Cargo Tank Maintenance Seminar Rear Underride, Tank Corrosion Are Among Topics Covered by Tank Experts
Feb 1, 1998 12:00 PM
REAR underride requirements have been mandated by the US government for most types of trailers, including tanks. That was one of many technical topics covered during the 1997 Cargo Tank Maintenance Seminar October 27-29 in Chicago, Illinois.
Pat O'Brien, E D Etnyre & Company, reviewed the new regulation, which took effect for new trailers manufactured after January 26, 1998. There is no retrofit requirement at this time.
The requirement applies to most trailers and semitrailers with a gross vehicle weight rating of 10,000 pounds or more. Among the trailers exempted from the rule are those with a wheels-back configuration in which the distance from the rear most point of the trailer to the back side of the tires is 12 inches or less.
The bumper can be fabricated from steel or aluminum but must support 11,240 pounds at three points and 22,480 pounds at two points. It must be able to absorb 4,167 foot-pounds of energy and can't deflect more than five inches. The bumper must be certified.
Maximum distance from the ground to the bottom of the bumper is 22 inches. Minimum guard height is four inches, and the guard must extend nearly the full width of the trailer. It will increase tare weight by 50 to 100 pounds and will add $130-$200 to the cost of the vehicle.
Larger tank trailer manufacturers are designing and building their own rear underride guards. Smaller builders that lack sufficient volume to justify the design and testing investment may have to turn to outside fabricators.
More Weight The extra metal in the new rear underride guards is just the latest weight challenge for tank truck operators. The industry still must operate under size and weight rules that were set 20 years ago. Dave Fellows, Heil Trailer International, suggested that it's time for a change.
"The rules give us tank trailers with high centers of gravity," he said. "Studies have shown that modest weight increases could improve the safety of tank trailers. We can get around a 26% improvement in rollover stability.
"It would not be hard to lower the center of gravity and make tanker rigs more stable. We can lower the CG with a 44-inch fifthwheel setting, wider-track axles, and offset wheels. Barrels can be made 102 inches wide, and we can adopt a flatter elliptical shape for petroleum tanks."
Shifting subjects, Fellows reminded fleet maintenance managers that tank modifications must be approved by a design-certifying engineer. Dave Cooper, Stuart Tank Sales Corp, added that accurate records must be kept when modifications are made to a specification tank.
In addition, the specification plate must be updated. Cooper said that repair operations may find it easiest to add a supplemental dataplate.
Modifications may be needed for many reasons. New hardware is required when a tank is put into a different type of service. A tank may undergo extensive repairs for serious damage that can be caused by corrosion.
Corrosion Threats Tank trailers can be damaged by at least eight types of corrosion, according to Ed Mansell, Polar Tank Trailer Inc. The tank truck industry has worked hard to combat the various forms of corrosion and has achieved considerable success. In fact, a Battelle Laboratories study found that the motor vehicle sector as a whole has made the greatest anti-corrosion effort of any single industry and has eliminated some 35% of avoidable corrosion.
"In the tank truck industry, we know most of the causes of corrosion," Mansell said. "Very few incidents are caused by phenomena that were previously unknown. Frequent causes include faulty design practices, poor loading and unloading procedures, improper selection of trailer materials, sensitized stainless steel welds, poor maintenance procedures, cross-contamination of loads through poor tank cleaning, overheating of loads, and carrying wash water heels.
"Shippers, carriers, and manufacturers do not need new technology to reduce or prevent tank corrosion. We need to make better use of the existing technology via communication. Tank corrosion often occurs for the following reasons: lack of a system of protocol that reliably generates information needed to efficiently handle chemical cargoes; inadequate operating discipline that results in variation from stated procedures; and inadequate levels of quality and technology in the design, fabrication, repair, and maintenance of cargo tanks."
Mansell detailed various forms of corrosion, starting with uniform attack. This type of corrosion normally is characterized by a chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface or over a large area.
