## 75 15 225 30 375 45 525 60 675 75 825 90

TABLE 7 Resistance coefficients for miter bends at reynolds number « 2.25 X 105

TABLE 7 Resistance coefficients for miter bends at reynolds number « 2.25 X 105

mary element provides pressure head recovery and determines the meter efficiency. The pressure differential between inlet and throat taps measure rate of flow; the pressure differential between inlet and outlet taps measures the meter head loss (an outlet tap is not usually provided). Of the three types, venturi meters offer the least resistance to flow, and orifice meters the most.

When meters are designed and pressure taps located as recommended,6 Figures 48-50 may be used to estimate the overall pressure loss. In these figures, the loss of pressure is expressed as a percentage of the differential pressure measured at the appropriate taps and values are given for various sizes of meters. This loss of pressure is also the meter total head, or energy loss, because there is no change in velocity head if the pipe inside diameters are the same at the various measuring points. The meter loss of head should be in units of feet (meters) of liquid pumped if other system losses are expressed this way.

FIGURE 47 Herschel-type venturi meter, showing locations of pressure taps (Reference 6)

FIGURE 47 Herschel-type venturi meter, showing locations of pressure taps (Reference 6)

AREA RATIO,

FIGURE 48 Overall pressure loss across thin-plate orifices (Reference 6)

AREA RATIO,

FIGURE 48 Overall pressure loss across thin-plate orifices (Reference 6)

Reference 6 should be consulted for information concerning formulas and coefficients for calculating differential pressure versus rate of flow.

Screens, Perforated Plates, and Bar Racks Obstructions to the flow of liquid in the form of multiple orifices uniformly distributed across an open or closed conduit may be used to remove solids, throttle flow, and produce or reduce turbulence. They may be used

DIAMETER RATIO,/3=d/D

DIAMETER RATIO,/3=d/D

DIAMETER RATIO (d/D)

FIGURE 50 Overall pressure loss across venturi tubes (Reference 6)

DIAMETER RATIO (d/D)

FIGURE 50 Overall pressure loss across venturi tubes (Reference 6)

upstream or downstream from a pump, depending on their purpose, and they therefore introduce a loss of head that must be accounted for. When an obstruction is placed upstream from a pump, a significant reduction in suction pressure and NPSH available can result.

The loss of head results from an increase in velocity at the entrance to the openings, friction, and the sudden decrease in velocity following the expansion of the numerous liquid jets. The total head loss is a function of the ratio of the total area of the openings to the area of the conduit before the obstruction, the thickness of the obstruction, the Reynolds number, and the velocities. Various investigators have determined values for resistance coefficients that can be multiplied by the approach velocity head to obtain the loss through these obstructions. According to Idel'chik,7 loss of head h in feet (meters) may be calculated from the equation h = K 21 (24)

where K1 = resistance coefficient

V1 = average velocity in the conduit approaching the obstruction, ft/s (m/s) g = acceleration of gravity, 32.17 ft/s2 (9.807 m/s2)

round-wire mesh screens For flow having Reynolds numbers equal to or greater than 400, the resistance coefficient for flow through a round-wire, plain square mesh screen (Figure 51a) may be estimated as a function of percentage of open area using the equations in USCS units in SI units

Re Re

VpWd v12 VoWd v1000

a 400 400

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