Small problems can lead to large impacts on a long-term basis. Heat rate surveys, improved instrumentation, and utilizing real-time monitoring and diagnostics can help recover plant efficiency and save money, with payback periods sometimes as short as weeks.
Muted orange light filtered through slowly drifting clouds of wet steam, illuminating two figures walking through the power plant. Marnie Surfaceblow, vice president and lead field engineer of Surfaceblow & Associates International, held out a protective arm at the edge of a large puddle of water to stop her lead field engineer and assistant Maya Sharma. As Maya noted the leak location on her tablet, Marnie shone her flashlight across the water. “Grandpa Marmaduke always said: ‘Don’t walk through water on the floor, unless you’re certain it’s free of hazards—like an electrical charge, an ankle-spraining hole, or Massasauga rattlesnakes.’ ”
“Ma’am, how many, um, snakes have you encountered at power plants?”
Detouring around the puddle, Marnie mused, “More than I care to mention. Only bitten once, but that was the time I brought home that baby copperhead when I was eight years old. I named her Penny, because, you know, copper. Don’t let anyone tell you that small things can’t cause a lot of trouble—that’s one hospital trip I’ll never forget.”
Maya suppressed a shudder, noting the film noir lighting made every cable and hose look decidedly snake-like. “Interesting coincidence, ma’am, since copper is one of the reasons we are here.”
Marnie continued, “Similarly, small condenser and feedwater heater issues can lead to big heat rate problems—like those condenser air leaks we fixed. That’ll reduce copper loss from the water tubes and deposits on the turbine blades, and improve their heat rate by about 50 Btu/kWh. And thanks to your noting those problems with their feedwater heaters by analyzing their DCS data, we have more work to do.”
“They did not notice it without an online system alerting them, but I noticed the drain cooler approach [DCA] of the #1 feedwater heater increased suddenly after a trip two months ago and has remained high,” Maya recalled. “Of more concern was that the DCA for their highest-pressure feedwater heater has increased steadily for months. However, examining their inspection reports, there was no indication of extraction line, heat transfer tube, or partition plate damage to cause this.”
“Feedwater heaters seem simple,” said Marnie, “but they require detailed design, careful maintenance, and consistent monitoring to maximize their efficiency. So, your investigation created a nice little scope adder. Hey, an adder is a type of snake, interesting coincidence!”
Maya, still imagining snakes in every air hose and flexible wire conduit, said a quick prayer, set her teeth, and followed her boss deeper into the plant.
Unstable Operation Has Consequences
“This plant was designed for 18 cold starts per year. Last year, it did 50, and some of those were after trips from unstable operation at loads much lower than design. That’s a serious problem, because a unit trip risks damaging plant equipment. Case in point, tell me my young apprentice, what happens to feedwater heaters during a trip?”
Maya thought for a moment and replied, “It is a highly unstable situation. Extraction steam and feedwater flows can change suddenly. The water level can rise and the emergency drain valves may open to protect the turbine from backflow damage.”
“Very good. Under those conditions, what happens to the emergency drain valves?” Marnie asked.
“Because they drain to the condenser, there is little backpressure, and the higher-pressure saturated water in the feedwater heater flashes to steam, accelerating past the valve, causing potential erosion and thermal shock. Both can damage the valve stem and seat, and the valve may fail to close and leak directly to the condenser, causing lost heat rate.”
“Continuing to damage the valve, steadily increasing the leakage,” Marnie added, as they arrived at feedwater heater #1.
An Open Door Is an Invitation
“The DCS indicates all emergency drain valves are fully closed, ma’am,” said Maya. Marnie turned and faced Maya, with her flashlight under her chin. “It may be telling the truth, or it may be turning the control room into a den of lies!” adding an evil laugh for effect. Maya stared, nonplussed.
“Moving on!” Marnie continued. “Examine this valve. The position indicator says it’s closed, but see here?” She pointed with her pen. “See that cleaner region a few millimeters wide on the valve stem? It tells me this valve isn’t normally in this position. That could be just due to normal wear, or seat adjustment. What would you say?”
Maya examined the valve, noting the position indicator and seeing there was more valve stem showing than should have been. “I agree. This emergency drain valve is indeed stuck open—maybe 5% or more. And as my auntie Natasha used to say, ‘An open door is an invitation to things both good and bad.’ In this case, an invitation to heat rate loss.”
