How New Orleans water pump control systems leapfrogged a generation of technology
This case study explores the challenges and lessons learned of installing automatic controls on a 60-year-old, mechanically operated drinking water distribution system for the City of New Orleans
- Gain awareness on designing systems that suit the plant itself and not just the manufacturer’s ideas or theories.
- Understand how old pump control systems and new technologies can work together for the best solutions.
Pump control insights
- When assessing a major overhaul of water system equipment that doesn’t entail starting completely from scratch, do not build expectations of a totally modern, digital system of controls. There might be some give and take and adapting the old to the new is necessary.
- It is imperative for plant operators to become familiar with the digital interface. Initial and ongoing training for both operations and maintenance staff is essential.
In a perfect world, municipalities and owners of public infrastructure systems would have the funds to continuously update equipment and hard assets to take advantage of improved technology and materials. In the real world, however, operators inherit older systems and do the best job possible.
That was the case with the Sewerage and Water Board of New Orleans (SWBNO) Carrollton Water Treatment Plant. The city, regularly embattled by extreme weather events, is served by a water system that shows its age. The plant had been experiencing damaging water hammer events from abrupt pressure changes due to power failures to its pump system.
Thanks to federal hazard mitigation funding, New Orleans first addressed its boil water advisory problem with two elevated storage tanks, a system designed by Stanley Consultants, that would give the city 40 minutes to address potential water pressure losses during interruptions such as loss of power to the pump system.
The second part of the federal plan was to upgrade huge, older pumps at the SWBNO’s Claiborne Pumping Station. The pumps at the treatment plant distribute treated drinking water out to the system and maintain the water system’s pressure at a level that keeps drinking water safe.
Replacing older pumps and installing modern controls
Even with the newly constructed water towers, water hammer events, which are essentially destructive waves of water inside the pipes, continued. The Claiborne station was 60 years old with pumps that manually started. The station also had an unreliable power source that was causing surge damage to the distribution system.
The Carrollton Water Treatment Plant has three individual pump stations that power the drinking water distribution system. Claiborne houses four 1,800-horsepower pumps, Panola Station has two 2,250-horsepower (hp) pumps and the High Lift Pump Station contains one 2,000-hp motor driven pump and one steam driven pump.
The second major phase of the SWBNO modernization was to update the older pumps at Claiborne Pumping Station as part of a less-expensive alternative to complete replacement. Stanley Consultants analyzed hydraulic models, pumping combinations, and simulations to produce a benefit cost analysis, plans, and specifications. Stanley later performed bidding, construction administration, and inspection.
The final design plan was to rebuild four 44 million gallons-per-day (mgd) pumps at the Claiborne Pumping Station and the two 45-mgd pumps at Panola Pumping Station, for a total of six, essentially new 45-mgd pumps. Four of them were designed with new variable speed drives and 60-cycle motors. In addition, there will be the installation of two rebuilt 45-mgd pumps with 60-cycle motors at the High Lift Pumping Station within the power plant along with the installation of slow opening and closing valves with battery backup for each.
Advancing pump controls
When controls engineers first visited the stations, they were surprised to find no digital controls. Instead, it was a labor intensive, largely mechanical operation to maintain the sensitive checks and balances of the city’s freshwater system. Pumps were started with two operators. If there was a problem when the pumps started, such as a pump cavitating, the operator listened for a particular sound (see Figure 1).
To modernize the pump starting process, controls engineers wrote system descriptions for programmers on how to program the system for controlling the pumps and coordinating that control system with the supervisory control and data acquisition (SCADA) system. With the new machinery and software, technicians would look at a computer screen rather than go down to the plant to observe how things were operating.
The design included the installation of four variable frequency drives (VFDs) to control the main pumps to a specific discharge pressure. An automated vacuum priming system was installed for holding prime on each pump. In addition, programmable logic controllers (PLCs) were installed to control the water pumps and minimize sudden pressure changes. A new frequency changer was designed to convert older-style 25- to 60-cycle power to allow dual source feeds for redundancy. The design also included a seal water system for the Claiborne Pumping Station to boost the water pressure for the main pump seals and the vacuum priming system pumps.
The controls design considered several questions about the overall operation and redundancy for the plant. What if this pump breaks down? What if power is interrupted or a breaker trips? How should the control system automatically react? This wasn’t a greenfield system where everything is new, as the plant renovation had to allow for the pump station to continue operating while being renovated (see Figure 2).
Pump control design challenges
Water hammer mitigation valves and controls: On the old system, each pump had a 30-inch weighted check valve on the discharge. When a pump tripped, the check valve slammed closed, which saved any water hammer wave effect on the pumps, but also sent shock waves outward to the distribution system. The new design added 30-inch automated or actuated ball valves to the pumps in place of the check valves. The valves needed to automatically close on a loss of power under any conditions, including a total loss of station power.
