Orifice meter example

Operating line, humidifying towers 778 Optimum pipe diameter, example 371 — water velocity, heat exchanger 505 Orifice meter 244,246, 248... 

In this chapter we will illustrate and analyze some of the more common methods for measuring flow rate in conduits, including the pitot tube, venturi, nozzle, and orifice meters. This is by no means intended to be a comprehensive or exhaustive treatment, however, as there are a great many other devices in use for measuring flow rate, such as turbine, vane, Coriolis, ultrasonic, and magnetic flow meters, just to name a few. The examples considered here demonstrate the application of the fundamental conservation principles to the analysis of several of the most common devices. We also consider control valves in this chapter, because they are frequently employed in conjunction with the measurement of flow rate to provide a means of controlling flow. 

The geometry of the present opposed-flow burner is identical to the one designed by Puri and coworkers (see [18] for example). The burner consists of two opposing ducts with 20-millimeter diameter separated by 15 mm. The exhaust is extracted by a vacuum pump though a water-cooled annulus mounted around the bottom duct and a guard co-flow of nitrogen is issued from an annulus around the top duct. Experiments were performed with methane (99% purity) and premixed air introduced from the bottom duct and air admitted from the top duct. The flow rates were monitored using choked orifice meters. 

Using the orifice meter as an example. Example 8.2 illustrates the sizing procedure. Calculating the orifice diameter requires assigning the pressine drop across the orifice. 

After the installation of the orifice meter of Example 8.5, the manometer reading at a definite constant flow rate is 45 mm. Calculate the flow through the line in barrels per day measured at 60 F. 

Example 2.6. Compare the relative uncertainty in the flow rate through an orifice meter for the case where the volumetric flow rate of afluid is 2.69 s , ... 

Example 6.4. An orifice meter is installed in a 4 in. Sch40 pipeline to measure the mass flow rate of butane up to 10000 kg h . Consider that butane is an incompressible fluid with a density of 600 kg m and a viscosity of 0.230 cP. The orifice diameter is exactly half of the pipe diameter ... 

Example 8.4. An orifice meter is used to measure the flow of liquid oxygen in a 5-cm-diam. tube. The orifice diameter is 2.5 cm, and the measured pressure drop is 240 Pa. The liquid oxygen is saturated fluid at 90 K. Find the mass flow rate and volumetric flow rate of the liquid oxygen. 

The proper installation of both orifice plates and Venturi-type flow tubes requires a length of straight pipe upstream and downstream of the sensor, ie, a meter mn. The pressure taps and connections for the differential pressure transmitter should be located so as to prevent the accumulation of vapor when measuring a Hquid and the accumulation of Hquid when measuring a vapor. For example, for a Hquid flow measurement in a horizontal pipe, the taps are located in the horizontal plane so that the differential pressure transmitter is either close-coupled or connected through downward sloping connections to allow any trapped vapor to escape. For a vapor measurement in a horizontal pipe, the taps should be located on the top of the pipe and have upward sloping connections to allow trapped Hquid to drain. 

The most difficult part of a field test is the flow meter, if it wasn t planned in the construction phase. There is no way to simulate a meter run if you don t have the proper pipe length. Figure 10-8 is an example of the requirements. An ASME long radius flow nozzle is preferred by the author, though a short throat venturi will do. The probability is that an orifice is all that will be available. It should be examined before and after the test to verify not only the bore diameter, but the finish. The bore should... 

Differential Pressure Meters Differential pressure meters or head meters measure the change in pressure across a special flow element. The differential pressure increases with increasing flow rate. The pitot tubes described previously work on this principle. Other examples include orifices [see also Eqs. (6-111) and (8-102), and Fig. 10-14], nozzles (Fig. 10-19), targets, venturis (see also Sec. 8 and Fig. 10-17), and elbow meters. Averaging pitot tubes produce a pressure differential that is based on multiple measuring points across the flow path. 

Instruments which measure the rate of flow (velocity) of liquids and gases are called flowmeters they may be broadly defined as being mechanical or electronic in operation. Examples of mechanical flowmeters are orifice plate and float meters (Fig. 5.8), venturi meters, and pitot tube meters, all of which depend on a constriction being introduced into the flow stream in order to produce a difference in pressure across the constriction. The rate of flow can then be obtained from the difference in pressure. 

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 8-2 shows an example of a flow control loop. Flow loops are typically designed so that a measurement of the flow rate is taken first and then the flow is interrupted or controlled downstream. Flow control loops start at the primary element. Flow control primary elements may include orifice plates, venturi tubes, flow nozzles, nutating disks, oval gears, or turbine meters. The most common primary element is the orifice plate, which artificially creates a high-pressure/low-pressure situation that can be measured by the transmitter. Primary elements are typically used in conjunction with a transmitter. 

The most frequent application of signal conditioning is linearisation. Many of the common functions may not be obvious to the control engineer since they are often built into the DCS or transmitter as standard features. For example, where cj is the discharge coefficient, d the orifice diameter, dp the pressure drop across the orifice and p the fluid density, the flow (F) through an orifice flow meter is given by... 

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