A PANEL of experts from the Truck Trailer Manufacturers Association (TTMA) addressed a range of cargo tank issues during the first full day of the 2014 Tank Truck Week in Houston, Texas. Panelists discussed baffled tanks, aluminum tank coatings, cargo tank suck-in, and pressure-relief vents.
Baffles can reduce the effect of a surge load or slosh load, but they also mean the tank can’t be cleaned from the center, there’s the potential to mix incompatible products that create corrosion, and they add additional weight to the tank, according to Steve McWilliams, chief engineer for The Walker Group.
In “Pros and Cons of Baffles in Tanks,” he defined a baffle using 178.345-1: “a non-liquid-tight transverse partition device that deflects, checks, or regulates fluid motion in a tank.”
McWilliams showed a short cargo tank rollover prevention video that described how cargo tank units have a high center of gravity. As the truck starts to lean and the center of gravity shifts toward the outside, the liquid in the tank moves sideways—sloshing. If it happens too suddenly and strongly, it can roll the vehicle. If the brakes are applied, liquid will surge forward, and that also can also cause problems.
He cited a number of regulatory codes that deal with baffles:
• 178.345-7, Circumferential reinforcements: (a) A cargo tank with a shell thickness of less than 3/8 inch must be circumferentially reinforced with bulkheads, baffles, ring stiffeners, or any combination thereof, in addition to the cargo tank heads. (c) When a baffle or baffle attachment ring is used as a circumferential reinforcement member, it must produce structural integrity at least equal to that prescribed in 178.345-3 and must be circumferentially welded to the cargo tank shell. The welded portion may not be less than 50 percent of the total circumference of the cargo tank and the length of any unwelded space on the joint may not exceed 40 times the shell thickness unless reinforced external to the cargo tank.
• NFPA 1901, Water tanks for fire equipment: This standard defines the requirements for new automotive fire apparatus and trailers designed to be used under emergency conditions to transport personnel and equipment and to support the suppression of fires and mitigation of other hazardous situations. In Chapter 18, it states that there must be “at least one baffle running longitudinal to the axis of the apparatus on all water tanks and at least one transverse baffle in tanks of 100 gallons or more.”
• International Maritime Dangerous Goods Code: For hazmat tank containers, a transverse baffle, also known as surge plate, is required.
The issue of coatings for aluminum tanks was discussed by Denis Grossen, vice-president of engineering for Polar Tank Trailer, who pointed out that TTMA is immersed in an initiative to develop a Recommended Practice (RP) to cover basic performance criteria for lining materials for a DOT407 tank trailer, and minimum criteria for lining contractors/repair facilities. Grossen said the goal is to complete the RP draft early this year.
“We are relying heavily on test data information that is in a confidential domain between lining manufacturers and trailer manufacturers, and hope it will become public, and once it’s public we can use it,” he said.
Grossen said the energy boom has created the need, and it’s widely spread between North Dakota (Bakken), Utah, Texas (Eagle Ford, Barnett), Pennsylvania (Marcellus), Louisiana (Haynesville-Bossier), and Canada (tar sands).
“The problem is that crude oil characteristics vary from area to area, and problems with transporting it vary from area to area,” he said. “There’s varying crude oil quality: sweet versus sour crude, based on sulfur content; and heavy versus light, based on API gravity.
“There’s still a lot of work being done in the lining of containers for crude, but the problem was everything was done for steel containers. We have not been able to find anything published for aluminum trailers.”
He said the Society for Protective Coatings was founded in 1950 as the Steel Structures Painting Council, a non-profit professional society concerned with the use of coatings to protect industrial steel structures, but it is not involved with aluminum trailers.
He said information can be obtained from:
• The American Institute of Steel Construction (AISC), headquartered in Chicago, a not-for-profit technical institute and trade association established in 1921 to serve the structural steel design community and construction industry in the United States. AISC’s mission is to make structural steel the material of choice by being the leader in structural-steel-related technical and market-building activities, including: specification and code development, research, education, technical assistance, quality certification, standardization, and market development.
• NACE International, which was established in 1943 by 11 corrosion engineers from the pipeline industry as the “National Association of Corrosion Engineers.” The founding engineers were originally part of a regional group formed in the 1930s when the study of cathodic protection was introduced. Since then, NACE International has become the global leader in developing corrosion prevention and control standards, certification and education.
