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City achieves consistent pressure and no overflows with tank fill valve upgrades
The city of Modesto, Calif., is situated 90 miles east of San Francisco and has approximately 200,000 residents relying on 55 million gal of water per day. In 2006, the city took on a project to upgrade three water storage tanks that hold approximately 750,000 gal each.
After years of using motor-operated valves to fill water storage tanks, the city recognized that there were just too many problems.
First, it experienced stem seal leakage after just a few years of operation. The cause was turbulence in the flow stream that would make the butterfly valve rock back and forth, resulting in premature wear.
Second, there was leakage due to high flow turbulence. When flowing a 16-in. valve wide open at about 10,000 gal per minute (gpm), there will be quite a bit of turbulence that causes wear on the stem and seals. If the valve is closed slightly to reduce the flow, the wear becomes worse. If a magnetic flowmeter and controller are used in an attempt to regulate flow, the valve will cycle constantly, causing even more wear.
Third, there were problems with influent pressure drop when opening the valve and pressure rise/water hammer when closing the valves. Both of these issues were a direct result of the valves not having adjustment capability to limit the speed of operation. Therefore, there was marginal flow control while limit switch and motor contactor failures were too frequent.
âThe bottom line is [that] for this application, motor-operated valves were incorrect, and it was time to look for alternative solutions,â said Steve Fassio, a SCADA technical support staff member for the city of Modesto.
In order to improve the distribution pressure to residential and industrial customers in south Modesto, the city needed replacement fill control valves with the following capabilities:
â¢ Adjustable flow control;
â¢ Adjustable opening and closing speeds (to minimize main line pressure fluctuations);
â¢ Manual override; and
â¢ Remote control interface to the SCADA system.
The city chose to go with Singer Valveâs 2SC-MV valve with MV1-TP electronic flow control and metering. The MV1-TP gave Modesto touchscreen control for local operation by providing intuitive connections and terminals that made it easy for the city to connect and integrate to its own control system. The PLC-based touchscreen controller allows on-screen calibration of process signals, graphic-trending displays for valve PID tuning, and manual or automatic mode selection for testing and maintenance.
âThis flexible package of controls was just what they needed for their system,â said Dave Burrell of Southwest Valve, representative for Singer Valve in California.
Eugene Bahia, instrumentation technologist for Singer Valve, was on site for the startup to ensure that the valves and panels were installed and wired correctly. Bahia also spent time observing valve and panel operations and simulating failures to ensure the system was performing to the cityâs requirements. At this time, Fassio requested modification of additional filtering functionality on the transmitter signals for stability and also modifications to the touchscreen display to make it more intuitive for Modesto operators. These modifications were made on the spot and took an hour, before placing the valve and panel in full automatic control.
â[Bahia] really knew his technology and was instrumental in commissioning our valves and training our maintenance staff,â Fassio said.
âThe three original valves have been in operation for five years now without one mechanical or electrical failure,â Fassio added. âWe have been so pleased with our new system in terms of product quality and service support that we have continued to purchase additional Singer flow control and pressure-reducing valves for many applications.â
The most notable of these occurred in 2010, with the installation of a fill control valve on a new 4-million-gal storage tank. In this application, there was another tank in close proximity on the same distribution pipeline. It was crucial that the mainline distribution pressure not be compromised in the event that both tanks attempted to fill at the same time.
Singer Valveâs solution called for adding an inlet pressure transmitter that was able to support a MCP-TP multi-process control panel. This meant that if inlet pressure dropped below an adjustable set point during filling, the controller would override and modulate (reduce) the flow to that tank until inlet pressure recovers. This upgraded control panel includes all of the functions of the original panels with some important customized
â¢ Inlet pressure monitoring and override. This prevents the inlet pressure from dropping too low, which would be detrimental to users in the system and the fire department. It also ensures that the valve does not erroneously open fully when the inlet pressure supply drops low and the set point is not reached. If it did, it would likely overflow the tank.
â¢ Selectable pressure control, flow control or level control functionality. The valve and panel, though primarily designed for level control, also can be used for pressure control or flow control. This additional functionality does not require further programming and is readily available to the city of Modesto when needed.
â¢ Dual-diaphragm (PGM) control valve with integral backup. The integral backup control valve is fitted with a hydraulic altitude pilot that senses tank overflow. If tank overflow is detected, the valve will close fully regardless of panel command, primary diaphragm failure, primarily pilot system failure, solenoid failure or plugged strainer.
With these solutions in place, the city of Modesto can fill both tanks simultaneously and still maintain a stable mainline pressure during peak summer season demands. The most important feature added, however, came about as a result of a near disaster in 2006 when a city of mechanic happened to be driving by and saw a tank beginning to overflow. The tank sits in a residential area, so with 10,000 gal of water releasing per minute, it would have flooded the neighboring houses if not detected and dealt with immediately.
When the problem was investigated, it was determined that there was a momentary loss (about five minutes) of influent pressure to a valve because of a failure in the booster pump system upstream that occurred on one of the old valves. Without sufficient inlet pressure, the differential pressure transmitter could not calculate the flow feedback correctly, thinking it is lower than it actually is. The PID loop in the PLC began to integrate positive, calling the open pilot solenoid to drain water off the diaphragm. When the influent pressure returned, there was no pressure on top of the diaphragm, so the valve was operating almost wide open with no chance of recovery. The only way to stop flow in this case was to close an isolation gate valve manually in order to close the valve.
The critical feature that Singer Valve put in place was a dual-diaphragm valve chamber with a level-sensing altitude pilot valve. This ensures a positive mechanical override in the event of an electronic failure. If inlet pressure drops, the valve starts closing to sustain inlet pressure.
âBased on our five-year history of outstanding performance from our previous Singer valves and control panels, I believe I may never use this feature,â Fassio said. âBut it still provides our department with additional peace of mind because a storage tank overflow at 5,000 to 10,000 gpm can make quite a messv