Tie Crane Operator Qualification

Correct: According to NFPA 30, the minimum distance between two adjacent aboveground storage tanks containing stable liquids is determined based on the diameter of the larger tank. This standard ensures sufficient space to prevent fire spread and allow for emergency response activities between the structures, with specific ratios applied based on the tank type and protection measures.

Incorrect: Using the maximum height of the taller tank is an incorrect metric because fire codes prioritize horizontal separation based on the footprint of the hazard rather than vertical elevation. Focusing on the specific gravity of the product in the smaller tank fails to address the primary risk factor, which is the volatility and volume of the larger hazard. Calculating the total surface area of the dike is relevant for spill containment capacity but does not dictate the required shell-to-shell separation distance between individual tanks.

Takeaway: NFPA 30 determines minimum tank spacing for stable liquids based on the diameter of the larger adjacent tank to mitigate fire spread risk.

Correct: In anhydrous ammonia service, carbon steel is highly susceptible to Stress Corrosion Cracking (SCC), particularly in the presence of high residual stresses from welding. Industry standards and API RP 571 specify that SCC can be effectively mitigated by performing post-weld heat treatment (PWHT) to reduce these residual stresses or by ensuring the ammonia contains at least 0.2% water, which acts as a chemical inhibitor against the cracking mechanism.

Incorrect: Focusing only on increasing shell thickness fails to address the high residual tensile stresses at the welds that trigger cracking regardless of the plate thickness. The strategy of using internal cathodic protection is generally ineffective for preventing SCC in ammonia and is typically reserved for bottom-side soil corrosion or aqueous environments. Choosing to apply internal linings may prevent general corrosion but does not eliminate the underlying susceptibility of the steel to cracking if the coating develops holidays or fails to provide a perfect barrier.

Takeaway: Stress Corrosion Cracking in anhydrous ammonia is primarily mitigated through post-weld heat treatment or the addition of water as an inhibitor.

Correct: According to API 651, the 100 mV polarization shift criterion is a recognized alternative to the -850 mV criterion. This method involves measuring the difference between the ‘instant off’ potential and the fully depolarized potential of the structure. If the shift is at least 100 mV, the tank bottom is considered to have adequate cathodic protection regardless of the absolute potential value.

Incorrect: The strategy of simply increasing rectifier output to reach a specific ‘on’ potential ignores the effects of IR drop and risks overprotection, which can lead to coating disbondment or hydrogen embrittlement. Opting for silver/silver chloride electrodes is technically incorrect for this scenario as those cells are primarily used in seawater environments rather than the soil conditions typical of tank farms. Choosing to install supplemental anodes is a remedial action that should only be taken after a deficiency is confirmed; it does not serve as a verification method for existing protection levels.

Takeaway: The 100 mV polarization shift is a valid alternative criterion for verifying cathodic protection when the -850 mV potential cannot be met.

Correct: API 653 Section 9 requires that all new materials used for repairs, alterations, or reconstructions must conform to the current applicable standard for new tank construction, which is typically API 650. The inspector must ensure the new material is compatible with the existing shell, particularly regarding weldability and toughness, to prevent issues like brittle fracture in colder design temperatures.

Incorrect: Attempting to match the original chemical composition exactly through custom forging is not required and does not guarantee that the material meets modern safety and toughness standards. Selecting material based solely on yield strength and thickness is insufficient because it ignores critical factors like notch toughness and compatibility with the existing heat-affected zone. Opting for historical ASTM specifications is incorrect because modern standards have superseded older material requirements to improve structural integrity and safety in the United States.

Takeaway: Replacement materials for tank repairs must meet current API 650 specifications and ensure compatibility with the existing structure.

Correct: According to API 653, new bottom plate welds must be tested for leaks using either a vacuum box and bubble solution or a tracer gas method. This is the standard industry practice to ensure the integrity of the bottom lining and prevent environmental release into the sub-grade, as lap welds cannot be effectively radiographed.

