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Mechanical Flowmeters

Discussed in this chapter are various types of mechanical flowmeters that measure flow using an arrangement of moving parts, either by passing isolated, known volumes of a fluid through a series of gears or chambers (positive displacement, or PD) or by means of a spinning turbine or rotor.

All positive displacement flowmeters operate by isolating and counting known volumes of a fluid (gas or liquid) while feeding it through the meter. By counting the number of passed isolated volumes, a flow measurement is obtained. Each PD design uses a different means of isolating and counting these volumes. The frequency of the resulting pulse train is a measure of flow rate, while the total number of pulses gives the size of the batch. While PD meters are operated by the kinetic energy of the flowing fluid, metering pumps (described only briefly in this article) determine the flow rate

while also adding kinetic energy to the fluid.

The turbine flowmeter consists of a multi-bladed rotor mounted at right angles to the flow, suspended in the fluid stream on a free-running bearing. The diameter of the rotor is very close to the inside diameter of the metering chamber, and its speed of rotation is proportional to the volumetric flow rate. Turbine rotation can be detected by solid state devices or by mechanical sensors. Other types of rotary element flowmeters include the propeller (impeller), shunt, and paddlewheel designs.

Positive Displacement Flowmeters

Positive displacement meters provide high accuracy (±0.1% of actual flow rate in some cases) and good repeatability (as high as 0.05% of reading). Accuracy is not affected by pulsating flow unless it entrains air or gas in the fluid. PD meters do not require a power supply for their operation and do not require straight upstream and downstream pipe runs for their installation. PD meters are available in sizes from in to 12 in and can operate with turndowns as high as 100:1, although ranges of 15:1 or lower are much more common. Slippage between the flowmeter components is reduced and metering accuracy is therefore increased as the viscosity of the process fluid increases.

The process fluid must be clean. Particles greater than 100 microns in size must be removed by filtering. PD meters operate with small clearances between their precision-machined parts; wear rapidly destroys their accuracy. For this reason, PD meters are generally not recommended for measuring slurries or abrasive fluids. In clean fluid services, however, their precision and wide rangeability make them ideal for custody transfer and batch charging. They are most widely used as household water meters. Millions of such units are produced annually at a unit cost of less than USD$50 U.S. In industrial and petrochemical applications, PD meters are commonly used for batch charging of both liquids and gases.

Although slippage through the PD meter decreases (that is, accuracy increases) as fluid viscosity increases, pressure drop through the meter also rises. Consequently, the maximum (and minimum) flow capacity of the flowmeter is decreased as viscosity increases. The higher the viscosity, the less slippage and the lower the measurable flow rate becomes. As viscosity decreases, the low flow

performance of the meter deteriorates. The maximum allowable pressure drop across the meter constrains the maximum operating flow in high viscosity services.

Liquid PD Meters

Nutating disc meters are the most common PD meters. They are used as residential water meters around the world. As water flows through the metering chamber, it causes a disc to wobble (nutate), turning a spindle, which rotates a magnet. This magnet is coupled to a mechanical register or a pulse transmitter. Because the flowmeter entraps a fixed quantity of fluid each time the spindle is rotated, the rate of flow is proportional to the rotational velocity of the spindle (Figure 3-1A).

Because it must be nonmagnetic, the meter housing is usually made of bronze but can be made from plastic for corrosion resistance or cost savings. The wetted parts such as the disc and spindle are usually bronze, rubber, aluminum, neoprene, Buna-N, or a fluoroelastomer such as Viton®. Nutating disc meters are designed for water service and the materials of which they are made must be checked for compatibility with other fluids. Meters with rubber discs give better accuracy than metal discs due to the better sealing they provide.

Nutating disc meters are available in 5/8-in to 2-in sizes. They are suited for 150-psig operating pressures with overpressure to a maximum of 300 psig. Cold water service units are temperature-limited to 120°F. Hot water units are available up to 250°F.

