Vent testing critical to tank safety
Feb 1, 2012 12:00 PM, By Rick Weber
BELIEVE Joe Evans when he says that a BLEVE isn't a pretty sight.
In his vent-testing presentation, Evans, Hazardous Materials Program Manager for the Federal Motor Carrier Safety Administration's (FMCSA) Eastern Service Center, showed a picture of an MC331 tank ablaze because it went into BLEVE (Boiling Liquid Expanding Vapor Explosion). Evans spoke during the 2011 Cargo Tank Maintenance Seminar October 24-26 in Louisville, Kentucky.
“It's a situation that is not common, but it does happen,” he said. “This is a situation where vents would come in handy to relieve pressure on the cargo tank. Eventually, the cargo tank can't vent enough with liquid in there, and will essentially explode.”
He said that normal vents function for pressure and vacuum and let the cargo tank breathe, while emergency vents limit the pressure inside the tank to prevent a catastrophic failure of the tank.
“Cargo tanks have different design pressures, so different vents have to be used on different, specific cargo tanks,” he said.
The HM 183 rule, which was published in the Federal Register on June 12, 1989, required self-closing pressure relief devices (PRDs) on all newly constructed cargo tanks and limited the use of frangible devices. It also required leak tightness of manhole closures. Manhole fill openings and washout assembly had to be structurally capable of withstanding, without leakage or permanent deformation that would affect its structural integrity, a static internal fluid pressure of at least 36 psig, or cargo tank test pressure, whichever is greater.
He gave some background on the regulation covering DOT400 Series Pressure Relief (178.345-10), which says that cargo tanks (CTs) must be equipped to relieve pressure and vacuum in accordance with this section of the CFR and the individual specification; pressure and vacuum relief system must be designed to operate and have sufficient capacity to prevent CT rupture or collapse due to over-pressurization, or vacuum resulting from loading, unloading, or heating and cooling of lading. Pressure relief systems are not required to conform to the ASME requirements.
“We know that 400 series cargo tanks are all designed in accordance with ASME code, and some are certified,” he said. “But designed and certified are two different things.”
Cargo tanks must be provided with: a primary pressure-relief system consisting of one or more reclosing pressure relief valves and a secondary pressure-relief system consisting of another pressure-relief valve that may be used to augment the total venting capacity. Non-reclosing pressure relief devices are not authorized in any CT except when in series with a reclosing PRD. Gravity-actuated reclosing valves are not authorized on any CT.
Pressure-relief systems must be designed to withstand dynamic pressure surges in excess of the design set pressure. PRDs must be able to withstand a dynamic pressure surge reaching 30 psig above the design set pressure and sustained above the design set pressure for at least 60 milliseconds (0.06 seconds).
“This is where it gets more complicated,” Evans said. “The total volume of liquid released should not exceed one liter before the valve recloses to a leak-tight condition.”
PRDs must be constructed and installed to prevent unauthorized adjustment of settings. No shut-off valve or other device that could prevent venting through the pressure-relief system may be installed. The pressure-relief system must be mounted, shielded, and drained to minimize the accumulation of material that could impair the operation or discharge capability of the system by freezing, corrosion, or blockage, and the PRD must be certified by the device manufacturer.
Each device must communicate with the vapor space above the lading as near as practicable to the center of the vapor space. Discharge from any device must be unrestricted. Protective devices which deflect the flow of vapor are permissible provided the required vent capacity is maintained.
According to MC306-178.341-1 of the 300 series, the tank must be equipped with a normal vent, with the pressure setting at 1 psig and the vacuum setting at 6 ounces. The loading and unloading vent/closed dome limits vacuum to 1 psig and pressure to 3 psig. The cargo tank must have minimum emergency venting capacity. The pressure-actuated venting device must open at no less than 3 psig and close at 3 psig or below.
According to MC307-178.342-4, the design pressure is no less that 25 psig, and a venting device should be equipped to limit the tank's internal pressure to 130% of the tank's design pressure. Venting devices are set to open at less than or equal to the tank's design pressure. Venting capacity of the PRD is calculated at 130% of the tank design pressure. Fusible vents may be added to meet the minimum emergency venting capacity requirements
40 series tanks
According to 178.345-10 of the 400 series, the set pressure of the pressure-relief system is the pressure at which it starts to open. For the primary system, the set pressure of each primary relief valve must be no less than 120% of the maximum allowable working pressure (MAWP), no more than 132% of the MAWP, and the valve must reclose at not less than 108% of the MAWP and remain closed at lower pressures. For the secondary system, the set pressure of each pressure-relief valve used as a secondary relief device must be not less than 120% of the MAWP.
According to 178.346-3 DOT 406, the MAWP range is 2.65 psig to 4 psig. The set pressure is not less than 110% of the MAWP or 3.3 psig, whichever is greater, and not more than 138% of the MAWP. The valve must close at not less than the MAWP and remain closed at lower pressures.
According to 178.345-10(d)-DOT 407 and 412, the 407 MAWP must be at least 25 psig and the 412 MAWP must be at least 5 psig.
The set pressure is no less than 120% of the MAWP, and no more than 132% of the MAWP. The valve must reclose at not less than 108% of the MAWP and remain closed at lower pressures. There are exceptions for the 412 in dedicated corrosive service.
MC331 requirements state that each tank must be equipped with one or more pressure relief devices which, unless otherwise specified, must be of the spring-loaded type. Each valve must be arranged to discharge upward unobstructed to the outside of the protective housing to prevent any impingement of escaping gas upon the tank. Each valve must be designed, constructed, and marked for a rated pressure not less than the cargo tank design pressure at the temperature expected to be encountered. The total relieving capacity is determined by the flow formulas contained in Section 5 of Compressed Gas Association S-1.2. Each PRD must prevent a maximum pressure in the tank of more than 120% of the design pressure.
Under Part 180.407(d) requirements for bench testing, an external visual inspection is done annually or every six months, unless you're dealing with full-opening rear heads designed to be loaded by vacuum. All reclosing PRD must be externally inspected for any corrosion or damage preventing safe operation. If the CT is carrying lading corrosive to the valve, the PRD must be removed from the CT for inspection and testing. Each PRD required to be removed and tested must open at no less than the required set pressure and no more than 110% of the required set pressure. Reseat should be done to a leak-tight condition at no less than 90% of the start-to-discharge pressure or the pressure prescribed for the applicable CT specification.
Under Part 180.407(g) requirements for bench testing, the pressure test is done every five years. All self-closing PRD, including emergency relief vents and normal vents, must be removed from the cargo tank for inspection and testing. A self-closing PRD that is an emergency relief vent must open at no less than the required set pressure and no more than 110% of the required set pressure. Reseat should be done to a leak-tight condition at no less than 90% of the start-to-discharge pressure or pressure prescribed for the applicable cargo tank specification.
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