LNG gasification station operation management

1 Basic requirements for LNG gasification station operation

The basic requirements for the operation of LNG gasification stations are:

① Prevent the leakage of LNG and gaseous natural gas to form explosive mixtures with air.

② Eliminate the basic conditions that cause combustion and explosion, and implement fire protection for the LNG process system and equipment according to the requirements of the regulations. ③Prevent overpressure and overpressure discharge of LNG equipment.

④Prevent LNG's low temperature characteristics and huge temperature difference from harming the process system and cold burns to operators.

2 LNG gasification station process system pre-cooling

 Before the LNG gasification station is completed and put into operation, liquid nitrogen should be used to dry, pre-cool, inert and passivate the equipment and process pipelines in the cryogenic system. During pre-cooling, the opening degree of the valve of the liquid nitrogen tanker is used to control the cooling rate of the pipeline or equipment ≤ 1℃/min. Every time the temperature of the pipeline or equipment drops by 20°C, stop the pre-cooling, and check the air tightness of the system and the displacement of the pipeline and equipment. After the pre-cooling is completed, the residual liquid nitrogen in the LNG storage tank is used to purify and replace the normal temperature equipment and pipelines. Finally, the liquid nitrogen in the storage tank is replaced with LNG, and the LNG can be formally filled for gas supply.

3 Operation management and safety protection of LNG gasification station

1) Pressure control of LNG cryogenic storage tank

During normal operation, the operating pressure of the LNG cryogenic storage tank must be controlled within the allowable range. The normal working pressure range of LNG storage tanks in South China is 0.3～0.7MPa. When the pressure in the tank is lower than the set value, the self-pressurizing vaporizer and self-pressurizing valve can be used to pressurize the tank. The lower limit of boost is determined by the opening pressure of the self-boosting valve, and the upper limit of boosting is determined by the automatic closing pressure of the self-boosting valve, and its value is usually about 15% higher than the set opening pressure of the self-boosting valve. For example: when LNG is used as the main source of city gas, if the opening pressure of the self-boosting valve is set to 0.6 MPa, the closing pressure of the self-boosting valve is about 0.69 MPa, and the boosting value of the storage tank is 0. ．09MPa.

 The maximum working pressure of the low-temperature storage tank is limited by the constant pressure value (front pressure) of the automatic pressure reducing valve set on the low-temperature gas-phase pipeline of the storage tank. When the maximum working pressure of the storage tank reaches the set opening value of the pressure reducing valve, the pressure reducing valve will automatically open to relieve pressure to protect the safety of the storage tank. In order to ensure that the pressure-increasing valve and the pressure-reducing valve do not interfere with each other when they work, the closing pressure of the pressure-increasing valve and the opening pressure of the pressure-reducing valve cannot overlap, and a pressure difference of more than 0.05 MPa should be ensured. Considering the manufacturing accuracy of the two valves, the appropriate pressure difference should be determined during equipment commissioning.

2) Overpressure protection of LNG storage tanks

 During storage, LNG will evaporate slowly due to the "environmental heat leakage" of the storage tank (daily static evaporation rate volume fraction ≤

0.3%), leading to a gradual increase in the pressure of the storage tank, and ultimately endangering the safety of the storage tank. In order to ensure the safe operation of the storage tank, the design adopts three-level safety protection measures of the storage tank decompression regulating valve, the pressure alarm manual release, and the safety valve jump-off to protect the storage tank from overpressure.

 The protection sequence is: when the pressure of the storage tank rises to the set opening value of the pressure reducing regulator, the pressure reducing regulator automatically opens to release gaseous natural gas; when the pressure reducing regulator fails, the pressure in the tank continues to rise and reaches the pressure alarm value , Pressure alarm, manual release and pressure relief; when the pressure reducing regulator fails and the manual release is not opened, the safety valve will trip and release the pressure to ensure the safe operation of the LNG storage tank. For an LNG storage tank with a maximum working pressure of 0.80 MPa, the design pressure is 0.84 MPa, the set opening pressure of the pressure reducing valve is 0.76 MPa, the alarm pressure of the storage tank is 0.78 MPa, and the opening pressure of the safety valve is 0. 80MPa, the discharge pressure of the safety valve is 0.88MPa.