Electric motor actuated valves were considered at first, but to close four of these valves simultaneously would have required a battery backed power source the size of a semi-tractor trailer. This was an expensive and impractical solution. Instead, the solution was a fail-safe design using electric hydraulic actuators, which would work even if power was lost. The valves would open and close on a specific timing curve determined by the water hydraulics study. In the case of a total loss of station power, the hydraulic system contains an accumulator that closes the valve on the same timing curve without electricity.
Too much technology too fast: Controls design must take the usability by an average operator. The client needed the water distribution system to operate around the clock daily and only report faults or shut down upon major failures. In this case, the team designed too many alarms that eventually overwhelmed operators.
For example, variable frequency drives generate a lot of heat and need cooling in the hot climate of New Orleans. Instead of using the SCADA computer, operators felt more comfortable entering the building and manually recording information from the VFDs. If building doors were open too long, heat could build and the drives could overheat. So open doors tripped intrusion switches and set off alarms, up to 30 per day. Stanley engineers asked the integrator to modify programming to not trip alarms unless doors were open more than two minutes — or five minutes on the main garage door.
Another example is the common alarm on the heating, ventilation and air conditioning (HVAC) units. It was originally set to go off if high temperatures were 80°F, it tripped numerous daily alarms. There were other sensitive alerts as well. There needed to be more critical alarms for certain instances, such as when the temperature in the room hit 100°F. Because the HVAC vendor couldn’t modify its programs in that way, independent temperature sensors were installed. If it hit 90°F in the building, operators would go and check it out.
Thirdly, to protect the pumps per manufacturers protocol, the pump must be tripped offline by the control system if temperatures rise above a given limit. This alarm condition triggered an immediate shutdown of that pump, lowering critical water pressure in the system. The client wanted the pumps to stay on even under high temperatures until they could start other pumps. In this situation, maintaining water pressure is more critical than pump failure. It’s necessary to weigh which failure scenario is the least damaging to the client, the client’s system or the client’s users.
Loss of power response: During commissioning, there was a loss of 480-volt power on the VFDs, which shut down all four. Alongside the provider, the drives were reprogrammed and hardware was added to keep the VFD control system on for three minutes without shutting the pumps down while working off battery backups. This change, coupled with a priority alarm to the operators, gives the system a chance to remain running, even in the event of a minor power loss.
Determining how much to automate: The client ultimately decided to refurbish and rebuild its freshwater system instead of building it from scratch. This decision is based on various considerations, including adapting to older equipment, funding and Federal Emergency Management Agency’s recommendations. Controls engineers incorporated automation into the pump priming process design.
However, during the research visit to the pump priming process, there were engineers mechanically starting and stopping pumps, adjusting based on how much water was exiting a seal. This on-the-job training involves instinct and intuitive judgment on an older, mechanical piece of equipment. Because of this, the decision was made to abandon fully automatic priming. It was partially automated with the vacuum priming, but there was no automating the seal water flow to the pump seals.
Pump backup power supply: Operators run one pump in times of low demand, two pumps in high demand and three pumps if demand is high and they need to increase tower water level. This is still an operations decision based on water demand. Starting and stopping pumps automatically was not a viable option to implement with this project because it would require a robust medium-voltage electrical system. The client has begun construction on a new substation, but it has yet to be implemented. In the future, automated starting and stopping of pumps is the goal (see Figure 3).
Pump controls lessons
Controls designers need to pay special attention to how the operations group works and operates. A controls system isn’t something that exists only on a computer screen. The controls engineer must adapt the design to the operation as it operates in reality, not by manufacturer’s specifications or system theory.
Operator interventions were worked into the controls design because even refurbished equipment still has mechanical operational requirements. The computer screen designs don’t always fit the real situation and models might not work to exact physical parameters.
In a design control system retrofit, controls engineers need to attend the factory acceptance test, which the COVID-19 pandemic prevented engineers from doing. As a result, Stanley Consultants missed the opportunity to collect input from operators. They were uncomfortable with the graphics on computers for the pumps. One wrong click could stop the all-essential pumps. Therefore, it is imperative for operators to become familiar with the digital interface. Initial and ongoing training for both operations and maintenance staff is essential.
When assessing a major overhaul of water system equipment that doesn’t entail starting completely from scratch, do not build expectations of a totally modern, digital system of controls. There might be some give and take and some adapting the old to the new. The system’s equipment and its human operators both must be adapted and trained to leapfrogging a generation of technology.