The corrosion is relatively uncommon in cargo tanks because operators usually know what chemicals are corrosive to stainless or mild steels. Preventive measures include selecting the proper tank coatings and linings, inhibitors, or cathodic protection.
Galvanic Corrosion Galvanic corrosion can occur when two dissimilar metals are in contact with each other. Usually, the corrosion is more pronounced on the less-resistant metal, whichbecomes the sacrificial metal actually protecting the other from corroding.
Galvanic corrosion can be seen when flanged connections of dissimilar metal are bolted together. A common error is to assume that the washers under the bolt heads and nuts insulate the two parts, Mansell said. Yet, the shank of the bolt touches both flanges. The problem can be solved by sliding non-metallic tubes over the bolt shanks and washers so the bolts are completely isolated from the flanges. This eliminates the electrolyte that carries the electrical current between the two electrode areas created by the flange assembly.
Crevice corrosion refers to the rapid attack that can occur between tightly fitting surfaces, such as bushing sleeves, flanges, gaskets, and lap joints. The corrosion also can be found in areas with surface deposits and in crevices under bolt and rivet heads.
The cause of crevice corrosion is small volumes of stagnant solution between the areas exposed to the tightly fitting faces and the more accessible surface areas. The crevice is an oxygen-depleted area that is enriched in metal ions. The steel is polarized toward the active potential.
Pitting Damage Pitting is an example of localized corrosion that may proceed rapidly in a small area, while leaving most of an exposed surface unaffected. It is one of the most destructive and insidious forms of corrosion, Mansell said.
"We've seen many more incidents of pitting corrosion since the trucking industry was deregulated," he said. "Fewer dedicated tanks are in service, and chemical fleets do a lot more backhauling. We're seeing cross-contamination, inadequate cleaning, and poor control of product temperature. All of this brings a risk of pitting corrosion."
Stress-corrosion cracking can be frustrating because it can occur without prior warning. In many cases, the tank is beyond repair. Stress corrosion is the combined action of stress and corrosion that results when the tank is exposed to certain media, particularly hot solutions containing chlorides. The stress cracks are microscopic and look like tree roots.
Mansell emphasized that corrosion of all types can be prevented in cargo tanks. First of all, it is imperative to find out as much as possible about the product that will be hauled.
Operators should recognize that corrosion threats can come from many sources. For instance, carbon tetrachloride can form hydrochloric acid when moisture is present. Cargo temperatures should be kept as low as possible. High-purity steam should be used for in-transit heat, and the system should be thoroughly drained after each trip. Mansell urged carriers to avoid using trailers for storage.
"Keep in mind that many products are not corrosive to the vessel if the trailer is moving and the load is kept in circulation," Mansell said. "Corrosion is more likely when the cargo is stagnant."
Good Communication A tank buyer should work closely with the builder and develop a good line of communication. The proper materials should be selected for new tanks. All operating parameters should be known before the fabrication work begins.
"It is important to recognize that the chemical composition of 316, 316L, 304, and 304L stainless steel can be modified to promote greater resistance to pitting," Mansell said. "For example, the addition of molybdenum (over 2%) promotes pitting resistance."
Steel should be purchased, traced, and handled in accordance with ASME (American Society of Mechanical Engineers) guidelines. Lax quality control poses a risk that lesser grades of stainless steel might be supplied by the mill or distributor.
A number of manufacturing steps can make a tank more resistant to corrosion. Welds should be well spaced, and butt welds are preferable to lap welds.
Stress potential can be reduced by eliminating notches, threads, grinding areas, abrasive scratches, corners, and grooves. Generous fillets at corners, smooth contour welds without undercutting, ground and polished welds, rounded corners, ground and polished edges, and flat surfaces will eliminate causes of stress concentration.
