Practical Estimation of Retorque Requirements

Abstract

Abstract The initial assembly of a bolted flanged joint (BFJ) commonly includes several ramped torque steps using various bolt torque patterns, final circular pass(es) at 100% of target torque, and then the choice of whether and when to come back and retorque the just-assembled joint. The value of retorquing a newly assembled bolted flanged joint varies significantly by which gasket material and type is used in the joint. The additional maintenance costs of labor, tool rental, scaffolding, crane usage, etc. to perform a retorque on a joint can be significant, but the larger cost is generally the additional process downtime. Gaskets based on sheet materials are often retorqued. Conventional practice indicates that BFJs with gaskets made from flexible graphite or fiber sheets typically do not benefit from retorques, or at least do not benefit enough to offset the additional costs. Conversely, industry experience with many polytetrafluoroethylene (PTFE) based gaskets indicates these joints benefit significantly from retorquing a BFJ after initial assembly. This paper explores some practical methods to estimate potential retorque benefit as a function of retained gasket stress without the retorque at temperatures normally “in range” for the gasket material being considered. That is, at temperatures that would not cause thermal damage to the gasket material. For flexible graphite and fiber materials known to experience mechanical property degradation with time and/or temperature (“aging”), a typical 5-year life is used. For PTFE gaskets known to relax primarily with temperature and that do not appreciably age, use of the Hot Blowout Thermal Cycling test (HOBTC) can provide a quick evaluation of gasket stress loss as a function of temperature. Retained gasket stress in both cases can then be used to estimate tightness for the gasket material using the Room Temperature Tightness (ROTT) data. If the joint is still at or above required tightness, then retorquing the joint soon after initial assembly is of little benefit. If the gasket has lost enough stress, then a retorque can bring the gasket stress back into range to provide sufficient tightness. Knowing this, the end user can make better choices for both gasket material selection and assembly procedures in order to reduce overall job costs.