These meters must meet American Water Works Association (AWWA) standards for accuracy. The accuracy of these meters is required to be ±2% of actual flow rate. Higher viscosity can produce higher accuracy, while lower viscosity and wear over time will reduce accuracy. The AWWA requires that residential water meters be re-calibrated every 10 years. Because of the intermittent use patterns of residential users, this corresponds to recalibrating 5/8 x 3/4 in residential water meters after they have metered 5 million gallons. In industrial applications, however, these meters are likely to pass this threshold much sooner. The maximum continuous flow of a nutating disc meter is usually about 60-80% of the maximum flow in intermittent service.

Rotating vane meters (Figure 3-1B) have spring-loaded vanes that entrap increments of liquid between the eccentrically mounted rotor and the casing. The rotation of the vanes moves the flow increment from inlet to outlet and discharge. Accuracy of ±0.1% of actual rate (AR) is normal, and larger size meters on higher viscosity services can achieve accuracy to within 0.05% of rate.

Rotating vane meters are regularly

used in the petroleum industry and are capable of metering solids-laden crude oils at flow rates as high as 17,500 gpm. Pressure and temperature limits depend on the materials of construction and can be as high as 350°F and 1,000 psig. Viscosity limits are 1 to 25,000 centipoise.

In the rotary displacement meter, a fluted central rotor operates in constant relationship with two wiper rotors in a six-phase cycle. Its applications and features are similar to those of the rotary vane meter.

Piston Meters

Oscillating piston flowmeters typically are used in viscous fluid services such as oil metering on engine test stands where turndown is not critical (Figure 3-2). These meters also can be used on residential water service and can pass limited quantities of dirt, such as pipe scale and fine (viz,-200 mesh or -74 micron) sand, but not large particle size or abrasive solids.

The measurement chamber is cylindrical with a partition plate separating its inlet port from its outlet. The piston is also cylindrical and is punctured by numerous openings to allow free flow on both sides of the piston and the post (Figure 3-2A). The piston is guided by a control roller within the measuring chamber, and the motion of the piston is transferred to a follower magnet which is external to the flowstream. The follower magnet can be used to drive either a transmitter, a register, or both. The motion of the piston is oscillatory (not rotary) since it is constrained to move in one plane. The rate of flow is proportional to the rate of oscillation of the piston.

The internals of this flowmeter can be removed without disconnection of the meter from the pipeline. Because of the close tolerances required to seal the piston and to reduce slippage, these meters require regular maintenance. Oscillating piston flow meters are available in 1/2-in to 3-in sizes, and can generally be used between 100 and 150 psig. Some industrial versions are rated to 1,500 psig. They can meter flow rates from 1 gpm to 65 gpm in continuous service with intermittent excursions to 100 gpm. Meters are sized so that pressure drop is below 35 psid at maximum flow rate. Accuracy ranges from ±0.5 % AR for viscous fluids to ±2% AR for nonviscous applications. Upper limit on viscosity is 10,000 centipoise.

Reciprocating piston meters are probably the oldest PD meter designs. They are available with multiple pistons, double-acting pistons, or rotary pistons. As in a reciprocating piston engine, fluid is drawn into one piston chamber as it is discharged from the opposed piston in the meter. Typically, either a crankshaft or a horizontal slide is used to control the opening and closing of the proper orifices in the meter. These meters are usually smaller (available in sizes down to 1/10-in diameter) and are used for measuring very low flows of viscous liquids.

Gear & Lobe Meters

The oval gear PD meter uses two fine-toothed gears, one mounted horizontally, the other vertically, with gears meshing at the tip of the vertical gear and the center of the horizontal gear (Figure 3-3A). The two rotors rotate opposite to each other, creating an entrapment in the crescent-shaped gap between the housing and the gear. These meters can be very accurate if slippage between the housing and the gears is kept small. If the process fluid viscosity is greater than 10 centipoise and the flowrate is above 20% of rated capacity, accuracy of 0.1% AR can be obtained. At lower flows and at lower viscosity, slippage increases and accuracy decreases to 0.5% AR or less.

