenge in small areas. However, some newer turbine meters reduce or eliminate the amount of straight pipe required upstream, by incorporating flow straighteners into the body of the unit.

Another disadvantage in some designs is a loss of linearity at the low-flow end. Low-velocity performance and calibration can be affected by the natural change in bearing friction over time. However, today's self-lubricated retainers, low-drag fluid bearings, and jeweled-pivot bearings all help to reduce the friction points, thereby allowing for greater accuracy and repeatability in lower-flow applications.

Applications:
Turbine flowmeters can be found in a wide variety of industries and applications:

  • Rotometer replacement
  • Pilot plants
  • Research and development facilities
  • Cooling water monitoring
  • Inventory control
  • Test stands
  • Water consumption
  • Makeup water

Go to Top



Oval-Gear Flowmeters

Design Overview: The design of the oval-gear flowmeter is relatively simple: oval-shaped, gear-toothed rotors rotate within a chamber of specified geometry (Figure 8). As these rotors turn, they sweep out and trap a very precise volume of fluid between the outer oval shape of the gears and the inner chamber walls, with none of the fluid actually passing trough the gear teeth. Normally, magnets are embedded in the rotors, which then can actuate a reed switch or provide a pulse output via a specialized, designated sensor (such as a Hall Effect sensor). Each pulse or switch closure then represents a precise increment of liquid volume that passes through the meter. The result is a high accuracy (usually ±0.5 percent of reading) and resolution, and almost negligible effects for varying fluid viscosity, density and temperature.
Figure 8
During operarion, each gear rotation in the oval-gear meter traps a pocket of fluid between the gear and the outer chamber walls. A designated sensor counts the pockets of fluids passing from inlet to outlet, and correlates this value to a flowrate.

When sizing an oval-gear flowmeter, keep in mind that the higher the fluid viscosity, the more pressure will be required to "push" the fluid into the flowmeter and around the gears. Essentially, the pressure drop is the only limiting factor when the application requires the metering of highly viscous liquids.

The general rule is that as long as the fluid will flow, and as long as there is enough system pressure, the oval-gear meter will be able to measure the flow. In applications where the lowest possible pressure drop is required, some manufacturers can replace the standard rotors with specially cut, high-viscosity rotors. The manufacturer will be able to provide a graph of flowrate versus pressure drop for various viscosities.

The oval-gear flowmeter works best when there is a little backpressure in the line; a throttling valve on the meter outlet usually works just fine. The oval-gear meter is not suitable for gases, including steam and multi-phase fluids.

More Details or Order Online:

Oval Gear Flowmeters with Integral Display

Advantages: The advantage of the oval-gear flowmeter is the it is, withing certain limits, largely independent of the fluid viscosity (users should just remain aware that higher pressures will be required to push higher-viscosity fluids through the meter). This opens up a whole range of applications, including the metering of oils, syrups and fuels.

Ease of installation is another advantage of th oval design. Because no straight pipe runs or flow conditioning is required, these meters can be installed in tight areas, allowing for more flexibility in application design.

Disadvantage: Oval-gear meters are generally not recommended for water or water-like fluids, because the increased risk of fluid slippage between the gears and chamber walls. Fluid slippage will cause a slight degradation in accuracy, with low-viscosity fluids being more prone to degradation. As viscosity increases, the wall slippage quickly becomes minimal, and the best accuracy is realized. Since the oval-gear meter is really designed for higher-viscosity fluids, it can be argued that running water through them is not a viable application anyway.

Applications:
Oval meters are best suited for the following applications:

  • Measurement of net fuel use in boilers and engines
  • Verification of proper bearing-lubricant delivery in hydraulic applications
  • Monitoring of paper-finishing chemicals
  • Monitoring the flow of wax finishes
  • Monitoring syrup injection in main beverage lines
  • Monitoring and batching volumes of thick candy coating
  • Monitoring and automating the dispensing of cooking oils

The specifications for the six flowmeter designs discussed above will vary widely from manufacturer to manufacturer, and the performance values provided represent an average. When selecting a flowmeter for a given attribute, the engineer should consider additional attributes—including velocity-profile deviations, the effect of non-homogeneous or pulsating flow, and cavitation, all of which will affect flowmeter choice, installation and operation. While beyond the scope of this article, a thorough discussion of these parameters can be found in Ref. 5.

Go to Top



References

  1. <%=session("company_name")%>, 1999-2000 catalog, Vernon Hills, IL, 1999
  2. Hammond, Michael, "Is a Turbine Flowmeter Right for Your Application?," Flow Control, Vol. IV, No. 4, 1998, Witter Publishing Corp., N.J.
  3. Patrick, D., and Fardo, S., "Industrial Process Control Systems," Delmar Publishers, N.Y., 1997
  4. Parr, E. A., "Industrial Control Handbook," 2nd ed., Butterworth-Heinemann, England, 1995
  5. Miller, R. W., "Flow Measurement Engineering Handbook," 2nd ed., McGraw-Hill, N.Y., 1983
  6. Reif, David, "Matching the Flowmeter to the Job," Flow Control, Vol. III, No. 5, 1997, Witter Publishing Corp., N.J.
  7. Swearingen, C., "New Differential Pressure Flow Controllers Offer Exciting Benefits," 1997, European Process Engineer, Volume 7, No. 1, Setform Ltd, England.
  8. Swearingen, C., "High Viscosity Flowmeters: Solution to a Sticky Problem," Flow Control, Vol. IV, No. 5, 1998, Witter Publishing Corp., N.J.
  9. Gilmont, R., and Roccanova, B. "Low-flow rotameter coefficient," Instruments and Control systems, Vol. 39, p. 89, 1966.
  10. Gilmont, R., and Wechsler, L., "Rotameter correlation," Measurements and Control, February 1992, p. 124.