API 650 Tank Design course equips engineers and professionals with practical expertise to design aboveground welded steel storage tanks as per API 650 standards. The program covers design philosophy, shell thickness calculation, bottom and roof design, material selection, joint efficiency, wind and seismic loads, anchorage, corrosion protection and inspection requirements. Through real-world examples and code-based calculations, learners develop the skills needed to deliver safe, economical and code-compliant tank designs for oil, gas and chemical storage applications.
API 650 Tank Design Training Interview Questions Answers - For Intermediate
1. What is the design temperature in API 650 and why is it important?
Design temperature is the maximum metal temperature that the tank components are expected to experience during operation. It is important because material strength, allowable stress, and suitability for service depend on temperature, and incorrect selection can lead to reduced strength or premature failure under operating conditions.
2. How is allowable stress defined in API 650?
Allowable stress in API 650 is the maximum stress permitted in the material during service and is derived from a fraction of the material’s specified minimum yield or tensile strength. It ensures that shell plates operate within safe limits under hydrostatic and environmental loads.
3. What is a frangible roof joint and its purpose?
A frangible roof joint is a deliberately weakened joint between the shell and the roof designed to fail preferentially in case of excessive internal pressure. Its purpose is to allow controlled release of pressure upward, reducing the risk of catastrophic shell rupture and improving overall tank safety.
4. What is the significance of minimum shell thickness in API 650?
API 650 specifies minimum shell thickness values to ensure adequate rigidity, corrosion resistance, and weldability, even when calculated thickness is lower. These limits help prevent buckling, handling damage, and excessive corrosion over the tank’s service life.
5. How does API 650 address tank settlement?
API 650 recognizes that some degree of foundation settlement is inevitable and provides guidelines for allowable settlement limits and inspection. It emphasizes proper foundation design and monitoring to prevent excessive shell distortion, bottom cracking, or roof misalignment.
6. What is the role of shell stiffener rings?
Shell stiffener rings are horizontal structural members attached to the shell to increase resistance against wind-induced buckling and external pressure. They improve shell stability for large-diameter or tall tanks exposed to high wind loads.
7. What is the difference between self-supported and column-supported roofs?
Self-supported roofs, such as cone or dome roofs, carry their loads directly to the shell without internal columns, reducing obstruction inside the tank. Column-supported roofs use internal columns to support the roof structure, which can be economical for very large diameters but may interfere with maintenance and product movement.
8. What is vacuum design in API 650 tanks?
Vacuum design considers the negative internal pressure that may develop during rapid emptying or cooling of vapors. API 650 requires tanks to be checked for vacuum conditions to prevent shell buckling and recommends proper venting systems to maintain safe internal pressure.
9. How are bottom plates designed under API 650?
Bottom plates are designed to support the weight of the stored liquid and shell loads while providing leak-tight containment. API 650 specifies minimum thickness, lap welding details, slope for drainage, and corrosion allowance, with special attention to the shell-to-bottom joint area.
10. What is the importance of venting systems in API 650 tanks?
Venting systems allow air or vapor to enter and exit the tank during filling and emptying, preventing overpressure or vacuum conditions. Proper vent design is critical to maintain structural integrity and is often coordinated with standards such as API 2000.
11. What are design liquid levels and freeboard?
Design liquid level is the maximum height of liquid used for shell design calculations, while freeboard is the space between this level and the roof or overflow point. Freeboard accommodates thermal expansion, wave action, and operational surges, reducing the risk of overflow or roof damage.
12. How does API 650 treat nozzles and openings in the shell?
Nozzles and openings are designed with reinforcement to compensate for removed shell material and maintain stress continuity. API 650 provides rules for reinforcement area, attachment welding, and spacing to ensure that openings do not weaken the shell excessively.
13. What is the role of lap-welded versus butt-welded joints?
Lap-welded joints are commonly used in bottom plates for ease of construction, while butt-welded joints are used in shell courses to provide full-strength joints. API 650 specifies joint types based on stress levels and inspection requirements to ensure structural integrity.
14. Why is radiographic or ultrasonic testing important for shell welds?
Non-destructive examination such as radiography or ultrasonic testing verifies internal weld quality by detecting defects like cracks, lack of fusion, or porosity. These inspections ensure that critical shell joints meet strength requirements and achieve the intended joint efficiency.
15. What is the importance of documentation and data sheets in API 650 projects?
Proper documentation, including design calculations, material test reports, welding procedures, inspection records, and as-built drawings, ensures traceability and regulatory compliance. It also supports future maintenance, audits, and modifications throughout the tank’s service life.
API 650 Tank Design Training Interview Questions Answers - For Advanced
1. How does API 650 treat the interaction between hydrostatic pressure and axial shell stress?
API 650 recognizes that shell plates are subjected simultaneously to circumferential tensile stress from hydrostatic liquid head and axial compressive stress from shell self-weight, roof loads, wind overturning, and seismic effects. The design approach ensures that circumferential stresses remain below allowable limits while also checking that combined axial compression does not approach elastic buckling thresholds. By enforcing minimum thickness, controlling height-to-diameter ratios, and requiring wind girders or anchorage where necessary, the standard maintains shell stability under combined stress states rather than treating each load in isolation.
2. What advanced checks are required for shell buckling under external vacuum conditions?
API 650 requires tanks to be evaluated for external pressure or vacuum that may occur during rapid emptying, cooling of vapors, or improper venting. Advanced checks involve calculating critical buckling pressure based on shell geometry, material properties, and boundary conditions, and comparing it with the maximum expected vacuum. When margins are insufficient, designers must increase shell thickness, add stiffener rings, or improve venting capacity. This ensures that the shell does not experience sudden inward collapse, which is one of the most dangerous failure modes for atmospheric tanks.
