Flow meters

Flow measurements using tracers are performed in all piping systems carrying oil, gas or water including separators, compressors, injector systems, and flares. Calibration of elsewhere difficult accessible flow meters is regularly performed by the tracer methods, which are based on international standards. Tracer flow measurements are also well suited for special purposes... 

Gas flaring in offshore installations and oil refineries represents a source of loss of energy making it important to operators and authorities to monitor the amounts of flared gas. In some countries the flare gas is subject to CO2 tax. Flow metering systems are installed on some but not all flare systems. 

This experiment describes the construction of an air sampler using an aquarium pump, a flow meter, a filter holder, and bottles that serve as traps for analytes. Applications include the determinations of SO2, NO2, HCHO, and suspended particulate matter. 

The most common mobile phases for GC are He, Ar, and N2, which have the advantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument s detector. With packed columns the mobile-phase velocity is usually within the range of 25-150 mF/min, whereas flow rates for capillary columns are 1-25 mF/min. Actual flow rates are determined with a flow meter placed at the column outlet. 

Fig. 14. OxyTech/Uhde HU-type cell a, cell bottom b, cathode c, anode d, cell cover e, bus bars f, brine level gauge g, brine flow meter h, bypass...

Most flow meters are designed and caHbrated for use on turbulent flow, by far the more common fluid condition. Measurements of laminar flow rates may be seriously in error unless the meter selected is insensitive to velocity profile or is specifically caHbrated for the condition of use. 

A.m blent Environment. The environment around the flow conduit must be considered in meter selection. Such factors as the ambient temperature and humidity, the pipe shock and vibration levels, the avadabiHty of electric power, and the corrosive and explosive characteristics of the environment may all influence flow meter selection. Special factors such as possible accidental flooding, the need for hosedown or steam cleaning, and the possibiHty of lightning or power transients may also need to be evaluated. 

Enough space must be available to properly service the flow meter and to install any straight lengths of upstream and downstream pipe recommended by the manufacturer for use with the meter. Close-coupled fittings such as elbows or reducers tend to distort the velocity profile and can cause errors in a manner similar to those introduced by laminar flow. The amount of straight pipe required depends on the flow meter type. For the typical case of an orifice plate, piping requirements are normally Hsted in terms of the P or orifice/pipe bore ratio as shown in Table 1 (1) (see Piping systems). 

Measurement Requirements. Any analysis of measurement requirements must begin with consideration of the particular accuracy, repeatabihty, and range needed. Depending on the appHcation, other measurement considerations might be the speed of system response and the pressure drop across the flow meter. For control appHcations repeatabihty may be the principal criterion conversely for critical measurements, the total installed system accuracy should be considered. This latter includes the accuracy of the flow meter and associated readout devices as well as the effects of piping, temperature, pressure, and fluid density. The accuracy of the system may also relate to the required measurement range. 

Accuracies of the flow meters discussed herein are specified as either a percentage of the full-scale flow or as a percentage of the actual flow rate. It may be convenient in some appHcations to compare the potential inaccuracies in actual volumetric flow rates. For example, in reading two Hters per minute (LPM) on a flow meter rated for five LPM, the maximum error for a 1% of full-scale accuracy specification would be 0.01 x 5 = 0.05 LPM. If another flow meter of similar range, but having 1% of actual flow rate specification, were used, the maximum error would be 0.01 x 2 = 0.02 LPM. To minimize errors, meters having full-scale accuracy specifications are normally not used at the lower end of their range. Whenever possible, performance parameters should be assessed for the expected installation conditions, not the reference conditions that are the basis of nominal product performance specifications. 

Economic Considerations. The principal economic consideration is, of course, total installed system cost, including the initial cost of the flow primary, flow secondary, and related ancillary equipment as well as material and labor required for installation. Other typical considerations are operating costs and the requirements for scheduled maintenance. An economic factor of increasing importance is the cost of disposal at the end of normal flow meter service life. This may involve meter decontamination if hazardous fluids have been measured. 

