Orifice meter tappings

A 2 in. sch 40 pipe carries a 35° API distillate at 50°F (SG = 0.85). The flow rate is measured by an orifice meter which has a diameter of 1.5 in. The pressure drop across the orifice plate is measured by a water manometer connected to flange taps. 

An orifice meter is used to measure the flow rate of CC14 in a 2 in. sch 40 pipe. The orifice diameter is 1.25 in., and a mercury manometer attached to the pipe taps across the orifice reads 1/2 in. Calculate the volumetric flow rate of CC14 in ft3/s. (SG of CC14 = 1.6.) What is the permanent energy loss in the flow above due to the presence of the orifice in ft lbf/lbm Express this also as a total overall unrecovered pressure loss in psi. 

An orifice meter is installed in a 6 in. ID pipeline that is inclined upward at an angle of 10° from the horizontal. Benzene is flowing in the pipeline at the flow rate of 10 gpm. The orifice diameter is 3.5 in., and the orifice pressure taps are 9 in. apart. 

Gasoline is pumped through a 2 in. sch 40 pipeline upward into an elevated storage tank at 60°F. An orifice meter is mounted in a vertical section of the line, which uses a DP cell with a maximum range of 10in.H2O to measure the pressure drop across the orifice at radius taps. If the maximum flow rate expected in the line is 10 gpm, what size orifice should you use If a water manometer with a maximum reading of 10 in. is used instead of the DP cell, what would the required orifice diameter be ... 

An orifice meter is installed in a vertical section of a piping system, in which SAE 10 lube oil is flowing upward (at 100°F). The pipe is 2 in. sch 40, and the orifice diameter is 1 in. The pressure drop across the orifice is measured by a manometer containing mercury as the manometer fluid. The pressure taps are pipe taps (2j in. ID upstream and 8 in. ID downstream), and the manometer reading is 3 in. What is the flow rate of the oil in the pipe, in gpm ... 

The coefficient of discharge Cd for a particular orifice meter is a function of the location of the pressure taps, the ratio of the diameter of the orifice to the inside diameter of the pipe dJdY, the Reynolds number in the pipeline Re, and the thickness of the orifice plate. 

The value of the coefficient of discharge Cd for orifice meters depends on the properties of the flow system, the ratio of the orifice diameter to the upstream diameter, and the location of the pressure-measuring taps. Values of Cd for sharp-edged orifice meters are presented in Fig. 14-55. These values apply strictly for pipe orifices with throat taps, in which the downstream pressure tap is located one-third of one pipe diameter from the downstream side of the orifice plate and the upstream tap is located one pipe diameter from the upstream side. However, within an error of about 5 percent, the values of Cd indicated in Fig. 14-55 may be used for manometer taps located anywhere between the orifice plate and the hypothetical throat taps. 

Steam is metered with an orifice meter in a 10-in boiler lead having an internal diameter of dp = 9.760 in. Determine the maximum rate of steam flow that can be measured with a steel orifice plate having a diameter of d0 = 5.855 in at 70°F (294 K). The upstream pressure tap is ID ahead of the orifice, and the downstream tap is 0.5D past the orifice. Steam pressure at the orifice inlet pp = 250 psig (1825 kPa) temperature is 640°F (611 K). A differential gage fitted across the orifice has a maximum range of 120 in of water. What is the steam flow rate when the observed differential pressure is 40 in of water Use the ASME Research Committee on Fluid Meters method in analyzing the meter. Atmospheric pressure is 14.696 psia. 

Although the orifice meter is simple to use, it is not infallible. First, it requires proper installation of the plate itself. If the plate is put in backwards, erroneous readings can result. In addition, the location of the pressure taps is critical. The locations must adhere to the strict guidelines. 

Figure 8.2.1 An orifice meter with corner pressure taps. Adapted from Ref. 26 with permission.

A venturi meter is a device to measure fluid flow rates, which in its operation resembles the orifice meter (Section 3.2b). It consists of a tapered constriction in a line, with pressure taps leading to a differential manometer at points upstream of the constriction and at the point of maximum constriction (the throat). The manometer reading is directly related to the flow rate in the line. 