The lubricating characteristics of the process fluid also affect the turndown of an oval gear meter. With liquids that do not lubricate well, maximum rotor speed must be derated to limit wear. Another way to limit wear is to keep the pressure drop across the meter below 15 psid. Therefore, the pressure drop across the meter limits the allowable maximum flow in high viscosity service.

Rotating lobe and impeller type PD meters are variations of the oval gear flowmeter that do not share its precise gearing. In the rotating lobe design, two impellers rotate in opposite directions within the ovoid housing (Figure 3-3B). As they rotate, a fixed volume of liquid is entrapped and then transported toward the outlet. Because the lobe gears remain in a fixed relative position, it is only necessary to measure the rotational velocity of one of them. The impeller is either geared to a register or is magnetically coupled to a transmitter. Lobe meters can be furnished in 2-in to 24-in line sizes. Flow capacity is 8-10 gpm to 18,000 gpm in the larger sizes. They provide good repeatability (better than 0.015% AR) at high flows and can be used at high operating pressures (to 1,200 psig) and temperatures (to 400°F).

The lobe gear meter is available in a wide range of materials of construction, from thermoplastics to highly corrosion-resistant metals. Disadvantages of this design include a loss of accuracy at low flows. Also, the maximum flow through this meter is less than for the same size oscillatory piston or nutating disc meter.

In the rotating impeller meter, very coarse gears entrap the fluid and pass a fixed volume of fluid with each rotation (Figure 3-3C). These meters are accurate to 0.5% of rate if the viscosity of the process fluid is both high and constant, or varies only within a narrow band. These meters can be made out of a variety of metals, including stainless steel, and corrosion-resistant plastics such as PVDF (Kynar). These meters are used to meter paints and, because they are available in 3A or sanitary designs, also milk, juices, and chocolate.

In these units, the passage of magnets embedded in the lobes of the rotating impellers is sensed by proximity switches (usually Hall-effect detectors) mounted external to the flow chamber. The sensor transmits a pulse train to a counter or flow controller. These meters are available in 1/10-in to 6-in sizes and can handle pressures to 3,000 psig and temperatures to 400°F.

Helix Meters

The helix meter is a positive displacement device that uses two radially pitched helical gears to continuously entrap the process fluid as it flows. The flow forces the helical gears to rotate in the plane of the pipeline. Optical or magnetic sensors are used to encode a pulse train proportional to the rotational speed of the helical gears. The forces required to make the helices rotate are relatively small and therefore, in comparison to other PD meters, the pressure drop is relatively low. The best attainable accuracy is about ±0.2% or rate.

As shown in Figure 3-4, measurement error rises as either the operating flowrate or the viscosity of the process fluid drops. Helical gear meters can measure the flow of highly viscous fluids (from 3 to 300,000 cP), making them ideal for extremely

thick fluids such as glues and very viscous polymers. Because at maximum flow the pressure drop through the meter should not exceed 30 psid, the maximum rated flow through the meter is reduced as the fluid viscosity increases. If the process fluid has good lubricating characteristics, the meter turndown can be as high as 100:1, but lower (10:1) turndowns are more typical.

Metering Pumps

Metering pumps are PD meters that also impart kinetic energy to the process fluid. There are three basic designs: peristaltic, piston, and diaphragm.

Peristaltic pumps operate by having fingers or a cam systematically squeeze a plastic tubing against the housing, which also serves to position the tubing. This type of metering pump is used in laboratories, in a variety of medical applications, in the majority of environmental sampling systems, and also in dispensing hypochlorite solutions. The tubing can be silicone-rubber or, if a more corrosion-resistant material is desired, PTFE tubing.

Piston pumps deliver a fixed volume of liquid with each "out" stroke and a fixed volume enters the chamber on each "in" stroke (Figure 3-5A). Check valves keep the fluid flow from reversing. As with all positive displacement pumps, piston pumps generate a pulsating flow. To minimize the pulsation, multiple pistons or pulsation-dampening reservoirs are installed. Because of the close tolerances of the piston and cylinder sleeve, a flushing mechanism must be provided in abrasive applications. Piston pumps are sized on the basis of the displacement of the piston and the required flow rate and discharge pressure. Check valves (or, on critical applications, double check valves) are selected to protect against backflow.

