David W. Spitzer

Which of the following can be located in the upstream straight-run of an orifice plate primary element
A. Elbow
B. Tee
C. Fully Open Full-Port Ball Valve
D. Fully Open Reduced-Port Ball Valve
E. Throttling Valve
F. None of the Above

The performance of most flowmeters can be adversely affected by a distorted velocity profile at the flowmeter inlet. One method to improve the velocity profile at the inlet of the flowmeter is to install straight run upstream that allows velocity profile distortions to dissipate prior to entering the flowmeter. To achieve this end, the upstream straight run should not contain any obstructions to flow.

Elbows, tees and throttling valves present an obstruction to the fluid flow and can distort the velocity profile. Because the objective of straight run is the opposite — reduce velocity profile distortion — elbows, tees and throttling valves should not be located in the upstream straight run. Answers A, B and E are not correct.

Although designed without turns or bends, fully open reduced-port ball valves do present a small obstruction to flow and should not be located in the upstream straight run. Answer D is not correct.

A fully open full-port ball valve should create minimal (if any) distortion of the velocity profile if proper gaskets are used, if the valve is properly aligned in the pipe, and if the valve is maintained in its fully open position. In a practical sense, this valve could be located within the upstream straight run, but it would be better if it were located upstream of the straight run. However, this valve would not be acceptable in the straight run when the flowmeter installation standard requires the upstream straight run to have a machined finish. Depending on application, Answer C or F might be correct.

Additional Complicating Factors
Fully open ball valves could be pragmatically used in the upstream straight run, such as when space is limited or when extensive piping modifications are necessary to install sufficient straight run. Full-port ball valves are preferred, but reduced-port valves could be used in a pinch.

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.