Estimation of head loss due to flow intrusion with inner ring spiral wound gaskets

The spiral wound gasket, developed after the turn of the 20^th century, has been a popular gasket choice in the chemical, petrochemical, power and other industries. Since that time, metal gasket manufacturers have developed various designs for this popular type of gasket, using different metals and fillers in the windings. As a result, some subsequent spiral wound designs have included a metal inner ring inserted inboard of the windings, with the primary objective of preventing inward radial buckling of this gasket as well as to cover more of the flange face and minimize erosion between flange faces. However, the cost of fabricating the spiral wound increases in accordance with the metallurgy of the inner ring, as does the assembly bolt force required to fully compress this now captured spiral winding. Additionally, another recently identified drawback of the inner ring is the head loss due to the contraction of the flow area caused by the inner ring intrusion into the flow for certain nominal pipe sizes (NPS) and pipe schedules. For example, standard ASME B16.20 spiral wound gaskets with inner rings designed for ASME B16.5 Class 150 raised face flanges extend inside the flange and pipe wall to varying degrees for most Schedule 10 pipe between NPS 1/2'' and 6''. Such intrusions can impact the flow in the form of minor head loss resulting from intrusion of the inner ring. Flow equations for an orifice can be used to estimate head loss; such an empirical equation has been applied to estimate head loss associated with an inner ring style spiral wound gasket in various pipe schedules and NPSs. Total head loss for multiple flanges in a pipe network has also been calculated as the head losses from individual "inner ring orifices" are accumulated. Several scenarios were identified to indicate where spiral wound gaskets with an inner ring can not only lead to pipe network head loss but substantial hidden energy costs.