Diaphragm pumps are the most common industrial PD pumps (Figure 3-5B). A typical configuration consists of a single diaphragm, a chamber, and suction and discharge check valves to prevent backflow. The piston can either be directly coupled to the diaphragm or can force a hydraulic oil to drive the diaphragm. Maximum output pressure is about 125 psig. Variations include bellows-type diaphragms, hydraulically actuated double diaphragms, and air-operated, reciprocating double-diaphragms.

Gas PD Meters

PD gas meters operate by counting the number of entrapped volumes of gas passed, similar to the way PD meters operate on liquids. The primary difference is that gases are compressible.

Diaphragm gas meters most often are used to measure the flow of natural gas, especially in metering consumption by households. The meter is constructed from aluminum castings with cloth-backed rubber diaphragms. The meter consists of four chambers: the two diaphragm chambers on the inlet and outlet sides and the inlet and outlet chambers of the meter body. The passage of gas through the meter creates a differential pressure between the two diaphragm chambers by compressing the one on the inlet side and expanding the one on the outlet side. This action alternately empties and fills the four chambers. The slide valves at the top of the meter alternate the roles of the chambers and synchronize the action of the diaphragms, as

well as operating the crank mechanism for the meter register.

Diaphragm meters generally are calibrated for natural gas, which has a specific gravity of 0.6 (relative to air). Therefore, it is necessary to re-calibrate the flow rating of the meter when it is used to meter other gases. The calibration for the new flow rating (QN) is obtained by multiplying the meter's flow rating for natural gas (QC) by the square root of the ratio of the specific gravities of natural gas (0.6) and the new gas (SGN):

Diaphragm meters are usually rated in units of cubic feet per hour and sized for a pressure drop of 0.5-2 in H2O. Accuracy is roughly ±1% of reading over a 200:1 range. They maintain their accuracy for long periods of time, which makes them good choices for retail revenue metering applications. Unless the gas is unusually dirty (producer gas, or recycled methane from composting or digesting, for example), the diaphragm meter will operate with little or no maintenance indefinitely.

Lobe gear meters (or lobed impeller meters, as they are also known), also are used for gas service. Accuracy in gas service is ±1% of rate over a 10:1 turndown, and typical pressure drop is 0.1 psid. Because of the close tolerances, upstream filtration is required for dirty lines.

Rotating vane meters measure the flow of gas in the same ranges as do lobe gear meters (up to 100,000 ft3/hr) but can be used over a wider 25:1 turndown. They also incur a lower pressure drop of 0.05 in H2O for similar accuracy, and, because the clearances are somewhat more forgiving, upstream filtration is not as critical.

High-Precision PD Systems

High-precision gas meters are usually a hybrid combining a standard PD meter and a motor drive that eliminates the pressure drop across the meter. Equalizing the inlet and outlet pressures eliminates slip flows, leakage, and blow-by. In high-precision gas flowmeter installations, high-sensitivity leaves are used to detect the pressure differential, and displacement transducers are used to measure the deflection of the leaves (Figure 3-6A). Designed to operate at

Figure 3-7: Click on figure to enlarge. ambient temperatures and at up to 30 psig pressures, this meter is claimed to provide accuracy to within 0.25% of reading over a 50:1 range and 0.5% over a 100:1 range. Flow capacity ranges from 0.3-1,500 scfm.

For liquid service, a servomotor-driven oval-gear meter equalizes the pressure across the meter. This increases accuracy at low flows and under varying viscosity conditions (Figure 3-6B). This flowmeter uses a very sensitive piston to detect the meter differential and drives a variable speed servomotor to keep it near zero. This design is claimed to provide 0.25% of rate accuracy over a 50:1 range at operating pressures of up to 150 psig. High precision flowmeters are used on engine test stands for fuel flow measurement (gasoline, diesel, alcohol, etc.). Flow ranges from 0.04-40 gph are typical. Vapor separators are usually included, to prevent vapor lock.

