Flow Measurement: Intelligent Flowmeters and Calibration

Intelligent Flowmeters

All the usual benefits associated with intelligent instruments are applicable to most types of flowmeters. Indeed, all types of mass flowmeters routinely have intelligence as an integral part of the instrument. For volume flow rate measurement, intelligent differential pressure measuring instruments can be used to good effect in conjunction with obstruction-type flow transducers. One immediate benefit of this in the case of the commonest flow restriction device, the orifice plate, is to extend the lowest flow measurable with acceptable accuracy down to 20% of the maximum flow value. In positive displacement meters, intelligence allows compensation for thermal expansion of meter components and temperature-induced viscosity changes.

Correction for variations in flow pressure is also provided for. Intelligent electromagnetic flowmeters are also available, and these have a self-diagnosis and self-adjustment capability.

The usable instrument range is typically from 3 to 100% of the full-scale reading, and the quoted maximum inaccuracy is _0.5%. It’s also normal to include a nonvolatile memory to protect constants used for correcting for modifying inputs and so on against power supply failures. Intelligent turbine meters are able to detect their own bearing wear and also report deviations from initial calibration due to blade damage, etc. Some versions also have a self-adjustment capability.

The ability to carry out digital signal processing has also led to emergence of the cross-correlation ultrasonic flowmeter. This is a variant of the transit time form of ultrasonic flowmeter in which a series of ultrasonic signals are injected into the flowing liquid. The ultrasonic receiver stores the echo pattern from each input signal and then cross-correlation techniques are used to produce a map of the profile of the water flow in different layers. Thus, the instrument provides information on the profile of the flowrate across the cross section of the pipe rather than just giving a measurement of the mean flow rate in the pipe.

The trend is now moving toward total flow computers, which can process inputs from almost any type of transducer. Such devices allow user input of parameters such as specific gravity, fluid density, viscosity, pipe diameters, thermal expansion coefficients, and discharge coefficients.

Auxiliary inputs from temperature transducers are also catered for. After processing raw flow transducer output with this additional data, flow computers are able to produce measurements of flow to a very high degree of accuracy.

Choice between Flowmeters for Particular Applications

The number of relevant factors to be considered when specifying a flowmeter for a particular application is very large. These include the temperature and pressure of the fluid, its density, viscosity, chemical properties and abrasiveness, whether it contains particles, whether it’s a liquid or gas, etc. This narrows the field to a subset of instruments that are physically capable of making the measurement. Next, the required performance factors of accuracy, rangeability, acceptable pressure drop, output signal characteristics, reliability, and service life must be considered. Accuracy requirements vary widely across different applications, with measurement uncertainty of _5% being acceptable in some and less than _0.5% being demanded in others.

Finally, economic viability must be assessed, which must take into account not only the purchase cost, but also reliability, installation difficulties, maintenance requirements, and service life.

Where only a visual indication of flow rate is needed, the variable area meter is popular. Where a flow measurement in the form of an electrical signal is required, the choice of available instruments is very large. The orifice plate is used extremely commonly for such purposes and accounts for almost 50% of instruments currently in use in industry. Other forms of differential pressure meters and electromagnetic flowmeters are used in significant numbers. Currently, there is a trend away from rotating devices, such as turbine meters and positive displacement meters.

At the same time, usage of ultrasonic and vortex meters is expanding.

Calibration of Flowmeters

The first consideration in choosing a suitable way to calibrate flow-measuring instruments is to establish exactly what accuracy level is needed so that the calibration system instituted does not cost more than necessary. In some cases, such as handling valuable fluids or where there are legal requirements as in petrol pumps, high accuracy levels (e.g., error _0.1%) are necessary and the expensive procedures necessary to achieve these levels are justified. However, in other situations, such as in measuring additives to the main stream in a process plant, only low levels of accuracy are needed (e.g., error _5% is acceptable) and relatively inexpensive calibration procedures are sufficient.

The accuracy of flow measurement is affected greatly by the flow conditions and characteristics of the flowing fluid. Therefore, wherever possible, process flow-measuring instruments are calibrated on-site in their normal measuring position. This ensures that calibration is performed in the actual flow conditions, which are difficult or impossible to reproduce exactly in a laboratory. To ensure the validity of such calibration, it’s also normal practice to repeat flow calibration checks until the same reading is obtained in two consecutive tests. However, it has been suggested that even these precautions are inadequate and that statistical procedures are needed.

