10 Tips For LNG Storage Tanks & FGSS Design On New ...

Queseas has prepared a list of guidelines for LNG storage tanks and FGSS design (Fuel gas Supply System) for dual-fueled (DF LNG) vessels.

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Queseas has prepared a list of recommendations and guidelines for LNG storage tanks and FGSS design (Fuel gas Supply System) for dual-fueled (DF LNG) vessels.

Concept

The fundamental philosophy of a fuel gas supply system (FGSS) in an LNG fuel fuelled vessel (excluding LNG carriers) is to manage the pressure of boil-off gas from the LNG fuel tanks and supply gas fuel (warmed-up LNG) to the main engine, auxiliary engines, and boilers. These fuel gas supply systems are designed and constructed in compliance with the IGF Code and Class requirements.

FGSS Design & Equipment

The design and equipment philosophy of the gas fuel handling system depends on the choice of the dual-fueled main engine by the shipowner. The main engine may operate on the 'Diesel' cycle, requiring high-pressure gas injection (around 300 bar), or the 'Otto' cycle, requiring low-pressure gas admission (around 15 bar). Dual-fueled auxiliary engines and boilers typically require low-pressure gas admission (less than 5 bar), and auxiliary boilers may also consume boil-off gas at very low pressures in free flow from the LNG fuel tanks.

The fuel gas supply system typically includes the following equipment:

• Glycol water system: A closed-loop system with a steam heater and two supply pumps that circulate glycol water solution as a heating medium in the high and low-pressure vaporizers.
• High-pressure and low-pressure vaporizers: Heat exchangers that warm up the LNG using glycol water solution, as needed.
• Low-pressure LNG fuel supply pumps: Usually submerged in the LNG fuel tanks, these pumps are used to supply LNG at low pressure to the fuel handling equipment or low-pressure gas consumers such as auxiliary engines or boilers.
• High-pressure LNG fuel supply pumps: Used to pressurize the LNG up to 300 bar in case of a main engine operating on the 'Diesel' cycle with high-pressure gas injection.
• Boil-off gas compressor: Used to consume boil-off gas in the main engine and auxiliaries. The boil-off gas compressor can operate in high-pressure and low-pressure modes.
• Centralized control system.
• Safety devices and automation in compliance with the IGF Code.
• Nitrogen generators for nitrogen purging (inerting) of the LNG fuel tanks, components, and piping in order to reach a safe gas concentration when the fuel gas supply is stopped

The above-mentioned equipment is typically housed in an independent fuel gas preparation room (except for the nitrogen generators) which should be designed and constructed in accordance with the IGF Code and Class requirements.

Everything you need to know about a Fuel Gas Supply System (FGSS): What? Why? How it works? Components and Design

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Guidelines for LNG Storage & FGSS Design

A list of recommendations for LNG storage tank and FGSS design can be summarized (but not limited) to the following:

1. Temperature Monitoring at Various Levels within LNG Storage Tanks: Install multiple temperature sensors within the tank to monitor temperature changes at different loading levels. This helps detect temperature discrepancies under various operational conditions.

2. Ensure Adequate BOG Handling Capacity: Design the fuel supply system to have sufficient capacity to handle both design and operational BOG. Operational BOG depends on the operating conditions of the vessel and may thus vary significantly. Neglecting operational BOG can lead to insufficient BOG handling capacity, increasing the probability of BOG dumping or venting.

3. Top Filling Line for Pressurized LNG Storage Tanks : Ensure the installation of a top filling line on pressurized LNG storage tanks to facilitate better fuel mixing and prevent LNG stratification. This assists in maintaining an optimal loading flow rate and helps control Boil-Off Gas (BOG) during bunkering.

4. Holding Time and Pressure Accumulation inside the LNG storage tanks : Ensure the LNG storage tank's holding time meets the IGF requirements (15 days) and provides sufficient pressure accumulation capacity to cover realistic operational conditions. Consider redundancy and alternative means of pressure management to prevent unwanted emergency venting.

5. Run Realistic Holding Time Calculations for Different Scenarios: Understand that operating holding times can significantly differ from theoretical calculations due to LNG ageing, the liquid level in the tank, etc. Designers and vessel operators should collaboratively define conditions for different scenarios early in the design process.

