Quiz Corner: Upstream and downstream straight-run allocation

A total of 10 diameters of straight run is available, but the flowmeter specification shows that 20 diameters of straight run upstream and 5 diameters of straight run downstream are needed to measure accurately. How much straight run should be installed upstream and downstream of the flowmeter?    

A.   5D upstream and 5D downstream

B.   6D upstream and 4D downstream

C.   7D upstream and 3D downstream

D.   8D upstream and 2D downstream

The purpose of straight runs is to develop good velocity profiles entering and leaving the flowmeter. Most flowmeters that require straight run are more sensitive to flowing conditions upstream of the flowmeter and less sensitive to flowing conditions downstream.

There is no definitive answer, but of the installations offered, 8D upstream and 2D downstream (Answer D) represents the longest upstream straight run, which should provide the best balance of velocity profiles — hopefully, without exhibiting the adverse effects of a short downstream straight run. 

Additional complicating factors

Manufacturers test their flowmeters in different installations and different operating conditions, which allows them to publish installation guidance to include straight run requirements. Therefore, the manufacturer can be contacted and perhaps use this test data to better determine how much straight run should be allocated for upstream and downstream straight runs in non-ideal installations.  

David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.