Liquefied natural gas station (LNG Station) and LNG storage tank will be under the supervision of DOEB: Department of Energy Business, Ministry of Energy. The engineering design, construction, installation, and safety must be strictly carried out under ministerial regulations and NFPA 59A.
Liquefied natural gas storage tank
LNG storage tank has its special function which stores liquid at very low temperatures -196 °C or LNG at -162 °C. The tank is designed by double-layered containers including inner containing LNG and outer vessels. Between Inner and Outer vessels, there is an annular space which is a vacuum working as an insulator. This vacuum layer is to protect heat transfer from outside. The tank is designed in both vertical and horizontal tank according to ASME SECTION VIII DIVISION1 as following.
ASME SECTION VIII (PRESSURE VESSELS), Division 1
Division 1 provides requirements applicable to the design, fabrication, inspection, testing, and certification of pressure vessels operating at either internal or external pressures exceeding 15 Psig. Such vessels may fire or unfire. This pressure may obtain from an external source or by the application of heat from a direct or indirect source, or any combination thereof. Specific requirements apply to several classes of material used in pressure vessel construction, and to fabrication methods such as welding, forging, and brazing. Division 1 contains mandatory and non-mandatory appendices detailing supplementary design criteria, nondestructive examination, and inspection acceptance standards. Rules pertaining to the use of the single ASME certification mark with the U, UM and UV designators are also included
Application of LNG storage tank
1. Filling circuit
The cycle of filling LNG into the tank begins with LNG passing a check valve through a separate pipe into Top Filling valve and Bottom Filling valve. These valves control LNG flow to regulate the pressure during filling
1.1 LNG flows into the upper of LNG tank which has compressed gas (Compressed Gas). Therefore, LNG with a lower temperature combines with compressed gas. As a result, gas pressure in the top of tank is lower.
1.2 LNG also flows into the lower of LNG tank which is liquefied gas state. Therefore, the amount of added LNG to the tank and is constantly increasing. This will cause higher gas compression in the top of the tank.
To fill LNG and control the pressure efficiently , the amount of LNG flow must be controlled by on-off valve.
2. Pressure build-up coil
Using LNG consistently, the decreasing volume of the liquid lower the tank pressure. The principle of enhancing pressure depends on changing state of LNG from liquid to gas. With flowing through pressure build-up unit, the liquid will be changed to gas phase. Then, it returns to the upper tank to compensate lost pressure. Another important equipment is Regulator which controls on-off valve relying on the set pressure. When the tank pressure drops lower than the set point, the regulator will turn on the LNG flow. In contrast, the tank pressure increases higher than the set point, the regulator will narrow the LNG flow.
To sum up , the heat exchanger (Pressure Build-up coil) must be designed appropriately.
3. Pressure and Level Gauge
Monitoring tank pressure and volume are critical to its usage. Therefore, this device will have the principle as below
The mostly used pressure gauge is the Bourdon type to measure the pressure directly from the cylinder head which has different sizes and measurement ranges according to each type of tank and capacity.
Liquid level gauge mostly useห in differential pressure type which applies the pressure difference between two points to measure the level of LNG tank. One is from the upper of the tank. Also, the other is from the bottom of the tank. This pressure difference (Delta P) converts to be in units of height, such as millimeters of water (mmH2O) or Inch H2O to represent the height of the volume of LNG tank. In order to find the LNG quantity, there is a content chart to convert the height to weight.
4. Pressure Relief Valve
LNG can vaporize due to the heat. Although it is stored in the tank, it can vaporize as well, called as Normal Evaporation. Moreover, if the gas phase rises, the pressure will also gather. Unless it reduces pressure from other factors such as application or pressure release, the excessive pressure will reach to set point of the pressure safety valve. The safety valve will open to relieve the pressure to the set point.
This safety valve will calculate the relief rate properly for the tank and usually have 2 sets (1 set = 2 each) to switch operation and to calibrate.
This process has a major device which is the Back-Pressure Regulator (BRP) or called Economizer. The economizer is set following the pressure outlet at the tank header. When the pressure reaches to the set point, the vapor will flow to liquefied gas circuit. Thus, the excessive pressure from economizer will be sent to customers instead of releasing out from the safety valve.
6. LNG supply circuit (Supply Circuit)
There are two types of LNG supply.
- Direct liquid application which will pass through the on-off valve and flow directly to the next process, e.g. Vaporizer.
- Applications connected to the Economizer system shall be applied for process that requires gas phase.