If you could lower freight loading costs by more than 90%, reduce the incidence of injuries and prevent the leakage of dangerous hazardous spills without changing anything - would you be interested? In this third blog of five, Parker's ISS Division will look at the main reason why manway nozzle gaskets fail.
Manway NAR related issues at all time high
Year on year data from the Association of American Railroads (AAR) has identified a pattern of human and engineering errors. In 2012, manway related issues accounted for 48% of all Non-Accidental Releases (NAR's), up from 39% in 2011. More importantly, 95% of those failures were caused by just four factors:
- 48% loose manway lid bolts
- 26% gasket deterioration
- 11% gasket misalignment
- 10% missing gasket
Insufficient bolt yield
Karl Alexy, Staff Director of the Hazardous Materials Division of the Federal Railroad Administration, an agency of the U.S. Department of Transportation notes, "even the most highly trained and conscientious loading technician cannot prevent loose bolts". Bolts are like springs, requiring a sufficient amount of torque to stretch to the point of creating spring back. In order to achieve and maintain this yield, the bolt requires a stable surface to pull against. Once attained, the nut is virtually locked in place and the bolt unable to loosen.
Conventional manway gaskets (all rubber or PTFE), do not provide a stable surface in which to attain sufficient bolt yield. Even when torqued using suggested best practices, the gaskets will compress causing the bolts to lose 25% or more torque compression within the first 24 hours. In an attempt to address this, loading technicians overcompensate by over tightening the bolts, especially when using impact wrenches. In doing so, one of two undesirable outcomes will result:
- cutting, cracking, splitting or other damage to the gasket - compromising its ability to seal
- extending bolt yield, either surpassing its ability to spring back or snapping altogether
Gasket material degradation
Existing gaskets are typically only recommended for one use. While all-rubber seals have limited ability to return to their original shape after being compressed, PTFE gaskets have no ability to revert. In other words, after their first use, both types of gasket are deformed and whatever forces were initially used, no longer apply.
Similarly, as with any material, there is a breaking point where too much force will cause it to break. Even if not immediately noticeable, once damaged, that material will thereafter under-perform and continue to deteriorate until it fails completely.
Currently, loading technicians are over compressing brand new gaskets, believing that tighter equals a better seal. This instantaneously and irreversibly damages the gaskets with the tanker car having never left the rail yard.
Industry best practices only dictate a visual inspection. As such, it is not unusual to hear of loading technicians reusing a gasket up to a dozen times. The alternative requires maintaining a large amount of inventory on chemically compatible gaskets.
Missing or misaligned gaskets
According to the AAR, 20% of loading technicians fail to adhere to best practices including; correct torquing procedures, properly aligning gaskets to the nozzle or complete failure to use a gasket.
Current sealing technologies are large, heavy and very cumbersome. They are difficult to install and in some instances, require technicians to use mild adhesives to aid installation, a method that compromises sealing integrity. Nevertheless, it is not unusual for these gaskets to fall into a manway, requiring manual removal by the technician.
Adding to the challenges already noted, the over compression and gasket deterioration make it difficult to loosen bolts and pry the gasket from the nozzle without damaging the bolts or the nozzle itself.
While current issues have done little to address the primary issue of inadequate bolt yield when securing manway covers, there is little room for excuse when examining other issues caused by human error. The resources to monitor adherence to safe loading have dramatically fallen behind the rampant growth in rail shipments (5,000 in 2009 to 240,000 in 2012), yet the measurable incidence of NAR's has increased. How many more incidences have slipped through unnoticed?
For more on this topic, read NAR's: Where the Feds Are Focusing