Explosion-proof structure and safety requirements for positive voltage electrical equipment

Positive voltage electrical equipment, according to the different structure, function and positive voltage protection mode of the equipment, is divided into static positive voltage electrical equipment and non-static positive voltage electrical equipment; For non-static positive voltage electrical equipment, it can be divided into positive voltage electrical equipment with a release source and positive voltage electrical equipment without a release source.

These different types of positive voltage electrical equipment should meet the specific requirements of their respective types, in addition to the general explosion-proof construction and safety requirements below.

1. Universal structure

(1) Structural materials and structural strength

In positive voltage electrical equipment, the housing of the equipment is called the so-called "positive pressure housing", which is an important structure of this explosion-proof type.

Normally, the positive pressure housing is made of metal plates such as ordinary steel plates (e.g., Q235A steel plates) or stainless steel plates (e.g., 1Cr18Ni9Ti steel plates), but of course, high-strength plastic materials can also be used. However, if plastic materials are used, designers of explosion-proof positive pressure cabinets should consider that the plastic materials used must have antistatic properties.

In addition to the normal structural strength required by the equipment, the shell, the protective gas transmission pipeline connected to it and the connecting parts should withstand the pressure value of 1.5 times the maximum positive pressure under the condition that all inlet and exhaust ports are closed, and the minimum should also withstand the pressure of 200Pa without deformation or damage.

If the internal pressure of positive pressure electrical equipment during operation may cause deformation of the shell, transmission pipeline and related connecting parts, then people should set up automatic safety devices in the positive pressure protection system, such as the maximum positive pressure monitoring device, to limit the overvoltage in the system to the extent that it does not adversely affect the explosion-proof type.

(2) Doors and lids

Doors and covers of positive pressure electrical equipment should be interlocked with the electrical circuit. This interlock, when the door or cover is opened, should automatically cut off the prefix power supply of the electrical components in the positive pressure protection system that are not explosion-proof according to the corresponding explosion-proof type; Power cannot be re-energized to these components until the door or lid has not been reliably closed and the system has not finished purging.

In the case of static positive pressure electrical equipment, the door or lid must be opened with a special tool and a warning sign should be placed in a conspicuous part of the housing: "Warning! Do not open in hazardous locations!".

In positive pressure electrical equipment, especially in Class I equipment, the fasteners used should be so-called special fastening elements. To prevent injuries when opening doors or lids due to excessive internal pressure, designers can use double-position fasteners or other special easing measures.

In addition, when a positive voltage electrical device contains a large capacitor and/or a heat-generating component, the door or cover must not be opened when the residual energy of the capacitor does not meet the requirements and/or when the temperature of the heat-generating component does not drop below the temperature value of the temperature group. Therefore, designers should mark the time interval between disconnecting the preamp and allowing the door and lid to open the device's housing, warning that the device should only be allowed to be opened after this time interval.

(3) Inlet and exhaust ports of protective gases

Positive pressure electrical equipment should be provided with protective gas inlets and/or gas tunnels. Their location should be determined according to the density of the protective gas.

When the density of the protective gas is greater than that of air, the air inlet should be opened in the upper part of the equipment shell, and the exhaust port should be opened in the lower part of the equipment shell; When the density of the protective gas is smaller than that of air, the air inlet should be opened in the lower part of the equipment casing and the exhaust port should be opened in the upper part of the equipment housing. It doesn't matter if the air intake or exhaust port is in the upper or lower part of the device housing. Both the intake and exhaust ports should be located on opposite sides of the device housing. This arrangement facilitates the flow of protective gases and the purging of air exchange.

In general, the area of the air inlet (or exhaust port) can be calculated as not less than 1 cm² per 1000 cm³ positive pressure shell volume. In this way, the calculated total area of the air inlet ensures that the positive pressure housing can be fully purged (exchanged) for a suitable period of time.

(4) Deflector plate in positive pressure shell

In order to ensure that the positive pressure electrical equipment is fully purged, the designer can set up some deflectors in the positive pressure housing, so that the purge air flow through every corner of the positive pressure housing.

In some cases, the mounting plates of the electrical components in positive voltage electrical equipment can also play a role in diversion.

If the positive pressure shell is divided into several small cavities, in addition to using the deflector plate to improve the purging effect of the protective gas, some air inlet holes can also be set separately for the small cavities to increase the permeable channel of the purge airflow.

