Discussion on Pressure-Relief Valves

Feb. 1, 1999
Regulations regarding pressure-relief valves have received significant changes, said Chad Betts, engineering manager at Betts Industries Inc. After August

Regulations regarding pressure-relief valves have received significant changes, said Chad Betts, engineering manager at Betts Industries Inc. After August 31, 1998, if an MC306 relief valve is replaced, it must be replaced with a relief valve that leaks less than one liter when tested. Pressure-relief devices are required to protect MC306 and DOT406 cargo tanks from over-pressurization or vacuum during normal operations, loading and unloading, and in emergency situations.

"In a rollover accident, a brief pressure spike occurs when the tank impacts the ground or other object. This pressure spike only lasts a few thousandths of a second, but because it exceeds the set pressure, the relief valve opens momentarily. Some of the lading is released because the relief valve is submerged during the rollover."

In the new DOT406 specifications, DOT reduced the allowable leakage. When the DOT406 requirements were originally adopted in 1990, DOT set some compliance deadlines for maximum allowable leakage, which over a period of years became more restrictive. In addition to the surge leakage limits imposed in the new regulations, formulas for determining the set pressure for DOT406 relief valves were introduced.

"As a result we have MC306 pressure-relief valves with a set pressure of not less than three psig and flow rated at five psig, while for DOT406 cargo tanks, we have a set pressure of 3.63 psig and a flow rating pressure of 6.25 psig," he said.

DOT requirements developed as: -Pressure-relief valves installed on or before August 31, 1995, must leak less than one gallon when tested.

-Pressure-relief valves installed after August 31, 1995, must leak less than one liter when tested.

"Therefore, any emergency pressure-relief valve installed on a DOT406 cargo tank today must leak less than one liter, regardless of when the tank was built," Betts said. MC306 requirements developed as:

-MC306 pressure-relief valves were grandfathered as long as they are main- tained in proper operating condition.

-Until August 31, 1998, MC306 relief valves could be replaced with an identical unit.

-Pressure-relief valves on MC306 cargo tanks may be replaced with relief valves conforming to DOT406 specifications. Note that you do not have to change specification plates on the tank or make any other changes to the cargo tank in order to make this change.

"You never have to replace a properly functioning pressure-relief valve on an MC306 cargo tank," he said. "In addition, faulty relief valves may be repaired and returned to service as long as they are tested and operating properly before being returned to service. This has been confirmed in a recent interpretation from DOT."

The regulations clearly state that at any time, relief valves on MC306 cargo tanks may be replaced with DOT406 relief valves without making any other changes to the cargo tank or specification plate, he said. "This has raised concerns with some people in the industry for several reasons," he said.

Set pressures and flow rating pressure are not the same for MC306 and DOT406 pressure-relief valves because MC306 valves are set to open at not less than three psig and flow rated at five psig. DOT406 relief valves are set at 3.63 and 4.55, respectively, and are rated at 6.25 psig. Putting a DOT406 relief valve on an MC306 cargo tank could subject the tank to higher internal pressures under an emergency situation.

"Based on the discussions I have had, tank builders are not overly concerned about the slight increase in pressure," he said. "Keep in mind that if the tank is operated properly, the emergency vent will never be actuated unless the tank is involved in a fire or accident. I have never heard anyone worry that the extra half psig pressure would result in a catastrophic failure. As far as I know at this point, none of the relief-valve manufacturers are offering a one-liter surge leakage relief valve with a three-psig pressure setting. It certainly can be done, but the benefits may not be significant due to the slight set pressure differences."

Another recent concern focuses on the height of the DOT406 relief valves relative to the MC306 valves. DOT406 relief valves are higher than the common MC306 relief valves.

"In our case, the DOT406 design is about a half inch higher than the older PAF9000, which is very common in the industry," he said. "So if you replace a PAF9000 with the new DOT406 design, you must ensure that the overturn protection is higher than the relief valve and manhole. At this point, I do not know for sure if any cargo tanks will have a problem due to the increased height, but you need to be aware of the potential problem. It is technically possible to produce a lower profile DOT406 style relief valve, but there will be a trade-off in terms of flow rating.

"Always keep in mind, however, that you do not have to replace an MC306 relief valve if it can be repaired," Betts said.

Pressure-relief valve and manhole requirements have been standardized in DOT regulations since 1990, he added. "It is fairly simple to determine what the requirements are," he said.

