Surge analysis in LNG bunkering systems is essential to ensure safe and efficient operations, particularly when fueling ferry ships. This analysis involves evaluating several worst-case scenarios to understand surge phenomena in a 600-meter long LNG bunkering line. The system utilizes a main pump and a booster pump to achieve flow, with connections to ships facilitated by a loading arm equipped with an emergency shutdown valve. Various solutions to mitigate surge pressures are explored, including considerations for unbalanced forces to protect the vacuum-insulated piping.
Assessment
The surge analysis focuses on four primary scenarios:
- Pump Trip of the Booster Pump: Evaluating the effects when the booster pump experiences a sudden trip.
- Rapid Closure (5s) of the Emergency Shutdown Valve with Booster Pump Active: Analyzing the surge when the emergency shutdown valve closes rapidly while the booster pump continues to run.
- Rapid Closure (5s) of the Emergency Shutdown Valve with Simultaneous Booster Pump Shutdown: Investigating the impact when both the emergency shutdown valve closes rapidly and the booster pump shuts down simultaneously.
- Water Hammer Effect During Initial Flow in an Empty LNG Loading Arm: Assessing the surge due to water hammer when initiating flow in an empty loading arm.
To mitigate high surge pressures caused by the emergency shutdown valve closure, the implementation of pressure relief valves is considered. Various valve settings and locations are discussed in close collaboration with the client. Additionally, the closure profile of the emergency shutdown valve is analyzed to find solutions that do not require costly system additions. In the event of a pump trip, the presence of cavitation within the system is examined. Recommendations are provided to reduce pressure surges due to the water hammer effect during start-up conditions.
Conclusions
The analysis yields the following conclusions:
- Throttling Precision: Precise throttling of the main valve is crucial for controlled start-up. Alternatively, a cool-down line for start-up should be considered.
- Emergency Shutdown Valve: Selecting a valve with an “equal percentage” closure curve is necessary to reduce surge pressures effectively.
- Pressure Relief Valves: Additional pressure relief valves should be installed no further than 70 meters from the loading arm to maintain surge pressure peaks within allowable limits. The relief valve backpressure should not exceed 3 barg.
- Pump Shutdown Coordination: If the pump shuts down properly during the emergency valve closure, surge pressures remain within allowable limits without additional precautions.
- Valve Pressure Drop: Depending on the emergency valve’s characteristics, it may cause a significant pressure drop. With a steady-state operating pressure of 14 barg, this valve may limit the maximum flow rate due to the additional booster pump pressure required to transport LNG at the desired high flow rate.
