Flow measurements Weirs

Eiquid Flow Measurement in Open Channels Using Thin Plate Weirs and Venturi Flumes, ISO 1438-1975(E), International Organization for Standardization, Geneva, Switzerland, 1975. 

Open-Channel Flow Measurement Open-channel flow measurements are usually based on measurement of liquid level in a flow channel constructed of a specified geometry. The two most common flow channels used are weirs and flumes. See Spitzer (2005, op. cit.). 

Information on other types of weirs can be obtained from Addison, op. cit. Gibson, Hydraulics and Its Applications, 5th ed.. Constable, London, 1952 Henderson, Open Channel Flow, Macmillan, New York, 1966 Linford, Flow Measurement and Meters, Spon, London, 1949 Lakshmana Rao, Theory of Weirs, in Advances in Hydroscience, vol. 10, Academic, New York, 1975 and Merritt, Standard Handbook for Civil Engineers, 2d ed., McGraw-Hill, New York, 1976. 

Flow measurement in open unpressurised channels is a requirement generally associated with waste water systems and sewers. The use of a weir or notch to measure the flow of a liquid presenting a free surface is described in Volume 1, Section 6.3.8. The flow through a rectangular notch (Fig. 6.7a) is given by (Volume 1, equation 6.42) ... 

This chapter discusses the unit operations of flow measurements and flow and quality equalizations. Flow meters discussed include rectangular weirs, triangular weirs, trapezoidal weirs, venturi meters, and one of the critical-flow flumes, the Parshall flume. Miscellaneous flow meters including the magnetic flow meter, turbine flow meter, nutating disk meter, and the rotameter are also discussed. These meters are classified as miscellaneous, because they will not be neated analytically but simply described. In addition, liquid level recorders are also briefly discussed. 

Flow meters fall into the broad category of meters for open-channel flow measurements and meters for closed-channel flow measurements. Venturi meters are closed-channel flow measuring devices, whereas weirs and critical-flow flumes are open-channel flow measuring devices. 

From the previous discussion, it can be gleaned that the meter measures rates of flow proportional to the cross-sectional area of flow. Rectangular weirs are therefore area meters. In addition, when measuring flow, the unit obstructs the flow, so the meter is also called an intrusive flow meter. 

A weir or other suitable flow-measuring device shall be provided for measure of reflux ratio, in the range of from 2 to 6 gph (liquid sp gr, 1.12). The weir will be calibrated by the purchaser. 

The obstruction of the air under the weir flow determines the flow trajectory and flow regime, so to make a thin-plate rectangular measuring weir, ventilation hole must set on the weir wall between the weir plate and the water tongue under the weir, to make sure the weir flow is freedom, and to guarantee the flow rate calculate accuracy by empirical formula. The diameter of the ventilation hole can be calculated according to the following formula (Sun, J.S. et al. 2003) ... 

BASIC FORMULAS FOR CALCULATION OF THIN-PLATE RECTANGULAR MEASURING WEIR FLOW... 

The measuring weir is a measuring water device commonly used in laboratories its measuring accuracy is associated with the attainment of certain technical requirements of its manufacture and installation. Currently the weir flow calculation formulas are more the empirical formula obtained through experiments under certain conditions, and the Rehbock formula for thin-plate equal-width rectangular weir is relatively simple and precision that always used in domestic hydraulic and physical model test. This paper theoretically deduced a weir flow formula by weir width, weir head and flume water depth, the weir flow calculation result of this formula is very close to of Rehbock empirical formula, it can be used for engineering application. 

Cox, G.N. (1928). The submerged weir as a measuring device A method for making accurate stream flow measurements. University of Wisconsin Milwaukee. 

In many instances in process engineering and in agriculture, liquids are flowing in open channels rather than closed conduits. To measure the flow rates, weir devices are often used. A weir is a dam over which the liquid flows. The two main types of weirs are the rectangular weir and the triangular weir shown in Fig. 3.2-5. The liquid flows over the weir and the height /iq (weir head) in m is measured above the flat base or the notch as shown. This head should be measured at a distance of about 3/iq m upstream of the weir by a level or float gage. 

Flow rate is typically measured in gallons per minute (gpm) or gallons per hour (gph). A variety of devices can be used to accomplish flow measurement. Common examples of flow measurement devices are orifice plates, venturi nozzles, nutating disc meters, turbine flow meters, oval gear meters, rotameters, pitot tubes, weir and flume, and flow transmitters. Figure 7-4 shows a few examples of flow-measurement devices. 

FIGURE 4.67 Nomographs for measuring flow over weirs. 

Measurement by Liquid Level. The flow rate of Hquids flowing in open channels is often measured by the use of weirs (see Liquid-LEVEL measurement). The most common type is the rectangular weir shown in Figure 22e. The flow rate across such a weir varies approximately with the quantity. Other shapes of weirs are also employed. Standard civil engineering handbooks describe the precautions necessary for constmcting and interpreting data from weirs. 

Weir A wall or plate placed in an open channel and used to measure the flow of water. 

The notch or weir, in which the fluid flows over the weir so that its kinetic energy is measured by determining the head of the fluid flowing above the weir. This instrument is used in open-channel flow and extensively in tray towers 3 where the height of the weir is adjusted to provide the necessary liquid depth for a given flow. 

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

Weirs are commonly used to measure the flow rate of liquids in open conduits. The theory is based on the Bernoulli equation for frictionless flow. From equation 1.13 with Ah = 0 and hf= 0... 

V-notch weirs are particularly useful for measuring flow rates that vary considerably. 

The clean water flows horizontally through the unn and falls into the clean-water outlet chamber overflowing the adjustable water-outlet weir plates. The height of this water weir as measured from tank inside bottom decides ihc working level of the coalescer unit A sheen baffle is located just before (lie water-outlet weir. The sheen baffle captures small oil droplets that might pass undet the oil-retention baffle. The buildup of oil film at the sheen baffle is so slow that no skimming device is required This oil buildup gets hack into the separation chamber when the unit is shut down for planned maintenance purposes... 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

Other sensors which are described in Volume 1 (Sections 6.3.7-6.3.9) are the variable area meter, the notch or weir, the hot wire anemometer, the electromagnetic flowmeter and the positive displacement meter. Some of these flowmeters are relatively less suitable for producing signals which can be transmitted to the control room for display (e.g. weir, rotameter) and others are used in more specialist or limited applications (e.g. magnetic flowmeter, hot wire anemometer). The major characteristics of different types of flow sensor are summarised in Table 6.1. Brief descriptions follow of the principles underlying the more important types of flowmeter not described in Volume 1. In many instances such flow sensors are taking the place of those more traditional meters which rely upon pressure drop measurement. This is for reasons of versatility, energy conservation and convenience. 

The zero-point analysis characterizes the rotor hydraulics in two ways. First, the zero-point value gives the liquid flow rate over the more-dense-phase weir just as the liquid in the separating zone rises to the edge of the less-dense-phase weir. Second, the slope of the curve above the zero point measures the liquid rise over the more-dense-phase weir relative to the liquid rise over the less-dense-phase weir. Thus, if all rotors in a set have about the same zero point and the same slope above the zero point, then they can all be expected to operate about the same in two-phase flow. 

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