Flowmeters

Flowmeters.— The measurement of gas-flow rate is usually carried out with a soap-bubble meter although other methods involving rotameters and capillary manometers are available. The soap-bubble meter is an inexpensive, easy to operate, direct method which has good accuracy. 

The flowmeter may be placed upstream or downstream of the column depending on the type of detection used. If thermal conductivity is used, then the flow- 

If the inlet flow rate Ui is measured, then the outlet flow rate Uo can be calculated from an approximate equation of state of carrier gas, knowing the temperature T of the flowmeter and the temperature T of the coluimi from 

For accurate flow-rate measurements, temperature T should be known to within 0.5 K. It is therefore advisable to place the flowmeter in an air thermostat and to ensure that the carrier gas is at this temperature before entering the flowmeter by using a long conditioning coil. 

The range of flow rates usually encountered in this application of g.l.c. is between 0.5 and 5 cm s. Even with a good stop watch capable of reading to 0.02 s, errors of 0.2 per cent are possible at the faster flow rates. Improved 

The following text will emphasize the flow detection devices used in connection with the operation of solar collectors wind turbines fuel cells and 

Principle of Flame Detection Rectification Infrared Visible Light Ultraviolet 

Type of Detector Flame Rod Photo-tube Photocell Photocell Tube 

Advantages Same detector for gas or oil flame Can pinpoint flame in three dimensions /  

Viewing angle can be orificed to pinpoint flame in two dimensions / /  

---

A separator is fed with a condensate/gas mixture. The condensate leaves the bottom of the separator, passes a flowmeter and is followed by a choke valve, after which the condensate is boiling. The flow can not be measured using the transit time method, due to the combination of short piping, the absence of a suitable injection point and the flow properties of the condensate, which is non-newtonian due to a high contents of wax particles The condensate can not be representatively sampled, as it boils upon depressuratioh... 

A development project has been initiated with the aim of providing a radiotracer method that ultimately can be implemented as a routine calibration method for permanently installed multiphase flowmeters. The project is supported by a research grant from the Danish Ministry of Energy. 

Example 3 Venturi Flowmeter An incompressible fluid flows through the venturi flowmeter in Fig. 6-7. An equation is needed to relate the flow rate Q to the pressure drop measured by the manometer. This problem can he solved using the mechanical energy balance. In a well-made venturi, viscous losses are neghgihle, the pressure drop is entirely the result of acceleration into the throat, and the flow rate predicted neglecting losses is quite accurate. The inlet area is A and the throat area is a. 

The usual measuring device for feed flow is a magnetic flowmeter, which is a volumetric device whose output F must be multiplied by density p to produce mass flowMo- For most aqueous solutions which are fed to evaporators, the product of density and the function of solid content appearing above is linear with density ... 

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

The principal classes of flow-measuring instruments used in the process industries are variable-head, variaBle-area, positive-displacement, and turbine instruments, mass flowmeters, vortex-shedding and iiltrasonic flowmeters, magnetic flowmeters, and more recently, Coriohs mass flowmeters. Head meters are covered in more detail in Sec. 5. 

Orifice Meter The most widely used flowmeter involves placing a fixed-area flow restriction (an orifice) in the pipe carrying the fliiid. This flow restriction causes a pressure drop that can be related to flow rate. The sharp-edge orifice is popular because of its simplicity, low cost, and the large amount of research data on its behavior. For the orifice meter, the flow rate for a liquid is given by... 

Vortex-Shedding Flowmeters These flowmeters take advantage of vortex shedding, which occurs when a fluid flows past a non-streamlined objec t (a Blunt body). The flow cannot follow the shape of the object and separates from it, forming turbulent vortices or eddies at the object s side surfaces. As the vortices move downstream, they grow in size and are eventually shed or detached from the objec t. 

Turbine flowmeters are probably the most common example where pulse inputs are used. Another example is a watt-hour meter. Basically any measurement device that invowes a rotational element can be interfaced via pulses. 

Linearizing the output of the transmitter. Functions such as square root extraction of the differential pressure for a head-type flowmeter can be done within the instrument instead of within the control system. 

Axial-Flow Transverse-Momentum Mass Flowmeter. 10-19... 

