Attività

The continuous flowmeter

Flow meters are classed as volumetric or inferential, the latter term referring to meters that determine velocity from other variables such as pressure differences across a device such as an orifice plate. There is a large variety of flow measurement device, using numerous physical principles. Full discussion of the whole range of flow measurement device is out of the scope of this book but the reader will find a comprehensive reference in the Flow Measurement Handbook (Baker, 2000). Table 18.4 gives typical information on some of the flow meters usually encountered in the water industry.

Mass magnetic flowmeter such as the Coriolis meter provide a more sophisticated metering device. Sometimes configured in a distinctive U-tube shape, an internal tube is set oscillating using an electric current supplied to coils at either end of the tube. The flow of liquid through the tube sets up a twisting force on the inner tube due to the naturally occurring Coriolis Effect. Sensors fitted along the length of the tube detect and measure the twisting force, which is a function of the mass flow rate; the processed data provides production and fluid density data.

The principles of orifice and venturi meters are discussed in Section 14.16. Two other kinds of inferential (or momentum) meter are the Dall tube and the V cone venturi. In both, flow accelerates through a constriction and leads to a pressure drop. The pressure difference is measured in the Dall tube and the V cone venturi as an indicator of velocity (and so flow) in the same way as for an orifice. The V cone venturi design is claimed to have a turn-down ratio of 25:1 and to be less affected by conditions upstream and downstream and can be fitted into shorter lengths of straight pipe than is recommended for other meter types. Further types of momentum meter are indicated in Table 18.5.

An ultrasonic flow meter as shown in Fig. 16.11 measures the velocity of a fluid to calculate volume flow. The vortex flowmeter can measure the average velocity along the path of an emitted beam of ultrasound by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect. Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates. They are often inexpensive to use and maintain because they do not use moving parts, unlike mechanical flow meters.

Insertion probe flow meters are installed for temporary measurement of flow for consumption surveys or for distribution networks analyses. These instruments are either the turbine or electromagnetic (EM) type, the latter becoming more common. Both are inserted into the pipe where flow measurement is required. The turbine type uses a small rotating vane at the end of a probe to record flow velocity. The vane is susceptible to damage, in which case the instrument has to be returned to the manufacturer for repair and recalibration. The turbine meter is inserted through a 40 mm diameter tapping in the pipe which has to be of at least 200 mm diameter. The EM probe (Plate 30(c)) uses an electromagnet at its end to apply a magnetic field to the water. Electrodes either side of the probe pick up the induced EMF in the water which is proportional to the velocity past the electrodes. The tapping for an EM insertion probe is 20 mm diameter and can usually be installed in pipes of diameter 150 mm and greater. EM probes are made up to 1 m long; therefore, they cannot be used for pipes of diameter greater than 900 mm and are restricted to flow with velocity less than about 1.75 to 2.0 m/s due to the flexibility of the probe.

Insertion probes measure the velocity at the position of the measuring device. This can be at the pipe centre line or at defined points along the diameter. The measured velocity has to be converted to mean pipe velocity of flow by relating the measured value to the average velocity across the whole pipe. For this a velocity profile for the pipe is used, determined by using the same instrument to record velocities at set points across the diameter from crown to invert. The recorded measurements are corrected to take account of the disturbance caused by the instrument itself (increased local velocity). The disturbance coefficients are unique to each instrument and are provided by its manufacturer. For the conditions usually encountered the ratio of the mean velocity to the centerline velocity is 0.83 but can range from 0.7 to 1.0. Values differing widely from 0.83 should be viewed with caution and the cause investigated. However, satisfactory results should be obtained if the internal diameter is measured accurately and if the number of flow profile readings is sufficient—five for pipes of DN 150, nine for DN 300 and 13 for larger pipes. The measurements should be repeated at least three times to ensure the ratio is consistent and repeatable; the flow must be relatively consistent during each profile run. In practice poor field conditions often make precise measurement difficult so that several attempts may be necessary. Once the profile is established satisfactorily the instrument is set at the pipe centre line and the data logger is attached. In pipes in poor condition the velocity profile changes with flow and can render very inaccurate measurements of flows significantly different from that at which the velocity profile was established.

Making tappings and installing insertion probes pose a risk to water quality. Although such risks can be managed, ultrasonic strap-on mass flowmetter are being used increasingly as an alternative and avoid the tedious exercise of velocity profiling. Versions of these meters can be installed on all sizes and materials of pipe used in distribution systems.

For an orifice meter Equation (14.24) equally applies. However, C varies much more, depending on the velocity and the ratio d/D. If the latter is in the typical range of 0.4–0.6 and for pipe diameters greater than 200 mm, the value of C will be in the range 0.60–0.61 for the usual velocities experienced in a pipeline.

The accuracy of measurement of both venturi and orifice meters depends on the lateral flow distribution through the device and can be severely affected by flow disturbances created by fittings in a pipe system. Detailed conditions for accurate measurement are laid down in BS EN ISO 5167-1:2003. These can generally be met by ensuring that for d/D ratios not exceeding 0.6, there are at least 20 diameters of straight pipe without a fitting upstream of the meter and 7 diameters of straight pipe downstream. The venturi is designed to minimize the headloss and this can be made very small with a well-designed expansion downstream of the throat providing good recovery of pressure head. The headloss through an orifice is substantially higher because of the sudden expansion of the diameter downstream. If headloss is an important consideration a venturi meter, such as the illustrated Dall tube, should be considered but venturis are more expensive and require greater space than an orifice meter.

Subsea manifolds have been used in the development of oil and gas fields to simplify the subsea system, minimize the use of subsea pipelines and risers, and optimize the fluid flow of production in the system. The manifold is an arrangement of piping and/or valves designed to combine, distribute, control, and often monitor fluid flow. Subsea manifolds are installed on the seabed within an array of wells to gather product or to inject water or gas into wells as shown in Figure 19-1. There are numerous types of manifolds, ranging from a simple pipeline end manifold (PLEM/PLET) to large structures such as an entire subsea process system. A PLEM is one of the most common manifolds in this range and it is detailed in the next section separately because it is directly connected to pipelines and its installation considerations are key factors in the design.

A subsea manifold system is structurally independent from the wells. The wells and pipelines are connected to the manifold by jumpers. The subsea manifold system is mainly comprised of a manifold and a foundation. The manifold support structure is an interface between the manifold and foundation. It provides necessary facilities to guide, level, and orient the manifold relative to the foundation. The connection between the manifold and the manifold supporting structure allows the manifold to retrieve and reinstall with sufficient accuracy so as to allow for the subsequent reuse of the production and well jumpers.

A manifold is a structural frame with piping, valves, control module, pigging loop, flow meters, etc. The foundation provides structural support for the manifold. It may be either a mudmat with skirt or a pile foundation, depending on seabed soil conditions and manifold size.

Any obstruction inserted into a duct or pipe that creates a measurable pressure difference can be used as a density meter. The three basic standardized flow measurement devices presented above are perhaps more suitable for laboratory work than installation as permanent ductwork instruments in ventilation applications. They are sensitive to flow disturbances, relatively expensive, require considerable space, and have a narrow measurement range and a high permanent pressure loss. For these reasons, numerous attempts have been made to develop instruments without these drawbacks. Some of them, like the Dall tube,47 which is a modification of the venturi, have even been standard instruments. Several other solutions based on plates, rings, or wing-type obstructions are commercially available. This wide variety of devices is not covered here. For further information, the reader should contact the manufacturers of such instruments.
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