Cold temperatures can be dangerous for personnel and often cause havoc for power plant equipment. Suitable preparation can mean the difference between a quiet shift and a nail-biting struggle.
Cold temperatures are nothing new, but extreme winter weather has been in the headlines more and more in recent years. Many of us had never heard the term “polar vortex” before last winter, but earlier this year it seemed as if the Arctic polar vortex was dipping down into the U.S. on a regular basis.
For coal-fired power plants, changing seasons bring a variety of challenges. Cold weather, snow, and ice each cause different problems. Coal deliveries via rail can be delayed due to train tracks being impassable as a result of winter storms. Once a train does arrive, frozen fuel can result in a large portion of the delivery being stuck inside the cars. When temperatures remain below zero for weeks on end, the problem can be nearly impossible to prevent.
Design Sets the Stage
Basin Electric Power Cooperative is one of the largest electric cooperatives in the U.S. The company’s core business is generating and transmitting wholesale bulk electric power primarily to its 137 member rural electric systems, which are located in nine states: Colorado, Iowa, Minnesota, Montana, Nebraska, New Mexico, North Dakota, South Dakota, and Wyoming.
Basin Electric gets about 60% of its nearly 4 GW of owned generation capacity from coal-fired power plants located mainly in North Dakota and Wyoming. Its first power plant—the 222-MW Leland Olds Unit 1 located near Stanton, N.D.—went online in January 1966 and was the largest lignite-based power plant in the Western Hemisphere at the time.
“We learned a lot from the facility in the early years,” John Jacobs, vice president of operations for Basin Electric told POWER. “Most of our design specifications were developed as a result of lessons learned at the Leland Olds Station. Contracts now call for control between –40F and 120F. We are very diligent in ensuring that those climate extremes are met.”
Heavier insulation, larger heating systems, and heat trace—use of an electrical heating element in direct contact with a component—can be added to protect against freezing temperatures in cold weather climates. But even when the proper design parameters are met, trouble can still arise. Basin Electric has learned where weak points exist through experience. The company uses its computer-driven preventive maintenance system to queue up projects and manage preparations.
“Once one winter is complete, we begin working on the next winter’s projects during the spring and summer,” said Jacobs. “Heat trace, insulation, siding, it runs the gamut of anything that might affect you that you’ve found out from the previous year. You put in work requests for those that come up year after year. Of course, you want to get those done when the weather is warm, so people don’t have to fight the extreme cold during the winter months.”
Xcel Energy performs a similar procedure. Each year its power plants review, update, and perform winterizing preparation tasks in accordance with a procedure and checklist. The checklist follows the North American Electric Reliability Corp. winterization guidelines (developed in response to rolling blackouts and natural gas delivery interruptions experienced in the Southwest in February 2011) and also has a section for plant-specific tasks. The company’s preventive maintenance crews regularly check all winter protection items, such as heat trace, heaters, and insulation, to ensure that they are in place and working as required.
Southern Regions Are Not Immune
A person might think that cold weather concerns are isolated to northern climates, but that is not the case. Plants in the southern U.S. also face challenges in the winter, as “THE BIG PICTURE: A Generation Freeze” in this issue shows. The Springerville Generating Station is a prime example. The plant—operated by Tucson Electric Power (TEP)—is a 1,600-MW four-unit coal-fired facility located in eastern Arizona. (For more on the plant, see “Springerville Generating Station Earns PRBCUG 2014 Honors” in the July issue of POWER, online at powermag.com.)
Sited at an elevation 7,000 feet above sea level, winter temperatures are regularly below freezing. Tracy Ortiz, fuel superintendent for Springerville Generating Station, gave a presentation during the 2014 Powder River Basin Coal Users’ Group Annual Conference, which was held in conjunction with the ELECTRIC POWER Conference & Exhibition. She noted several improvements that the plant had made to deal with cold winter weather.