Testing, Calibration & Provers

All meters with moving parts require periodic testing, recalibration and repair, because wear increases the clearances. Recalibration can be done either in a laboratory or on line using a prover.

Gas systems are recalibrated against a bell-jar prover--a calibrated cylindrical bell, liquid sealed in a tank. As the bell is lowered, it discharges a known volume of gas through the meter being tested. The volumetric accuracy of bell-jar provers is on the order of 0.1% by volume, and provers are available in discharge volumes of 2, 5, 10 ft3 and larger.

Liquid systems can be calibrated in the laboratory against either a calibrated secondary standard or a gravimetric flow loop. This approach can provide high accuracy (up to ±0.01% of rate) but requires removing the flowmeter from service.

In many operations, especially in the petroleum industry, it is difficult or impossible to remove a flow- meter from service for calibration. Therefore, field-mounted and in-line provers have been developed. This type of prover consists of a calibrated chamber equipped with a barrier piston (Figure 3-7). Two detectors are mounted a known distance (and therefore a known volume) apart. As the flow passes through the chamber, the displacer piston is moved downstream. Dividing the volume of the chamber by the time it takes for the displacer to move from one detector to the other gives the calibrated flow rate. This rate is then compared to the reading of the flowmeter under test.

Provers are repeatable on the order of 0.02%, and can operate at up to 3,000 psig and 165°F/75°C. Their operating flow range is from as low as 0.001 gpm to as high as 20,000 gpm. Provers are available for bench-top use, for mounting in truck-beds, on trailers, or in-line.

PD Meter Accessories

PD meter accessories include strainers, filters, air/vapor release assemblies, pulsation dampeners, temperature compensation systems, and a variety of valves to permit dribble cut-off in batching systems. Mechanical registers can be equipped with mechanical or electronic ticket-printers for inventory control and point-of-use sales. Batching flow computers are readily available, as are analog and intelligent digital transmitters. Automatic meter reading (AMR) devices permit the remote retrieval of readings by utility personnel.

Turbine Flowmeters

Invented by Reinhard Woltman in the 18th century, the turbine flowmeter is an accurate and reliable flowmeter for both liquids and gases. It consists of a multi-bladed rotor mounted at right angles to the flow and suspended in the fluid stream on a free-running bearing. The diameter of the rotor is very slightly less than the inside diameter of the metering chamber, and its speed of rotation is proportional to the volumetric flow rate. Turbine rotation can be detected by solid state devices (reluctance, inductance, capacitive and Hall-effect pick-ups) or by mechanical sensors (gear or magnetic drives).

In the reluctance pick-up, the coil is a permanent magnet and the turbine blades are made of a material attracted to magnets. As each blade passes the coil, a voltage is generated

in the coil (Figure 3-8A). Each pulse represents a discrete volume of liquid. The number of pulses per unit volume is called the meter's K-factor.

In the inductance pick-up, the permanent magnet is embedded in the rotor, or the blades of the rotor are made of permanently magnetized material (Figure 3-8B). As each blade passes the coil, it generates a voltage pulse. In some designs, only one blade is magnetic and the pulse represents a complete revolution of the rotor.

The outputs of reluctance and inductive pick-up coils are continuous sine waves with the pulse train's frequency proportional to the flow rate. At low flow, the output (the height of the voltage pulse) may be on the order of 20 mV peak-to-peak. It is not advisable to transport such a weak signal over long distances. Therefore, the distance between the pickup and associated display electronics or preamplifier must be short.

Capacitive sensors produce a sine wave by generating an RF signal that is amplitude-modulated by the movement of the rotor blades. Instead of pick-up coils, Hall-effect transistors also can be used. These transistors change their state when they are in the presence of a very low strength (on the order of 25 gauss) magnetic field.

