In the context of bolted flanged joint installations , it is important to note that the significance of dwell time varies across gasket technologies . While ASME PCC- 1-2019 emphasizes the importance of a minimum four hour dwell time to address short-term creep relaxation and embedment losses , it is essential to recognize that this practice is not universally applicable to all gasket technologies . Despite its crucial role , the implementation of a four hour dwell time can contribute to increased overall downtime during maintenance activities , incurring a ( sometimes significant ) cost .

A study published by three engineers from Teadit , Carlos D . Girão , Igor Meira , and José Carlos Veiga in 2021 , sought to investigate the significance of dwell time in bolted flanged joint installations to determine if the amount of time between installation and retorque could be minimized and / or optimized for different gasket materials . In their study , Girão , Meira , and Veiga explored varying dwell times ( 15 minutes , one hour , four hours , and 24 hours ) across different gasket styles and materials , including PTFE , Compressed Fiber Sheet Gaskets ( CFG ), and metallic gaskets . The aim was clear : to strike a balance between minimizing plant downtime and ensuring gasket effectiveness .

Experiment Setup

Their study employed an NPS 4 , class 150 ASME B16.5 flange outfitted with eight strain-gauged bolts , correlating bolt elongation with applied stress . This comprehensive approach allowed them to measure the impact of different dwell times on joint relaxation and bolt preload loss . The goal was to determine , based on the different gasket types , the point at which dwell time begins producing diminishing gains . This would hopefully provide insight for plant personnel seeking to minimize downtime .

ket type , as the impact varied among the tested materials . For instance , in the case of expanded PTFE ( ePTFE ), a 15-minute dwell time demonstrated a significant improvement in gasket stress .

Extending the dwell time to one hour , then four hours , and eventually to 24 hours produced a nearly consistent incremental increase ( i . e . constant trendline slope ), however the most advantage seen of a retorque is after 15 minutes consistent incremental increase ( i . e . constant trendline slope ). Conversely , restructured PTFE ( rPTFE ) showed steeper increases at one hour and , to a slightly lesser extent , four hours before flattening out dramatically , indicating very little benefit in extending dwell time beyond the ASME recommendations . Expanded PTFE boasted the lowest overall preload loss with retorque at 24 hours among the ‘ soft ’ gaskets tested .

Skived PTFE ( sPTFE ), a material inherently known to be susceptible to high levels of relaxation resulting in preload loss , experienced the highest level of loss ( 20 % loss of total preload ) without retorquing . However , the difference in the results between dwell times of 15 minutes ( 12.3 % loss ), one hour ( 11.7 % loss ), and four hours ( 11.3 % loss ) was negligible . This indicates that skived PTFE materials relax and lose their load quickly after installation , so retorquing after 15 minutes could be a viable option compared to waiting the recommended 4 hours per ASME PCC-1 . Notably , it is worth mentioning that ePT- FE and rPTFE exhibit the same amount of relaxation after 15 minutes of dwell time as virgin PTFE does after 24 hours . Extending the dwell time to a full 24 hours does dramatically decrease the overall preload loss . Likewise , compressed fiber gaskets ( CFG ) exhibited only slight overall improvements from one hour to 24 hours , suggesting that for CFG gaskets , a one hour dwell time could be a pragmatic compromise to expedite operations without sacrificing overall performance .

Figure 1 .

to consider that relaxation is influenced by temperature , and these results reflect ambient conditions .

Additionally , the study looked at the impact of under and over-tightening on relaxation and load retention for two of

Figure 2 .

the materials , spiral wound gaskets and ePTFE . In Figure 4 , one can see the ePT- FE stress loss results for torque levels of 60 ft-lbs ( 81 N-m ), 120 ft-lbs ( 162 N-m ), and 158 ft-lbs ( 214 N-m ). These torques equate to approximately 37 %, 75 %, and 98 % of bolt yield , respectively .

The gaskets were installed per ASME guidelines and a retorque was applied at the determined dwell times . After 20 hours , the retained gasket stress was measured to determine the overall loss from preload . A control sample that was not retorqued at all was used as a baseline to measure against .

Gasket Material Matters

The results of their study revealed intriguing insights into the role of dwell time , particularly in relationship to gas-

Metallic gaskets , inherently less susceptible to relaxation effects , as indicated by the study , showed only marginal stress loss differences across various dwell times ( including no retorque at all ) at ambient conditions . It is important to note that Figure 2 and 3 illustrate this phenomenon for spiral wound gaskets ( SWG ) and kamprofiled gaskets ( KAM ) under these conditions . Kamprofiled gaskets , with a 24-hour dwell time before retorque , exhibit less than 1 % overall preload loss , making them one of the tightest overall sealing options . However , it is essential

Figure 3 .