3. Explain the role of wind girders in large-diameter API 650 tanks.
Wind girders act as circumferential stiffening members that increase shell resistance to buckling under wind loads and external pressure. API 650 specifies when wind girders are required based on shell height, diameter, and thickness, and provides design rules for their section modulus and placement. In large-diameter tanks, these girders significantly enhance shell rigidity by reducing unsupported shell height, thereby preventing elastic instability and limiting shell ovalization during high wind events.
4. How does API 650 ensure roof structural adequacy under live and environmental loads?
API 650 requires roofs to be designed for self-weight, live loads from maintenance personnel, snow loads where applicable, and wind uplift forces. The roof structure must safely transfer these loads to the shell or internal columns without excessive deflection or overstress. For self-supported roofs, the standard controls roof slope, plate thickness, and rafter design, while for column-supported roofs, it addresses column stability and load distribution. This ensures roofs remain structurally sound throughout service, even under extreme weather conditions.
5. What are the advanced design implications of corrosion under insulation (CUI)?
Corrosion under insulation is a critical concern for insulated API 650 tanks, especially in humid or coastal environments. Advanced design considers increased corrosion allowance, selection of coatings compatible with insulation systems, proper insulation jacketing, and inspection access points. API 650 highlights the need for protective measures even though it does not directly govern insulation design, as CUI can significantly reduce shell thickness over time and compromise structural integrity if not addressed during design.
6. How does API 650 manage the design of shell-to-bottom joints for high-cycle service?
The shell-to-bottom joint is one of the most highly stressed and fatigue-sensitive regions of the tank. API 650 specifies joint geometry, weld details, and annular plate requirements to ensure smooth stress transfer and minimize stress concentrations. In high-cycle service, such as tanks with frequent filling and emptying, advanced practice focuses on high-quality full-penetration welds, better inspection levels, and thicker annular plates to reduce the risk of crack initiation and propagation at this critical junction.
7. Describe the advanced considerations in floating roof seal system design.
Floating roof seal systems must accommodate roof movement while maintaining vapor tightness and minimizing emissions. API 650 requires primary and sometimes secondary seals, designed to remain effective under wind-induced roof tilt, shell irregularities, and thermal expansion. Advanced considerations include seal material compatibility with stored products, resistance to weathering, and ability to tolerate shell out-of-roundness, ensuring long-term emission control and operational reliability.
8. How is foundation flexibility accounted for in API 650 tank design?
API 650 assumes that tank foundations exhibit some degree of flexibility and settlement over time. Advanced design integrates geotechnical data to predict settlement patterns and ensures that shell and bottom details can tolerate these movements without overstressing. This includes proper annular plate design, bottom lap joint orientation, and sometimes the use of flexible membrane liners. The intent is to allow controlled deformation rather than rigid restraint, which could otherwise lead to cracking.
9. Explain the treatment of uplift and compressive shell stresses in anchored tanks.
In anchored tanks, API 650 requires evaluation of shell uplift due to overturning moments from wind or seismic loads and compressive stresses on the opposite side of the shell. The design ensures that uplift does not exceed the capacity of anchor bolts and that compressive stresses remain below allowable limits to prevent shell buckling. This balanced approach ensures that anchorage enhances stability without introducing excessive localized stresses in the shell.
10. How does API 650 support high-temperature tank applications?
For elevated temperature service, API 650 requires material selection that maintains adequate strength and toughness at operating temperatures, along with adjusted allowable stresses. Advanced design also considers thermal expansion effects, roof distortion, and increased corrosion rates. Proper detailing of shell joints and appurtenances ensures that thermal strains do not lead to cracking or loss of containment over long-term high-temperature operation.
11. What challenges arise in designing very large diameter tanks exceeding 100 meters?
Very large tanks introduce challenges such as increased shell flexibility, higher susceptibility to buckling, greater settlement sensitivity, and complex seismic liquid-structure interaction. API 650 provides the framework, but advanced engineering judgment is often applied to refine shell thickness distribution, stiffener placement, anchorage systems, and foundation design. These tanks demand rigorous analysis and quality control to ensure that scale effects do not compromise safety.
12. How does API 650 address shell openings near the bottom course in seismic zones?
Openings in the lower shell courses are located in regions of highest hydrostatic and seismic stress. API 650 requires careful reinforcement design and sometimes limits nozzle size or location in such zones. Advanced practice involves placing critical nozzles higher when possible, increasing local shell thickness, and ensuring reinforcement pads effectively distribute stresses, thereby reducing the risk of crack initiation during seismic events.
13. Explain the importance of roof-to-shell attachment details in wind uplift scenarios.
Roof-to-shell attachments must resist wind uplift forces without transferring excessive loads into the shell that could cause distortion. API 650 defines attachment strength based on roof type and wind design, while also preserving frangible behavior where required. Advanced detailing balances sufficient strength for normal loads with controlled failure under extreme pressure, ensuring both operational integrity and safety relief capability.
14. How does API 650 integrate quality control into advanced tank construction?
API 650 embeds quality control through strict requirements for welding procedures, welder qualification, material traceability, dimensional tolerances, and staged inspections. Advanced projects often extend these with additional non-destructive testing, third-party inspections, and real-time documentation. This systematic control ensures that design assumptions are faithfully realized in construction, which is critical for large or high-risk tanks.
15. What is the role of engineering judgment in applying API 650 to non-standard designs?
While API 650 provides comprehensive rules, non-standard designs such as unusual geometries, special products, or extreme environments often fall beyond prescriptive limits. In such cases, experienced engineering judgment is applied to interpret intent, supplement calculations, and sometimes perform finite element analysis. This ensures that even when designs deviate from typical configurations, they still meet the fundamental objectives of safety, reliability, and regulatory compliance embedded in API 650.
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