In terms of control performance, sealing life, and cost, quarter turn valves incorporating soft, non-metallic components have been the gold standard for decades.
Unfortunately, these soft seals have a somewhat limited ceiling for temperature and are easily damaged when used in applications where the process fluid contains abrasive materials.
Metal seals address these deficiencies, but getting metal surfaces to consistently seat and seal can be challenging. Our article in the August 2025 issue of Hydrocarbon Engineering (pages 35-38), titled “The art of metal seal design in quarter turn valves,” discusses the intricacies associated with metal seals, providing end users with the information needed to choose the best solution for their application.
Benefits and drawbacks of soft seals
Soft seals use polymeric materials to provide a malleable surface for the disc to press against. These seals are the first choice for most valve applications because they typically achieve extremely low leakage rates over an extended service life, and they require relatively little actuator torque.
However, most soft seals begin to break down as process temperatures approach 400°F, and abrasive fluids can erode these seals each time the valve closes.
Metal alternatives
Metal seals are better suited to handling high temperatures and slurry-type fluids, but they have their own set of constraints. Unlike soft seals, metal seals rely on precision fits to achieve tight shutoff, and they may require higher seat loads.
Surface damage can occur on the mating regions of the seal and disc as the valve is cycled, causing an increase in leakage over time. This damage will also create additional friction, increasing the torque required to initiate rotation and leading to erratic performance.
Metal seat design enhancements
Metal seated valve designs that slow the accumulation of abrasive wear can approach the performance and longevity associated with soft seals.
Eccentric valve designs reduce wear by offsetting the shaft from the flow centerline, creating a disc path that causes the seal to quickly lose contact with the disc. However, larger offsets also increase the camming torque required to turn the valve stem, so a balance between these two features is essential.
Hardening the surface of critical components by using special coatings or penetrative treatments is an effective way to preserve the surface finish and maintain its coefficient of friction.
Seat load should be adjusted so that it is high enough to pass the factory acceptance leakage test, but not so high that it needlessly accelerates wear.
Finally, elevated temperature cycle testing provides an opportunity to observe the impact of design decisions, and to gather experimental data to determine an appropriate safety factor for actuator sizing.
Careful specification
A successful valve installation starts with an accurate description of process conditions. End users should take care not to overstate the maximum design temperature because doing so can result in excessive seat load. More generally, overstated requirements typically raise the cost of the valve, and they may hinder performance.
When process conditions exclude the use of soft components, valves with metal trim are a natural choice. Many Fisher™ valves, such as the Fisher 8580 High Performance Butterfly Valve, are available with either a soft or metal seal. Having such options allows end users to select a valve with the appropriate trim for their application to support reliable performance over an extended service life.
