Overview
At present, China still has not issued a mandatory specification for small size LNG station design. In the LNG station project that has already been built or is now being built, the common practice usually is that the professional staff use foreign standards and draw on the experience of the experience from both domestic and abroad, design and construct the project in combination with the actual situation of the project. Commonly used abroad standard mainly include the national standard of the United States Liquefied Natural Gas (LNG) Production, Storage, Loading and Unloading Standard, Japanese standard Generally High Pressure Gas Security Law and so on, main domestic standard are Standards of fire protection design for petrochemical enterprises, Standards of the town gas design, Standards of building extinguisher disposition design, Standards of automatic fire alarm system design etc.

For the sake of discussion, we divided LNG accident into two types [4]. The first type is that when a dangerous medium is leaking and overflow, there is no fire, so no fire disaster happened, in other words, no fire disaster emergency; The other type is when the leaking dangerous medium exposed to fire, and the fire disaster occurred immediately.

Or the overflow of liquefied gas spread to a certain extent, and after the exposure to fire a large area burning fire thus occurred, in other words, this is called "fire disaster emergency".

No fire disaster emergency and its security countermeasures
In case of fire disasters, the security measures should be taken for the major equipment are:

(1) The LNG tank
Material Performance

The insulation layer of the material properties coated in the outer layer of the storage tank shall have the performance of fireproof, flame retardant, water resistance and waterproof, and will not move under the impact of firefighting water, and have sufficient fastness. Incombustible insulating materials should be filled between the inner tank and the external tank to fit with the performance of LNG and natural gas. The storage and filling materials should be able to adapt to the low temperature performance requirements , and can work normally in the temperature of LNG and under cold steam temperature.

Safety spacing
The distance between the fire equipment and other sources of fire to any cofferdam or storage tank is at least 15m.

The filling volume

LNG tank design has a maximum liquid limit value, which is used to prevent excessive liquid level from being discharged from the overflow pipe. In filling the LNG, the expansion of the volume of the liquid when the liquid is heated should be taken into account. If the injected LNG is bloated LNG relative to the storage pressure, then it can be filled to the highest level, whereas the appropriate space should be left for the expansion of liquids.

Purification treatment

Before storage tank's first filling of LNG it shall undertake an internal overhaul, inerting disposal shall be conducted when the tank is stopped. The aim is to replace the air or natural gas in the tank with inert gas, avoiding the mixture of natural gas and air.

Valve configuration
In the absence of other effective measures to prevent the stratification, the hydraulic system of the storage tank into is supposed to be equipped with top inlet valve and bottom inlet valve, when the filling liquid and the original liquid thermal performance showed difference , through different parts of the conveyor, make uniform liquid, reduce the possibility of a liquid layer. In addition, mechanical agitation or imported injection can be used to achieve uniform mixing purpose.

Pressure control device should be equipped to allow the pressure of the tank is within an allowable range. Set up safety valve with enough pressure so as to prevent overpressure of the storage tank; Set up vacuum safety valve and determine whether there is vacuum in the tank so as to prevent negative pressure of the storage tank; And a cut-off stop valve between the pressure safety valve, the vacuum relief valve and the tank must also be equipped so as to provide convenience for the inspection and maintenance of the safety valve.

Each tank should be equipped with an emergency shut-off valve when accidents happen cut off the connection between the tank and outside, so as to prevent the leaking of the LNG inside the storage tank.

The tank should be equipped with a safety air release valve to connect the torch; release pressure facility should be equipped with the external tank, discharging the air to the highest point.

Operating distance should be kept between 0.9 m for multiple storage tanks.

Measuring meter
Each storage tank should be equipped with two sets of the independent liquid level measuring devices, two independent sounding high-liquid alarms and pressure measuring instrument.

Install the instrument and joint inside the storage tank, the external cylinder vacuum jacket to measure the absolute pressure in the interlayer space. When the on-site storage tank is to put into operation, the temperature detection device shall be set to control the temperature or as a tool for inspecting and correcting the liquid level meter.

Anti-Seismic performance
According to the small size station and its surrounding area's geological and meteorological data, to analyze the potential and features of the natural disasters such as earthquake and blizzard, take the anti-seismic ability and the load ability of windproof and snow proof into consideration. (2) Cofferdam area
When considering the plan of LNG storage tank, the storage and loading and unloading of LNG should be fully considered, if any accidents occur, LNG is likely to spill or leak, which has a potential harmfulness to the nearby personnel and important equipment. For small LNG tanks, the common method is to set up barriers such as compacted soil, concrete, metal and other low-temperature materials such as embankments, protective walls or Liquid storage pit. In the case leakage of the storage tank occurred, the flammable liquid can be confined to the cofferdam area, which will not flow everywhere, this is a method to prevent the spread of LNG. In addition, if a fire disaster breaks out, it can prevent the flames from spreading to the surrounding area and minimize the damage to the surrounding facilities. Cofferdam in the volume, therefore, should be big enough to accommodate to the storage tank of liquefied natural gas (LNG), it should includes the discharge area of any available volume, considering the allowance for the snow replacement, tanks and other equipment as well.

