In my last four “Applications Corner” columns (Nov. page 14, Oct. page 14, Sept. page 12, Aug. page 14), I described the decision process used in the mid-1980s to apply a Coriolis mass flowmeter in ammonia service operating at a relatively low density and the apprehensions about using said flowmeter in this application. The 1” Coriolis mass flowmeter installation created such a large obstruction to flow that the flowmeter would not pass (even close to) the operating flow. The installation with a 1.5” Coriolis mass flowmeter installation was barely able to pass the design flow while operating at almost double the nominal operating pressure. Then the manufacturer told me that the 1.5” Coriolis meter was properly sized and should exhibit approximately 400 mbar (6 PSID) of pressure drop at design conditions. I was now on my own to solve the problem.
I asked one of the chemical engineers (who also did piping design) for a piping handbook to calculate the pressure drop through fittings. Analysis of the inlet piping revealed the pressure drop across the inlet spool-piece was significant, but should not have created much of a problem. However (to my surprise) the pressure drop across the outlet spool-piece was huge. Now that calculations confirmed the problem, I started to work on a solution.
New upstream and downstream spool pieces were made from concentric pipe reducers that resembled the converging and diverging sections of a Venturi flowmeter. The new spool pieces were designed to reduce the pressure drop of the gas entering and exiting the flowmeter. The piping people added a nice touch by installing pressure gauges upstream and downstream of the flowmeter.
The Coriolis mass flowmeter was installed using the new spool pieces, and it worked fine. Can you guess what the pressure drop was across the flowmeter at design flow? If you guessed 400 mbar (6 PSID)—you would be correct.
Success — Finally!
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC offering engineering, seminars, strategic marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 250 articles about flow measurement, instrumentation and process control.