Marnie nodded agreement. “Now, given this is the lowest-pressure heater, the loss is likely small, maybe 1–3 Btu/kWh? Still, it’s definitely something they should fix during their next scheduled outage.”
A Tale of Two Feedwater Heaters
Even with heavy insulation blankets installed, the heat was oppressive near the highest-pressure feedwater heater. “Feel that heat! I’ll bet snakes love basking here! Maya, examine the emergency drain valve if you please.”
Maya, muttering myriad maledictions toward Marnie regarding snakes, bravely examined the valve. “I am not certain this is the problem, ma’am. All indications show the valve is fully closed.”
“Maybe,” replied Marnie, “but is there a way we can quickly check for leakage into the emergency drain line?”
Maya thought, then opened her tablet and scrolled through a spreadsheet of DCS data. Meanwhile, Marnie reached into her battered Prada power plant purse, retrieved an ornate fan, and waved it to cool herself. “I hope you find out before I melt, Maya,” she said.
“Perhaps you could find some snakes to play with to distract yourself, ma’am,” Maya quipped.
“No, thanks. I gave that up after my arm swelled to the size of 6-inch schedule 80 pipe.” Marnie shuddered at the memory.
“And what ultimately happened to Penny?”
“Grandpa Marmaduke set her free by the creek that ran through his property. He said it wasn’t the snake’s fault that I was—and I quote—a darned foolish granddaughter.”
“That explains much,” replied Maya. As Marnie opened her mouth to protest, Maya continued, “I can find no temperature sensors on this drain line. Shall we retrieve our thermal imager to scan the piping?”
“That’s a good way to tell, since these lines normally aren’t insulated, but there’s another way to detect leaks that grandpa showed me,” Marnie said, removing her largest screwdriver from her belt (Figure 1). She placed the tip against the emergency drain line and placed her ear against the end of the screwdriver grip. She nodded and handed the screwdriver to Maya. “Try it yourself and tell me what you hear.”
1. Maya can find no data in her tablet to confirm a problem. Marnie will try listening for leakby using a screwdriver. Source: POWER
Hesitantly, Maya copied her boss’s actions and was rewarded with the sound of a loud “HISSSSSSS” conducted through the screwdriver. “It sounds like an enormous snake!”
Marnie smiled. “Maybe the valve seat eroded from too many trips. With the high oxygen levels we found in the feedwater, maybe a bit of scale or corroded metal stuck on the valve seat, keeping it slightly open, allowing for long-term erosion. Given the long, steady climb of DCA you noticed, that’s the most likely scenario.”
Maya tapped calculations into her tablet. “If this leak caused the entirety of the DCA rise, then fixing this problem will yield us 58 Btu/kWh. That alone allows us to achieve our goal for the extra work!”
“A good day’s effort,” Marnie said, stowing her screwdriver safely on her belt. “The heat rate savings we’ve found in less than eight hours will pay the plant owner back for the repairs, plus our always reasonable consulting fees, in less than three months. If this plant is going to keep cycling this much, they’d clearly benefit from additional sensors and anomaly detection logic here. But let’s depart this steamy swamp of hissing feedwater heaters, head back to the hotel, and type our report.”
As they walked through the plant together toward the engineering offices, Maya, uncertain at first, spoke up. “I must say, ma’am, that dangerous wildlife is a fact of life at many power stations, and one should not joke about the presence of it. If nothing else, it makes the workers feel uneasy, and this could lead to lapses in concentration, mistakes, and injuries.”
Stopping suddenly, Marnie turned and gave Maya a look filled with contrition. “You’re absolutely right. In the plant environment, any hazard, even an unlikely one, should not be joked about. Once again, the student teaches the teacher. Thank you for the correction, and I apologize.” Maya smiled and nodded gratefully.
Speaking seriously, Maya added, “However, you must understand, ma’am, that as soon as we arrive at the hotel, I am going to find some truly awful method to tease you back.” Laughing, Marnie replied, “I wouldn’t have it any other way.”
—Una Nowling, PE is an adjunct professor of mechanical engineering at the University of Missouri-Kansas City.