When the working pressure of the storage tank is less than or equal to 0.1 MPa, the height of the fence or the wall of the cofferdam and the distance from the storage tank shall be determined according to figure 1.

Note:
1. The size X should be equal to or greater than the sum of the size Y and the head of air supply and depressurization;
Exception: when the height of the fence or the wall of the cofferdam is equal to or greater than the maximum liquid level, X can take any value.
2. Dimensions X represents the distance between the inner wall of the tank to the fence or to the inside of the cofferdam wall.
3. Dimension Y means the maximum level of liquid level to the top of the fence or to the wall of the cofferdam.

In addition, it is necessary to set up baffles for the valve is likely to have leakage problem to prevent the injection of LNG. At the bottom part a liquid collecting tray should be set up to collect leakage of LNG, and introduce it to liquid storage pit through the drain tube.

(3)Vaporizer
Valve configuration
Each vaporizer of the vaporizer group shall be equipped with the cut off valve at the inlet and outlet. To prevent leakage of LNG enter into the idle vaporizer group, two inlet valves shall be installed and other safety measures shall be adopted to empty the LNG or gas between the two valves. A cut off valve should be installed on the LNG pipe of 15m of the carburetor, which can be operated on site or remotely controlled, and there are protective measures to prevent the valve from being invalidated due to external freezing conditions. Every vaporizer should install safety pressure reducing, and when the heating vaporizer is working normally, a place where the temperature less than 60 ℃ should be set in. Install automatic control cut off valve in the fluid pipeline, the distance between the valve to the vaporizer is at least 3m, and it can automatically shut down when the pipeline pressure loss (flow), carburetor surrounding area temperature anomaly (fire) or low temperature happens at the exit of the vaporizer.Other equipment

Each heating vaporizer should be equipped with a device that can cuts off the heat source, which can be operated on site or remotely. Install automatic equipment of piping valves for the use in emergency to prevent LNG or steam from being delivered to the system at higher or lower temperature to the temperature of external delivery system. The temperature detection device is configured to measure its LNG, vaporizing gas and the outlet and inlet temperature of the heat exchange fluid medium so as to ensure the heat transfer efficiency.

Safety spacing

The distance between the vaporizer and its main heat source and any other source is at least 15m unless the heat-conducting fluid medium is not combustible. The distance between the overall heating vaporizer and the land line is of at least 30m, and the distance to the following location is at least 15 m: ① any LNG, flammable refrigerant or flammable liquid in cofferdam, or carrying these liquid in any other accident leakage between the source and the cofferdam area; ② LNG, combustible liquid, combustible refrigerant or combustible gas storage tank, or the unfired process that contains of these liquid, or the unloading equipment connector of these liquid; ③ Controlling the buildings, offices, stores, residential or other important buildings. The remote controlled heating vaporizer and environmental vaporizer can be installed in the cofferdam area. When there are multiple vaporizers, the distance between the vaporizers is at least 1.5 m.

(4) Compressor and pump

Material selection

The compressor and pump should be able to ensure normal operation under the possible working temperature and pressure.

Valve configuration

The valve configuration should make each compressor and pump the valve is available to be repair and maintenance alone, and it should be equipped with voltage reducing device at the exit of the pipe. Pressure relief valve with enough capacity should be installed, when conveying liquid below - 29 ℃ temperature it should be equipped with precooling device, to ensure the pump will not be damaged or cause temporary or permanent failure. The compressor that handles combustible gas should set up the exhaust duct at the point where the gas leak is possible to happen and lead the gas to the safe place.

(5) LNG piping and its components

Material selection
When design piping and its components, the fatigue effect of thermal cycling on the material should be taken into account. For pipes, valves, fittings and other fittings that are connected to each other should give special attention if the thickness of the pipe wall is not at the same. Therefore, it is necessary to cover waterproof, heat insulating, flame retardant and anti-extreme temperature pipe insulation materials to reduce the hazard of accidents such as fire. It is not recommended to use aluminum, copper, copper alloys or other low refractory materials. Cast iron pipes are not allowed.

Connection mode
Pipeline that is not greater than 50mm in diameter adopt sealing structure by a thread, welded or flange joint; Pipeline with a diameter greater than 50mm is connected by welding or flange.

