Pumps & Systems, September 2008
The town of Walden rests on a high glacial plain in Colorado, just 20 miles from the Wyoming border in Jackson County. Situated in the center of a large open valley called North Park, the 734 residents (based on the 2000 census) of the town and the surrounding area refer to themselves as "North Parkers."
Problem
As is the case with many municipalities, the needs of Walden's water treatment plant have changed over time. The town found out recently it had to measure finished water, but the original pump station design did not include a flowmeter. The town also had two common problems municipalities face when specifying and installing a retrofit flowmeter for a finished water application: 1) Limited real estate and space requirements and 2) A need for high accuracy flow measurement. Another challenge, common to local governments, was the need to minimize both installation costs and ongoing maintenance costs for any new technology solution.
Traditional liquid flowmeters typically require a minimum upstream and downstream straight-pipe run to operate effectively and accurately. Most traditional flowmeter installations require 10 to more than 40 straight-pipe diameters upstream from the meter and five or more straight pipe diameters downstream. These straight-pipe runs are required to eliminate the effects of swirl and other pipeline disturbances caused by pumps, elbows, valves and other devices that negatively affect liquid flow measurement accuracy.
Often, as in this town, the required plant real estate for a standard liquid flow measurement solution was simply not available in this system retrofit. The town's existing piping configuration had no space for installation of a traditional liquid flowmeter given the lay length requirements and the typical upstream and downstream piping space needed for the accuracy of most flow measurement devices.
The town's engineers consulted with Canyon Systems, Inc. (Lakewood, Colo.), McCrometer's municipal representative, about a solution with the best overall flow measurement and accuracy. The engineers stressed the need for a flowmeter solution that would also minimize installation costs given their space constraints.
Solution
The town's engineers and Canyon Systems evaluated multiple flow measurement options suited for municipal water system applications. The McCrometer V2 SystemTM municipal flowmeter was considered because of its accuracy, flexible installation requirements and versatile capabilities. It is a differential pressure (DP) type flowmeter that includes a self-conditioning cone within the meter.
This DP type flowmeter is useful when irregular or crowded piping and equipment are in place because its self-conditioning cone design greatly reduces the straight-pipe run required for accurate measurement. Other flow technologies, such as orifice plate, turbine or venturi tube meters, often require complex or expensive construction for the upstream and downstream straight piping required to achieve the desired measurement accuracy.
This meter's cone design actually conditions fluid flow to provide a stable flow profile for accuracy. The centrally-located cone inside a tube interacts with the fluid flow and reshapes the velocity profile to create a lower pressure region immediately downstream. The cone's central position in the line optimizes the velocity of the liquid flow at the measurement point. The result is a highly stable flow profile for measurement accuracy.
Prepackaged and ready to install, this self-conditioning meter features a built-in three-way valve that isolates the transmitter from the process fluid flow for easy maintenance without shutting down the pipeline. It arrives from the factory wet-flow calibrated, eliminating many problems that arise when adding a flowmeter to any existing installations.
The self-conditioning DP flowmeter selected by the town reduces flowmeter straight-pipe run requirements by up to 70 percent or more, and needs only zero to three straight-pipe diameters upstream and zero to one downstream to operate effectively. This flowmeter can be installed in close proximity to pumps in crowded plant retrofit projects by reducing the number of straight-pipe runs required by other meters. This installation flexibility results in real estate, pipe and labor cost savings that can exceed the cost of the instrument.
This flowmeter also meets the town's accuracy needs. This flowmeter operates over a wide flow range of 10:1 with low head loss and supports line sizes from 4- to greater than 18-in. The range of accuracy is +1 percent over the 10:1 turndown for between a 40- to 7500-gpm liquid flow rate. The engineers appreciated that the flowmeter requires virtually no recalibration or maintenance throughout an exceptionally long life. This low maintenance requirement allows the town to keep its operational costs low.
The town's city engineers installed an 8-in self-conditioning DP flowmeter (see Figure 2). To ease installation, the 8-in flowmeter was equipped with a flange connection on the inlet and a plain end connection on the outlet. These connections made it easier to mate the existing flange by Dresser sleeve piping connections. Due to space limitations, the lay length of the flowmeter was shortened by 13-in from the standard lay length of 34-in to 21-in. The last consideration was to install the flowmeter downstream of a "Y" fitting with no straight-run of pipe and upstream of an ell and a valve with no straight-run of pipe (see Figure 3).
Results
This self-conditioning DP flowmeter fit the town's water plant retrofit and finished water application. The flowmeter allowed the town's engineers to measure finished water effluent accurately. The meter's transmitter also produces a 4-20 mA signal for easy integration with the plant's SCADA system (see Figure 4). The completed retrofit project provides the town with better water plant process control and accounting of water usage.
Pumps & Systems, September 2008