Once the tank is completed, the interior should be thoroughly cleaned to remove all contamination, such as oxides, iron dust, weld spatter, welding flux, dirt, and organic matter. Final cleaning should be with a solution of specific acids, such as nitric acid.
Passivation Priority Passivation of new tanks should be a priority, according to Mansell. While 300-series stainless steels are self-passivating upon exposure to air and moisture (due to chromium content in excess of 11%), induced passivation has a number of advantages.
The enhanced protective oxide film achieved through induced passivation is somewhat thicker and more persistent than the layer formed by auto-passivation. In addition, nitric acid dissolves iron particles, which ensures complete formation of the protective oxide film.
It's important to recognize that passivation is not a constant state, Mansell said. It exists only in certain environments or under certain conditions. The range over which stainless steels exhibit passivity may be broad or narrow, and passivity can be destroyed by slight changes in conditions.
Tank operators should periodically repassivate tanks that are in corrosive service. This should be a key step in a detailed corrosion-prevention program.
Tanks should be inspected regularly for evidence of corrosion. Wash racks should have anti-corrosion procedures that include drying the tank after cleaning. All moisture must be removed.
Corrosion Repairs When signs of corrosion are detected, mechanics must determine the extent of the damage and whether repairs are needed. Type and degree of corrosion will help determine the method of repair.
Deep pits usually show up on or close to weld seams and on the lower half of heads. Minor pits are extremely fine and may appear as a pale dot or give the tank surface an appearance like a pin cushion. Minor pits often are difficult to see.
Linear pits are found in weld seams and usually are caused by an inherent defect in the weld. Spider cracks indicate that stress corrosion has occurred. Uniform attack can cover the entire exposed surface.
Corrosion pits can go unrepaired if the remaining thickness below the pit is above the in-service minimum, according to Mansell. However, there can be serious consequences when pits aren't repaired.
Corrosion will continue, and tank life can be significantly reduced. Unrepaired pits can contaminate loads and prevent natural passivation.
In general, pitting should be repaired as soon as possible after discovery. Thickness testing is a must to determine the extent of the damage. Another thickness test should be done when repairs are complete, and the repairs should be checked by a qualified inspector.
Pit Welding Buffing can remove minor pitting, but deep pits usually are welded over. If pitting has occurred over a large area, it may be necessary to install a new tank section. All welds should be polished smooth to help avoid future problems.
When pitting occurs on a weld, mechanics usually Vee out the existing weld material and re-weld. Depending on the depth of the pits and the quality of the existing weld, it may be necessary to weld from both sides.
Stress corrosion offers greater challenges for tank repair operations. This form of metal failure usually is beyond weld repair and sometimes beyond repair altogether, Mansell said. At best, it may be possible to remove just the damaged area if the entire tank isn't affected. After any corrosion-damage repair, the tank should be repassivated, according to Mansell. At the very least, the repaired section should be passivated.
Pressure Concerns Pressure is another factor that can affect cargo tank performance. Chad Betts, Betts Industries Inc, said that pressure factors impact tank design and operation. Understanding and respecting pressure is the key to a safe operation.
Cargo tanks are designed with a wide range of pressure capabilities. Low-pressure tanks-for example DOT406-are used to transport petroleum products and such. Pressure tanks include DOT407 and DOT412 units and are used for general chemicals and acids.All of the tanks have venting systems tha t protect against over-pressurization. The pressure-relief systems are designed to activate during loading and unloading and in the event of an emergency.
Emergency venting limits internal pressure to test levels during fire or overloading. Venting capacity is calculated from the exposed area of the tank, according to Betts. More area requires more venting capacity.
Venting for loading and unloading allows the tank to breathe. In some locations, vapor recovery must be combined with the venting. Betts reminded maintenance managers that the vents must be actuated before loading or unloading commences. Otherwise, the tank could be damaged.
Don Kilgore, Fort Vale Inc, added that tanks built to different codes have different venting requirements. This applies even to tanks in the same family, such as MC307 and DOT407 chemical units. A DOT407 vent can be used on an MC307 tank, but it must be set at MC307 performance levels.