In these turbine flowmeters, very small magnets are embedded in the tips of the rotor blades. Rotors are typically made of a non-magnetic material, like polypropylene, Ryton, or PVDF (Kynar). The signal output from a Hall-effect sensor is a square wave pulse train, at a frequency proportional to the volumetric flowrate.

Because Hall-effect sensors have no magnetic drag, they can operate at lower flow velocities (0.2 ft/sec) than magnetic pick-up designs (0.5-1.0 ft/sec). In addition, the Hall-effect sensor provides a signal of high amplitude (typically a 10.8-V square wave), permitting distances up to 3,000 ft. between the sensor and the electronics without amplification.

In the water distribution industry, mechanical-drive Woltman-type turbine flowmeters continue to be the standard. These turbine meters use a gear train to convert the rotation of the rotor into the rotation of a vertical shaft. The shaft passes between the metering tube and the register section through a mechanical stuff

Figure 3-9: Click on figure to enlarge. ing box, turning a geared mechanical register assembly to indicate flow rate and actuate a mechanical totalizer counter.

More recently, the water distribution industry has adopted a magnetic drive as an improvement over high maintenance mechanical-drive turbine meters. This type of meter has a sealing disc between the measuring chamber and the register. On the measuring chamber side, the vertical shaft turns a magnet instead of a gear. On the register side, an opposing magnet is mounted to turn the gear. This permits a completely sealed register to be used with a mechanical drive mechanism.

In the United States, the AWWA sets the standards for turbine flowmeters used in water distribution systems. Standard C701 provides for two classes (Class I and Class II) of turbine flowmeters. Class I turbine meters must register between 98-102% of actual rate at maximum flow when tested. Class II turbine meters must register between 98.5-101.5% of actual rate. Both Class I and Class II meters must have mechanical registers.

Solid state pickup designs are less susceptible to mechanical wear than AWWA Class I and Class II meters.

Design & Construction Variations

Most industrial turbine flowmeters are manufactured from austenitic stainless steel (301, 303, 304SS),

This innovative turbine meter trades out a transmitted signal for local LCD indication.

whereas turbine meters intended for municipal water service are bronze or cast iron. The rotor and bearing materials are selected to match the process fluid and the service. Rotors are often made from stainless steel, and bearings of graphite, tungsten carbide, ceramics, or in special cases of synthetic ruby or sapphire combined with tungsten carbide. In all cases, bearings and shafts are designed to provide minimum friction and maximum resistance to wear. Some corrosion-resistant designs are made from plastic materials such as PVC.

Small turbine meters often are called barstock turbines because in sizes of 3/4 in to 3 in. they are machined from stainless steel hexagonal barstock. The turbine is suspended by a bearing between two hanger assemblies that also serve to condition the flow. This design is suited for high operating pressures (up to 5,000 psig).

Similar to a pitot tube differential pressure flowmeter, the insertion turbine meter is a point-velocity device. It is designed to be inserted into either a liquid or a gas line to a depth at which the small-diameter rotor will read the average velocity in the line. Because they are very sensitive to the velocity profile of the flowing stream, they must be profiled at several points across the flow path.

Insertion turbine meters can be designed for gas applications (small, lightweight rotor) or for liquid (larger rotor, water-lubricated bearings). They are often used in large diameter pipelines where it would be cost-prohibitive to install a full size meter. They can be hot-tapped into existing

pipelines (6 in or larger) through a valving system without shutting down the process. Typical accuracy of an insertion turbine meter is 1% FS, and the minimum flow velocity is about 0.2 ft/sec.

Turbine Meter Accuracy

Figure 3-9 shows a typical turbine-meter calibration curve describing the relationship between flow and K-factor (pulses/gallon). The accuracy of turbine meters is typically given in percentage of actual rate (% AR). This particular meter has a linearity tolerance band of ±0.25% over a 10:1 flow range and a ±0.15% linearity in a 6:1 range. The repeatability is from ±0.2% to ±0.02% over the linear range.

