Millcreek Township connects to neighboring township with a pressure-reducing valve with integral backup.
The Millcreek Township Water Authority (MTWA) near Erie, Penn., was created in 1974 to provide water to the rapidly growing Millcreek Township customer base. The MTWA purchased several small water systems that have been upgraded and grown to 112 miles of pipeline ranging in size from 2 to 24 inches with 10 pump stations to maintain water pressure, five water towers for storing a total of 5 million gallons and one groundwater well. The water distribution system has elevation changes of about 400 feet and an average daily flow of approximately 2.5 millions of gallons per day (MGD). With 7,300 customers and growing, it is essential that the Township of Millcreek continues to maintain an effective and reliable water system.
Taxed Capacity
Recent growth in the Peach Street commercial district, the main shopping and restaurant area of Erie County, had taxed the capability of MTWA's Peach Street Pump Station. This station provides water to the neighboring communities of Summit Township and McKean Township. MTWA retained the consulting firm of Hill Engineering, Inc., to evaluate the future demands of the Peach Street pressure district and to develop a long-term plan to meet those needs.
Figure 1. Double-diaphragm PRV in the heated enclosure
The Long-Term Plan
The Hill Engineering report recommended construction of a larger Peach Street Pump Station with greater capacity and the construction of more distribution system storage to meet fire demands. Construction of these improvements would take years to plan, design and build. Interim upgrades were essential to make it through the next few years.
The owners of the Millcreek Mall, (the largest shopping center in the area) approached MTWA, requesting to be connected to Millcreek's system to avoid costly repairs to its own private water tower, which was in need of an upgrade. MTWA's Peach Street Station could provide the Mall with sufficient water to meet its daily demands, but the availability of adequate fire flow from the station could not be guaranteed.
Short-Term Solution
The solution was to develop a reliable interconnection between the Millcreek system and the Summit system, so that the Summit Township Water Authority's (STWA) existing Waterview water storage tank could automatically backfeed into the Millcreek system in the event of a fire or loss of electric power at MTWA's Peach Street station. A manually operated pressure reducing valve (PRV) had been installed years before to provide emergency backflow, but it was not 100 percent reliable.
In addition to being a single (non-redundant) device, the valve was located in a buried vault. Its activation required an operator to be present to open a gate valve. With a pressure differential of about 100 psi at the interconnection point, an automatically operated, fail-safe PRV was needed to avoid damage in the event of valve failure. Installing two PRVs as a safety backup was considered as a safety backup, but that would add cost and double the maintenance required. A pressure reducing valve with integral backup (PR-SM), was the best solution. The new control valves have a second and independent operating system superimposed upon the standard primary system. Under normal pressure reducing conditions, the primary pilot senses the downstream pressure through a connection at the valve outlet. Under flowing conditions, the pilot reacts to small changes in pressure to control the valve position by modulating the pressure above the diaphragm in the lower operating chamber. The downstream pressure is steadily maintained at the pilot set point that is adjustable.
Should the primary pilot system and/or main valve fail to control the downstream pressure, the independent back-up pilot system will begin to operate. It controls the pressure above the diaphragm in the second operating chamber. The back-up pilot is set slightly higher than the primary pilot. The forces now operating in the top chamber assume control of the inner valve assembly and maintain pressure reducing control.
The secondary pilot continually senses the downstream pressure. Should there be a rapid rise in downstream pressure, the secondary pilot will respond quickly and pressurize the top chamber. This will complement the primary pressure reducing controls and provide faster response. The installation took about two weeks to complete once the materials were on hand.
This project has allowed MTWA to maintain water pressure to its customers and the Millcreek Mall continuously, even during power outages.
Pumps & Systems, October 2011