Automation Feeds the Tap

The community of Palmyra, near the Mississippi River in Northeastern Missouri, with a population of about 3,500 people, last underwent a major upgrade to its water treatment and supply facilities in the early 1970s. To address its needs for clean water at the time, the community added an elevated tank and a lime-softening water treatment plant.

03/01/2005


AT A GLANCE
 
  • Elevation differs on 4 tanks

  • Control system upgrade

  • 46% decrease in pumping time

  • 21% increase in pressure


The community of Palmyra, near the Mississippi River in Northeastern Missouri, with a population of about 3,500 people, last underwent a major upgrade to its water treatment and supply facilities in the early 1970s. To address its needs for clean water at the time, the community added an elevated tank and a lime-softening water treatment plant. It already had an existing, elevated tank (on Dickerson Street) and a ground storage tank.
As the century drew to a close, the growth of an industrial park at the edge of town required adding another water tower. In 2003, superintendent of utilities Don Lloyd and plant supervisor Jeff Schneider initiated a major project to take better advantage of the system's available capacity and improve its overall efficiency.
A major issue for Lloyd and Schneider was how the elevation and geographical dispersion of the water tanks complicated the task of filling them. In the current configuration, the water plant (on the northeastern side of the city) has an elevated tank adjacent to it; the Dickerson Street elevated tank resides approximately in the center of the distribution system; and the industrial park tank is on the southeastern edge of the city. The ground storage tank is approximately half-way between the water plant and the Dickerson Street tower. Although the elevated tank at the industrial park added capacity to the system, it created a few problems.

Instrumentation & Sensors, Process Control & Advanced Control
The schematic representation shows the entire Palmyra water-treatment system.


The entire water distribution network, including the water levels in all four tanks, was driven directly from the plant's high-service pumps. To maintain the necessary water level at the industrial park tank, the plant had to pump enough water to fill the plant tank, the ground storage tank, and the Dickerson Street tank, as well. During the day, the water plant pumped water into the distribution system until the water plant's elevated tank was ready to overflow. The water plant pumps cycled until they had pumped one day's demand—usually eight to 10 hours.

The original control installation operated manually. When the plant was pumping, the operator opened a valve to fill the ground storage tank. When the day's water pumping had completed, the electric generator crew (located adjacent to the ground storage tank) manually turned on the pumps to take water from the ground storage tank to fill the elevated tanks during late afternoon or night.

A drop in system water pressure prompted an operator to turn on the high service pump(s). Because of the distance between the industrial park's tank and the rest of the system, completely filling the tank proved impossible, wasting part of the system's total capacity. Although the water plant tank was full, the Dickerson Street tower remained 2-4 ft below maximum, and the industrial park was always 4-6 ft below maximum. In addition, the industrial park's demand for water reduced the pressure all over that part of town. Because the ground storage tank's elevation was lower than that of the other tanks, its water supply did not "turn over"—that is, the operation left old water in the tank while new water passed over it on its way to the industrial park. The challenge was to automate the filling and pumping from the ground storage tank to fill all the water towers.

The city's planners remained unconvinced that updating and upgrading the system could yield enough benefits to justify its cost in money and time. The prevailing opinion was that the distant towers could not be filled completely—that the ground storage tank could not operate to keep them full. In addition, when workers expressed reluctance to give up familiar procedures that had worked well for so many years, planners were unwilling to force the issue. Workers also believed that implementation of greater levels of automation would lead to a loss of jobs, further fueling their resistance to the change.

Creating a solution

To address these challenges, the city of Palmyra engaged the Quincy, IL-based engineering firm of Poepping, Stone, Bach, & Associates (PSBA). Charlie Bach at PSBA devised a plan to automate tank filling, add control valves at each tank, remove existing booster pumps at the ground storage tank, and install a new one in their place. The new system also would control the filling and periodic use of the ground storage tank to keep its water fresh. Hydro-Kinetics of St. Louis would furnish the valves, pumps, and the Healy-Ruff control system.

The existing Healy-Ruff control infrastructure, installed in 1973, still ran the distribution system. The existing water plant had Healy-Ruff controls for the chemical feeds, to operate the well pumps and the system pumps, to operate the treatment process, and to shut off the system pumps when the elevated tower next to the water plant was full. Parts of the existing system could even be re-used in the new configuration.

Bach proposed that the new system provide sufficient flexibility to allow an operator to adjust how, when, and whether to fill the ground storage tank and at what levels the automatic valves would open and close. The pump booster activities needed to be flexible as well, running the ground-storage-tank pump at night to top off the system and allowing the plant to catch up in the morning. He suggested making sure that automatically filling the ground tank would not also allow water pumped out of the tank to return to it immediately. Instead, water in the elevated tanks would wait for one cycle before refilling the ground storage tank.

Implementation

The new control system consists of a Micro versatile process automation controller (V-PAC) remote unit and a level transducer at each tank regulating the valve and controlling water levels. Each Micro V-PAC—built using miniOCS (Operator Control Station) from Horner APG—includes a user interface, control, I/O module, and communication back to the plant. A Micro V-PAC also controls the new booster pump station. At the plant, a Master V-PAC graphical touch-screen interface is used for control of the high service pumps and serves as a central control unit to provide supervisory control and communication with the rest of the system. The Master V-Pac is an OCS with embedded Ethernet, also supplied by Horner APG.

With the new system, when the water plant is operating, a pressure transducer on the adjacent elevated tower measures line pressure to determine water elevation. When the tank is full, it signals the central controller to close an automatic valve at the tank feed line.

A pressure transducer, radio, and motor-operated valve at the Dickerson Street tower monitor the level of that tank. When the tank is full, a signal from the Master V-PAC closes the motorized valve. At the industrial park, a pressure transducer and radio monitor the tank water level. When it is full, a signal notifies the central controller to shut off the system pumps and operate all elevated water-tank valves.

While the elevated tanks are filling, a valve at the ground storage tank opens to fill that, as well. The valve and an attached meter inform the central processor by radio about the tank's level and the volume flowing into it. The valve closes when that tank is full. When the water plant is shut down, the elevated tanks fill in the same way from the ground storage tank pump. The water level is monitored at all times by the central controller.

Convincing the team

On the first day of operation, with the new system in place and the plant process controls communicating with the water tank controls, the elevated towers filled in just four hours. When operators came to came to work the next day, all elevated tanks were full and the ground storage tank needed to receive water. The plant can now achieve a water pressure of 74 psi (previously, 61 psi was maximum). In addition, the pressure is uniform throughout the city and demand cycle. The new equipment also provides 150,000 gallons of additional water capacity to allow for future growth.

Substantial cost savings were also achieved. Previously, keeping the system "fully charged" required that the treatment plant pump water an average of 9.5 hours per day. By individually controlling each tank, the new system accomplishes the same task in 6.5 hours, reducing energy consumption and the need for overtime, while increasing staff time available for other projects. Shorter runtime also reduces equipment maintenance requirements and promises extended equipment life.

As for testimonials, Schneider says he was surprised the system could get water out of the overflow pipe at the industrial park tank. Lloyd added, "The morning after we switched over, several people called from around town, telling me about the improvement in the water pressure for their morning showers. That made me feel good."






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