Hydraulic Maintenance Moving on to the product handling systems, two speakers addressed the hydraulic drive systems that have become increasingly popular for use with product pump systems. Both speakers stressed the importance of regular service.
Larry Taylor, Drum Industries, said that 60% of all hydraulic system leakage is due to improper installation. Mechanics need to check the entire installation. Often, this includes a PTO-driven hydraulic pump and a hydraulic oil cooler.
"Good maintenance will extend the service life of a hydraulic system," he said. "It reduces overall maintenance cost and improves performance and reliability."
Oil cleanliness is a must, and the filler cap gasket on the oil cooler must be inspected regularly to ensure that it still functions properly. The system is designed to operate under pressure.
The radiator on the cooler should be cleaned regularly with a low-pressure spray, Taylor said. Relief valves should be checked every couple of months, and the same goes for hydraulic hoses and fittings.
Oil Changes Mike Knox, Muncie Power Products, said the most important maintenance step with a hydraulic system is to change the oil regularly. The change schedule depends on level of use. Mechanics should look at the condition of the oil. Is it cloudy? Does it have dirt in it?
Contaminants are anything other than hydraulic oil. This includes air, water, diesel fuel, motor oil, transmission fluid, dust, dirt, sand, rust, paint, metal shavings, nuts, bolts, and shop rags. "You'd be surprised how often rags used to plug the fill opening get sucked into hydraulic pumps," Knox said.
Minute contaminants can even be found in new hydraulic oil. Knox recommended that mechanics filter the oil before pouring it into the hydraulic system.
Contamination will damage the system, and the amount depends on the size of the contaminant, Knox said. Contamination can create grooves in the wear plates and in the walls of the pump body. Pump efficiency can be degraded significantly.
Quality Control Quality control is one of the keys to making any tank maintenance program a success, according to Rick Katzung, Quality Management Institute. Companies performing specification tank repairs are required to have a quality control manager and a quality control manual.
The most effective quality control programs are those where the shop management is thoroughly knowledgeable about DOT rules and code tank requirements, Katzung said. Problems result when management doesn't send the right message and doesn't provide adequate training.
Smaller shops seem more prone to quality control deficiencies, Katzung said. They don't do a lot of code repairs and lack qualified weld procedures for all types of repairs.
New requirements call for repair shops to assemble drawings or sketches of tank work that is underway. The sketches don't have to be elaborate, but they should provide a good outline of the project, Katzung said.
Quality control procedures should verify that the proper materials were ordered for a repair job. Katzung suggested that this could prevent mistakes because material suppliers aren't always as professional as they should be.
Frequently, tanks are repaired and returned to the owner without the required final inspection. Quality control managers are not inspecting all of the work that leaves a shop. Some shops are failing to document leakage or pressure testing following repairs.
>TI Cargo Tank Maintenance Seminar Girard General Manager Reviews >Closed-Loop Loading Systems
AS SHIPPERS prepare for expected closed-loop chemical unloading regulations, they should consider the safety and cost-effectiveness of wide-mouth vacuum breakers mounted on the tank, said Patrick A Remmert, general manager at Girard Equipment, Rahway, New Jersey.
Closed-loop unloading and the inherent hazards of inadequate pressure adjustment were among the topics he discussed October 28, 1997, during the 14th annual Cargo Tank Maintenance Seminar in Chicago, Illinois. Remmert also provided information on the benefit of lined vents and the use of Department of Transportation (DOT) vents on American Society of Mechanical Engineers (ASME)-code tanks.
"To ensure safety, a large emergency vacuum breaker with an opening as large or larger than the vapor recovery line must be installed on the tank. When we closed-loop unload, we must remember that most tanks are not built to withstand high vacuums," he said.
