In previous columns, we discussed the three situations that should be carefully considered: operation under vacuum, measuring downward liquid flow and gravity flow. In part 1 of measuring gravity flow, flowmeter sizing in gravity flow applications was discussed. In particular, the pressure drop across the flowmeter often needs to be low in order to pass the desired flow lest the liquid accumulate upstream. This can be accomplished by increasing the flowmeter size, which tends to degrade accuracy because the larger flowmeter is operated lower in its flow range.
Consider a 1-meter-high liquid separator fed by a pump operated with a variable speed drive with a tangential inlet, gravity underflow and gravity overflow with flow rates of 100, 98 and 2 flow units respectively, where it is desired to measure the inlet and underflow flow rates for operational purposes. A 1½-inch inlet flowmeter was selected based upon operation at a reasonable flow velocity and pressure drop. As a side note, the variable speed drive will increase the inlet pump speed to generate more pressure as needed to provide the desired flow to the separator.
These inlet and underflow flow rates are effectively the same (100 and 98 respectively) so one might expect to use identical flowmeters in both applications — but, not so fast. The pressure drop across the 1½-inch inlet flowmeter was approximately 1 meter of water column at the design flow. However, the liquid must also pass through a control valve that develops an additional pressure drop. Therefore, the total pressure drop associated with the flowmeter and control valve is greater than the available head in the separator so the underflow flowmeter and control valve cannot pass the entire design flow without backing up liquid in the separator and causing an inappropriately high overflow of improper composition.
Increasing the size of the underflow flowmeter to 2 inches reduced the pressure drop sufficiently to allow the flowmeter and control valve to operate properly without backing up liquid into the separator. Note that different size flowmeters were used to measure effectively the same flow rates.
Be careful … and remember to install flowmeters and control valves in piping that ensures that both remain full of liquid at all times.
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.
