Machinery's Handbook, 31st Edition
2758
Flow Measurement
A 1
2 ∆ p ρ -----
A 1 A 2 --- 2 –1 --------------
Q C =
where D p is the measured differential pressure, A 1 is the area at the upstream pressure tap, and A 2 is the area at the throat. Coefficient C is generally 1 for ideal fluids, and 0.90 to 0.98 for viscous fluids. Orifice Plates and Meters: These are the most widely used method of flow measure - ment in industry. Similar to venturi meters, these flow- through devices measure pressure drop across a restriction. The restriction is generally a hole in a plate placed across the flow path. This causes sudden contraction and expansion. The placement of the pressure measurement point is critical and dependent on the shape of the orifice. These meters generally create a larger energy loss than venturi meters due to the turbulence created by sudden changes in diameter, but are very cost-effective. The relevant standard covering orifice plates is ISO 5167. Flow through an orifice meter can be calculated using the fol - lowing equation. Q C f A o 2 ∆ p ρ ----- = where A o is the area of the orifice. The coefficient C f is generally between 0.6 and 0.9, and depends on the Reynolds number as well as pipe and orifice diameter. Tabulations of coef ficient C f can be found in various reference sources. Flow Nozzle Meters: Similar to venturi and orifice meters, these flow-through devices measure pressure drop caused by flow restriction. Flow nozzles have a smooth elliptical inlet leading to a throat section with a sharp outlet. The geometry of these nozzles has been highly standardized by ASME and ISO. The nozzle tends to have a well-rounded intake, and therefore creates a much lower energy loss than an orifice meter. This higher efficiency means greater flow capacity when compared to most other differential meters of the same size. Energy loss does tend to be higher than with venturi tubes. Nozzle meters are available in standard and long radius configurations, with different discharge coef - ficients. Flow is calculated similarly to orifice and venturi meters, using Bernoulli’s equa - tion and the appropriate coefficient. Variable Area Flow Meters: These devices are extremely common in industry, and are available for measuring liquids or gases. There are tube and vane types. Tube meters have a weighted float that is suspended in the flowing fluid. The clearance space between the float and the tube forms an annular passage or orifice. Since the tube is tapered, the area of this orifice is larger when the float is near the top than it is when the float is near the bottom. Volumetric flow is measured based on the height the float is carried to. As a result, these meters must be installed vertically. Vane type meters use a vane to obstruct a bend in the pipe. It is normally closed, and fluid flow swings it open. Both tube and vane meters are for use with clean fluids at moderate pressures. Some types allow the float or vane to be viewed against a calibrated scale. In those cases, the fluid must be clear enough to see the float. Other types of variable area flow meters are suitable for high pressures and opaque fluids, and use sensors to detect the position of the float or vane and display the result re - motely. Due to the restriction caused by the float or vane in the fluid stream, some energy is lost by fluid flowing through these meters. Elbow Meters: These meters are simply a bend in a pipe through which the fluid flows. These meters are sometimes the only choice where space constraints do not allow the other types of meters. Pressure differential is measured between a point on the outside radius and a point on the inside radius. Because the fluid is moving faster along the outside radius, it will be generate higher pressure.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online