By Maria Garcia , MONTERO FYE S . A .
Column
Can optimal seating stress improve valve packing performance ?
In technical papers , information is available about low emissions and how aiming for the best torque on a valve packing gland is essential to optimise economic resources and environmental impact . The correct balance between sealing force and stem friction must be reached . Inadequate torque methods result in several problems , including increased maintenance , higher leakage and reduced packing life with the consequent waste of money and energy . Reducing energy consumption , being competitive and minimising costs means being efficient . The best results are needed . So , once the correct packing material , geometry and design have been chosen , the right torque choice and its controlled application on the bolts are required to reach an optimised compressive load . This is one of the most important considerations to be taken . Appropriate load and an accurate transmission to the packing must be achieved . At this point , two questions must be addressed : torque value and tightening method .
Torque value & tightening
Usually , the packing manufacturer will recommend the torque value for their particular packing . The optimum torque value is calculated by considering several parameters : number of bolts , quality , diameter , packing composition and friction factors . Regarding the tightening method , some variation occurs in the information given by manufacturers , but in general best practice will involve the following elements :
• Clean and inspect the housing and counter face
• Use hardened flat washers
• Tighten the gland bolts ( often recommended to be done in several stages )
Torquing of gland nuts
• Use a calibrated torque wrench
• Measure bolts distance
• Readjust gland nuts after a short working period ( sometimes it is recommended at least 5 times with the valve in the closed stroke position ).
• Proper lubrication These practices will minimise the changes and losses due to variations in the torquing process . All of them affect the sealing system .
Can we adjust the packing to minimise deviations in the torquing process ?
The first contact between the packing and the valve stem is produced by a seating torque value . During assembly , the first torque value required to seat the packing will affect packing behaviour during operation . Plastic deformation in the packing set takes place , and all the gaps between the stem / housing and packing gradually
About the author Maria Garcia is the R & D Manager of MONTERO FYE S . A . and a member of the ESA Packings Division . She has a degree in Physical Chemistry ( Basque Country University ) and a postgraduate qualification as ‘ Expert in Plastics and Rubber ’.
Emission testing at Babcock Valves installation . π www . valve-world . net Valve World February 2023
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