Do not use threaded connections or flange connections as much as possible except for the necessary location of tube, equipment connection or maintenance location. The pipeline system is not allowed to use F type takeover, spiral welding structure and butt welding structure.

Valve configuration
Set cut-off valve and isolation valve to minimize leakage of dangerous liquid volume if any accident happen, and with enough quantity to ensure that the site control and remote control can realize smooth stop process and transfer system, or completely shut off system in emergency; Set up thermal expansion valve to prevent excessive pressure; Set up the safety pressure relief valve to minimize the damage to the pipeline and its accessories.

Set the safety valve between two shutoff valves of the liquid phase, in case that the two shutoff valves are off. When the liquid in the pipe is heated and gasified, the relief valve automatically starts to jump, so as to prevent accident caused by the overpressure. An emergency discharge device is installed in the gas phase so as to protect the safety of the gas phase in case of incorrect operation or overpressure relief valve of the equipment take off.

Support system
Pipeline support system should have the ability of fire resistance and low-temperature resistance.

The support system of low temperature pipeline should have cold insulation measures to prevent the pipes freeze or support steel embrittlement that is caused by heat transfer.

Cleaning treatment
Pipeline system shall be treated with nitrogen or carbon dioxide gas prior to the first filling of LNG.

(6) LNG handling equipment

Safety distance

The distance between the LNG, flammable refrigerant handling interface to the uncontrolled ignition process area, storage tank, control of buildings, offices, shops, residential buildings and other structures is at least 15 m.

Fire emergency and safety measures

In case of a LNG fire accident, the following principles can applied.
①Try to cut off the gas source and control the leakage. ②To protect the fire tank and adjacent cans and equipment, avoiding heat overpressure of storage tank equipment to cause greater disaster. ③The leaking LNG can be brought to the safety zone such as the accident collecting tank, which can be covered with high power foam to safely gasify it and avoid combustion expansion.

Generally, fire safety design includes the following parts:

(1) Distribution Control System, DOS

The DOS system is used to display and control the temperature, pressure, liquid level and flow of the LNG plant and important control parameters [6].

(2) Emergency Shut Down system, ESD

ESD system and DOS system are independent of each other, when the LNG device open under an emergency circumstance, the Emergency Shut Down system is used in isolation and shut off LNG, or other flammable sources, and shut down the equipment that may maintain or increase the damage if keep on running. The system should be equipped with automatic remote control and manual button at the same time. The manual button should be installed at a certain distance from the relevant production unit, the quantity shall be more than 2 pieces, and the measures shall be taken to prevent it from being affected by the fire thermal radiation.

(3) Fire and leak detection alarm system

Fire and leak monitoring equipment should be set up in places and enclosed buildings where combustible gas accumulation, LNG or flammable refrigerant can occur. Places near the natural gas production equipment and the storage tank should set up the flammable gas concentration detection alarm device, when the gas concentration reaches the lower explosive limit of 10-25%, namely sends out sound and light alarm, in order to rapidly take emergency measures.

In addition to the design of flammable gas detection, flame detector and fire alarm system in the production unit area, the cold detection system should also be set up to detect whether the LNG is leaking or not. All the detection signals in the device are sent to the DCS system and alarm values are set, meanwhile, important signals are sent to the ESD system to carry out the unit cutting or the interlock stop.

(4) Firefighting water system

Fire water system of fire pump house and fire water tank, fire protection and fire tied pipeline pumps, firefighting water cannons, etc. used for fire control, cooling tank, equipment and piping, control of unburned leak or overflow hazards.
The design of the fire water supply and distribution system shall meet the needs of all fixed firefighting systems. Ensure that the supply of water under design pressure and flow is ensured during the maximum single incident that may occur, ensuring the normal operation of the handheld hose. The continuous water supply time is not less than 2h.

It is important to note that fire water is not used to control and extinguish the flame in the LNG plant, because the water on the surface of the liquid will increase the steam generation speed and the combustion rate will be accelerated. But the jet of water can cooling the equipment, containers and pipes in the fire area around that are not burning, absorb and control the heat generated by the fire, to prevent the fire from spreading, and it can also protect the fire fighters and the personnel attempt to enter to the fire to cut off the necessary air supply valve.

(5) Dry powder extinguishing system

Fire caused by small amount of gas leakage can be put out by using dry chemical, carbon dioxide, halon extinguisher, but after the fire is going out, gas source shall be immediately cut off to prevent gas overflow, otherwise it may cause resurgence, even explode. The powder extinguishing agent is the most effective way to deal with high pressure large volume gas fire.