Incorrect: Relying solely on radiographic examination is technically inappropriate because lap-welded joints on tank bottoms do not allow for the effective placement of film and source required for standard radiography. The strategy of using a high-pressure pneumatic test on the interstitial space is not a recognized standard for single-bottom lap welds and lacks the specific leak-detection sensitivity of vacuum testing. Focusing only on liquid penetrant examination of weld starts and stops is insufficient as it fails to verify the leak-tightness of the entire weld length as mandated by the code.

Takeaway: API 653 requires vacuum box or tracer gas testing for all new tank bottom plate welds to ensure leak-tight integrity.

Correct: API 653 stipulates that replacement shell plate material must conform to the current edition of API 650. This requirement ensures that any new components integrated into the existing structure meet modern safety and material standards, particularly regarding notch toughness and chemical composition, which may have evolved since the original construction date.

Incorrect: Relying on the original material specification from the 1970s is incorrect because it fails to incorporate modern safety improvements and material standards found in the current code. The strategy of selecting material based solely on yield strength is insufficient as it ignores critical properties like ductility and fracture toughness required by API 650. Choosing to exempt repairs from toughness requirements based on service history is a violation of the mandatory compliance standards for structural integrity during tank alterations.

Takeaway: Replacement shell plates must comply with the current API 650 material and toughness requirements to ensure structural integrity.

Correct: According to API 650 and API 653, the tell-tale hole is designed to reveal any leakage in the interior nozzle-to-shell weld during the pneumatic test and throughout the service life of the tank. It ensures that any breach in the primary pressure boundary is immediately detectable before it can cause significant structural damage or hidden corrosion behind the reinforcement plate.

Correct: According to API 650, which is referenced by API 653 for design considerations, a frangible roof-to-shell joint is designed to fail before the shell-to-bottom joint or the shell itself fails. This safety mechanism requires the cross-sectional area of the joint (the top angle and the participating roof and shell plates) to be limited. If the joint is too strong, internal overpressure could cause a catastrophic failure at the base of the tank, leading to a massive spill, rather than simply blowing the roof off to vent the pressure.

Incorrect: Relying on double-sided seal welds at the top angle is incorrect because frangible joints typically require a single-pass weld on the outside only to facilitate the intended failure during overpressure. The strategy of adding heavy reinforcement or stiffening rings at the roof-to-shell junction is counterproductive for frangibility as it increases the joint’s strength, potentially forcing failure to occur in the shell or bottom plates. Focusing only on the roof slope or center column support ignores the primary safety mechanism of the frangible joint, which is specifically engineered through the weld and top angle geometry rather than the roof pitch.

Takeaway: A frangible roof-to-shell joint must be designed to fail preferentially to prevent catastrophic shell or bottom failure during internal overpressure.

Correct: Caustic Stress Corrosion Cracking (CSCC), also known as caustic embrittlement, occurs in carbon steel exposed to sodium hydroxide at elevated temperatures. The mechanism is driven by the combination of a corrosive environment and residual tensile stresses. Performing Post-Weld Heat Treatment (PWHT) is the standard industry practice to reduce these residual stresses to a level where cracking is unlikely to initiate or propagate.

Incorrect: Attributing the damage to hydrogen-induced cracking is inaccurate because that mechanism is typically found in sour service environments containing hydrogen sulfide rather than caustic service. The strategy of using cathodic protection is ineffective here as it targets electrochemical corrosion rather than the stress-driven cracking mechanism. Choosing to replace carbon steel with 300-series stainless steel is often counterproductive because austenitic stainless steels are highly susceptible to chloride-induced cracking and may still fail in caustic environments. Relying on internal linings and holiday testing for sulfide cracking fails to address the specific temperature and chemical concentration factors that define caustic embrittlement.

Takeaway: Mitigating Caustic Stress Corrosion Cracking requires eliminating residual welding stresses through Post-Weld Heat Treatment in susceptible temperature and concentration ranges.

Correct: According to API 653, when corrosion is identified on a tank component, the inspector must evaluate the extent of the metal loss. Performing ultrasonic thickness measurements allows the inspector to quantify the remaining metal and verify that it still meets the minimum thickness requirements (t-min) for the current service conditions and design liquid level. This data is critical for determining if repairs are necessary or if the tank can safely continue to operate until the next scheduled inspection.

