CHAPTER 2 - BASIC CONCEPTS RELATED TO FLOWING WATER AND MEASUREMENT

17. Approach Flow Conditions

Water measurement devices are generally calibrated with certain approach flow conditions. The same approach conditions must be attained in field applications of measuring devices.

Poor flow conditions in the area just upstream from the measuring device can cause large discharge indication errors. In general, the approaching flow should be subcritical. The flow should be fully developed, mild in slope, and free of curves, projections, and waves. Pipeline meters commonly require 10 diameters of straight pipe approach. Fittings and combinations of fittings, such as valves and bends, located upstream from a flowmeter can increase the number of required approach diameters. Fluid Meters (American Society of Mechanical Engineers, 1983) and International Organization for Standardization (1991) give requirements for many pipeline configurations. By analogy and using a minimum of 10 pipe diameters of straight approach, open channel flow would require 40 hydraulic radii of straight, unobstructed, unaltered approach.

A typical example approach criteria as specified by Bos (1989) follows:

• If the control width is greater than 50 percent of the approach channel width, then 10 average approach flow widths of straight, unobstructed approach are required.
• If the control width is less than 50 percent of the approach width, then 20 control widths of straight, unobstructed approach are required.
• If upstream flow is below critical depth, a jump should be forced to occur. In this case, 30 measuring heads of straight, unobstructed approach after the jump should be provided.
• If baffles are used to correct and smooth out approach flow, then 10 measuring heads (10h₁) should be placed between the baffles and the measuring station.

Approach flow conditions should be continually checked for deviation from these conditions as described in chapter 8 of this manual.