A typical elbow, or elbow tap, is shown in Figure 500-14. The pressure taps are located along a radius that is at a 45-degree angle to the face of the flange. Generally speaking, elbows that measure 4 inches and larger are of the “short radius” type, in which the radius of the elbow equals the inside diameter of the elbow. Elbows that measure less than 4 inches are usually the “long radius” type, in which the elbow radius equals one and one-half (1.5) times the inside diameter of the elbow.
Because the elbow depends on centrifugal force to develop a differential pressure, measuring the flow of gas, with its inherent low density, is not practical. Liquids, with their much higher densities, develop reasonable differentials. The radius of curvature has a considerable effect on the differential. Thus, a short radius elbow will develop more differential than a long radius elbow.
Elbows are not adversely affected by changes in viscosity because both pressure taps are located along the same diameter.
Uncalibrated elbows typically measure flow within ± 5% of upper range value. Better accuracy can be obtained for calibrated elbows.
The minimum Reynold’s number (based on pipe diameter) for elbows is typically 10,000.
Although elbows are often sized by vendors, a simple formula for measuring flow through an elbow is provided below.
Q = 5.663 K D2 (h/gf)
where:
Q = flow rate in gallons per minute (gpm) at flow temperature
K = flow coefficient for given elbow
D = pipe inside diameter of elbow in inches
h = differential pressure in inches of water
gf = specific gravity at flow conditions
Figure 500-15 gives the flow coefficient K for various elbow sizes and radii of curvature. These are nominal values, subject to a tolerance of up to ±5%.