If on-site calibration is not feasible or is not accurate enough, the only alternative is to send the instrument away for calibration using special equipment provided by instrument manufacturers or other specialist calibration companies. However, this is usually an expensive option. Furthermore, the calibration facility does not replicate the normal operating conditions of the meter tested, and appropriate compensation for differences between calibration conditions and normal use conditions must be applied.

The equipment and procedures used for calibration depend on whether mass, liquid, or gaseous flows are being measured. Therefore, separate sections are devoted to each of these cases. It must also be stressed that all calibration procedures mentioned in the following paragraphs in respect to fluid flow only refer to flows of single phase fluids (i.e., liquids or gases).

Where a second or third phase is present, calibration is much more difficult and specialist advice should be sought from the manufacturer of the instrument used for measurement.

Calibration Equipment and Procedures for Mass Flow-Measuring Instruments

Where the conveyor method is used for measuring the mass flow of solids in the form of particles or powders, both mass-measuring and velocity-measuring instruments are involved.

Suitable calibration techniques for each of these are discussed in later sections.

In the case of Coriolis and thermal mass flowmeters, the usual method of calibrating these while in situ in their normal measurement position is to provide a diversion valve after the meter.

During calibration procedures, the valve is opened for a measured time period to allow some of the fluid to flow into a container that is subsequently weighed. Alternatively, the meter can be removed for calibration using special test rigs normally provided by the instrument manufacturer.

Calibration Equipment and Procedures for Instruments Measuring Volume Flow Rate of Liquids

Calibrated tank:

Probably the simplest piece of equipment available for calibrating instruments measuring liquid flow rates is the calibrated tank. This consists of a cylindrical vessel with conical ends that facilitate draining and cleaning of the tank. A sight tube with a graduated scale is placed alongside the final, upper, cylindrical part of the tank, which allows the volume of liquid in the tank to be measured accurately. Flowrate calibration is performed by measuring the time taken, starting from an empty tank, for a given volume of liquid to flow into the vessel.

Because the calibration procedure starts and ends in zero flow conditions, it’s not suitable for calibrating instruments affected by flow acceleration and deceleration characteristics. This therefore excludes instruments such as differential pressure meters (orifice plate, flow nozzle, Venturi, Dall flow tube, pitot tube), turbine flowmeters, and vortex-shedding flowmeters.

The technique is further limited to the calibration of low-viscosity liquid flows, although lining the tank with an epoxy coating can allow the system to cope with somewhat higher viscosities.

The limiting factor in this case is the drainage characteristics of the tank, which must be such that the residue liquid left after draining has an insufficient volume to affect the accuracy of the next calibration.

Pipe prover:

The commonest form of pipe prover is the bidirectional type, which consists of a U-shaped tube of metal of accurately known cross section. The purpose of the U bend is to give a long flow path within a compact spatial volume. Alternative versions with more than one U bend also exist to cater for situations where an even longer flow path is required. Inside the tube is a hollow, inflatable sphere, which is filled with water until its diameter is about 2%larger than that of the tube. As such, the sphere forms a seal with the sides of the tube and acts as a piston. The prover is connected into the existing fluid-carrying pipe network via tappings either side of a bypass valve. A four-way valve at the start of the U tube allows fluid to be directed in either direction around it. Calibration is performed by diverting flow into the prover and measuring the time taken for the sphere to travel between two detectors in the tube. The detectors are normally of an electromechanical, plunger type.

Unidirectional versions of the aforementioned also exist in which fluid only flows in one direction around the tube. A special handling valve has to be provided to return the sphere to the starting point after each calibration, but the absence of a four-way flow control valve makes such devices significantly less expensive than bidirectional types.

Pipe provers are particularly suited to the calibration of pressure-measuring instruments that have a pulse type of output, such as turbine meters. In such cases, the detector switches in the tube can be made to gate the instrument's output pulse counter. This enables not only the basic instrument to be calibrated, but also the ancillary electronics within it at the same time.

The inaccuracy level of such provers can be as low as _0.1%. This level of accuracy is maintained for high fluid viscosity levels and also at very high flow rates. Even higher accuracy is provided by an alternative form of prover, which consists of a long, straight metal tube containing a metal piston. However, such devices are more expensive than the other types discussed earlier and their large space requirements also often cause great difficulties.