According to foreign experience in the fight against gas fire, the continuous discharge time of dry powder is 25 ~ 60 seconds. After the initial fire extinguishing, dry powder must be used intermittently.

(6) Foam extinguishing system

The high expansion foam extinguishing system is used to cover the LNG of the tank area, pipeline, unloading platform leakage and the accident collecting tank, so as to safely gasify it and avoid the occurrence of danger. A fixed low-expansion foam fire extinguishing system can also be set up according to the standard requirements, and foam sprinkler system can be installed at the top of storage tank, pipeline, unloading platform and important equipment.

(7) Mobile fire extinguishing equipment

The key position of the LNG equipment and the car tanker is the serious hazard level, and the portable or wheel type fire extinguisher should be installed to extinguish the fire. Control room, variable distribution room should be equipped with portable co2 fire extinguisher to ensure rapid and effective early fire. The motor vehicles entering the plant should be equipped with at least one portable dry chemical fire extinguisher with a capacity of no less than 9kg.

Others
The layout of the general drawing
In order to meet the requirements of the process, the functional zoning shall be reasonably arranged, and the storage area, production and auxiliary areas and office areas shall be set apart. Give a comprehensive consideration to the fire spacing, fire lane and fire protection requirements.

The power system

The safety and reliability of the power supply system should be ensured and the electrical equipment suitable to the explosion-proof zone level should be selected. The cable should also be treated with fire, which can still function after a fire.

High-security tossing

The dedicated blowdown flare can be equipped in the station, overpressure relief with the height of 30m ~ 40m, the LNG storage tank, the BOG storage tank, the process pipe and the overpressure relief of each production section should all introduce dedicated blowdown flare to avoid the explosive mixture in the station.

Ventilation measures

Ventilating measure should be adopted to deal with the construction of LNG, combustible refrigerants and combustible gases to minimize the possibility of combustible gas or steam accumulation. If evaporation gas density is greater than air, ventilation should be installed at the evaporative gas deposition.

Personnel Security

Staff members are trained regularly to learn about the characteristics of LNG and the hazards and effects of LNG exposure, protective effects and proper use. When it is needed to enter into the harmful atmosphere in an accident, the staff should be armed with a helmet, mask, gloves, boots and breathing apparatus, besides from the necessary protective clothing, and these outfits should be placed in the accessible place. Due to the lack of smell of LNG and steam in the process equipment, a suitable combustible gas indicator is required to be installed in case of smell detection.

Lightning and static electricity protection

The auxiliary facilities and industrial buildings are equipped with lightning protection and lightning rods to prevent lightning strike. The enclosed metal canister, tower and equipment piping in the installation area should be equipped with lightning protection grounding as per specification requirement.

Building fire resistance rating

Station building and structures shall be designed according to The Code for Fire Protection of Buildings Design, building, structures and important equipment joint platform, and shall be set up with more than two safety evacuation mouths; The load-bearing steel frame, pedestal, skirt seat and pipe rack in the production equipment should coating fire-resistant layer protection according to criterion, and the fire resistance limit of the refractory layer is no less than 1.5h.

Fire services and communications

Fire services and communication facilities within the station should be ensured.

The main power equipment such as fire control system, fire water pump, air supply and water equipment should be equipped with standby power supply, and internal control center should have an outside line alarm call or a direct line with fire department.

Conclusion

Throughout the world LNG industry development course, our country's is still in its infancy, especially in domestic when there is still not yet promulgated the industry standard of the design and construction of LNG station, More commonly, we usually use the standards from the United States, Europe, Japan and other counties in engineering design as a reference, which somehow has a lot of problems.

The main performance is: ① In the overall situation, the quality of the equipment produced by domestic manufacturers is slightly lower than that of foreign countries. Therefore, the commonly used American fire prevention association standard NFPA 59A is uncertain to in line with the current situation of urban gas industry in China or not. ② The vacancy of compulsory industry standard makes the LNG station in the design and construction of the project to leave hidden danger to the future operation of the gas supply system due to various factors such as capital input. ③ There are already enterprises in China that are capable of producing LNG and related equipment, but there is still no basis for how to conduct safety quality assessment of its products before it is put into use. ④ Some manufacturers are just blindly pursue advanced technology, high quality, whether waste can be caused by the product quality process is still not so sure.

In my opinion, when we are paying attention to imported LNG project in China, discussing the LNG technological process and energy saving measures, and develop new LNG products, new technology, new equipment at the same time, we should focus on the city gas industry in our country the status quo in the first place, based on the principle of safe gas supply, formulate and promulgate a reliable, feasible industry standard and a standard that can be in accordance with, this is what should be the top priority.