Because there are minor inconsistencies in the manufacturing process, all turbine flowmeters are calibrated prior to shipment. The resulting K-factor in pulses per volume unit will vary within the stated linearity specification. It is possible, however, to register several K-factors for different portions of the flow range and to electronically switch from one to the other as the measured flow changes. Naturally, the K-factor is applicable only to the fluid for which the meter was calibrated.

Barstock turbine meters typically are linear to ±0.25% AR over a 10:1 flow range. The linearity of larger meters is ±0.5% AR over a 10:1 flow range. Turbine meters have a typical nonlinearity (the turbine meter hump, shown in Figure 3-9) in the lower 25-30% of their range. Keeping the minimum flow reading above this region will permit linearity to within 0.15% on small and 0.25% on larger turbine meters. If the range of 10:1 is insufficient, some turbine flow- meters can provide up to 100:1 turndowns if accuracy is de-rated to 1% of full scale (FS).

Sizing & Selection

Turbine meters should be sized so that the expected average flow is between 60% and 75% of the maximum capacity of the meter. If the pipe is oversized (with flow velocity under 1 ft/sec), one should select a Hall-effect pick-up and use a meter smaller than the line size. Flow velocities under 1 ft/sec can be insufficient, while velocities in excess of 10 ft/sec can result in excessive wear. Most turbine meters are designed for maximum velocities of 30 ft/sec.

Turbine flowmeters should be sized for between 3 and 5 psid pressure drop at maximum flow. Because pressure drop increases with the square of flow rate, reducing the meter to the next smaller size will raise the pressure drop considerably.

Viscosity affects the accuracy and linearity of turbine meters. It is therefore important to calibrate the meter for the specific fluid it is intended to measure. Repeatability is generally not greatly affected by changes in viscosity, and turbine meters often are used to control the flow of viscous fluids. Generally, turbine meters perform well if the Reynolds Number is greater than 4,000 and less than or equal to 20,000.

Because it affects viscosity, temperature variation can also adversely affect accuracy and must be compensated for or controlled. The turbine meter's operating temperature ranges from -200 to 450°C (-328 to 840°F).

Density changes do not greatly affect turbine meters. On low density fluids (SG < 0.7), the minimum flow rate is increased due to the reduced torque, but the meter's accuracy usually is not affected.

Installation & Accessories

Turbine meters are sensitive to upstream piping geometry that can cause vortices and swirling flow. Specifications call for 10-15 diameters of straight run upstream and five diameters of straight run downstream of the meter. However, the presence of any of the following obstructions upstream would necessitate that there be more than 15 diameters of upstream straight-pipe runs

20 diameters for 90° elbow, tee, filter, strainer, or thermowell;

25 diameters for a partially open valve; and

50 or more diameters if there are two elbows in different planes or if the flow is spiraling or corkscrewing.

In order to reduce this straight-run requirement, straightening vanes are installed. Tube bundles or radial vane elements are used as external flow straighteners located at least 5 diameters upstream of the meter (Figure 3-10).

Under certain conditions, the pressure drop across the turbine can cause flashing or cavitation. The first causes the meter to read high, the second results in rotor damage. In order to protect against this, the downstream pressure must be held at a value equaling 1.25 times the vapor pressure plus twice the pressure drop. Small amounts of air entrainment (100 mg/l or less) will make the meter read only a bit high, while large quantities can destroy the rotor.

Turbine meters also can be damaged by solids entrained in the fluid. If the amount of suspended solids exceeds 100 mg/l of +75 micron size, a flushing y-strainer or a motorized cartridge filter must be installed at least 20 diameters of straight run upstream of the flowmeter.

New Developments

Dual-rotor liquid turbines increase the operating range in small line size (under 2 in) applications. The two rotors turn in opposite directions. The front one acts as a conditioner, directing the flow to the back rotor. The rotors lock hydraulically and continue to turn as the flow decreases even to very low rates.

The linearity of a turbine meter is affected by the velocity profile (often dictated by the installation), viscosity, and temperature. It is now possible to include complex linearization functions in the preamplifier of a turbine flowmeter to reduce these nonlinearities. In addition, advances in fieldbus technology make it possible to recalibrate turbine flowmeters continuously, thereby correcting for changes in temperature and viscosity.

