Emerson’s Rick Belin presented a session, DP Flow for Dummies, to share some of the best practices and pitfalls to avoid in differential pressure (DP) flow measurement. He cited many reasons from being well understood to being accepted practice in many applications, such as pipeline custody transfer applications. Accuracy can be 0.55% over a 14:1 turndown ratio.
Some mistakes include insufficient straight-run pipe lengths. A standard orifice plate requires 44 x the pipe diameter of straight pipe length leading to the measurement to avoid turbulence. The Rosemount Conditioning Orifice Plate reduces this pipe length requirement to 2 pipe diameters.
Another common mistake is overspending of orifice plates on large line sizes. For larger line sizes, an averaging pitot tube flow meter may be a better option. For and averaging pitot tube, a common mistake to avoid is incorrect averaging pitot tube measurement. All of the sensing tubes must be exposed to the fluid flowing through the pipe. Rick noted that there are sizing tools to help get the correct sensing tube length.
Another pitfall is the misalignment of the orifice plate. The plate must be within 3% of the inside pipe diameter per ANSI/API 2530. Centering mechanisms help avoid any misalignment issues. With proper alignment, accuracy is improved and installations and ongoing support costs are reduced.
Rick shared how to do DP flow measurement correctly. The first is picking the pipe location for the measurement. The length of straight run and location of the control valves can impact the measurement and orifice plate type selection. By understanding the available technology options you can select the right measurement technology for your application.
The Instrument Toolkit sizing software can help you with the calculation to make the right selections.
Rick concluded by noting how a DP flow measurement device can replace multiple devices and complex impulse lines.