Straight-Run Requirements for Flow Accuracy

Have you ever seen the inside of a carbon steel pipe used for sodium chloride brine service? The inside pipe wall has ridges, mountains, boils and other strange shapes. How can you accurately measure flow in such a pipe? The upstream and downstream piping might be straight in your installation. However, the inside of the pipe can be far from straight/smooth. In this application, the strange shapes on the inside of the pipe can affect the velocity profile and, in turn, the flow measurement.

From a system design perspective, it is important to ensure the fluid being handled will not affect the geometry of the selected flowmeter. For example, if it is found that sodium chloride brine affects carbon steel, the flowmeter should not have any carbon steel wetted parts.

As for flowmeter technology selection, one possible solution in an application where the piping does not permit a well formed velocity profile is to select a flowmeter that does not require straight run. This might be possible, but many such flowmeters are only available in limited sizes and they can be expensive. Another possibility is to alter the piping such that the strange shapes are not formed and the inside of the pipe remains straight/smooth.

In the application noted above, replacing the carbon steel metering runs with plastic pipe suitable for the pressure in a vertical run with flow up would increase the probability of maintaining straight/smooth pipe in the straight run. However, plastic pipe has limited pressure ratings and strength, so it might be more pragmatic to use glass-lined carbon steel pipe. Stainless steel pipe might also be an option if the grade of stainless steel is compatible with the fluid.

It is important to note that the ideas described above are related to the flow measurement system – the operative word being "system." The entire system must function properly to implement an accurate flow measurement. The straight run is part of the system. Changing the geometry of the straight run can affect the flow measurement. Designing the straight run to be unaffected by the process may not be common, but it is sometimes necessary.


David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.