Liquefied Petroleum Gas (LPG) storage has evolved over the years to enhance safety, efficiency, and cost-effectiveness. Historically, spherical pressure vessels were widely used for LPG storage due to their structural advantages. However, modern design trends have shifted towards horizontal cylindrical tanks, commonly referred to as LPG bullets, particularly in underground configurations. This article explores the key pressure vessel design considerations for these storage systems and the reasons behind this transition.
Spherical Tanks: Design Advantages and Limitations
Advantages
Optimal Pressure Distribution: A spherical shape is ideal for withstanding internal pressure, as stress is uniformly distributed across the entire surface. This reduces material fatigue and allows for thinner shell walls compared to cylindrical tanks.
Minimized Surface Area: A sphere has the lowest surface area per unit volume, which helps reduce heat transfer from the environment. This is beneficial in minimizing boil-off losses and maintaining LPG in a stable liquid state.
Large-Scale Storage Efficiency: Spherical tanks are suitable for high-capacity storage, making them ideal for refineries, petrochemical plants, and distribution terminals.
Limitations
High Construction Cost: Fabrication of spherical tanks requires complex welding techniques and specialized materials, increasing manufacturing costs.
Land and Space Constraints: These tanks require large open areas and are typically elevated, demanding additional structural support.
Safety Risks in Above-Ground Placement: Exposure to external hazards like explosions, extreme weather, and mechanical impacts increases safety risks.
Underground LPG Bullet Tanks: Design Advantages and Challenges
Advantages
Enhanced Safety and Security: Being underground, these tanks are less vulnerable to external hazards, reducing explosion risks and exposure to mechanical damage. In the event of a leak, LPG is less likely to accumulate in dangerous areas, as the surrounding soil can help contain and disperse gas safely.
Efficient Land Use: Unlike spherical tanks, underground bullet tanks free up surface space for other industrial or commercial use.
Cost-Effective Construction and Maintenance: Horizontal tanks are easier and cheaper to manufacture, transport, and install compared to spherical tanks.
Temperature Stability: The natural insulation provided by soil minimizes temperature fluctuations, reducing boil-off losses and improving efficiency.
Challenges: Engineering Verification
The Engineering Equipment and Materials Users Association (EEMUA) provides best practice guidelines for underground storage tanks. EEMUA 190 outlines design, inspection, and maintenance recommendations to ensure structural integrity and operational safety. Compliance with these standards ensures that underground LPG tanks meet industry safety and environmental requirements, however applying the standard is very challenging.
While EEMUA 190 presents significant design challenges, the industry continues to persist in applying more rigorous calculations and engineering solutions for underground bullets rather than reverting to spherical tanks. This shift highlights the industry’s commitment to enhanced safety, efficiency, and regulatory compliance despite the complexities involved.
Why the Transition from Spheres to Bullets?
While spherical tanks were initially preferred due to their structural benefits, advancements in material technology, welding techniques, and safety regulations have made underground LPG bullet tanks a more practical choice. The shift has been driven by:
Lower installation and maintenance costs
Better safety and environmental compliance
Increased land-use efficiency
Conclusion
The choice between spherical and horizontal LPG tanks depends on factors such as cost, safety, storage capacity, and regulatory requirements. While spherical tanks remain relevant for large-scale storage in refineries, underground LPG bullets have emerged as the preferred solution due to their enhanced safety, cost-effectiveness, and space efficiency. Compliance with industry standards like EEMUA 190 further ensures the reliability and longevity of these underground storage solutions, even though its strict requirements pose engineering challenges.