Flow computers are capable of linearization, automatic temperature compensation, batching, calculation of BTU content, datalogging, and storage of multiple K-factors. The batching controller is set with the desired target volume and, when its totalizer has counted down to zero, it terminates the batch. Such packages are equipped with dribble flow, pre-warn, or trickle-cut-off circuits. Whether functioning through a relay contact or a ramp function, these features serve to minimize splashing or overfill and to accurately terminate the batch.

Gas Turbine & Shunt Meters

Gas meters compensate for the lower driving torque produced by the relatively low density of gases. This compensation is obtained by very large rotor hubs, very light rotor assemblies, and larger numbers of rotor blades. Gas turbine meters are available from 2" to 12" and with flow ratings up to 150,000 ft3/hr. When operating at elevated gas pressures (1,400 psig), a rangeability of 100:1 can be obtained in larger size meters. Under lower pressure conditions, typical rangeability is 20:1 with ±1% linearity. The minimum upstream straight pipe-run requirement is 20 pipe diameters.

Shunt flowmeters are used in gas and steam service. They consist of an orifice in the main line and a rotor assembly in the bypass. These meters are available is sizes 2 in. and larger and are accurate to ±2% over a range of 10:1.

Other Rotary Flowmeters

Other types of rotary element flowmeters include propeller (impeller), shunt, and paddlewheel designs.

Propeller meters are commonly used in large diameter (over 4 in) irrigation and water distribution systems. Their primary trade-off is low cost and low accuracy (Figure 3-11A). AWWA Standard C-704 sets the accuracy criterion for propeller meters at 2% of reading. Propeller meters have a rangeability of about 4:1 and exhibit very poor performance if the velocity drops below 1.5 ft/sec. Most propeller meters are equipped with mechanical registers. Mechanical wear, straightening, and conditioning requirements are the same as for turbine meters.

Paddlewheel flowmeters use a rotor whose axis of rotation is parallel to the direction of flow (Figure 3-11B). Most paddlewheel meters have flat-bladed rotors and are inherently bi-directional. Several manufacturers, however, use crooked rotors that only rotate in the forward direction. For smaller pipes (1/2" to 3"), these meters are available only with a fixed insertion depth, while for larger pipe sizes (4" to 48") adjustable insertion depths are available. The use of capacitively coupled pick-ups or Hall-effect sensors extends the range of paddlewheel meters into the low-flow velocity region of 0.3 ft/sec.

Low-flow meters (usually smaller than 1 in.) have a small jet orifice that projects the fluid onto a Pelton wheel. Varying the diameter and the shape of the jet orifice matches the required flow range and provides a flowmeter that is accurate to 1% FS and has a rangeability of 100:1. Higher accuracy can be achieved by calibrating the meter and by lowering its range. Because of the small size of the jet orifice, these meters can only be used on clean fluids and they incur a pressure drop of about 20 psid. Materials of construction include polypropylene, PVDF, TFE and PFA, brass, aluminum, and stainless steel.

References & Further Reading OMEGA Complete Flow and Level Measurement Handbook and Encyclopedia®, OMEGA Press, 1995. OMEGA Volume 29 Handbook & Encyclopedia, Purchasing Agents Edition, OMEGA Press, 1995. Flow Measurement Engineering Handbook, Miller, McGraw-Hill, 1982. Flow Measurement, D. W. Spitzer, ISA, 1991. Flowmeters in Water Supply, Manual M33, AWWA, 1989. Industrial Flow Measurement, D. W. Spitzer, ISA 1984. Instrument Engineer's Handbook, Bela Liptak, editor, CRC Press, 1995. "Turbine Flowmeter Extends Flow Range", E. Piechota, Flow Control, February, 1997. Water Meters--Selection, Installation, Testing and Maintenance, Manual M6, AWWA, 1986.

Source: www.omega.de

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