Wouldn’t you want to reduce both the incidence of railcar Non-Accidental Releases (NAR's) and cost of railway shipping without making significant changes?
In this fourth of five blogs, Parker’s ISS Division examines why rail manway gaskets fail and how current shipping procedures are needlessly expensive and cumbersome to manage.
Durlon 9000 (PTFE Gaskets) – Better in Some Ways, Worse in Others
PTFE is quickly rising in popularity and currently represents about 25% of all gaskets being used on railcar manways due to its excellent chemical resistance. Unfortunately, there are disadvantages to using PTFE such as the tendency to "creep" (cold flow) under compression. While elastomeric, or rubber, gaskets will stretch under the same compression, both cases result in a loss of bolt torque. In addition to being very expensive at about $140-200+ each, PTFE gaskets also have a number of other drawbacks:
- they never recover their original shape, therefore are limited to just one use, or torque
- they are difficult to remove, often requiring time consuming chiseling and resulting in gouged sealing surfaces, risking the decommissioning of the tank car for up to 6 weeks
- although stiffer than standard rubber, PTFE is still flexible enough to fall into the tank
- bolt torque is difficult to achieve and likely leads to over torquing, exceeding the bolts yield by either overstretching or snapping
Failure to torque bolts in a star pattern
In order to optimize even load/pressure distribution, the Association of American Railroads recommends a minimum of three passes using a calibrated torque wrench (10-20 ft-lbs/ 50% of target torque/ 100% of target torque) to close manways. In practice, most loading technicians use impact wrenches and fail to check bolt tension requirements.
Restrictions of beveled manway nozzle geometries
In an attempt to reduce the incidence of NAR's, manway manufacturers introduced peaked nozzle designs. These 'pinch points' were designed to create a focused point of contact pressure to ensure better sealing. While this has helped to create a better seal, the focused pressure point places undue burden on the seal, making it less resilient.
Constraints of single-bolt manways
Single-bolt manways were developed to mitigate a number of practical issues by:
- eliminating the need to apply the star-patterned closing sequence
- eliminating the differences in bolt torque from one tank car to the next
- moderate uneven seating of the gasket and lid
While this design does address and alleviate some of the incidences of NARs, it still requires a gasket.
The issues with manways, as with many other bolt/clamp sealed appliances, has little to do with the actual gasket and everything to do with the mechanism securing it.
The technology and engineering required to fix these issues has been successfully applied in numerous other industries. Nevertheless, railcar gasket companies have ignored all major underlying issues, choosing instead to provide new materials, elaborate procedures and specialized tools, all of which fail in equal measure.
Using its automotive and aerospace expertise, Parker's Integrated Sealing Systems Division (ISS), has engineered a stainless steel, over molded, controlled compression gasket that maintains correct bolt torque and uses computer designed geometry to ensure a continued and repeatable seal that can be used up to 30 times.
The Parker Manway Nozzle Gasket is the only engineered solution that resolves all of the above issues while also reducing labor costs and the incidence of NAR's and injuries. For more information visit this support link and watch a short video here.
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