Self-pressurization of LNG storage tanks

It is necessary to consider what purpose the self-pressurization of your LNG storage tank serves: 1. If it is used to prevent negative pressure in the tank, then the pressure behind the control valve will be the normal operating pressure of the tank, or slightly higher than the low-pressure alarm value. 2. If it is used for exporting LNG, for example to downstream equipment or tank trucks, then it is necessary to determine the pressure required by those downstream devices. For filling tank trucks, the pressure is usually around 4–6 bar; at the same time, the design pressure of the storage tank must also be taken into account.

So I would like to ask, what is the working principle of this self-pressurizing control valve?

Reply to 3# MintLife: Pressurization of storage tanks is only applied to small tanks under pressure in the LNG industry. The self-pressurization valve of the storage tank is linked to the tank pressure system; when the tank pressure falls below the set value, the pressurization valve ahead of the booster opens, and the tank booster starts to operate ； When the pressure in the storage tank reaches the set value, the pressure increase valve before the booster closes, and the tank booster stops operating ； When the pressure in the storage tank exceeds the set value, it is vented for safety reasons. The boost valve before the turbocharger is generally controlled by compressed air based on a interlock signal.

So what medium is being supplemented? Nitrogen? Is it still a gas-phase material?

Low-temperature liquid storage tanks are generally equipped with a self-pressurization system, which consists of a circuit made up of a booster and pressure control valves; the liquid within the tank enters the booster, vaporizes, and then returns to the upper gas phase space of the tank. It has two main functions: 1 is to stabilize the pressure in the storage tank during liquid discharge, and 2 is to allow the tank to be kept at a lower pressure under normal conditions; when discharge is needed, a booster is used to raise the pressure to the required level for discharging, thereby enhancing safety. Pressure boost valves include self-acting pressure boost valves and pneumatic diaphragm control valves, etc. . Generally, for cost reasons, self-acting pressure boost valves are used in storage tanks. The answer given by the student on the 4th floor was not quite correct; pressure control valves are usually installed after the booster, that is, in the gas phase space. In some foreign countries, they are also installed in the liquid phase. In principle, installation in the liquid phase allows for more accurate control, but domestic valves generally cannot achieve this, as it would be too costly to use foreign valves. This is determined by taking both performance and cost into consideration; therefore, a manual shut-off valve is installed in front of the booster. The manual valve must be opened when boosting is required, and once the gas space in the tank reaches the desired pressure, the booster valve closes automatically. Generally, this pressure is set to be slightly higher than the operating pressure, but it must be lower than the design pressure. Due to the lag in valve closure, in order to maintain the discharge pressure and stability, it is common practice to set the closing pressure of the pressure-regulating valve slightly above the design pressure.

Small cryogenic liquid storage tanks are generally pressurized, allowing the liquid to flow by gravity during distribution, thus eliminating the need for expensive and complex cryogenic liquid pumps. Large cryogenic liquid storage tanks are generally operated at atmospheric pressure, as large tanks under pressure are too expensive to construct and pose safety risks; therefore, a pressure enhancement system for the tanks is not required. In the several small LNG storage and distribution stations, CNG filling stations, and gasification plants I have worked on, control valves are installed in front of the boosters to regulate the flow rate of liquid entering the boosters, while pressure regulators are installed behind the boosters.

What the student on the 7th floor said is basically correct; let me add a few more points: 1. Small low-temperature liquid storage tanks are generally pressure vessels. As mentioned earlier, they can use their own pressure to generate energy, thereby reducing the need for pumps. The downside is that due to limitations related to road transportation and processing techniques, their capacity is relatively small – at present, the maximum capacity is around 200 m3. The initial investment is high, and the costs for valves and maintenance are also high. 2. Large cryogenic liquid storage tanks are generally at atmospheric pressure. The pressure used for storing cryogenic liquids is generally below 100 KPa. Large tanks also have pressurization systems, and these are essential for cryogenic liquid storage tanks as well. Under various operating conditions such as large amounts of liquid discharge and rapid changes in external temperature, the inner tank is likely to develop negative pressure, thereby posing a risk of the inner cylinder being sucked in. Therefore, a pressurization system is necessary to maintain the pressure in the gas phase space. As far as I know, none of the large cryogenic liquid storage tanks built in the initial stages of operation were without such a pressurization system. The purpose of the pressurization system is not to increase the pressure in the storage tank, but to ensure the proper operation of the tank and the safety of its interior. 3. The valves installed in front of the turbocharger, whether they are manual or pneumatic control valves, or electric shut-off valves. All of these are intended to regulate the flow rate of liquid entering the booster. For small storage tanks, manual stop valves are typically used to keep costs down. At filling stations for gas or liquids, control valves can be installed to enable unified control and regulation from the DSC, but manual valves are also essential; they function as root valves and are usually kept open, allowing them to be closed promptly when maintaining the control valves. However, foreigners like to install an additional emergency shut-off valve at the base of the storage tank, mainly for safety reasons.

Reply to 4# Windy Lord of the World: What is the difference between a self-pressurizing valve and a self-regulating valve?

All domestic pressure-rated storage tanks of this type involve this: there are manual valves before and after the pressure regulating valve as well as in the bypass system for control, but they are basically not used. It involves using the ENG or LNG inside the tank, which is controlled by valves to be vaporized in a heat exchanger, thereby regulating the pressure within the tank; my workplace uses this type of device.

I would like to ask the experts: What are the operating costs of LNG trucks compared to diesel trucks? Is the modification complicated?