Water & Sewer Main Case Studies
EPWU has installed a series of nineteen TP-1’s to reduce the volume of data by about 3 orders of magnitude, converts it to graphic form, and improves the detection of transient events. Prior to the installation of the TP-1 monitoring devices, EPWU had undergone a progressive program attempting to reduce pressure transients without having any real knowledge of where they were occurring or what equipment was causing them. These efforts include the purchase and installation of soft starts on electric pump motor controls, valve actuators hard-wired for long opening and closing times and some globe style check valve installations. Where gas engine drive pumps were installed, they were programmed to start from idle and to ramp up to full speed over a period of several minutes. El Paso Water Utilities had incorporated more than 40 pressure sensors into the SCADA system, each with a data sample rate of once per second. That generated 86,400 data points daily from each of the sites, in tabular format. A brief review of that data demonstrated that there is no such thing as a brief review of this much data, and still sharp transients were not detected.
In spite of these measures main breaks continued to occur due to transient pressures. The purchase and installation of the TP-1 units allowed EPWU for the first time to correlate transients with actual changes in system configuration. Several important discoveries came from the effort to install TP-1s and the subsequent analysis of the information produced.
For example, in installations where globe-type check valves had been installed to control pressure transients related to pump starts, some TP-1s showed pressure transients were still being produced. Analysis of the data showed significant and regular spikes. After a field visit to the site, it was discovered that the check valves had been replaced at some point with conventional swing checks. Further investigation revealed that maintenance crews routinely replaced globe-type check valves with swing checks because the globe-type were leaking and the swing checks were thought to be maintenance-free.
Pump stations equipped with soft-starts were shown not to be immune to pressure transients. Field investigation revealed that while soft-starts improved performance, that is to say they reduced the number and amplitude of pressure transients, they did not eliminate them. Conversely, pump installations with variable speed drives were shown to be transient free. The variable speed drives were clearly doing a better job of eliminating pressure transients. As a result, nearly all pump systems are specified for variable speed drives. Many soft start installations have been retrofitted with variable speed drives and many pump stations with neither soft starts nor variable speed drives will be so equipped. EPWU’s experience with their extensive network of TP-1 systems has been a great benefit to the agency, providing a unique capability to deal with transient pressures that has not existed before. The program of transient pressure monitoring has revealed or confirmed a number of important aspects of pipeline operation and has reduced damage caused by pressure transients.
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