New Drives Reduce Water Main Breaks

By Ron Rabbitt

The city of Sandusky, OH, boasts a population of 29,000 from its perch on the edge of Lake Erie. With 22 miles of shoreline and one of the finest, best-protected harbors on the Great Lakes, it is easy to see why American Demographics named Sandusky one of the "Top Ten Best Small Cities in America" in 1990 and The New Rating Guide to Life in America's Small Cities rated it sixth in the nation in 1998.

In 1998, the city of Sandusky Water Department's water lines were frequently breaking because of pressure surges, causing recurrent disruptions in water service, road closings and an increased risk of system contamination. With public safety and health at stake, department officials sought a permanent solution.

The department's Filtration Division and an outside engineering firm worked with Rockwell Automation to update antiquated finished water pump controls that supply water to the city's distribution system. The new design uses Allen-Bradley variable frequency drives to control water output more efficiently and has lowered the city's electricity and maintenance costs.

In 1998 and 1999, the Sandusky Water Department rebuilt four high-service pump units, and installed a new control system for the pumps.Click here to enlarge imageBackgroundLike many communities, Sandusky's water demand fluctuates according to time of day and season. The department operates six high service pumps with capacities that range from 3 mgd to 8 mgd. Prior to the installation of variable frequency drives, achieving the 12 mgd flow required at peak times was costly, time-consuming and stressful on the water distribution system. To maintain system pressure under varying demand scenarios, the department operated its high service (HS) pumps in various combinations.Operators used a labor-intensive pump changing process to meet fluctuating pressure demands. During the HS pump changes, water pressure increased and decreased in the distribution system, causing water hammer a phenomenon where sudden pressure fluctuations can cause pipes to shake, rattle and possibly break. Frequent pump start-ups also increased electrical costs considerably, due to the increased voltage that pump motors require at start-up.

Water hammer was a frequent occurrence in the distribution system and was a common cause of water main breaks, according to Sandusky Water Services Superintendent Douglas Keller.

Pump RehabIn 1998 and 1999, the Sandusky Water Department (SWD) rebuilt four high-service pump units. Because of the city's previously successful experience with Allen-Bradley products, Keller decided to work with distributor Brohl & Appell and Rockwell Automation engineering experts to design and install a new control system for the pumps.

Rockwell Automation staff installed Allen-Bradley 1336 Plus II variable frequency drives to control the new motors on Sandusky's two largest high-service pumps.Click here to enlarge image

A 5 mgd pump rebuilt in the project received a soft-starter system for its existing motor. For the city's two largest pumps, 6 mgd and 8 mgd horizontal split case pumps, the rebuild project included replacing the pumps' 250 and 300 hp 1940s-vintage squirrel-cage motors with new high efficiency motors. Rockwell Automation staff specified and designed Allen-Bradley 1336 Plus II variable frequency drives to control the new motors.

The new AC drives provide the city with more precise water pressure control, leading to significant energy savings. Over a 12-month period, Sandusky's water works facility has saved over $31,000 in electrical costs, which can be attributed to reduced demand charges because of fewer pump start-ups, more constant flow to satisfy fluctuating demand and more efficient motors.

By controlling motor speed and torque, the Allen-Bradley 1336 Plus II drives allow the HS pumps to have an adjustable speed range the 8 mgd pump can be adjusted from 4 mgd to 8 mgd and the 6 mgd pump can be adjusted from 5 mgd to 6.8 mgd. Most importantly, the drives allow the pumps to gradually increase system pressure and reduce the number of cumbersome pump changes.

Keller noted that the two large pumps now can serve most of the city's needs and are used the majority of the time.

"The variable frequency drives have decreased the need for pump changes, reduced electrical cost and reduced water hammer, leading to reduced water main breaks," Keller said.

ConclusionThe new drives-centered system has decreased the need for pump changes, and has reduced water main breaks by 76 percent. By reducing main breaks the potential for back flow, system contamination and fire protection interruption has been greatly reduced. Water service interruptions also have decreased, which means service to residential and industrial customers has been more reliable.

"Allen-Bradley variable frequency drives have been a tremendous value-add to the Sandusky Water Department," said Keller. "It also has been important to us to have a supplier that will support us and answer questions as we continue to expand and improve our system. Rockwell Automation has been there for us, and the city of Sandusky plans to incorporate variable frequency drives in all future motor-related applications."

About the Author: Ron Rabbitt is a Drives Specialist for Brohl & Appell, where he is responsible for advising customers on how to specify, install and maintain drives. Brohl & Appell is an affiliated distributor who has been meeting the needs of the industrial automation, PVF and electrical markets for over 110 years