As more closed-loop unloading is utilized as a result of expected federal regulations forecast for the very near future, Remmert predicts many cargo tanks will lack the necessary equipment to contend with the high differential pressure involved in the process.
Vapor Recovery The typical vapor recovery device in use today is a two-inch full-port ball valve fitted to a three-inch clean-out nipple, he said. Another fitting is a two-inch male quick-fit with a spring-loaded poppet in the center fitted to a clean-out nipple. New hydraulically operated fittings also are available.
"As we all know, air under lower pressure requires a large flow area. You cannot use a three-fourths-inch Chicago fitting and three-fourths-inch ball valve for vapor recovery. It does not work, period. Let us not forget Murphy's Law: whatever can go wrong, will go wrong," he said.
Ordinarily, a two- to three-inch pump-off hose is connected to a truck-mounted product pump. As the liquid is offloaded into a storage tank, vapors return to the tank truck via the vapor recovery line and fitting, replacing the product that was removed.
A two-inch male quick-fit is used for connection of the vapor hose to the storage tank. The two-inch male quick-fit with a spring-loaded poppet in the center and fitted to a clean-out nipple requires opening with a special hose-mounted vapor fitting that has an internal probe. In hydraulically operated fittings, the vapor recovery valve opens at the same time as the internal valve.
Troubles arise when ball valves are not opened, vapor recovery lines are not hooked up, and fittings and hoses are clogged with product.
Specific Examples Hardware problems can create an internal vacuum that can cause serious damage to a cargo tank. Remmert reviewed some of the basics relating to vacuum pressures in cargo tanks. As an illustration, one pound per square inch absolute (psia) pressure equals two inches of mercury. If the tank can withstand four pounds per square inch (psi) pressure differential from the outside in, the vacuum breaker should open before eight inches of mercury.
To further explain the vacuum phenomenon that can crush a tank, Remmert talked about gauge pressure and absolute pressure. When a DOT407 dome lid is open, the inside and outside pressures are equal at 14.7 psia. If the dome is closed with a pressure gauge attached to the tank, the pressure on the gauge is 0 psig, meaning gauge pressure at that moment. Equal pressure is on both sides of the tank wall.
When the tank is pressurized to 25 psig, the measurement is 39.7 psia inside the tank and 14.7 psia outside the tank with a positive inside pressure of 25 psi.
"If you don't have vapor recovery lines or a large vacuum breaker, problems arise when we remove products from the tank, which expands the space available for the vapor in the tank, and the tank internal pressure drops lower than the earth's atmospheric pressure," Remmert said.
Lined Tank Benefits After Remmert discussed vacuum hazards in depth, he also touched on the importance of lined tanks in order to keep harmful products away from the shell. In addition to the tank, the vents also should be lined.
"It keeps the corrosive product away from the vent. That's where the problem begins. Rupture discs last one day, one load, or until you leave the yard. It did not take a rocket scientist to figure out that if a lined tank worked, a lined vent would work, too," he said.
The DOT regulations state that a rupture disc can be used in series with a vent, but the regulation does not say a rupture disc must be used. Adding linings to the pressure-relief vents eliminates the need for a rupture disc, he said.
"The lining is there to protect the vent from getting corroded. Over the years, I have seen vents completely eaten away by product when mounted over a rupture disc," he said. When a disc does fail, downtime for repairs can be costly," he said. "In my opinion, rupture discs are not meant for over-the-road cargo tanks. The way to eliminate costly maintenance problems is to specify lined vents with lined tanks."
DOT Vents on ASME-Code Tanks Regarding the use of DOT vents on new DOT412 code tanks, Remmert said DOT vents can be used alone. Rupture disc or ASME vents do not have to be added, despite what some ASME inspectors say.
"The DOT specification supersedes ASME," he said. "I have been receiving calls about the specification since HM 183 was adopted. If you ask DOT, you will get the same answer that DOT venting is what is required on DOT tanks and you need not install any ASME vents."
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