6. Redundancy in BOG Pressure Management Equipment : Implement at least two fully independent active systems of BOG pressure control, especially for tanks with limited pressure accumulation capability, to ensure availability and reliability in managing BOG.

7. Free Flow BOG to Consumers: Utilize free flow BOG systems for efficient handling, especially in simpler systems or as a backup in more complex ones. Free flow of BOG means feeding consumers with excess gas by simply using the pressure difference between the tank and the consumer. Ensure the machinery operates in the full gas system flow and consider the installation of heat exchangers for the BOG temperature increase for consumption.

8. Emergency BOG Line: Install a dedicated emergency BOG line for specific tank types to maintain BOG consumption and control tank pressure in emergency scenarios, as required by IGF standards (paragraph 6.9.1.1.)

9. Select Appropriate Pressure Management Means: Choose active pressure management means for consumers based on their intended use, considering the entire pressure range, gas flow rate, and nitrogen content.

10. Crew-Operated Tank Pressure Control Valves: Install tank pressure control valves that can be controlled by the crew in emergency venting vents to prevent their uncontrolled opening/venting.

Read the IGF Code and SGMF Guidelines Carefully: Familiarize yourself with the IGF Code and the latest SGMF Guidelines on LNG Fuel Tanks ' Pressure and Temperature Management Strategies for Gas Fueled Vessels.

Source: Queseas

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If you are looking for more details, kindly visit lng storage tanks.

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  Queseas is an online platform that provides maritime professionals with a structured and reliable source of information and knowledge.

Liquefied natural gas (LNG) storage tanks are critical components in the natural gas supply chain. They present unique engineering challenges due to the extremely low temperatures (-160°C to -196°C) at which LNG is stored. Designing LNG storage tanks involves a comprehensive understanding of various factors including safety, international standards, materials selection, engineering features, and operational considerations. Here's a detailed breakdown of LNG storage tank design, incorporating the aspects you mentioned:

International Standards and Regulations:

• American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) Section VIII-2: This sets the standards for the design, fabrication, inspection, and testing of cryogenic pressure vessels, for low-temperature service, including LNG storage tanks.
• National Fire Protection Association (NFPA) 59A: This standard focuses on the production, storage, and handling of LNG, including  tank siting, construction, fire protection and emergency response procedures and operation for storage facilities.
• International Organization for Standardization (ISO) : This standard specifies requirements for low-temperature,  cryogenic  insulated  storage tanks used for the storage of refrigerated, liquefied gases, including LNG.
• API 620 and 625: American Petroleum Institute (API) standards are widely used in LNG storage tank design. API 620 covers design and construction of large, welded, low-pressure storage tanks while API 625 focuses specifically on LNG tanks.This standard provides guidelines for the design, construction, operation, maintenance, and inspection of aboveground, cylindrical, flat-bottom, vertical storage tanks for liquefied petroleum gas (LPG). (Note: While API 625 is for LPG, some aspects apply to LNG tanks as well).

Safety Features:

• Double-Walled Design:  LNG tanks are typically double-walled with a pre-stressed concrete outer tank and a nickel steel inner tank. The space between the walls is filled with insulating materials like perlite to minimize boil-off and thermal stresses. These tanks consist of two concentric shells - an inner steel tank containing the LNG and an outer concrete or steel shell. The space between the shells is filled with thermal insulation to minimize boil-off and maintain the LNG's low temperature.
• High-Nickel Steel: The inner tank is typically constructed from 9% nickel steel. This special steel retains adequate ductility (resistance to fracture) at cryogenic temperatures, preventing brittle failure.
• Thermal Insulation: A thick layer of insulation material (e.g., perlite, polyurethane foam) fills the space between the inner and outer tanks. This minimizes heat transfer from the environment, reducing LNG boil-off and vaporization losses.
• Impounding Area: A secondary containment structure surrounds the outer tank. In case of a leak from the inner tank, this area captures the spilled LNG, preventing environmental contamination. 
• Pressure Relief Systems: Safety relief valves are incorporated into the tank design. These valves vent excess pressure buildup in the tank due to boil-off or other emergencies, preventing tank rupture.  Pressure relief valves (PRVs) are installed on the inner tank to vent excess pressure in case of emergencies, preventing tank rupture.
• Fire Protection Systems: LNG storage facilities typically have comprehensive fire protection systems, including water spray systems, firewalls, and fire hydrants, to mitigate fire risks.  Water spray systems and firewalls are implemented to mitigate fire hazards.

