Selco seal gaskets replace traditional spiral wound gaskets

By Joe Zwers, Freelance Writer

Think thin for better gaskets. Doctors are continually telling us that we are too fat; we need to slim down to maximize our health, longevity and energy levels. That advice applies to power plants as well where retrofitting turbines with lighter, thinner blades can boost power output. But, while many believe in the slogan “you can never be too thin or too rich,” some engineers have been reluctant to apply that same adage to their pressure seals. They opt to go with older, thicker spiral wound or camprofile gaskets, rather than switching to slimmer Selco gasket technology from Sealing Corp. which are more pliable, longer lasting, stable under temperature and pressure shifts, less prone to leakage and require a much lower bolt load.

“The power industry is reluctant to move away from spiral wound gaskets,” says Mike Perkins, sealing device specialist for A.W. Chesterton Co., an industrial fluid sealing firm headquartered in Woburn, Mass. “Even though it is an older technology and not as good as others, it is the what they are familiar with.”

If the switch is made, it can make a major difference. TransAlta Corp.’s 1,376 MW coal-fired plant in Centralia, Wash., for instance, had been using spiral wound gaskets on its boiler manways, but seal failure was causing unplanned shutdowns. Switching to more streamlined gaskets solved the problem.

“The Selco Seal gaskets have been working very well,” said Stuart Bussman, who retired after three decades at the plant and now works as a project manager for TransAlta. “Ever since we put them in, we have had a lot fewer problems than anything else we used before.”

The Old Standard

When looking at investment it takes to build and operate a power plant, the cost of buying gaskets is pocket change. So, even though more modern designs are often more expensive than spiral wound, the price difference can be justified by lower maintenance costs or a decrease in downtime.

Selecting the right gasket, then, requires analysis of a combination of factors including the material making up the joint to be sealed, the operating temperature range, the pressure class required and the characteristics such as pH of the material the seal is designed to keep in or out. One other factor to consider when looking at the temperature is whether it is constant or whether the equipment is frequently cycled causing flange bolt loosening over time.

For high-pressure applications in the power generation industry, maintenance crews have three basic options: spiral wound, camprofile or Selco Steeltrap gasket.

The earliest types were commonly made from either a fibrous material such as asbestos or punched from a sheet of metal. These sheet gaskets, even if made from a softer material than the flange, still required a high load on the bolts to compress the gasket material and provide a seal. This high stress leads to flange warping and loss of seal.

Spiral wound gaskets have been around for nearly a century and are a definite improvement on sheet gaskets. They have justifiably earned a stable position in the maintenance marketplace. Consisting of alternate layers of a filler (typically graphite) and a metal (generally high carbon or stainless steel) wound in a spiral. They are more expensive than sheet gaskets, but do not need as high of a bolt load since some of the flange surface is in contact with the more compressible filler, rather than being fully in contact with the metal that provides support for the filler. While lower, the bolt load is still significant and can lead to warping and other problems associated with high bolt loads.

David Pierce, manager of Design Engineering at Vogt Power International Inc., a Babcock Power Inc. company that builds heat recovery steam generators (HRSGs), said his firm primarily uses spiral wound gaskets for manways and some flanges.

Vogt Power uses the gaskets on joints from as low as 50 lbs. all the way up to the 2,000 to 3,000 lb. range. They try to limit the use of gaskets on high-pressure systems, mostly opting for welded joints. Since the pipes carry only water and steam, they don’t have to look for specialized gaskets for dealing with exotic materials. The only gaskets the company supplies for the aftermarket are for the manways, so Pierce said he doesn’t know what type of flange replacement gaskets are being selected by its customers. He said the company doesn’t have any current plans to investigate other gasket types.

Single-piece Support

Camprofile (or kammprofile) gaskets have been used in Europe for about 20 years and are gaining more of a foothold in the U.S. Like spiral wound, they use metal to support the softer sealing material. The camprofile gaskets, however, use a solid metal core surrounded by two layers of sealing material. The surface of the metal core has a series of concentric grooves to hold the sealing material in place.

Camprofile has two advantages over spiral wound. The first is that the metal support consists of a single piece rather than a thin wound layer, which can come unraveled. Bussman, for example, said the spiral wound gaskets on the manways frequently failed.

