LNG cryogenic storage tank protection system

In the normal production process of LNG storage tanks, the process parameters such as liquid level, pressure, and temperature are mainly controlled. The computer control system is used to monitor whether the process parameters are within a reasonable range in the control room. The LNG cryogenic storage tank liquid level is the main parameter controlled by the LNG plant. The LNG injection into the storage tank and the loading pump and the external delivery of LNG can cause the storage tank liquid level to change. LNG storage tanks will generate flash vapor (BoilOffGas, BOG), which is recovered by the compressor to control the pressure of the storage tank. Temperature parameters belong to the auxiliary parameters of LNG cryogenic storage tanks, which are mainly used to assist the analysis of LNG evaporation and leakage of storage tanks.

The above parameters are controlled within a reasonable range. When the control range is exceeded, corresponding protective measures should be taken to prevent the parameters from continuing to exceed the alarm range and causing accidents. LNG storage tanks are usually equipped with the following protection systems:

1) Liquid level protection system. It is used by the operator to monitor the liquid level of the pressure vessel, understand the liquid level of the storage tank in time and calculate the LNG storage capacity, to prevent the liquid overflow of the tank and the LNG loading pump from evacuating. 2) Pressure protection system. The design pressure of the LNG storage tank is generally -0.6 ~ 14kPa (part of the design pressure of the storage tank reaches 20kPa). The BOG gas cannot be discharged in time, and the cryogenic storage tank may be overpressured; otherwise, if the compressor pumping capacity is too large, the storage tank will experience negative pressure. Both overpressure and negative pressure will cause serious threats to the storage tank and even cause explosion and other dangers. Therefore, a pressure protection system must be set up.

3) Leak alarm system. Multi-point thermometers with uniform distribution are set in the annular zone at the bottom of the pressure vessel sandwich, the inner wall of the outer tank, and the ceiling space. The internal tank can promptly alert and take corresponding measures after the LNG leaks from the inner tank.

4) Others should also be equipped with flammable gas leak detection and flame detection alarm systems and automatic fire protection systems to protect the normal operation of pressure vessels.