|David W. Spitzer|
The following article is the latest in a series based on the troubleshooting of a mysterious unit shutdown due to flowmeter performance problems.
If you have been following along for the past few months, ammonia bubbles affected an ultrasonic flowmeter that caused a reactor to scram (shut down) on warm days. The problem was traced to the formation of ammonia bubbles caused by a pressure control valve located 200 meters from the unit at the exit of the liquid ammonia tank farm.
The control valve (the source of bubbles) was eliminated by modifying the self-contained differential pressure control valve to control the backpressure of the ammonia leaving the tank farm by bypassing flow back into the ammonia tank. In this configuration, the liquid upstream would flow directly to the plant and bubbles formed downstream of the self-contained valve would return harmlessly into the ammonia tank.
The parts were put on order and installed about two weeks later. We could not remove the pressure control valve without a major outage, so the local pressure controller was put in manual with the control valve and its bypass wide open.
There were no scrams related to the liquid ammonia flowmeter during that summer—even though there were a number of days when conditions were significantly warmer than the day in June that presented the original problem.
Was the problem solved? Not quite yet.
I was working in my office on a warm day in June of the next year when I received a call that the reactor scrammed. I went directly to the pressure control valve. Someone had closed the bypass valve during the winter! There were no more problems that summer after the bypass valve was re-opened.
The pressure control valve and its bypass were removed one or two years later after which there were no more spurious scrams in warm weather.
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. Spitzer and Boyes is also the publisher of the Industrial Automation Insider. Mr. Spitzer can be reached at 845 623-1830 or www.spitzerandboyes.com.