October 15, 2006

Focus on O&M (October 2006)

PLANT MAINTENANCE

Nuts about Superbolt

Located between Lake Ontario and Lake Erie on the U.S./Canadian border, Niagara Falls is the site of the world's first hydroelectric plant. It went live on November 16, 1896, and was built by Nikola Tesla and Westinghouse Electric Co. Since then, several other power plants have been built on both sides of the river.

One of those plants is the 2,400-MW Niagara Falls Power Project, which went on-line in 1961 as the world's largest hydroelectric plant at the time. It diverts 375,000 gallons of water per second from the Niagara River through a tunnel underneath the City of Niagara Falls and into a 164-billion gallon forebay. When demand is low, 12 pump-generators move the water into a 1,900-acre holding pond. During the day, when demand is high, the turbines switch from pumping to generating, adding to the available output. Each unit has six 2-inch bolts in its large shaft couplings that are difficult to remove and replace using traditional methods.

"We used to use a big, heavy hydraulic wrench to stretch the bolts, but that was cumbersome," explained Richard Halas, senior engineer at the Niagara Falls Power Project. "Even worse, the wrench would often score the surface underneath the nuts and sometimes damage the threads of the bolts themselves. As strike three, we had no way of knowing how much torque to apply."

To address these problems, about four years ago Halas switched from standard nuts to multijackbolt tensioners (MJTs). An MJT (Figure 10), which can be used to retrofit existing nuts or bolts, is installed hand-tight against a hardened washer. Instead of turning the entire nut, though, an MJT uses a series of jackbolts threaded through the body of the tensioner. The jackbolts can be tightened by a garden-variety handheld torque wrench or air gun. For this particular job, Halas ordered the type of jackbolts with lock-wire holes in them (called Superbolts) as an extra precaution against loosening.

10. Tight fit. A nut-style multijackbolt tensioner can be tightened by a handheld torque wrench. Courtesy: Superbolt Inc.
 

"Our mechanics really enjoyed installing the Superbolts since it takes just one person with a small ratchet wrench," said Halas. "They're very easy to install, very easy to disassemble and—because they use pure tension—there's no chance of damaging the bolt threads."

Since Halas began using the MJTs on the 12 pump-generators, he's thought about using them for maintenance of the plant's much larger main generators as well. Current practice is to use a hydraulic wrench that is so large it takes two or more men and chain pawls to lift and operate it, and special tools to remove it. "It's not just that it's heavy and cumbersome," said Halas. "It's also dangerous, because of the very high hydraulic pressures and hoses involved. Although we've have never had any accidents, our mechanics definitely prefer the Superbolt nut."
 

Tension by torque

You may remember from Shop class that the strength of a screw fastener increases with the square of its diameter. A 6-inch bolt can handle 36 times as much load as a 1-in. bolt at the same bolt stress. Unfortunately, however, the torque it takes to tighten such a fastener increases with the cube of its diameter. So a 6-inch bolt requires 216 times as much force to tighten as a 1-incher.

Who but Popeye has the forearms to apply such torque? According to Allan Steinbock, a VP of Superbolt Inc. (an MJT manufacturer based in Carnegie, Pa.), "A number of alternative methods were developed because it's impossible to tighten large bolts by hand."

Those methods include sledgehammers, crane wrenches, hydraulic wrenches, and thermal tightening. However, these "traditional" methods present a host of potential problems, including worker safety, equipment damage, time consumption, inaccuracy, insufficient bolt load, and—in the case of hydraulic methods, as represented by the wrench used on the main generators at Niagara Falls—expensive tooling.

MJTs eliminate these bolting problems by providing a safe, fast, and easy alternative that makes use of simple hand tools (see table). Another plus of MJTs is that they can be used in cramped areas where there may not be enough room to turn a wrench. In many cases, using an MJT may even improve equipment reliability. By adding elasticity to the fastener portion of a bolted joint, they typically help improve fatigue life and reduce leakage problems on gasketed joints. MJTs can be custom-designed to fit any size application.

Torque comparison. Superbolt vs. hydraulic wrench. Source: Superbolt Inc.
 

Deciding whether to upgrade to MJTs requires evaluating several factors in tandem. Obviously, an MJT's capital cost is higher than that of a standard nut-and-bolt combo. But it may be possible to justify that cost premium against the total savings from avoided equipment downtime, the cost of hydraulic wrenches, and the elimination of problems such as stripped bolts or frozen studs. Another advantage of MJTs is improved worker safety, which is literally invaluable.

—Contributed by Superbolt Inc. (www.superbolt.com).