• American Society for Testing and Materials (ASTM), which was formed in 1898 by chemists and engineers from the Pennsylvania Railroad. At the time of its establishment, the organization was known as the American Section of the International Association for Testing and Materials. Charles B. Dudley, Ph.D., a chemist with the Pennsylvania Railroad, was the driving force behind the formation of the Society. In 2001, the Society became known as ASTM International.
Jack Rademacher, chief engineer, The Walker Group, reviewed cargo tank vacuum damage, more often referred to as tank suck-in. “If you want to see what it looks like when a tank trailer vacuum collapse is orchestrated in a controlled demonstration, go to youtube.com, Google “Wabash National” and click on the second entry,” he said.
It was staged as the concluding event of the 2014 Walker Group Expo.
Why destroy a cargo tank by vacuum implosion?
• Verification of engineering structural integrity vacuum design calculations and FEA.
• Educate tank operators to potential catastrophic damage caused when a cargo tank is not properly vented during unload.
• Educate the industry to the importance of proper training.
The tank was a 22-year-old Brenner model with little pitting inside. It was 637/8 inches in diameter, with 0.103” minimum thickness and 54” maximum ring space.
Rademacher said that in Boyle’s Law, the pressure exerted on the vessel is inversely proportional to the volume if the temperature remains the same. Example: assume 60 gallons of vapor space at 0 psig (14.7 psia) and pumping off 100 gallons with no venting.
In the Ideal Gas Law, the pressure exerted on the vessel is proportional to temperature if volume remains the same. Example: assume an empty tank that has just been steam cleaned with an inside temperature of 200°F that is closed up tight with no available venting and cools to 30°F. Condensation could more than double the pressure.
“Where might lower vacuum pressures be a factor?” he asked. “In a single bulkhead design where you might have the possibility of reverse angles. The DOT406 single bulkheads are probably more tested and true. The old MC306 single bulkhead had issues with reversing in the past.
“Wide ring/baffle spacing could factor in. The tank we did was 54 inches, but you’re allowed to go up to 60 inches. Some non-code foodgrade cargo tank manufacturers go beyond 60 inches. So then you have even more likelihood of vacuum collapse.”
Dave Girard, vice-president of sales and manager of Midwest Region for Girard Equipment, reviewed factors related to converting pressure-relief venting on MC307 tanks to DOT407 vents. Conversion is allowed federal rules, but is not mandated.
In 49 CFR 173.33, which deals with hazardous materials in cargo tank motor vehicles, it says:
• Non-reclosing pressure relief devices are not authorized in any cargo tank except when in series with a reclosing pressure relief device. However, a cargo tank marked or certified before August 31, 1995, which is fitted with non-reclosing pressure relief devices may continue to be used in any hazardous material service for which it is authorized. The requirements in this paragraph do not apply to MC330, MC331 and MC338 cargo tanks.
• Each cargo tank motor vehicle used to transport a liquid hazardous material with a gas pad must have a pressure relief system that provides the venting capacity prescribed in 178.345-10(e) of this subchapter. The requirements in this paragraph do not apply to MC330, MC331 and MC338 cargo tanks.
• A cargo tank motor vehicle made to a specification listed in the regulation may have pressure relief devices or outlets conforming to the applicable specification to which the tank was constructed, or the pressure relief devices or outlets may be modified to meet the applicable requirement for the upgrade specification without changing the markings on the tank specification plate. The venting capacity requirements of the original DOT cargo tank specification must be met whenever a pressure relief valve is modified.
“You don’t have to change the dataplates,” Girard said. “So it’s a pretty simple proposition. We get calls all the time. People want a document to put in their file telling them it’s OK, but it’s written right in the code.”
In addition, higher integrity tanks can be substituted for lower integrity tanks. Section F of the regulation states, “An MC331 type cargo tank may be used where MC306, MC307, MC312, DOT406, DOT407 or DOT412 type cargo tanks are authorized. An MC307, MC312, DOT407 or DOT412 type cargo tank may be used where MC306 or DOT406 type cargo tanks are authorized. A higher integrity tank used instead of a specified tank must meet the same design profile (for example, an MC331 cargo tank must be lined if used in place of a lined MC312 cargo tank). ♦
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