Ortiz explained that keeping the heat inside of plant buildings had been troublesome. Keeping doors closed was half the battle. The plant was built with a lot of swinging doors that were easily caught by the wind. One big improvement was changing all exterior doors to sliding doors (Figure 1).
|1. Heavy duty slider. Sliding doors hold up better in windy conditions. Courtesy: Tucson Electric Power (TEP)|
The plant chose to install a model made by PS Doors. The company’s industrial sliding door is designed specifically to withstand the abuses of industrial applications—especially the effects of positive and negative draft or wind loads—making it a good choice for use in power generation facilities, which require sturdy, maintenance-free doors.
“The little things that we have done have really made a difference,” said Ortiz. “The doors have helped keep the heat in the buildings while at the same time keeping dust out.”
According to Brad Watson, senior director for corporate communications with Luminant, the Dallas-based utility has literally thousands of items on its winter weather preparedness checklist too. Luminant increases management and craft staffing on all shifts for the duration of cold weather events, and it coordinates closely with the Electric Reliability Council of Texas, the independent system operator for the region that manages the flow of electric power to around 24 million Texas customers.
“In anticipation of extreme temperatures, we stockpile fuels, erect windbreaks, and utilize large radiant heaters to supplement existing freeze protection and insulation,” said Watson.
Use What You’ve Got
In some cases, the people most familiar with a problem can help develop a unique solution. At Basin Electric’s Antelope Valley Station (AVS), the cooling towers presented a source of recurring trouble during the winter. The original design included a heat trace system around the periphery of the cooling towers to prevent ice from forming and interfering with the blades of the cooling tower fans (Figure 2). Although the load on the fans is minimal during winter months, they must continue to rotate to keep air flowing nonetheless.
|2. Ice is the enemy. Employees at the Antelope Valley Station in North Dakota developed a solution to keep ice from building up around the plant’s cooling tower fans. Courtesy: Basin Electric|
The original heat trace was difficult to maintain and stopped working after only a few years. Replacement was problematic due to the location and time constraints on the work. Several employees who had worked on the system got together and collaborated on a solution.
The purpose of the cooling tower is to remove heat from the cooling water used to condense steam exhausted from the plant’s turbine generator. The water is typically returned to the cooling tower at a temperature of about 80F.
The answer was to pipe some of that relatively warm water into a ring surrounding the fan blades. The warm water keeps ice from building up and eliminates the need for heat trace. It’s more economical too. The new design utilizes waste heat that needs to be removed anyway and saves energy that had been consumed by the electrical heat trace.
At the Springerville plant, conveyors offered an opportunity for improvement. Because some were completely exposed to weather, ice frequently developed on walkways in the winter, creating the potential for injuries from slips and falls. The solution was to install elevated grating (Figure 3) to provide traction for workers, thus reducing the hazard.
|3. Traction makes a difference. Walking surfaces exposed to weather were frequently icy. Elevated grating increased safety. Courtesy: TEP|
Additionally, some of the Springerville buildings were constructed with no heating system at all. Ortiz explained that TEP’s maintenance crew installed diesel heaters in these buildings (Figure 4) to keep areas above freezing. From December through the end of March, the heaters are used regularly.
|4. Adding heat. Unheated buildings are bound to be cold. Using portable heaters at least keeps them above freezing. Courtesy: TEP|
While heat trace offers vital freeze protection in cold weather climates, at one of Xcel’s facilities, workers found that some installation techniques were resulting in electrical shorts. Originally, the heat tape was doubled back on itself any time the tube trace ended at an instrument enclosure. In time, continuous heating and cooling of the heat trace caused the insulation to break down and short out.
Similar to Basin’s experience, the people performing the repairs came up with a solution to the problem. The electricians drilled a hole through the wall of the enclosure and mounted the power connection box directly to the enclosure instead of to the tube, which eliminated the short radius turn in the cable. The jacketed cabling was taped and sealed inside the enclosure to help prevent escaping moisture from potentially contaminating and freezing the insulation. Problem solved!
