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

Queseas has compiled a set of essential strategies for the effective design of LNG storage tanks and Fuel Gas Supply Systems (FGSS) specific to dual-fueled (DF LNG) vessels.

CIMC ENRIC provides professional and reliable services in this field.

This guide focuses on recommendations and best practices for the design and configuration of LNG storage tanks and FGSS.

Understanding the Concept

At the heart of a Fuel Gas Supply System (FGSS) for LNG-powered vessels (excluding LNG carriers) is the control of boil-off gas (BOG) pressure from the LNG storage tanks, providing heat-processed LNG to main engines, auxiliary engines, and boilers. The design and construction of these systems adhere strictly to the IGF Code and Class regulations.

Design Philosophy for FGSS and Equipment

The structural and equipment choices for a gas fuel handling system hinge on the type of dual-fueled main engine selected by the vessel's owner. The engine may function on the Diesel cycle requiring high-pressure gas injection (approximately 300 bar), or the Otto cycle which demands low-pressure gas supply (around 15 bar). Auxiliary engines and boilers typically utilize low-pressure gas (below 5 bar) and may directly consume boil-off gas at very low pressures from the LNG storage tanks.

Critical components of a fuel gas supply system usually consist of:

• Glycol water system: A closed-loop system featuring a steam heater and two supply pumps to circulate a glycol water solution through high and low-pressure vaporizers for heating.
• High-pressure and low-pressure vaporizers: Heat exchangers that utilize glycol water for warming LNG as required.
• Low-pressure LNG fuel supply pumps: Typically submerged in LNG storage tanks, these pumps provide low-pressure LNG to fuel handling systems or low-pressure gas users like auxiliary engines or boilers.
• High-pressure LNG fuel supply pumps: Essential for pressurizing LNG to 300 bar for main engines employing the Diesel cycle.
• Boil-off gas compressor: Designed to utilize boil-off gas in main and auxiliary engines, capable of operating in both high-pressure and low-pressure modes.
• Centralized control system.
• Safety equipment and automation compliant with IGF Code standards.
• Nitrogen generators for purging or inerting LNG tanks and piping, ensuring safe gas concentrations when fuel gas supply is halted.

This equipment is generally situated in a dedicated fuel gas preparation room (excluding nitrogen generators), constructed and designed following IGF Code and Class standards.

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Recommended Practices for LNG Storage and FGSS Design

Below is a compilation of recommendations regarding the design of LNG storage tanks and FGSS:

1. Implement Temperature Monitoring: Equip tanks with numerous temperature sensors to monitor temperature variances at different levels. This aids in identifying temperature discrepancies under various operational scenarios.

2. Guarantee Adequate BOG Management Capacity: Ensure the fuel supply system is sufficiently equipped to handle both design and operational BOG, as operational demands fluctuate based on vessel conditions.

3. Top Filling Line Installation for Pressurized LNG Storage Tanks: Incorporate a top filling mechanism to enhance fuel mixing and mitigate LNG stratification. This ensures optimal loading rates and controls boil-off gas during bunkering processes.

4. Holding Time and Pressure Management: Verify that LNG storage tank hold times meet the required IGF standards (15 days) while offering sufficient pressure management capacity. Consider redundancy to prevent unwanted emergency venting.

5. Perform Realistic Holding Time Calculations: Recognize the discrepancies that often arise between practical holding times and theoretical calculations due to factors like LNG aging and varying liquid levels.

6. Incorporate Redundant BOG Pressure Control Equipment: Utilize at least two separate systems for managing BOG pressure, particularly in tanks with limited pressure accumulation capabilities.

7. Facilitate Free Flow of BOG to Consumers: Employ free flow systems for BOG management, especially in simplified systems. Ensure machinery operates effectively within the complete gas flow system.

8. Establish an Emergency BOG Line: Set up dedicated emergency BOG lines for specific tank types to maintain BOG management during emergencies, adhering to IGF regulations.

9. Choose Suitable Pressure Management Tools: Select active pressure management apparatus based on the consumers’ intended applications, factoring in the full pressure range and gas flow rates.

10. Crew-Controlled Pressure Control Valves: Implement tank pressure control valves that crew members can regulate manually to prevent inadvertent venting during emergencies.

Familiarize yourself with the IGF Code and SGMF Guidelines concerning LNG Fuel Tanks' pressure and temperature management strategies.

Source: Queseas

See Also

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