Incorrect: The strategy of immediately applying a spot primer focuses on cosmetic or preventative maintenance without first assessing the structural damage that may have already occurred. Relying solely on a follow-up visual inspection in one year is insufficient because it fails to quantify the current metal loss, potentially allowing the shell to thin below safe limits. Choosing to recommend a full recoating of the entire shell may be an unnecessary and costly overreaction if the failure is localized and the remaining coating system is still performing effectively.

Takeaway: When external coating failures lead to corrosion, the inspector must quantify the metal loss using thickness measurements to ensure structural integrity.

Correct: API 620 is the standard specifically designed for large, welded, low-pressure storage tanks that operate at pressures between 2.5 psig and 15 psig.

Incorrect: Relying on API 650 is incorrect because its scope is limited to atmospheric tanks with internal pressures not exceeding 2.5 psig. The strategy of applying ASME Section VIII, Division 1 is misplaced as that code is generally reserved for pressure vessels operating above 15 psig. Focusing on ASME B31.3 is inappropriate because that standard regulates process piping rather than the construction of the tank shell.

Takeaway: API 620 governs the design of low-pressure storage tanks operating between 2.5 psig and 15 psig.

Correct: Radiographic Testing is a volumetric examination method that relies on the differential absorption of radiation. For a planar defect like a crack to be detectable, it must be oriented nearly parallel to the radiation beam so that it creates a sufficient change in the thickness of the material the beam passes through. If a crack is oriented perpendicular or at a significant angle to the beam, the change in density on the film may be too slight to be perceived by the inspector.

Incorrect: The strategy of claiming that film density requirements are insufficient is incorrect because API standards specify density ranges specifically designed to ensure adequate contrast for flaw detection. Suggesting that intensifying screens obscure cracks is a misunderstanding of the technology, as lead screens are actually used to improve image quality by reducing the effects of scattered radiation. The idea that RT is limited to surface-breaking defects is fundamentally wrong, as RT is primarily used for internal volumetric inspection, whereas surface defects are better handled by visual or magnetic particle testing.

Takeaway: Radiographic Testing effectiveness for detecting planar flaws like cracks is strictly limited by the orientation of the defect relative to the beam path.

Correct: According to API 653 and ASME Section V, which governs NDE procedures, surfaces must be prepared by removing any materials such as scale, dirt, grease, or loose coatings that could interfere with the examination. For Magnetic Particle Examination, the surface must be clean enough to allow the particles to migrate to the leakage fields and to ensure the inspector can clearly see any resulting indications without obstruction.

Incorrect: Requiring a White Metal finish for all NDE is an unnecessary over-specification that exceeds the standard requirements for most routine tank inspections. Relying solely on a solvent wipe after general cleaning is often insufficient because it does not remove mechanical barriers like tightly adhered scale or delaminated liners. The strategy of performing preparation only after a visual defect is found is incorrect because the purpose of NDE is to identify flaws that are not visible to the naked eye, which requires a clean surface from the start.

Takeaway: Effective non-destructive examination requires surfaces to be free of contaminants and loose materials that could mask or interfere with defect detection.

Correct: Acoustic Emission testing is a passive non-destructive examination method that detects transient elastic waves generated by the rapid release of energy from localized sources within a material. For a defect like a crack or active corrosion to be detected, it must be ‘active,’ meaning it must undergo some form of movement or growth. This typically requires an external stimulus or stress, such as the hydrostatic pressure change during a fill cycle, to cause the flaw to emit the detectable sound waves.

Incorrect: Relying on the replacement of product with demineralized water is unnecessary and contradicts the primary benefit of AE, which is the ability to test tanks in-service with their actual contents. The strategy of placing sensors internally via robotic crawlers is not the standard application for AE, as sensors are typically mounted on the external shell or via waveguides to monitor the entire structure. Opting for a completely static environment for 48 hours would likely fail to produce the necessary stress levels required to ‘activate’ the flaws, resulting in a lack of detectable acoustic signals from growing cracks or leaks.

Takeaway: Acoustic Emission testing requires an active stress or stimulus to trigger detectable energy releases from structural defects or leaks within the tank environment.