• Leak Detection Systems: Continuous monitoring systems using pressure, temperature, and level sensors are employed to detect even minor leaks.

Material Selection:

• Inner Tank: 9% nickel steel is the preferred material for its low-temperature ductility. In some cases, austenitic stainless steel may also be used.  9% nickel steel is the most common material for the inner tank due to its excellent low-temperature toughness and ductility. Austenitic stainless steels are also used in some cases.
• Outer Tank:  Pre-stressed concrete is preferred for the outer tank due to its strength, durability, and fire resistance. Concrete offers good insulation properties and fire resistance, while steel provides structural strength.
• Piping and Equipment: All materials used in piping and equipment that come into contact with LNG must be compatible with cryogenic temperatures and resistant to embrittlement.   Materials compatible with LNG's low temperature and resistant to embrittlement are chosen. These include austenitic stainless steels, special nickel alloys, and some aluminum alloys.

Other Design Considerations:

• Tank Size and Capacity: LNG storage tanks come in various sizes, ranging from small tanks for peak shaving to large facilities for long-term storage. The size is determined by factors like consumption needs, import/export volumes, and available space.
• Foundation Design: The foundation must be able to support the weight of the filled tank and withstand seismic loads. Soil properties and potential for ground movement are crucial factors in foundation design.
• Hydrostatic Testing: Before commissioning, the tank undergoes rigorous testing to ensure its leak-proof integrity. This involves filling the tank with water and pressurizing it to a specific level.
• LNG Tank Operation and Maintenance: Strict operational procedures are followed to ensure safe and efficient LNG storage. Regular maintenance is crucial to detect and address any potential issues before they escalate.

• Thermal Insulation: A crucial aspect to minimize boil-off losses and maintain LNG in its liquid state. Multi-layer insulation systems with perlite, cellular glass, and fiberglass are commonly used.
• Hydrostatic Testing: Tanks undergo rigorous hydrostatic pressure testing to ensure their structural integrity before operation.
• Seismic Design: In earthquake-prone areas, tanks are designed to withstand seismic loads.
• Risk Management: Comprehensive risk assessments are conducted to identify potential hazards and implement mitigation strategies.
• Training and Operation: Personnel responsible for operating and maintaining LNG storage tanks undergo specialized training to ensure safe and efficient operation.

Additional Points:

• Size and Capacity: LNG storage tanks come in various sizes, with capacities ranging from tens of thousands to hundreds of thousands of cubic meters.
• Types: There are two main types of LNG storage tanks: aboveground and full-containment. Aboveground tanks are more common, while full-containment tanks offer an extra layer of safety by capturing any potential leaks within the outer structure.

Engineering Design Features Ensuring Safety:

• Double Containment: Many LNG storage tanks feature double containment systems to prevent leakage and enhance safety.
• Heat Insulation: Thick layers of insulation are applied to prevent heat ingress, maintaining LNG at cryogenic temperatures.
• Roof Design: Tanks often have a domed or cone-shaped roof to accommodate thermal expansion and contraction.
• Emergency Shutdown Systems: Automated systems are installed to shut down operations in case of emergencies.
• Vapor Recovery Systems: To minimize emissions and ensure safety, vapor recovery systems capture and process LNG vapors.

Operational Considerations:

• Hydrostatic Testing: Tanks undergo rigorous hydrostatic testing to ensure structural integrity.
• Cryogenic Temperature Management: Specialized materials and design considerations are employed to withstand extremely low temperatures.
• Risk Assessment: Comprehensive risk assessments are conducted to identify potential hazards and mitigate risks.
• Training: Personnel receive extensive training on safety protocols, emergency procedures, and operational guidelines.
• Regular Inspections: Scheduled inspections and maintenance are crucial to ensure ongoing safety and operational efficiency.

Understanding these design aspects is essential for ensuring the safe, reliable, and efficient storage of LNG.

Conclusion: Designing LNG storage tanks requires adherence to stringent international standards, meticulous engineering, and careful consideration of safety, materials, and operational factors. By incorporating robust design features, selecting appropriate materials, and implementing rigorous safety protocols, LNG storage facilities can operate safely and efficiently, mitigating risks associated with handling liquefied natural gas.

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