The City of Camas, Wash., couldn’t afford the environmental risk of sewage backing up during an electric power outage.
“We had nine older lift stations, all designed differently, making them time-consuming to maintain and repair,” says Jim Dickinson, wastewater operations supervisor for the city. Camas is located on the north bank of the Columbia River next to Portland, Ore., and Vancouver, Wash. “Since 2003, we’ve been retrofitting those stations, while installing pre-engineered lift stations in new subdivisions and developments.”
Looking for consistency of design and equipment, Camas engineers selected Romtec Utilities in Roseburg, Ore., to fabricate the lift stations. Romtec, in turn, chose Cummins Power Generation in Minneapolis, Minn., to custom-build standby electric power systems. The combination enables Camas to keep pumping, even when utility power is unavailable.
All-in-one reliability
Pre-engineered lift stations include a wet well, two or three submersible pumps, piping, liquid-level sensors, underground valve vault, electric pump controls, standby power generator, automatic transfer switch, and communication equipment. Installation takes one week.
“Our systems pump lots of water against friction losses, dynamic head and other factors, requiring a sharp analysis to specify the proper size generator,” says Mark Sheldon, Romtec vice president for marketing and sales. “For example, the Hunter Ridge lift station has twin pump motors requiring 39.6 kW for starting and 22.5 kW for running. If necessary, the pumps must run simultaneously.”
Besides pump horsepower and voltage, other critical performance parameters include starting current, motor efficiency rating and required auxiliary loads, such as generator set controls, lights, heaters and odor control. Ambient temperature range and elevation also enter into the equation, as do anticipated population growth and pollution-control requirements.
Sizing the generators
Jim Stalnaker, sales manager with Cummins Northwest LLC, Portland, Ore., uses those parameters to determine the engine, alternator and excitation system for each lift station. “Some generator sets must be oversized to handle a motor’s higher starting current,” he says. “However, we can sometimes avoid oversizing by specifying variable-frequency drives or solid-state starters to reduce the inrush of current during starting.”
The 20- to 200-kW diesel-powered standby generator sets are permanently installed or on portable trailer-mounted units. Pump size, 11 to 35 hp, determines the power output specification.
The lift stations include Cummins OTEC open-transition transfer switches, which provide safe “break-before-make” power transfer from the grid to the generator and back again for testing and power outages. A programmable gap of several seconds between power sources allows the back-electromotive force (EMF) generated by the pump motors to dissipate, protecting the alternator. The time gap also enables the generator set to attain operating speed and stabilize its output voltage.
“Besides arriving in sound-attenuated enclosures, the generator sets have to fit in small spaces, sometimes just 1,000 square feet,” says Dickinson. “We installed diesel units where possible, as their efficiency reduces footprint and fuel storage requirements.”
Cummins trained the Camas wastewater staff to maintain the lift stations. “We found them reliable and easy to service, which helped reduce the time spent in the field,” says Dickinson. Camas has 20 lift stations. Seven are pre-engineered units.
