Commonwealth of Virginia Department of State Police — Hazardous Materials Safety Interpretation

Page 1U.S. Department of Transportation 400 Seventh St., S.W. Special Programs Research and APR 7 2004 Washington, D.C. 2059( Administration Sergeant David P. Feather Ref. 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), 178.345-1(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 vi sid or dorio any no euranterise fom the top ent kill be abrid and beating. 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 C /John A. Gate Chief, Standards Development - Office of Hazardous Materials Standards 5178.345-1 040055#