To handle growing water demands, the City of Cottonwood in the arid central highlands of Arizona devised a strategic water management plan that included creating a modern municipal water system with monitoring, control and pumping technology. The unusually high well-to-customer ratio presented piping and pumping challenges in accommodating more than 28 wells. Previously a 20 psi pressure change constantly stressed the pipe, creating leaks and water hammer on more than 100 miles of pipe.
Instead of turning pumps on and off, causing the 20 psi differential, the city converted nine sites from a fixed-speed hydropneumatic system into a duplex variable-speed, constant-pressure system using Danfoss VLT AQUA variable-frequency drives (VFDs). Using only the built-in Smart Logic Controller (SLC), the drives were programmed to operate in a lead, lag and duplexing sequence. When the lead pump reaches maximum speed, the lag pump starts, maintaining pressure within 2 to 3 psi.
With the constant-pressure design, the average pipe pressure stays in a 60 to 65 psi range. One or more pumps are online at all times to keep the system pressurized, cutting down on leaks by reducing pipe fatigue and system damage. The design stabilized pipe pressure and nearly eliminated water hammer. Water main leakage decreased substantially, which means less time spent digging up and repaving roads to make repairs.
This article first appeared in the February 2014 issue of Treatment Plant Operator, published by COLE Publishing, Three Lakes, Wis. It is reprinted by permission. The original article can be found here