(5) Spark and red-hot particle baffles

When the exhaust port of positive pressure electrical equipment is set in an explosive gas atmosphere, some baffles should be provided in its housing. Prevents red-hot particles and possible discharge sparks from inside the enclosure from escaping out of the enclosure through the exhaust port of the enclosure and into the surrounding explosive atmosphere.

If no hot particles are generated in the housing, this baffle can be dispensed with. In the other case, when the working voltage of the contact in the housing is not more than 275V (AC) or 60V (DC), the working current is not more than 10A, and the contact is also installed with an arc extinguishing cover, the exhaust port of the positive pressure shell can also be free of sparks and hot particles baffle.

When setting up spark and red-hot particle baffles, such baffles should allow at least eight 90° directional changes in the direction of the exhaust airflow in its flow direction. Because the air flow changes direction several times in the direction of its circulation, it can "precipitate" the red-hot particles and reduce the energy carried by the sparks.

(6) Protection requirements

In order to maintain the positive pressure that the internal pressure is higher than the external pressure when the positive pressure electrical equipment is operating normally, the positive pressure housing should be kept sealed as much as possible, except for the necessary air inlet and exhaust ports, so as to prevent the leakage of protective gas and reduce the pressure inside the housing.

In general, the degree of protection of the positive pressure enclosure should not be lower than IP5X from the point of view of protection class (IP), and the degree of protection should not be lower than IP54 for electrical equipment used on wet and dust-filled mining faces.

(7) Warning signs

In the conspicuous part of the exterior of positive pressure electrical equipment, people should set up a warning sign: "Warning! positive pressure housing!".

This warning sign tells people that the enclosure of positive voltage electrical equipment is a special kind of housing. It is an important structure to maintain the internal positive pressure pressure, should have a proper sealing effect, and should have a reliable interlock function. Therefore, people must pay enough attention when installing, operating, maintaining, and repairing positive voltage electrical equipment.

2. Clearance, creepage distance and limit temperature

(1) Clearance and creepage distance

There is no principle difference between the insulating materials used in positive voltage electrical equipment and those used in other explosion-proof types of explosion-proof electrical equipment, so the clearance and creepage distance should also be the same.

However, in the case of Class I electrical equipment, if the rated current is greater than 16A, such as in circuit breakers, contactors or disconnectors, which may cause arcing when on and off, then the insulation used should meet at least one of the following requirements:

(1) The CTI is not less than the CTl400M (equivalent to material grade II. or I) specified in GB/T 4207-2022 "Methods for Determining the Tracking Resistance Index and the Tracking Resistance Index of Solid Insulating Materials".

(2) The creepage distance between exposed live conductors of different potentials on the surface of the insulating material shall conform to the value applicable to the material under Class 3 pollution and Class III overvoltage specified in GB/T 16935.1-2008 "Insulation Coordination of Equipment in Low-voltage System Part 1: Principles, Requirements and Tests".

(2) Temperature limit

The temperature group of positive voltage electrical equipment should comply with the unified regulations of various explosion-proof electrical equipment, but the method of determining the temperature group is different due to the different equipment protection levels of positive voltage electrical equipment.

1) "PB" class positive voltage electrical equipment

When determining the temperature group of a "PB" class positive voltage electrical device, the designer should consider the maximum surface temperature of both the outer surface of the device housing and the maximum surface temperature of the internal components of the device, and use the highest of the two temperatures as the temperature value for the temperature group.

However, for some small components (surface area not exceeding 100m²) in positive pressure electrical equipment, when the maximum surface temperature is lower than a certain value of the ignition temperature of the corresponding flammable gas, the maximum surface temperature can be higher than the temperature value of the temperature group; In addition, when the internal heating components of the device can be cooled below the specified temperature value of the temperature group before opening the door or lid, the maximum surface temperature can also exceed the specified value of the temperature group.

In addition, when the positive pressure protection of positive pressure electrical equipment is suddenly interrupted, the flammable gas around the equipment will enter the equipment, so the electrical equipment should also have a certain protection ability to prevent the internal heating components from coming into contact with the flammable gas before the internal heating components are cooled to the maximum allowable temperature.

