Background

Spiral wound gaskets are common commodities used in piping systems across several industries , and maintenance and reliability staff are constantly on the lookout for products offering the promise of better performance . The ‘ low-stress ’ spiral wound gaskets are marketed as an alternative to standard spiral wound gaskets , requiring less torque to seat the gasket . This makes for an enticing offering , since the most common cause of bolted joint failures is a lack of load . The problem with this terminology , is that it is a very loose designation for gaskets that can , and often do , have a range of differences , making it difficult to judge the gaskets objectively .

market . Multiple types of plants use these designs in their piping systems in specialty applications , or even as replacements for ASME B16.20 standard spiral wound gaskets . In general , ‘ low-stress ’ spiral wound gaskets have various design elements , from additional graphite in the filler , to a stress relief anti-buckling design , which are all marketed as requiring less initial preload to seat . While no one can argue that it is possible to make spiral wound gaskets more compressible , questions remain about the proposed benefits of ‘ low-stress ’ technology and its effectiveness in providing a reliable seal . In other words , is a softer , more compressible sealing element a good thing for a spiral wound gasket ?

Figure 1 .

V Shape

W Shape

Many manufacturers of spiral wound gaskets have made subtle changes over the years to try to differentiate themselves , and the ‘ low-stress ’ technology has become a common theme throughout the

Spiral Wound Construction

For the most part , the spiral wound has undergone only subtle changes since its initial introduction over 100 years ago ,

with the overall concept remaining the same . This fact highlights the versatility and effectiveness of the gasket in general . It can withstand high temperatures and pressures , and can be used with a

It is important to recognize that only gaskets that meet all of the requirements of ASME B16.20 ( construction , dimensional , compressibility , and sealability ) should be stamped with this designa-

variety of chemicals : the manufacture can

tion . Other designations , such as the

change the materials of construction to enhance compatibility with the process .

manufacturer name , size , pressure class of the gasket , and materials of construc-

A standard spiral wound gasket has three major components : a solid metal outer

tion will commonly appear on all spiral wound gaskets .

ring , a solid metal inner ring , and the wound sealing element . The sealing element is formed by using alternating plies of metal wire and a soft filler material , which are wound together on a mandrel into a gasket shape of the desired size . During the process , the flat winding metal strip is formed into a “ V ”, or chevron shape , as depicted in Figure 1 . This allows the windings to flex like a spring under load . Common filler materials are flexible graphite and polytetrafluoroethylene ( PTFE ), though other options are readily available for specialty services . The materials and methods of construction will

It is also important to know that prior to 2008 , inner rings on spiral wound gaskets were not considered the standard . Industry feedback was consistent that buckling of the windings on spiral wound gaskets with only outer rings was a common issue . One solution was to machine relief ports into the outer ring to allow for outward expansion of the winding element . However , more commonly , inner rings were used . In May 2008 , ASME B16.20 standardized the use of inner rings on spiral wound gaskets .

Low-Stress Meaning

vary based on the operating conditions , such as temperature and pressure , of the service . These materials must also be chemically compatible with the process media . Additionally , the density of the winding is a key factor in determining how the gasket will respond and perform under load .

The designation of ‘ low-stress ’ spiral wound gaskets refers to those that have a lower density ( i . e . softer ) winding , which is created by utilizing a thicker filler strip , and less tension on the winding metal during manufacturing . This produces a softer , more loosely wound gasket with fewer metallic wraps , enabling the seal-

The outer ring serves as a centering ring ,

ing element to compress under a lower

or guide ring , to ensure that the sealing

load . Standard spiral wound gaskets

element is properly located between

should have a higher density , stiffer

the flanges ; while the inner ring , a com-

winding constructed with thinner strips

paratively recent addition to the design ,

of soft filler , and are wound tighter during

serves to prevent the winding ’ s natural

the forming process . This ensures a gas-

proclivity to radially buckle at the inside

ket with higher filler density , and more

diameter . However , neither of these ele-

metallic wraps , as shown in Figure 4 .

ments are designed or intended to assist

Testing has shown that a gasket without

in the functional sealing of the gasket .

this higher density winding cannot meet

That task falls solely on the winding ele-

the current compressibility and sealabil-

ment itself . ASME B16.20 also specifies

ity requirements of ASME B16.20 , hence

the identification markings required to be

the reason they cannot be stamped as

printed on spiral wound gaskets .

being manufactured per the standard .