A turbine flowmeter consists of a straight flow tube containing a turbine which is free to rotate on a shaft supported by one or more bearings and located on the centerline of the tube. Means are provided for magnetic detection of the rotational speed, which is proportional to the volumetric flow rate. Its use is generally restric ted to clean, noncorrosive fluids. Additional information on construction, operation, range, and accuracy can be obtained from Holzbock (Instruments for Measurement and Control, 2d ed., Reinhold, New York, 1962, pp. 155-162). For performance characteristics of these meters with liquids, see Shafer,y. Basic Eng., 84,471-485 (December 1962) or May, Chem. Eng., 78(5), 105-108 (1971) and for the effect of density and Reynolds number when used in gas flowmetering, see Lee and Evans, y. Basic Eng., 82, 1043-1057 (December 1965). 

When the pulsation amplitude is such as to result in a greater-than-permissible metering error, consideration should be given to installation of a pulsation damper between the source of pulsations and the flowmeter. References to methods of pulsation-damper design are given in the subsection Unsteady-State Behavior. ... 

Rotameters The rotameter, an example of which is shown in Fig. 10-21, has become one of the most popular flowmeters in the chemical-process industries. It consists essentially of a plummet, or float, which is free to move up or down in a vertical, slightly tapered tube having its small end down. The fluid enters the lower end of the tube and causes the float to rise until the annular area between the float and the wall of the tube is such that the pressure drop across this constriction is just sufficient to support the float. Typically, the tapered tube is of glass and carries etched upon it a nearly linear scale on which the position of the float may be visually noted as an indication of the flow. 

General Principles There are two main types of mass flowmeters (1) the so-called true mass flowmeter, which responds directly to mass flow rate, and (2) the inferential mass flowmeter, which commonly measures volume flow rate aud flmd density separately. A variety of types of true mass flowmeters have been developed, including the following (a) the Maguus-effect mass flowmeter, (b) the axial-flow, transverse-momentum mass flowmeter, (c) the radial-flow, transverse-momentum mass flowmeter, (d) the gyroscopic transverse-momentum mass flowmeter, aud (e) the thermal mass flowmeter. Type b is the basis for several commercial mass flowmeters, one version of which is briefly described here. 

Axial-Flow Transverse-Momentum Mass Flowmeter This type is also referred to as an augiilar-momeutum mass flowmeter. One embodiment of its principle involves the use of axial flow through a driven impeller aud a turbine in series. The impeller imparts augiilar momentum to the fluid, which in turn causes a torque to be imparted... 

Inferential Mass Flowmeter There are several types in this category, including the following ... 

Additional information on mass-flowmeter principles can be obtained from Yeaple (Hydraulic and Pneumatic Power and Control, McGraw-HiU, New York, 1966, pp. 125-128), Halsell [In.strum. Soc. Am. J., 7, 49-62 (June I960)], and Flanagan aud Colmau [Control, 7, 242-245 (1963)]. Information on commercially available mass flowmeters is given in the latter two references. 

Goldberg and Boothroyd [Br Chem. Eng., 14, 1705-1708 (1969)] describe several types of solids-in-gas flowmeters and give an extensive bibhography. 

Liquid-Solid Mixtures Liptak [Chem. Eng., 74(4), 151-158 (1967)] discusses a variety of techniques that can be used for the measurement of sohds-in-liquid suspensions or slurries. These include metering pumps, weigh tanks, magnetic flowmeter, ultrasonic flowmeter, gyroscope flowmeter, etc. 

Shirato, Gotoh, Osasa, and Usami [J. Chem. Eng. Japan, 1, 164— 167 (January 1968)] present a method for determining the mass flow rate of suspended sohds in a liqiiid stream wherein the liquid velocity is measured By an electromagnetic flowmeter and the flow of sohds is calculated from the pressure drops across each of two vertical sections of pipe of different diameter through which the suspension flows in series. 

Flowmeters These are used to measure flocculant addition, underflow, and feed flow rates. For automatic control, the more commonly used devices are magnetic flowmeters and Doppler effect flowmeters. 

Density Gauges These are used to measure the density or suspended solids content of the feed and underflow streams. Gamma radiation devices are the most commonly used for automatic control, but ultrasonic devices are effective in the lower range of slurry density. Marcy pulp density scales are an effective manually operated device. A solids mass flow indication is usually obtained by combining a density gauge output with the output from a flowmeter. 

Flow measurements using nonintrusive or low mechanical ac tion principles are desired, such as magnetic, vortex-shedding, or Coriolis-type flowmeters. Orifice plates are easy to use and reliable but have a limited range and may not be suitable for streams which are not totally clean. Rotameters with glass tubes should not be used. 

Flowmeter Reading mm. Rate of Flow I./hr. Secondary Voltage ... 

A wet gas meter such as No. S-39465, E. H. Sargent and Company, is satisfactory. This meter need not be a permanent part of the apparatus. It is used only to calibrate the flowmeter. 

---