Fluid flow orifice meter

The simplest and most common device for measuring flow rate in a pipe is the orifice meter, illustrated in Fig. 10-7. This is an obstruction meter that consists of a plate with a hole in it that is inserted into the pipe, and the pressure drop across the plate is measured. The major difference between this device and the venturi and nozzle meters is the fact that the fluid stream leaving the orifice hole contracts to an area considerably smaller than that of the orifice hole itself. This is called the vena contracta, and it occurs because the fluid has considerable inward radial momentum as it converges into the orifice hole, which causes it to continue to flow inward for a distance downstream of the orifice before it starts to expand to fill the pipe. If the pipe diameter is D, the orifice diameter is d, and the diameter of the vena contracta is d2, the contraction ratio for the vena contracta is defined as Cc = A2/A0 = (d2/d)2. For highly turbulent flow, Cc 0.6. 

An orifice meter with a hole of 1 in. diameter is inserted into a l- in. sch 40 line carrying SAE 10 lube oil at 70°F (SG = 0.93). A manometer using water as the manometer fluid is used to measure the orifice pressure drop and reads 8 in. What is the flow rate of the oil, in gpm ... 

The flow rate in a 1.5 in. line can vary from 100 to lOOObbl/day, so you must install an orifice meter to measure it. If you use a DP cell with a range of 10 in. HzO to measure the pressure drop across the orifice, what size orifice should you use After this orifice is installed, you find that the DP cell reads 0.5 in. H20. What is the flow rate in bbl/day The fluid is an oil with an SG = 0.89 and tx = 1 cP. 

The primary element of an orifice meter is simply a flat plate containing a drilled hole located in a pipe perpendicular to the direction of fluid flow as shown in Figure 8.2. 

Orifice meters, Venturi meters and flow nozzles measure volumetric flow rate Q or mean velocity u. In contrast the Pitot tube shown in a horizontal pipe in Figure 8.7 measures a point velocity v. Thus Pitot tubes can be used to obtain velocity profiles in either open or closed conduits. At point 2 in Figure 8.7 a small amount of fluid is brought to a standstill. Thus the combined head at point 2 is the pressure head P/( pg) plus the velocity head v2/(2g) if the potential head z at the centre of the horizontal pipe is arbitrarily taken to be zero. Since at point 3 fluid is not brought to a standstill, the head at point 3 is the pressure head only if points 2 and 3 are sufficiently close for them to be considered to have the same potential head... 

Fluid Flow. See under Fluid Mechanics or Dynamics and die following Refs Refs 1) R.F. Steams et al, Flow Measurements eith Orifice Meters , VanNostrand, NY (1951), 384pp 2) J.R. Caddell, "Fluid... 

Orifice Meter The most widely used flowmeter involves placing a fixed-area flow restriction (an orifice) in the pipe carrying the fluid. This flow restriction causes a pressure drop which 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 Qa for a liquid is given by... 

The coefficients for venturi meters, flow nozzles, and orifice meters vary with Reynolds number as shown in Figs. 10.7 to 10.10. The curve for an orifice meter shown in Fig. 10.10 covers an unusually wide range of both viscosity and Reynolds number. The fluids used were water and a series of oils up to a very viscous road oil, and for each fluid a number of different velocities were used, so that the curve represents points for many combinations of velocity and viscosity. Although the orifice plate may not be a standard beveled form, the value of C for high Reynolds numbers agrees closely with the value of C in Fig. 10.10 for a diameter ratio of 0.75. 

Patel, B. R. and Sheikoholeslami, Z., Numerical modelling of turbulent flow through the orifice meter, International Symposium on Fluid Flow Measurement, Washington, D.C., November 1986. 

Both gas flow rates and liquid flow rates can be measured by a wide variety of devices such as bellow meters, Venturi nozzles, nutating disk meters, orifice meters, rotameters, weirs (for liquids), Pitot tubes, and magnetic meters among others. Some devices measure volumetric flow directly as with meters in which the space between rotating paddles incorporates small volumetric displacements of fluid. Other device measure the flows indirectly by measuring the pressure drop caused by an orifice between two different sites in the pipe, or the change in voltage of a heated wire. 

Basic equations for the design and operation of orifice meters, venturi meters, and rotameters can be derived from the total energy balances presented at the beginning of this chapter. The following equations apply when the flowing fluid is a liquid, and they also give accurate results for the flow of gases if the pressure drop caused by the constriction is less than 5 percent of the upstream pressure ... 

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. 

An orifice meter (see Figure 3.2-1) is to be calibrated for the measurement of the flow rate of a stream of liquid acetone. The differential manometer fluid has a specific gravity of 1.10. 

The indicator dilution method is a technique used to determine flow rates of fluids in channels for which devices like rotameters and orifice meters cannot be used (e.g., rivers, blood vessels, and large-diameter pipelines). A stream of an easily measured substance (the tracer) is injected into the channel at a known rate and the tracer concentration is measured at a point far enough downstream of the injection point for the tracer to be completely mixed with the flowing fluid. The larger the flow rate of the fluid, the lower the tracer concentration at the measurement point. 

The flow rate required to yield a specified reading on an orifice meter varies inversely as the square root of the fluid density that is, if a fluid with density pi(g/cm- ) flowing at a rate V (cm /s) yields a meter reading 0, then the flow rate of a fluid with density p2 required to yield the same reading is... 

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. 

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. 

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