In this case, the heat-generating components can be protected by other explosion-proof types to prevent them from coming into contact with incoming flammable gases, or for some special equipment, a backup ventilation system (protective gas supply system) can be used to prevent external flammable gases from entering the housing when the main positive pressure protection system is suddenly interrupted (once interrupted, the backup system will be automatically activated immediately).

2) "PC" class positive voltage electrical equipment

The temperature group of a "PC" class positive voltage electrical device is determined only on the basis of the maximum surface temperature on the outer surface of the positive pressure housing.

This is because, since the "PC" class positive pressure electrical equipment is only allowed to operate in Zone 2 of the explosive hazardous location, the probability of the occurrence of explosive gas-air mixture is small, coupled with the obstruction of the positive pressure shell, the probability of contact with the internal components is smaller, so the determination of the equipment temperature group is not only based on the maximum surface temperature of the outer surface of the housing.

In addition, designers should also consider the protection of the internal live parts when the positive voltage protection is abruptly interrupted.

3. The explosion-proof type of automatic safety device in the positive pressure protection system

For positive pressure electrical equipment, in addition to the positive pressure shell, the monitoring of the working conditions of the positive pressure protection system is a very important safety measure to ensure the explosion-proof safety performance of this explosion-proof type. This is an important feature of positive voltage electrical equipment.

In the positive pressure protection system, the monitoring unit (device) that detects and controls the working conditions of the positive pressure protection system is called an automatic safety device. It should not be an ignition source for flammable gases in itself, should meet the requirements of an explosion-proof type, or be installed in a non-explosive hazardous location. This is something that designers must pay attention to.

When selecting the explosion-proof version of an automatic safety device, the designer should follow the following principles:

(1) For "PB" class positive voltage electrical equipment, the explosion-proof type of the automatic safety device can adopt various explosion-proof types with equipment protection level of Ga (Ma) or Gb (Ma) level.

(2) For "PC" class positive voltage electrical equipment, the explosion-proof type of the automatic safety device can adopt various explosion-proof types of all equipment protection levels.

In addition, it should be pointed out that various automatic safety devices are the necessary servo units of the positive pressure protection system; They should provide a reliable "service" before the positive pressure protection system is put into operation, during operation, and after it has been stopped. Therefore, the power supply of the automatic safety device should not share the same power supply with the main circuit, but should at least be set before the disconnector or power switch of the main circuit, so that it can still provide reliable "service" when the main circuit is de-energized.

4. Protective gas

(I) TYPE OF GAS

The protective gas used in positive pressure electrical equipment should be a non-flammable gas and should not be ignited on its own. In addition, the protective gas should not affect the reliability of the positive pressure housing, transmission pipes and connecting components, and should not affect the normal operation of the electrical equipment.

Therefore, clean air, as well as some inert gases such as nitrogen, can act as protective gases.

It should be noted here that when using inert gas as a protective gas, people should be warned that there is a risk of suffocation with inert gas.

(2) Gas temperature

At the air inlet of the positive pressure housing, the temperature of the protective gas, as a rule, should not exceed 40°C.

However, in general special cases, the temperature of the protective gas can be higher or lower, in this case, people should mark the maximum or minimum temperature on the housing of positive pressure electrical equipment. Sometimes people should also consider how to avoid the "breathing" effect caused by the temperature increase affecting the operation of electrical components, or the condensation or freezing phenomenon caused by the temperature being too low, and the alternating change of temperature from low temperature.

5. Positive pressure protection technology

According to the explosion-proof principle of positive pressure electrical equipment, the pressure inside the positive pressure housing must be higher than the pressure outside the housing (atmospheric pressure or a certain pressure) in order to prevent flammable gases from the external environment of the housing from entering the housing. GB3836.5 "Electrical Equipment for Explosive Gas Atmospheres Part 5: Positive Pressure Enclosure Type "P"" points out that the pressure of any pool square in the enclosure of positive pressure electrical equipment: for "pb" level, greater than or equal to 50Pa; For the "pc" level, greater than or equal to 25Pa.

When the positive pressure electrical equipment is purged and officially started, if the corresponding technical measures are not taken, the pressure inside the positive pressure housing will be reduced because the leakage will inevitably occur, so that it will be reduced to below the required pressure value, which is not allowed.

