LNG Bunkering System
In the analysis, four main scenarios were evaluated:
- A pump trip of the booster pump.
- Rapid closure (5s) of the emergency shutdown valve on the loading arm while the booster pump maintains power.
- Rapid closure (5s) of the emergency shutdown valve on the loading arm with simultaneous shutdown of the booster pump.
- The water hammer effect in the case of initiating flow in an empty LNG loading arm.
To reduce high surge pressures, caused by the closure of the emergency shutdown valve, the application of pressure relief valves was taken into account. Therefore, the valve settings and location were varied in close discussion with the client. Secondly, the closure profile of the emergency shutdown valve was analysed in order to provide a solution in which no costly additions to the system were necessary. In case of a pump trip, the presence of cavitation in the system was analysed. Several recommendations were made to reduce pressure due to a water hammer effect in the LNG arm during start-up conditions.
Based on the analyses the following may be concluded:
- Throttling of the main valve needs to be very precise to start-up controllably. Alternatively, the use of a cool-down line for the start-up should be considered.
- The emergency shutdown procedure requires that a valve is chosen that has an “equal percentage” closure curve to reduce the surge pressures.
- Additional pressure relief valves at a distance no further than 70m from the loading arm are able to keep the surge pressure peaks within the allowable value. The relief valve backpressure may be no higher than 3 barg.
- In the case the pump shuts down properly during the closure of the emergency valve, the surge pressure stays well within the allowable values without additional precautions.
- Depending on the characteristics of the specific emergency valve it may have a considerable pressure drop. Since the steady-state operating pressure of this system was set to 14 barg, the valve may therefore put a limit on the maximum flow rate due to the additional booster pump pressure needed to transport the LNG at the high desired flow rate.