Feb 1, 2006 12:00 PM
PROPER maintenance of venting systems on chemical trailers starts with a clear understanding of the hardware. Sometimes it's necessary to separate the myth from the reality when working on cargo tank vents.
John Freiler, Girard Equipment Inc, dispelled five of the most common vent misconceptions during the 2005 Cargo Tank Maintenance Seminar, which was held November 7-9 in Louisville, Kentucky.
The first vent myth concerns the “hammer” lugs. “Those lugs look so solid and beefy that many mechanics believe they can use a hammer to tighten the vent when there is a leak,” he said. “That's wrong. Those are wrenching lugs, and the proper wrench must be used to avoid damage to the vent.”
Myth number two is the belief that ASME (American Society of Mechanical Engineers) venting is required on ASME-stamped cargo tanks and those that are designed and constructed in accordance with the ASME code. Actually, V-stamped vents are allowed on these cargo tanks but are not mandated.
Only the old MC312 specification has any requirement for ASME venting, and the code allows a rupture disc in its place. Further, the ASME code allows manufacturers to build and stamp a vessel without a safety valve.
A Department of Transportation regulation interpretation issued in 1998 stated the following: The intent of the Hazardous Materials Regulation is not to require ASME “V” stamped pressure relief devices on DOT407 or DOT412 cargo tanks.
Cargo tanks on which the vent is welded to the manway can pose testing challenges. Many mechanics believe the only way to test such a vent is to disassemble its upper section and test only that on a stand with its own lower vent housing.
“The vent on the cargo tank must be tested intact,” Freiler said. “Most vent designs have critical features in the welded pieces. Without testing those parts, it's hard to say that you've really tested the vent. Blank off the bottom of the vent as is and test the whole thing. It's the only way to get an accurate reading.”
Many tank mechanics still believe that MC300-series tanks must be upgraded with DOT400-series vents. Freiler said that this is seldom the case. “Most MC307 and MC312 vents that can pass the leak test will also meet surge requirements,” he said.
Finally, there is a belief that upgraded vents must be flow-rated at 130% of the maximum allowable working pressure (MAWP). The fact is upgraded vents are rated like any other DOT400-series vent.
“The flow-capacity caution published by DOT specifically applies to upgrades from MC306 to DOT406 because some “smart vent” PAFs had a lower flow rate even at the DOT406 pressures,” Freiler said.
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