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The importance of load in maintaining seal: low compressive loads present challenges for gasket seals, but newly designed gaskets can solve these problems.

Many factors affect the performance of the ubiquitous gaskets that provide critical seals between pipe segments and ensure satisfactory operation of a broad range of industrial equipment. But no factor is more important than compressive load. The compressive load provided by the bolts must be sufficient to cause the gasket to flow into and fill any irregularities in the mating surfaces being sealed. It also provides the gripping force between the gasket and flange surfaces to mitigate against lateral gasket movement and blowout. The higher the compressive load, both initially and during service, the better the seal.

More than 70% of all gasket failures can be attributed to lack of load, according to studies conducted by Garlock Sealing Technologies, Palmyra, N.Y. Maintenance engineers, fitters, and mechanics must understand the various problems associated with low compressive load and the gasket solutions that have been developed to address these challenges.

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Lack of load

Low load conditions derive from several different causes. Frequently, the problem is caused by structural deficiencies of the flange system. Generally, non-metallic flanges are used in situations where operating temperatures, pressures, and mechanical shock and vibration are less severe. Plastic flanges can be very brittle, and some metals, like cast iron, have poor ductility. These flanges tend to be less robust and require softer, more easily compressed gasket material that can be seated under a lower gasket compressive stress. Glass-lined flanges are mechanically weak and prone to cracking and are unable to sustain the increased bolt load typically associated with thick gasket applications. Glass-lined flange applications therefore require special gasket solutions that meet the conflicting requirements of filling relatively large gaps and performing with low compressive load. Flange geometry can also limit a system's ability to sustain high bolt load. The flange geometry not only can reduce the system's capacity to supply load, but it can also increase the demand for compressive load. Problems with flange imperfections and alignment require gasket solutions that often depend on higher compressive load. When the flange joint contains irregularities, with areas of high and low compression, the areas of low compression are prime candidates for leakage or blowout.

Operating conditions can also influence the effective compressive load. Operational torque loss, also called stress relaxation, can be caused by elongation of bolts, thermal cycling, flange distortion, and vibration. Another common problem is creep relaxation. The result of creep relaxation is loss of thickness of a gasket, which causes compressive load loss, resulting in leakage. Progressive weakening of the gasket itself can be caused by aging and by fatigue due to the continuous application of stress that causes strain.

Solving low load issues

A variety of technology and design strategies have been developed to address the special requirements of low load gasket applications. Three approaches that have proven effective are restructured PTFE (polytetra-fluoroethylene, or Teflon) gaskets, swelling/self-loading gaskets, and engineered molded rubber gaskets.

For applications where the flange and bolt system cannot support high compressive loads, Garlock has developed the GYLON series of gaskets that employs a highly compressible and chemically-resistant PTFE material that conforms to surface irregularities under low bolt load. Gaskets are available with a rigid PTFE core, sandwiched between outer layers of the highly-compressible PTFE material, to reduce cold flow and creep normally associated with conventional PTFE gaskets. The soft and highly compressible characteristics of the restructured PTFE outer layers allow the gasket to maximize sealing performance despite minimal bolt load.

A second strategy, employing a material that swells when exposed to water or oil, takes essentially the opposite approach. Rather than being extra compliant to accommodate low bolt load, these swelling gaskets expand and push back, exerting force against the flanges. Garlock has created a self-loading general service gasket specifically designed for low-load applications

called the MULTI-SWELL gasket, which is twice as soft as conventional gaskets and readily conformable to irregular flanges. The new material provides extremely high crush strength and can be safely installed in applications that would typically crush elastomeric gaskets.

The third strategy takes advantage of the natural benefits of rubber, a material that offers exceptional softness and nearly 100% resiliency in the absence of heat. Garlock has built upon these advantages with the Stress Saver sealing rings by encasing the rubber in PTFE-molded envelopes to provide excellent chemical resistance. The raised, molded-in sealing rings seal with 75% less surface area than full faced gasket designs for high performance in low compression applications.

Although low compressive load presents a significant challenge for gasket seals, three types of gaskets--restructured PTFE gaskets, swelling gaskets, and engineered molded rubber gaskets--have proven highly effective in solving these problems. In turn, uniquely engineered gasketing solutions cut maintenance and repair costs, lessen safety concerns associated with blowouts, and reduce emissions and leakage.

For the full text of this article, go online to: www.rdmag.com

RESOURCES

Garlock Sealing Technologies, Palmyra, N.Y., 800-448-6888, www.garlock.com

--Jim Drago, Manager of business development, Garlock Sealing Technologies
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Title Annotation:LABORATORY EQUIPMENT/DESIGN
Author:Drago, Jim
Publication:R & D
Date:Oct 1, 2007
Words:825
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