Interpretation Response #04-0055 ([Commonwealth of Virginia Department of State Police] [Sergeant David P. Feather])
Below is the interpretation response detail and a list of regulations sections applicable to this response.
Interpretation Response Details
Response Publish Date:
Company Name: Commonwealth of Virginia
Department of State Police
Individual Name: Sergeant David P. Feather
Location State: VA Country: US
View the Interpretation Document
Response text:
Apr 7, 2004
Sergeant David P. Feather Reference No. 04-0055
Safety Division, Motor Carrier Safety
Commonwealth of Virginia
Department of State Police
P.O. Box 10900
Fairfax Station, Virginia 22039-0900
Dear Sergeant Feather:
This serves as a retraction of our September 25, 2003, response to your October 24, 2002 (Ref. No. 02-0287) letter requesting clarification on venting for double bulkheads or void spaces for MC 300 and DOT 400 series cargo tanks under the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180). Specifically, you asked whether it was permissible to plug the vent on top of the cargo tank at the double bulkhead, provided that the bottom drain opening was left open. This follow-up letter is to inform you that our previous response was incorrect.
Sections 178.340-7(c), 1 78.345-1 (i)(2) and 178.345-3 require any MC 300 and MC 400 cargo tank void or airspace to be vented and equipped with a drain, located on the bottom centerline, that is accessible and open at all times. In our previous response, we indicated that it was permissible to plug the vent on top of the cargo tank, provided the bottom opening or drain is open and operational at all times. This is not correct.
The correct response to your question is that both the top vent and bottom drain must be kept open. Vents are normally located in the top centerline of the tank and are required to communicate with the vapor space in order to allow any gases that have accumulated in the void area to escape through the vent located at the top of the tank. In the case of the void area, if the vent is plugged and there is a leak in the bulkhead, the gas will drain out of the drain located at the bottom centerline of the tank, but the vapors will have no means of escape if the vent is plugged. Further, if the bottom drain is closed or plugged with mud or debris, any moisture entering from the top vent will be trapped and become a source of corrosion at the bottom of the tank. Use of a hat device to keep out debris while maintaining an open vent and drain, thus allowing for vapors to escape, would be permissible.
We apologize for any inconvenience and hope this answers your inquiry.
Sincerely,
John A. Gale
Chief, Standards Development
Office of Hazardous Materials Standards
178.345-1