A wedge element is nothing more than a “V” shaped restriction welded to the top of the pipe with a segmental opening at the bottom of the pipe to allow for free passage of any solids present in the process fluids.
Wedge flow meters come in various designs. They are used in liquid or gas service where orifice meters are not suitable because of entrained solid or gas; for example, they are used in production field gathering systems. The chemical tee type (Figure 500-16) with its associated remote seal transmitter is used for liquids with solids, dirty viscous fluids, and hot fluids which tend to solidify when cooled. The pipe tap type is used for clean viscous fluids which can be dead-ended in conventional lead lines.
The smaller integral elements, typically limited to ½ to 1½ inch size, are also used for clean viscous fluids. They are usually mounted directly to conventional d/p transmitters.
Wedge flow meters are of the d/p type, in which the square root of the differential is linearly proportional to volume flow rate within a certain range (or Reynold’s number range). Wedge flow meters maintain this square-root relationship over much lower values of Reynold’s numbers than many other differential producing elements.
The flow coefficient for wedge flow meters remains relatively constant in the lower Reynold’s number ranges. For example, a 1½-inch segmental wedge flow coefficient is constant from Reynold’s numbers of 40,000 down to 2,000, as compared to the change on flow coefficient (K) for a 1½-inch orifice plate with flange taps (Figure 500-17).
The accuracy of uncalibrated wedge flow meters is typically ±3% of upper range value. Calibrated meters may achieve ±0.5% accuracy.
CPTC tested a 4-inch Combustion Engineering/Taylor Wedge meter in 1988 with a high gravity natural gas. The results revealed the following:
• Taylor Wedge meters should be properly calibrated against a suitable reference flow device before use. The flow coefficients supplied by the manufacturer cannot be relied upon to provide accurate (±0.5%) flow rates. Rather, they were found to undermeasure flow by 2 to 8%.
• The meter showed acceptable repeatability and its flow coefficients have a weak dependency on flow rate, making flow rate calculation relatively simple.
• The wafer-type wedge meter should be avoided if frequent change of the wedge element is required, because this is time-consuming.
Included in the following figures are a few typical sizing charts and equations, based on product brochures from vendors. For help in sizing and specifying a wedge flow meter, consult the manufacturer.
• Figure 500-18: Determination of Integral Wedge Element H/D Ratio and Differential Pressure (in three parts)
• Figure 500-19: Integral Wedge Capacity Table
• Figure 500-20: Wedge Element Capacity Tables (½ inch to 12 inches)