Protective bellows installed on the damper shafts of the bag house at Xcel Energy’s Sherburne County (Sherco) Generating Plant are extending the life of the shafts by minimizing corrosion from exposure to condensing flue gas and other contaminants. To fit the plant’s four different shaft lengths, the bellows are made in stock length modules and joined with backing plates for specific applications to reduce spare parts stocks and expedite repairs.
The Sherco plant is a coal-fired facility with two 750 MW units and one 940 MW unit. The 940 MW Babcock & Wilcox (B&W) unit, built in 1985, includes a reverse air-type baghouse. In the reverse air design, flue gas from the boiler is filtered as it passes through the middle of the bags. While the cleaned gas passes through an outlet duct, the ash is trapped by the bags. Lawrence Glass, an Xcel senior production engineer said the compartments have two sets of dampers, which are basically large steel discs about six feet in diameter. One is an outlet damper, and the other is a reverse air damper. These are positioned in ducts that make it possible to isolate the compartment and control the air flow. When the outlet dampers are closed and the reverse air dampers are opened, the cleaned flue gas flows backward and cleans the ash out of the bags.
The dampers are activated on a rotating cycle by pneumatic cylinders controlled by a programmable computer. Although the shafts only move approximately three or four times an hour, the repetitive motion along with insufficient clearance between the bellows and the shaft caused the previous bellows to wear. “These are dry scrubbers,” Glass says. “We spray lime slurry into the flue gas and keep the temperature as close as possible to the dew point of 125 F so it doesn’t condense. If one of the bellows develops a hole and the air leaks in, it cools the gas below the dew point, and it condenses. Then it can cause the shaft to corrode so much that it breaks off.” When this happens, it is necessary to retrieve the damper from the bottom of the flue gas duct about 30 feet below and re-install it with a new shaft.
Shafts and Bellows Upgraded
To reduce repairs and minimize future problems, Xcel began replacing the carbon steel shafts in 2005 with stainless steel and are now using a modular approach to stocking and installing the bellows. He explains that the easiest way to replace the corroded 2-inch diameter shafts is to cut them off at the poppet, slide a hollow shaft over the stub and weld it in place. The new stainless steel shafts have an outside diameter of 2.5 inches, so it was necessary to redesign the bellows to accommodate the larger size.
“It takes about 45 minutes to replace a bellows,” Glass says. But replacement of the actual bellows is only part of the job since the damper shaft/air cylinder “knuckle” has to be broken loose, which can take more time than replacement of the bellows itself.
 This view inside the baghouse shows some of the 200 cylinders that need bellows protection. |
In the process of redesigning the bellows, Glass worked with A&A Manufacturing Co. Inc., New Berlin, Wis., a specialist in the design and manufacture of bellows, boots, way covers and many other protective components for machinery. He uses the company’s Gortiflex Molded Bellows, which are manufactured from a continuous sheet of elastomer-coated fabric formed into a cover with only one diagonal seam. This provides a completely sealed design similar to a molded bellows but without tooling or die charges.
According to Glass, the new bellows’ inside diameter is 4.5 inches to allow more clearance over the shaft and prevent abrasion. In addition, he orders them in two different lengths that can be joined as needed to accommodate various shaft lengths, which eliminates the need to stock quantities of many different sizes. They are made with flanged ends that allow the sections to be bolted together with backing plates or mounted to the dampers.
Glass notes that the new bellows design helps eliminate the wear that can cause holes and lead to corrosion, but says they also are easier to replace if a hole were to develop. He explains, “The holes always occurred at the bottom end, and we had to throw the whole bellows away. Now, we can save money by just taking off the bottom piece and replacing it, which is another reason for buying them in sections.”
Pressure differentials between the environment and the bellows are relatively low, possibly two or three inches of water, according to Glass. However, the pressure is positive at startup and then becomes negative during operation. To maintain the shape of the bellows under these changes, wire rings are inserted inside each convolution.
Although the reverse air bag house design is not as widely used in power plants as the pulse-jet type, primarily because of its higher capital cost, Glass concludes, “It is a good design with very low pressure drop, and the bag life is twice as long as on a pulse jet.”
Xcel doesn’t yet know exactly how much the plant will save in maintenance costs, which Glass expects will occur due to the new bellows’ longer service life. “The jury is still out since the new bellows have not yet exceeded the expected life of the old style. If we can get six years out of the new bellows-and I expect we can-I’d say we’ve made an improvement.” He feels the new bellows design will minimize the previous shaft corrosion problem and help maintain the value of the system.