Correct: According to API 653 Section 7, all new materials used for shell plate replacements must conform to the current edition of API 650. This requirement ensures that the replacement material possesses the necessary notch toughness and mechanical properties to withstand the design metal temperature, thereby reducing the risk of brittle fracture.

Incorrect: Attempting to match the original 1972 metallurgy is often impractical and may result in using materials that do not meet modern safety and toughness standards. The strategy of adhering only to the original construction code is incorrect because API 653 specifically directs inspectors to modern API 650 standards for new materials. Focusing only on yield strength and thickness ignores the critical factor of material toughness, which is essential for preventing catastrophic brittle failure in cold weather conditions.

Takeaway: New shell plates must meet current API 650 material and toughness standards regardless of the tank’s original construction date or code version.

Correct: Eddy Current Testing utilizes electromagnetic induction to identify surface and near-surface discontinuities and is uniquely capable of scanning through non-conductive coatings. This allows the inspector to verify weld integrity without the time and expense associated with stripping and reapplying the epoxy lining, provided the probe is calibrated for the specific coating thickness.

Correct: Atmospheric storage tanks are designed to operate with internal pressures very close to atmospheric levels. When high-capacity pumps withdraw liquid faster than the venting system can replace the volume with air, a partial vacuum is created. This internal loading condition results in a net external pressure on the shell; because the upper shell courses are typically the thinnest, they are highly susceptible to inward buckling or collapse under even minor vacuum conditions.

Incorrect: Attributing the damage to internal vapor pressure is incorrect because positive internal pressure would result in outward bulging or tension, potentially failing the frangible roof-to-shell joint rather than causing inward buckling. Focusing on hydrostatic pressure is a common misconception, as hydrostatic head primarily affects the lower shell courses through hoop stress and would not cause the top course to buckle inward. The strategy of blaming foundation settlement is also misplaced, as settlement typically manifests as shell tilting, planar tilt, or localized depressions in the tank bottom rather than inward buckling of the upper shell rings.

Takeaway: Atmospheric tanks are extremely vulnerable to inward buckling caused by vacuum conditions when venting capacity is insufficient for liquid withdrawal rates.

Correct: Out-of-plane settlement occurs when the tank shell does not settle in a single plane, creating localized stresses and distortions in the shell plates. This condition is critical because it can lead to shell buckling, weld fractures, or floating roof binding, necessitating a detailed evaluation as described in API 653 Annex B.

Incorrect: Assuming uniform settlement is the primary concern is incorrect because it generally does not induce structural stress in the tank itself, although it may affect external piping. Focusing on rigid body tilt is misleading as this planar movement typically only impacts liquid level gauging and floating roof clearances without stressing the shell. The strategy of monitoring global subsidence is insufficient because broad soil movement does not necessarily indicate the specific differential settlement patterns that threaten the structural integrity of the tank.

Takeaway: Out-of-plane settlement is the most critical settlement type because it induces structural stresses that can lead to shell distortion or failure.

Correct: API 653 Section 4.3.2.2 provides specific allowances for widely scattered pitting. It states that such pitting can be ignored if the remaining thickness at the bottom of the pit is at least fifty percent of the minimum required shell thickness (tmin). Additionally, the sum of the pit dimensions measured along any vertical line must not exceed 2 inches within any 8-inch length of that line.

Incorrect: The strategy of averaging pit depths over a square area is incorrect because API 653 requires a linear evaluation along a vertical path to ensure vertical shell stability. Focusing only on the total percentage of surface area affected fails to account for the localized structural weakness caused by deep individual pits. Choosing to rely on the original design corrosion allowance is insufficient because API 653 evaluations are based on the calculated minimum required thickness for current service conditions rather than original design margins.

Takeaway: API 653 allows scattered pitting if depths remain above half of tmin and vertical clusters do not exceed two inches per eight-inch segment.

Correct: API 650 Section 7.2.1.1 states that when the base metal temperature is below 32 degrees Fahrenheit, the area within 3 inches of the weld must be heated to at least 50 degrees Fahrenheit.