Compact prover:

The compact prover has an identical operating principle to that of the other pipe provers described earlier but occupies a much smaller spatial volume. It’s therefore used extensively in situations where there is insufficient room to use a larger prover. Many different designs of compact prover exist, operating in both unidirectional and bidirectional modes. Common features of compact provers are an accurately machined cylinder containing a metal piston that is driven between two reference marks by the flowing fluid. The instants at which the reference marks are passed are detected by switches, of optical form in the case of the version. Provision has to be made within these instruments for returning the piston back to the starting point after each calibration and a hydraulic system is used commonly for this. Again, measuring the piston traverse time is made easier if the switches can be made to gate a pulse train, and therefore compact provers are also most suited to instruments having a pulse-type output such as turbine meters. Measurement uncertainty levels down to _0.1% are possible.

The main technical difficulty in compact provers is measuring the traverse time, which can be as small as 1 second. The pulse count from a turbine meter in this time would typically be only about 100, making the possible measurement error 1%. To overcome this problem, electronic pulse interpolation techniques have been developed that can count fractions of pulses.

Positive displacement meter:

High-quality versions of the positive displacement flowmeter can be used as a reference standard in flowmeter calibration. Such devices can give measurement inaccuracy levels down to _0.2%.

Gravimetric method:

A variation on the principle of measuring the volume of liquid flowing in a given time is to weigh the quantity of fluid flowing in a given time. Apart from its applicability to a wider range of instruments, this technique is not limited to low-viscosity fluids, as any residual fluid in the tank before calibration will be detected by the load cells and therefore compensated for. In the simplest implementation of this system, fluid is allowed to flow for a measured length of time into a tank resting on load cells. As before, the stop-start mode of fluid flow makes this method unsuitable for calibrating differential pressure, turbine, and vortex-shedding flowmeters. It’s also unsuitable for measuring high flowrate because of the difficulty in bringing the fluid to rest.

These restrictions can be overcome by directing the flowing fluid into the tank via diverter valves. In this alternative, it’s important that the timing system be synchronized carefully with operation of the diverter valves.

All versions of gravimetric calibration equipment are less robust than volumetric types and so on-site use is not recommended.

Orifice plate:

A flow line equipped with a certified orifice plate is sometimes used as a reference standard in flow calibration, especially for high flow rates through large-bore pipes. While measurement uncertainty is of the order of _1% at best, this is adequate for calibrating many flow-measuring instruments.

Turbine meter:

Turbine meters are also used as a reference standard for testing flowmeters. Their main application, as for orifice plates, is in calibrating high flow rates through large-bore pipes.

Measurement uncertainty down to _0.2% is attainable.

Calibration Equipment and Procedures for Instruments Measuring Volume Flow Rate of Gases

Calibration of gaseous flows poses considerable difficulties compared with calibrating liquid flows. These problems include the lower density of gases, their compressibility, and difficulty in establishing a suitable liquid/air interface as utilized in many liquid flow measurement systems.

In consequence, the main methods of calibrating gaseous flows, as described later, are small in number. Certain other specialized techniques, including the gravimetric method and the pressure—volume--temperature method, are also available. These provide primary reference standards for gaseous flow calibration with measurement uncertainty down to _0.3%.

However, the expense of the equipment involved is such that it’s usually only available in National Standards Laboratories.

Bell prover:

A bell prover consists of a hollow, inverted, metal cylinder suspended over a bath containing light oil. The air volume in the cylinder above the oil is connected, via a tube and a valve, to the flowmeter being calibrated. An air flow through the meter is created by allowing the cylinder to fall downward into the bath, thus displacing the air contained within it.

The flow rate, which is measured by timing the rate of fall of the cylinder, can be adjusted by changing the value of counterweights attached via a low-friction pulley system to the cylinder.

This is essentially laboratory-only equipment and therefore on-site calibration is not possible.

Positive displacement meter:

As for liquid flow calibration, positive displacement flowmeters can be used for the calibration of gaseous flows with inaccuracy levels down to _0.2%.

Compact prover:

Compact provers of the type used for calibrating liquid flows are unsuitable for application to gaseous flows. However, special designs of compact provers are being developed for gaseous flows, and hence such devices may find application in gaseous flow calibration in the future.

Reference Standards

Traceability of flow rate calibration to fundamental standards is provided for by reference to primary standards of the separate quantities that the flow rate is calculated from. Mass measurements are calibrated by comparison with a copy of the international standard kilogram, and time is calibrated by reference to a cesium resonator standard. Volume measurements are calibrated against standard reference volumes that are themselves calibrated gravimetrically using a mass measurement system traceable to the standard kilogram.

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