By Brian K. Schimmoller, Managing Editor
For years, the city of Mount Pleasant, in southeast Iowa, has had an interruptible power purchase agreement with its wholesale power supplier, buying power at wholesale rates and re-selling it at retail rates to its customers. In the summer, as temperatures rose, the city’s electric supply would get interrupted by the wholesale power supplier and Mount Pleasant Utilities would crank up an aging 7.5 MW steam turbine and 1 MW diesel gen-set to satisfy a portion of the demand.
As local power demand grew throughout the 1990s, reaching nearly 20 MW during peak periods, the city’s generation assets could not keep pace. In fact, in prior years, Mount Pleasant had to pay almost $1 million in capacity charges to cover the city’s generation shortfall. The city decided to retire the steam turbine and install twelve new 2 MW diesel generator sets, using the savings in capacity charges to service the project debt. Altorfer Power Systems, the local Caterpillar Dealer, served the role of general contractor on this turnkey project and assembled the team that constructed this power plant in less than nine months. “We are very happy with the plant Altorfer provided us and we found them very easy to work with,” said Lori Glanzman, General Manager of Mount Pleasant Utilities.
“We looked at natural gas engines and at combined diesel/natural gas engines, but ultimately settled on diesel generator sets for economic reasons,” said Glanzman. “After comparing capital costs, we decided diesel was the best option for us.” The Caterpillar 3516B generator sets are used for peaking duty and are available for emergency backup as well. Excess power is put back on the grid, with Mount Pleasant Utilities receiving a credit from their wholesale power supplier. The operating permit allows 472 hours of run-time per engine, although the interruptible power contract is only for 200 hours. In 2002, the engines each accumulated 103 hours of operation, and cumulative run-time for 2003, through June, is about 60 hours.
Flexibility and responsiveness have greatly improved with the new style generator sets. Rather than waiting hours to build steam pressure for use in the steam turbine, the diesel gen-sets require less than one-half hour to get on-line, and the bulk of that time is reserved for notification of proper authorities. In fact, for a blackstart exercise last January, the engines were providing full power to the city in under 90 seconds, according to Glanzman. Unless a unit is down for inspection and maintenance, all 12 gen-sets are operated concurrently, although Mount Pleasant strives to keep run-time hours about the same for each engine.
The separate-circuit aftercooled Cat 3516B engines are set for optimum emissions levels with unit-mounted radiator cooling systems. The 3516B engine is offered at three different emission ratings, based on the temperature of cooling water delivered to the aftercooler circuit. The installation at Mount Pleasant features the third level of emission reduced ratings and, although not the lowest rating, it is easily able to comply with local and state regulation requirements. “The allowable NOx emissions limit is 36 lb/hr, but we’re far below that,” said Glanzman. Mount Pleasant relies on a local diesel fuel supplier, who can deliver fuel within two hours at any time of day. Fuel storage consists of two double-walled 20,000-gallon above ground bulk tanks manifolded to twelve individual daytanks with rupture basins.
Six hourly operators plus the electric supervisor manage operations at the power plant. Each employee can run the plant, although two of the hourly operators focus more exclusively on daily maintenance activities. The twelve engines can be brought on-line in one of three ways: automatically from the main computer in the generator building, automatically from the remote computer in the operator’s office, or manually via the control panel in front of each generator set. There is currently no outside remote control of the generator sets, although this feature is available through a dial-up connection.
Engine maintenance is provided through a long-term maintenance agreement with Altorfer. Each month, Altorfer performs a detailed inspection and maintenance check-up on three of the engines, and a general check on the other nine. “This scheduling provides many benefits,” said Ron Henle, Electric Supervisor at Mount Pleasant Utilities. “Because the maintenance plan can be plotted far into the future, and because Altorfer is in here every month, they’re able to perform multiple service functions per site visit and anticipate future maintenance needs. For example, they can coordinate technicians and work load so that unscheduled service calls are not needed for items that are not critical.” Factory updates to the engines can also be seamlessly incorporated. To date, all required repairs have been implemented prior to failure.
To consolidate maintenance activities further, Mount Pleasant recently contracted with Altorfer to provide switchgear electrical testing and maintenance services. These comprehensive services include infra-red photography inspections, relay testing, circuit breaker testing, metering calibration, and other services necessary to ensure the most reliable operation possible. “We’ve truly got one-stop shopping now, for both mechanical and electrical maintenance,” said Henle. “We’ve eliminated the duplication of activities that occasionally came up when Altorfer and an electrical contractor were on-site together.”
Community acceptance and aesthetics played a big role in designing the new power plant. The plant is right in the middle of town — bordering residential areas and within a block of a middle school and a nursing home. To address noise concerns, Mount Pleasant Utilities installed a ten-foot wooden fence and sound packs on the east side of the property, which borders a residential area. “The features that I’m most proud of are the exhaust stacks and the building design,” said Glanzman. “The facility was designed with a higher pitched roof, so that the roofline covers all the bracing of the 55-foot stacks (see photo). From outside the plant, it just looks like an attractive metal building with only a small portion of the stacks showing above the roofline.”
Mount Pleasant Utilities also turned to neighboring utility installations for input during the design phase. Visits to two Iowa municipal utilities with Cat engines convinced Glanzman that the building size should be increased to provide easier access to the engines. Mount Pleasant decided to increase the width of the building another 15 feet to allow for more service area around each of the generator sets.
This site is a testimonial to a cooperative development effort between Altorfer Power Systems and Mount Pleasant Utilities to provide the City of Mount Pleasant a long-term economical source of electricity and emergency protection for its citizens.