Every Fuel Supply Situation Is Unique
Although Basin Electric’s four largest plants are all coal-fired, each is unique in its layout. Two are lignite-based (AVS and Leland Olds) while the other two (Dry Fork Station and Laramie River Station) use Wyoming subbituminous coal. Dry Fork and AVS are mine mouth plants, but Leland Olds and Laramie River rely on rail delivery for their fuel supply, so none are alike.
Lignite coal is roughly 35% water, so during long periods of subzero temperatures, coal in the railcars sent to the Leland Olds facility will freeze along the sides and start to build up in the cars. In some cases, the train will end up back hauling 10% to 15% of the coal delivered, because it won’t dump out of the car. Using a release agent that can be sprayed into the cars prior to loading is an option, but even that isn’t foolproof.
“The Wyoming plants are far enough south where you don’t typically have long, extreme cold snaps. They also have less moisture in the coal, so it’s not as big of an issue for them,” said Jacobs.
Fuel stockpiles are another item that must be considered going into winter. Mine mouth plants have less to worry about, but for plants that rely on rail transport, beginning the winter with an adequate stockpile of coal is very important.
“You make hay when you can,” Jacobs said. “You’re less loaded in the spring and fall of the year, so there is some surge to the stockpile during that timeframe. In the summer and winter months, your stockpile will drop to less than optimum because you’re running hard.”
Basin Electric targets around 500,000 tons of coal in ready stockpile at Leland Olds. Laramie River is a larger facility—three 560-MW units—so the target for that plant is typically about 1 million tons. Although AVS is a mine mouth plant, it still keeps 100,000 tons in its stockpile, but Dry Fork doesn’t maintain a stockpile at the facility because there is adequate reserve storage between the plant and the mine.
According to Xcel Energy’s Energy Supply business group, it also sets inventory goals as a precautionary measure because they anticipate weather-related disruptions to fuel delivery. During the coldest time of the year, an anti-freeze treatment is used on both the coal cars and on the coal to prevent buildup. By using coal car shakers—devices that clamp onto coal cars to help get the coal out—Xcel says that most of the coal is successfully removed from the cars.
People Make the Difference
Plants don’t operate without people, and people can be affected by winter storms even more than equipment is in some cases. Xcel noted that it has a robust program to reduce risk to personnel. The company typically starts each cold weather season with crew briefings on cold weather and the hazards involved so that the issue is top of mind for all employees.
Xcel’s program includes such measures as overhangs and metal roof “ice busters” to prevent falling ice near doorways from striking workers. Signage is also important in falling ice areas, and barricades are installed where necessary. Salt and sand resources are staged near plant doorways and walkways to address icing issues, but boot cleats are also important gear provided to employees, because ice is inevitable.
At Basin Electric, when the weather forecast indicates that a significant storm is approaching that is expected to be a major event, management will either call the on-coming operations shift in early or hold the off-going shift late so the crew doesn’t find themselves shorthanded.
“A 12-hour shift can be very long, especially if it’s a really busy time when the weather’s not cooperating and the equipment’s not behaving,” said Jacobs. “We’ll pay the overtime to keep people there and accrue that cost because safety is a priority. We want the employees’ families to know that they’re being fed, they’re being kept warm, and being taken care of.”
Basin Electric stocks food items at each power plant, specifically for extreme weather events. The company also has cots on site to make sure stranded employees have a place to rest.
“The most important aspect of a utility is its people to run these multi-million dollar machines, so we take care of our people. It’s not just that we don’t want people driving out in the winter weather—that’s very important—but if they are scheduled to work over a weekend because we expect a severe weather incident to come, we take care of them in that case too,” said Curt Pearson, media and community relations manager for Basin Electric.
Planning Ahead Saves Pain
In many parts of the country—and world—winter is bound to bring challenges. The trick is to prepare as much as possible to reduce the likelihood of problems that could take a unit offline or expose personnel to unnecessary risks. A little planning goes a long way toward making for an uneventful season. ■
— Aaron Larson is a POWER associate editor (@AaronL_Power, @POWERmagazine).