Flow Control: Technology information & new products for fluid handling engineers

November 2007

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Effluent Flow with Suspended Solids
Facing Rising Treatment Costs, Paper Mill Reconsiders Flowmeter Installation

David W. Spitzer, P.E.

Sometimes you have so much information that you actually have to start from the beginning in order to determine what is happening. Such was the case when the effluent with suspended solids from a paper mill was measured with an existing magnetic flowmeter. A new clamp-on ultrasonic flowmeter was installed when the existing magnetic flowmeter could not be calibrated per manufacturer specifications or taken out of service. I was invited to the plant to investigate the flowmeters after the measurements from the new ultrasonic flowmeter increased effluent treatment costs by over one million dollars per year.

The existing magnetic flowmeter was installed immediately between reducers with approximately one diameter of straight run upstream. The new ultrasonic flowmeter was installed at the high point in the line approximately one diameter from a 30-degree inlet. Neither flowmeter was installed with sufficient straight run to measure properly.

A temporary ultrasonic flowmeter in another location provided measurements that were similar to those of the new ultrasonic flowmeter. A spot check of the sum of the flows through the raw water flowmeters approximated the magnetic flowmeter measurement. Most of the raw water flowmeters did not appear to have
sufficient upstream straight run and their tubing was not properly installed. An ultrasonic flowmeter on the incoming pipe into the plant provided measurements between the existing magnetic flowmeter and the new ultrasonic flowmeter. Four effluent pumps were installed, but only one of the pumps operated continuously at reduced speed.

At this point, my head was spinning because I did not know which flowmeter to believe. The situation was so confusing with seemingly contradictory information that it was decided to install a new magnetic flowmeter in the effluent pipe in order to accurately measure the effluent flow so the billing would be correct. From a piping perspective, it was convenient to tie the discharge of the four pumps together and install the magnetic flowmeter in a horizontal pipe. This installation garnered support from just about everyone.

But remember, the effluent flow contains solids. If the velocity in the pipe is low, solids can accumulate in the horizontal pipe and flowmeter. This accumulation can alter the cross-sectional area of the pipe and affect the flow measurement. Therefore, the flow measurement would be in question if this were not taken into account.

You have probably been in this situation before — trying to do something correctly while others (in good faith) bow to expediency. It took a few meetings and much explanation to convince everyone that this installation was important enough to be implemented correctly so the measurements (and bills) would not be questioned down the road.

In the end, the flowmeter element was installed with sufficient straight run in a vertical pipe with flow upwards to avoid the solids problem. Physical obstructions required a pair of 180-degree bends. The installation was not pretty, but the flowmeter element was installed correctly. Sometimes, you just have to stick with it.

David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached 845 623-1830.

www.spitzerandboyes.com

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