Figure 5-1. Orifice pressure tap locations. 2 1/2D and 8D pipe taps are not recommended in iSO 5167 or ASME fiuid meters. D and D/2 taps are now used in place of...

The principle of the orifice meter is identical with that of the venturi. The reduction of the cross section of the flowing stream in passing through the orifice increases the velocity head at the expense of the pressure head, and the reduction in pressure between the taps is measured by the manometer. Bernoulli s equation provides a basis for correlating the increase in velocity head with the decrease in pressure head. 

One important complication appears in the orifice meter that is not found in the venturi. Because of the sharpness of the orifice, the fluid stream separates from the downstream side of the orifice plate and forms a free-flowing jet in the downstream fluid. A vena contracta forms, as shown in Fig. 8.19. The jet is not under the control of solid walls, as is the case in the venturi, and the area of the jet varies from that of the opening in the orifice to that of the vena contraeta. The area at any given point, e.g., at the downstream tap, is not easily determinable, and the velocity of the jet at the downstream tap is not easily related to the diameter of the orifice. Orifice coefficients are more empirical than those for the venturi, and the quantitative treatment of the orifice meter is modified accordingly. 

As shown in Fig. 5.11, the orifice meter consists of a flat orifice plate with a circular hole drilled in it. There is a pressure tap upstream from the orifice plate and another just downstream. If the flow direction is horizontal and we apply Bernoulli s equation, ignoring friction from point 1 to point 2, we find Eq. 5.30, exactly the same equation we found for a venturi meter. However, in this case we cannot assume frictionless flow and uniform flow across any cross section of the pipe as easily as we can in the case of the venturi meter. 

Figure 5.12 is based on a standard location of the upstream and downstream pressure taps. When the taps are in some other location, the value of is different [2]. In comparison with venturi meters, orifice meters have high pressure losses— high — and correspondingly high pumping costs, but because they are mechanically simple, they are cheap and easy to install. For small-size lines, orifice meters are much more common than venturi meters. 

When energy dehvered was measured by the flow computer the same orifice meter ran was used. A Rosemont differential pressine transducer, however, was used across the flange taps to obtain an electrical signal for input to the flow computer. A pressure transducer was also inserted in the line at the orifice meter run to provide a continuous electric input to the flow computer. The flow computer was manufactured by... 

The orifice meter is the simplest to manufacture and occupies the least space. It consists of a thin plate with a round hole perforated such that the hole is in the center of the pipe. Upstream, the hole has a sharp edge downstream, the edge may be bevelled. The position of the taps is somewhat arbitrary—the... 

A more subtle but frequently encountered problem is attributed to plugged orifice-flow meter taps. 

An orifice meter is used to measure the flow of liquid oxygen through a 100-mm-diam. tube. The orifice diameter is 50 mm and the measured pressure drop across the orifice (with D and Djl pressure taps) is 500 Pa. If the temperature of the liquid oxygen is 100 K, determine the volumetric flow rate and the mass 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. 

Segmental and eccentric orifices are frequently used for gas metering when there is a possibility that entrained liqiiids or solids would otherwise accumulate in front of a concentric circular orifice. This can be avoided if the opening is placed on the lower side of the pipe. For hquid flow with entrained gas, the opening is placed on the upper side. The pressure taps should be located on the opposite side of the pipe from the opening. 

Figure 6.19. Pressure distribution using orifice plate, venturi meter, and Dali tube. Pressure falls by 10% from upper pressure tapping to throat in each case...

A series of tap connections in an annular pressure ring gives a mean value for the pressure at point 1 in the approach section and also at point 2 in the throat. Although Venturi meters are relatively expensive and tend to be bulky, they can meter up to 60 per cent more flow than orifice plates for the same inside pipe diameter and differential pressure [Foust et al. (1964)]. The coefficient of discharge Cd for a Venturi meter is in the region of 0.98. Venturies are more suitable than orifice plates for metering liquids containing solids. 

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