Sharp-edged orifice

The orifice coefficient deviates from its value for sharp-edged orifices when the orifice wall thickness exceeds about 75 percent of the orifice diameter. Some pressure recovery occurs within the orifice and the orifice coefficient increases. Pressure drop across segmental ori-fiees is roughly 10 percent greater than that for concentric circular orifices of the same open area. 

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... 

FIG. 10-18 Square -edged or sharp-edged orifices. The plate at the orifice opening must not be thicker than one-thirtieth of the pipe diameter, one-eighth of the orifice diameter, or one-fourth of the distance from the pipe wall to the edge of the opening, (a ) Pipe-line orifice, (h ) Types of plates. 

The discharge coefficient of sharp-edged orifices was shown by Benedict, Wyler, and Brandt [J. Eng. Fower, 97, 576-582 (1975)] to increase with edge roundness. Typical as-purchased orifice plates may exhibit deviations on the order of 1 to 2 percent from ASME values of the discharge coefficient. 

Sharp-edged weirs have edges like those of square or sharp-edged orifices (see subsection Orifice Meters ). Notched weirs are or(iuarily sharp-edged. Weirs not in the sharp-edged class are, for the most part, those described as broad-crested weirs. 

Gas-Liquid Mixtures An empirical equation was developed by Murdock [J. Basic Eng., 84, 419 33 (1962)] for the measurement of gas-liquid mixtures using sharp-edged orifice plates with either radius, flange, or pipe taps. 

An equation for use with venturi meters was given by Chisholm [Br Chem. Eng., 12, 454—457 (1967)]. A procedure for determining steam quahty via pressure-drop measurement with upflow through either venturi meters or sharp-edged orifice plates was given By Colhus and Gacesa [J. Basic Eng., 93, 11-21 (1971)]. 

Reynolds number for the fluid being released. For sharp edged orifices with Reynolds numbers approaching 30,000, it approaches 0.61. For these conditions, the exit velocity of the fluid is independent of the size of the hole. For a rounded opening, Cj approaches 1. For a short section if pipe (length/diameter > 3), Cj = 0.81. For cases where is unknown or uncertain use C. = I i[Pg.338]

The pressure loss associated with this type of exhaust opening is the sum of two pressure losses. The slot hood is usually thought of as a sharp-edged orifice and the duct entry (from the slot plenum) is a flanged opening. The rec ommended hood entry loss is given by Eq. (10.56) - ... 

RP3.2 Flange Mounted Sharp Edged Orifice Plates for Flow Measurement... 

Table is based on 100% coefficient of flow. For well rounded entrance multiply values by 0.97. For sharp edged orifices a multiplier of 0.65 may be used for approximate results. 

Example 2-10 Gas Flow Through Sharp-edged Orifice... 

Condensate, Ibs/hr (Equation 2-133) or for pipe, Williams and Hazen constant for pipe roughness, (see Cameron Table 2-22 and Figure 2-24) or flow coefficient for sharp edged orifices... 

A small hole has been deliberately placed in a vessel near the top to provide a controlled vent for a nitrogen purge/blankeL The hole is 0.2-inch diameter witli the vessel operating at 150 psig at 100°F. Determine the flow through this vent hole. Assume it acts as a sharp edged orifice. 

Cq = discharge coefficient for sharp-edged orifice = 0.61 for Reynolds Number > 30,000 and not sonic... 

A Standard valve with integral legs used for most services, utilizing a sharp-edged orifice in the tray floor. 

P = pressure in drum, psia Cq = orifice or short pipe coefficient = 0.62 for sharp-edged orifice = 0.81 for short pipe h = allowable pressure drop, psi V = specific volume, fP/lb, at 14.7 psia and 60°F f = frequency, cycles/sec rpm... 

A gas cylinder containing air discharges to atmosphere through a valve whose characteristics may be considered similar to those of a sharp-edged orifice. If the pressure in the cylinder is initially 350 kN/m2, by how mnch will the pressure have fallen when the flowrate has decreased to one-quarter of its initial value ... 

For sharp-edged orifices and for Reynolds numbers greater than 30,000, CQ approaches the value 0.61. For these conditions the exit velocity of the fluid is independent of the size of the hole. 

For sharp-edged orifices with Reynolds numbers greater than 30,000 (and not choked), a constant discharge coefficient C0 of 0.61 is indicated. However, for choked flows the discharge coefficient increases as the downstream pressure decreases.9 For these flows and for situations where C0 is uncertain, a conservative value of 1.0 is recommended. 

For liquid reliefs through rupture discs without significant lengths of downstream piping the flow is represented by Equation 9-2 or by Equation 9-3 for flow through a sharp-edged orifice. No corrections are suggested. 

Orifice Meters A square-edged or sharp-edged orifice, as shown in Fig. 10-14, is a clean-cut square-edged h<5e with straight walls perpendicular to the flat upstream face of a thin plate placed crosswise of the channel. The stream issuing from such an orifice attains its minimum cross section (vena contracta) at a distance downstream of the orifice which varies with the ratio p of orifice to pipe diameter (see Fig. 10-15). 

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