The requirements for MC306 and DOT406 cargo tank manhole covers are basically the same. All DOT406 manholes must be structurally capable of withstanding a hydrostatic test pressure of at least 36 psig. MC306 tanks certified after December 30, 1990, must meet the same pressure test. MC306 tanks certified before that date may have original manholes if they meet the 36 psig test. Otherwise, new ones must be installed.

"Manholes must be marked with the manufacturer name, test pressure, and a statement certifying compliance," he said. "This marking is required on all manholes installed after December 30, 1990, for both MC306 and DOT406 tanks. However, those certified prior to that date are exempt from the marking requirement as long as the carrier has written certification from the manhole manufacturer, or through inhouse testing that certifies the manholes comply with structural requirements."

Manholes and relief valves need to be properly maintained in order to ensure that they can properly protect the integrity of the cargo tank and its contents. Regulations require periodic testing of the cargo tank and its components, and manhole/relief-valve manufacturers can provide information and manuals to assist this process.

Rules require tanks to be visually inspected and leak tested at 80% of the tank design pressure of MAWP annually. "For the manhole and relief valves, look for evidence of seeping gaskets, corrosion or other problems, and fix them if they are found," he said. "Betts recommends a visual examination of manholes and pressure-relief valves monthly."

Requirements call for all relief valves to be bench tested as part of the five-year pressure test. This test is important to verify that the relief valves are operational.

Vapor recovery equipment may get even more attention in the near future. The Environmental Protection Agency (EPA) has been applying pressure to all groups involved in the manufacturing, storage, distribution, and transportation of organic chemicals in an effort to reduce air pollution, said Michael Girard, southern region sales representative for Girard Equipment Inc.

"The EPA, with cooperation from the Chemical Manufacturers Association (CMA), has a goal to reduce and recover harmful air pollutants that are the result of handling activities," he said. "In the Kyoto, Japan, environmental summit, the United States pledged, along with most other industrialized nations, to reduce greenhouse gases that are believed to be causing a disruption in the climatic conditions over most of the world. Fugitive emissions from volatile organic compounds, which are just one of the components of these greenhouse gases, are on the hit list."

Organic chemicals and carbon-based substances develop a vapor pressure when confined in a storage vessel. Unconfined, they evaporate and float into the atmosphere.

"The goal of the EPA is for all transporters of organic chemicals to transfer these liquids to and from storage with the 'Best Available Control Technology' in terms of on-board vapor recovery equipment," Girard said. "The goal is to have the equipment in place by the year 2000.

"We don't need a repeat of the confusion that occurred in the early 1970s when petroleum transporters were forced to install vapor recovery on their transport equipment."

In Texas, the state's Natural Resource Conservation Commission (TNRCC) oversees EPA regulations. Some facilities there are in what is called an EPA "non-attainment zone," which means the facilities fail to attain the Clean Air goals mandated by EPA regulators.

The TNRCC has authority to grant or withhold permits to facilities that transfer organic chemicals. Facilities must prove compliance with the emission standards and are subject to a review board. There is intense pressure to comply with the mandates. Not only would a facility be forced to suspend activity, but the region may also be in jeopardy of losing crucial federal funds.

"Clearly, there is also pressure within a chemical company to avoid negative media coverage and maintain a clean image, both to shareholders and their CMA brethren," Girard said.

"Many tank truck carriers, as a condition of doing business with proactive CMA members, are installing vapor recovery equipment," he said. "Some are ordering new trailers with factory-installed equipment."

Girard posed some questions to prompt consideration for issues that might arise as a result of vapor recovery equipment use:

-What are the challenges facing the cleaning rack people with this equipment? Can the vapor recovery equipment be cleaned of all residue from the former product so as not to contaminate or react with the next liquid that will be hauled? Can the equipment be taken apart easily and reassembled by regular cleaning rack people?

-What danger is posed from static electricity?

-What about steam tracing of the vapor line? It may be a requirement of some shippers.

"The loading rack needs to supply some sort of secondary overfill prevention device. It won't be possible to see the rising liquid in the tank. When the dome cover is opened, doesn't that present vapor release? I don't care how fast the lid is opened and closed, some vapor is going to be released."

-Will all trailers need vapor recovery?

"Well, I guess if both your front haul and your back hauls are not organic chemicals, you might be OK," he said. "But how many of you can afford to have drivers and equipment idle while waiting for inorganic loads to return? Trailers without the vapor recovery equipment become hot potatoes. None of your terminal operators will want them."