Best Practice: Boiler-feed pump variable-frequency-drive project
Klamath Cogeneration Plant
Challenge. In today’s energy market our plant is routinely required to cycle load and, often times, is completely off. Most plant equipment has been designed to run at 100% capacity which is not always required and thus wastes energy. The plant staff was challenged to assess all plant equipment and functions to find energy savings sustainable throughout the life of the plant.
Solution. A search of energy conservation projects was conducted by site personnel and aided by an independent engineering group that facilitated the qualifying of energy saving incentives from the state. Numerous items were identified and put through a cost-benefit analysis and parasitic-load studies. The BFPs were designed such that they operate only at full output, recirculating feedwater when the unit is less than full load. Consuming 17,547 MWh/yr, the feed pumps were identified as the lead consumer in the parasitic-load study. Additionally, the project was eligible for state financial incentives which brought the payback time to an attractive duration.
Two state agencies were involved with this energy-saving project. The first agency reviews projects statewide and selects those that provide the greatest rewards. In this case, we qualified for a cash incentive from the agency to allow a reduction in the payback period from 2.82 to 1.5 years. The second agency uses the assessment from the first agency and provides a 35% tax credit that is distributed over a five-year period.
The facility is configured with two HRSGs, each having two 100%-capacity boiler feedwater pumps which were designed to operate continuously at full load. Each BFP is driven by a 2300-hp, 4160-V electric motor and the flow of feedwater to the high- and intermediate-pressure steam drums is regulated by independent feedwater regulating valves. The BFP also supplies water to the fuel-gas heater and numerous desuperheaters throughout the plant.
One pump from each HRSG was selected to receive a VFW with the redundant pump allowed to operate at its 100% operating point when called upon. Normal operation is with the VFD controlled BFP as the primary pump.
A tremendous amount of research was required to find a manufacturer that could provide the necessary equipment as well as the customer service and assistance we desired. Because of the complexity of the plant systems, a great deal of control logic and testing was also necessary. The plant personnel developed a detailed specification of what was required of the VFD manufacturer. Toshiba was selected as the VFD of choice and work began on the electrical, instrumentation, and installation of the drives.
Results. The drives were installed in spring 2006 and have been integrated into the plant’s DCS system. With the help of our independent engineering group, we estimated an annual saving of 7870 MWh and we are well on our way to meeting that target. At the plant’s minimum load (60%) the hourly saving per pump is 1 MWh and at base load (100%) the saving is 0.20 MWh. As noted above, the payback prior to any energy-saving incentive was 2.82 years. With the combination of two incentives from state agencies the current payback in years is 0.78. Work is in progress to evaluate VFD retrofits on other large motor-driven pumps and their possible qualification for state incentives.