Flow measuring equipment must generally be wet caHbrated to attain maximum accuracy, and principal flow meter manufacturers maintain extensive facihties for this purpose. In addition, a number of governments, universities, and large flow meter users maintain flow laboratories. 

Liquid Displacement Gas Meter Provers. The Hquid displacement prover is the most prevalent standard for the caHbration of flow meters at low to moderate gas flow rates. The method consists of displacing a known volume of Hquid with gas (Fig. 2). Gas entering the inverted beU causes it to rise and a volume increment can be timed. Typical prover capacities are 1 m or less although capacities as large as 20 m are available. Accuracies can be on the order of 0.5% of actual flow rate. 

Fig. 3. Comtrack 921 pipe prover. Liquid flow through the Comtrak s closed loop is created by the movement of a sealed piston. Flow meters being tested are installed in the loop upstream from the piston. As the piston advances, the caUbration fluid travels through the meters and returns to the back side of...

Flow meters have traditionally been classified as either electrical or mechanical depending on the nature of the output signal, power requirements, or both. However, improvement in electrical transducer technology has blurred the distinction between these categories. Many flow meters previously classified as mechanical are now used with electrical transducers. Some common examples are the electrical shaft encoders on positive displacement meters, the electrical (strain) sensing of differential pressure, and the ultrasonic sensing of weir or flume levels. 

The flow meters discussed herein are divided into two groups based on the method by which the basic flow signal is generated. The first group... 

Meters can be further divided into three subgroups depending on whether fluid velocity, the volumetric flow rate, or the mass flow rate is measured. The emphasis herein is on common flow meters. Devices of a highly specialized nature, such as biomedical flow meters, are beyond the scope of this article. 

Fig. 4. Operating sequence for a two-lobed rotary gas flow meter where the shaded area represents the flowing fluid.

Wedg e Meters. The wedge flow meter consists of a flanged or wafer-style body having a triangular cross section dam across the top of the fluid conduit. Pressure taps are on either side of this restriction. Overall meter sizes range from 10 to 600 mm. Within each size several restrictions are available to provide the range of differential pressure desired for the appHcation. 

La.mina.r Flow Elements. Each of the previously discussed differential-pressure meters exhibits a square root relationship between differential pressure and flow there is one type that does not. Laminar flow meters use a series of capillary tubes, roUed metal, or sintered elements to divide the flow conduit into innumerable small passages. These passages are made small enough that the Reynolds number in each is kept below 2000 for all operating conditions. Under these conditions, the pressure drop is a measure of the viscous drag and is linear with flow rate as shown by the PoiseuiHe equation for capilary flow ... 

Variable-Area Flow Meters. In variable-head flow meters, the pressure differential varies with flow rate across a constant restriction. In variable-area meters, the differential is maintained constant and the restriction area allowed to change in proportion to the flow rate. A variable-area meter is thus essentially a form of variable orifice. In its most common form, a variable-area meter consists of a tapered tube mounted vertically and containing a float that is free to move in the tube. When flow is introduced into the small diameter bottom end, the float rises to a point of dynamic equiHbrium at which the pressure differential across the float balances the weight of the float less its buoyancy. The shape and weight of the float, the relative diameters of tube and float, and the variation of the tube diameter with elevation all determine the performance characteristics of the meter for a specific set of fluid conditions. A ball float in a conical constant-taper glass tube is the most common design it is widely used in the measurement of low flow rates at essentially constant viscosity. The flow rate is normally deterrnined visually by float position relative to an etched scale on the side of the tube. Such a meter is simple and inexpensive but, with care in manufacture and caHbration, can provide rea dings accurate to within several percent of full-scale flow for either Hquid or gas. 

Gup and Vane Anemometers. A number of flow meter designs use a rotating element kept in motion by the kinetic energy of the flowing stream such that the speed is a measure of fluid velocity. In general, these meters, if used to measure wind velocity, are called anemometers if used for open-channel Hquids, current meters and if used for closed pipes, turbine flow meters. 

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