“They come apart,” said Bussman. “When you finally open the door on them they unravel just like a spring.”

The other is that the sealing material completely covers the surface of the metal ridges. Ideally, even when the seal is compressed, the metal ridges do not come in contact with the flange surfaces. Unfortunately, real-world plant operating conditions are less than ideal, which brings up the limitations of this type of gasket design. The compressibility is limited to the thickness of the sealant layer as it passes over the highest point of the ridges, rather than the full thickness of the sealant. Any further compression must come from the elasticity of the metal. As a result, camprofile gaskets still require a higher clamping force than if the full thickness of the sealant could be compressed. In addition, since the metal backing is rigid, vibration or water hammer eventually leads to destruction of the soft graphite fibers. When severe, this can cause the metal ridges themselves to break through and damage the flange surface.

Aeroderivative Design

GE, Rolls Royce and other turbine manufacturers incorporate lighter components developed for their jet engines into their industrial combustion turbine generator sets. Lessons in jet engine seal designs can also benefit the power industry. The engines on fighter jets need to be brought up to full speed almost instantly and shift from 100+ degrees on the desert floor to the subfreezing temperatures at their flight altitude in a matter of minutes. And, while a seal failure on a power turbine means an unplanned shutdown for repairs, on an F-22 Raptor flying Mach 2 at 30,000 feet it can mean loss of a $150 million plane and its pilot.

Technology originally developed for use on fighter aircraft has been incorporated into Selco gaskets. But it differs from its competitors in several other ways. The gasket base consists of a thin layer of convoluted stainless steel with a 0.015” graphite layer within the grooves on both sides. As a result, it is thinner and more flexible than either the spiral wound or camprofile designs.

This design has several advantages over the older gaskets. Since the metal backing is a single piece, it doesn’t have come unwound like the spiral wound gaskets. Bussman said that while the spiral wound manway gaskets had to be replaced every 3 to 4 shutdowns, and frequently caused unscheduled shutdown due to leakage during startups, the Selco’s lasted at least twice as long and have not failed during use.

Another point to consider is that a Selco gasket is designed to operate at a wide range of chemicals, temperatures and pressures. With the “Self Locator” product line, the same model can accommodate materials with a pH of 0 to 14, air temperatures from cryogenic to 932 F, steam temperatures up to 1,200 F and pressures from 150 lb. to 2,500,lbs. This reduces the amount of inventory needed on hand and the odds that you won’t have the one you need in an emergency.

Finally, the flexible nature of the metal base means that the metal itself does not need to be compressed to achieve a seal. Instead, the metal acts like a spring, keeping the graphite tight against the flange. This greatly reduces the bolt load required, as well as the need for retorquing and possible warping of the flange.

The facilities engineer for a major oil and gas producer uses more than 100 Selco gaskets on the steam generators and distribution systems used for heavy oil recovery techniques. He appreciates being able to keep just one size of gasket on hand for all flange ratings up to the 1,500 lb. class. He has also been able to eliminate the use of Belleville washers—spring washers used to keep the load on the bolts during thermal variations—since the new gaskets take a lower bolt load and maintain a seal even if the bolts or pipes relax when cold. And since they are more pliable, the metal absorbs the energy of vibration and water hammer, preserving the life of the gasket.

“If I have a choice, I am going to put a Selco in rather than a spiral wound because it is a much better gasket,” said the facilities engineer.

Leak-free Joints

While the intricacies of gasket design may be fascinating to certain engineers, what really matters most is not the type of seal used, but how the joint performs in operation. The object is not to have the most state of the art gasket, but to ensure that failures do not result in costly shutdowns, environmental or safety hazards, or add to maintenance costs. The bottom line is this:

a) 70 percent of scheduled gasket replacement costs are labor related

b) One unplanned shutdown or a single leak exceeds the dollars spent on a gasket.

When existing models are performing adequately, people are reluctant to take a chance and make a switch. Vogt Power, for example, continues to use spiral wound gaskets on its HRSGs.

Bussman on the other hand, said that spiral wounds were causing trouble on the manways and that opened the door to trying something new.

“I said this had better work because it is quite expensive to start up and then have to shut back down to change out a gasket,” said Bussman. “But it worked out well and we have been using Selco gaskets ever since.”