For this purpose, the so-called "serial dilution" positive pressure protection technology or "leakage compensation" positive pressure protection technology can be used to maintain the required pressure value inside the positive pressure well shell.

(1) Serial dilution positive pressure protection technology

Continuous dilution positive pressure protection refers to the continuous input of a certain flow of protective gas into the positive pressure shell after the purging of positive pressure electrical equipment, so that the concentration of flammable gas in the shell is always below the lower limit of its explosion limit.

This protection technology is characterized by allowing the protective gas to be discharged to the outside of the housing through the exhaust duct after the purge of the positive pressure electrical equipment, but not allowing the pressure inside the positive pressure housing to be reduced below the specified positive pressure value.

Therefore, for this positive pressure protection technology, the designer should set up a so-called "choke valve" at the outlet of the exhaust pipe of the positive pressure electrical equipment, so that the outlet of the exhaust pipe suddenly becomes smaller here, and artificially give a resistance to the protective gas flow through the positive pressure housing. In the case of a constant flow of protective gas through the positive pressure housing, the choke creates a higher pressure inside the positive pressure housing than on the outside of the housing, the so-called "positive pressure".

In addition, in order to keep the pressure inside the positive pressure housing not lower than the corresponding positive pressure specified value, the positive pressure protection system must continuously deliver a certain flow of protective gas to the positive pressure housing after the positive pressure electrical equipment is started.

The shielding gas used in this protection technology can be either air or inert.

(2) Leakage compensation positive pressure protection technology

Leakage compensating positive pressure protection refers to the supply of a certain amount of protective gas to the positive pressure housing after the purging of positive pressure electrical equipment to compensate for any leakage that occurs in the positive pressure housing and its connected transmission pipes, so as to maintain the required positive pressure value inside the housing.

For this form of protection of positive pressure electrical equipment, the designer should set up a shut-off valve at the outlet of the exhaust pipe of the positive pressure electrical equipment; After purging, the valve seals the exhaust pipe. However, the positive pressure shell and its transmission pipe will inevitably leak under the action of internal positive pressure, so the pressure inside the housing will drop, or even drop below the specified value of positive pressure.

So. In order to maintain the specified positive pressure inside the positive pressure electrical equipment, the positive pressure protection system must compensate for any leakage of protective gas in the positive pressure housing in real time after the positive pressure electrical equipment is started.

The designer sets up a flow control device at the population of the air inlet pipe of the positive pressure electrical equipment, and with the cooperation of other automatic safety devices of the system, a certain amount of protective gas is added to the positive pressure housing in real time to maintain the specified pressure inside the positive pressure housing. This is the characteristic of this positive pressure protection technology.

The shielding gas used in this protection technology can be either air or inert.

6. Safety measures and safety requirements for static positive voltage electrical equipment

Static positive pressure electrical equipment is an electrical equipment that does not need to be supplemented with protective gas during normal operation in explosive hazardous locations and can maintain positive pressure in the positive pressure housing. In general, this kind of electrical equipment does not need to be completely fixed.

In addition to meeting the general requirements of positive voltage electrical equipment, static positive voltage electrical equipment should also meet the following special requirements and regulations.

(1) The positive pressure housing should not contain an internal source of release of flammable substances.

(2) The protective gas filled in the positive pressure shell should be inert gas, and the oxygen concentration inside the shell should not exceed 1% (volume fraction) after the inert gas is charged. Work filled with inert gas should be carried out in a non-hazardous location.

(3) An automatic safety device, such as a pressure sensor, should be set on the positive pressure housing. For equipment of the "PB" class, two such units should be set; For "PC" class devices, just one can be set up. When the pressure inside the positive pressure wellshell drops to the specified value, the automatic safety device should reliably send out an alarm signal (sound and light signal) and automatically cut off the power supply of the main circuit.

(4) Under the specified unfavorable operating conditions, the positive pressure value inside the positive pressure shell must be higher than the external pressure by 50Pa.

7. Safety measures and safety requirements for non-static positive voltage electrical equipment

Non-static positive voltage electrical equipment is relative to static positive voltage electrical equipment. This electrical equipment needs to be replenished with protective gases in real time to maintain positive pressure values in the event of normal operation in explosive hazardous locations.

Non-static positive pressure electrical equipment, according to the different positive pressure protection technology, is divided into continuous dilution positive pressure electrical equipment and leakage compensation positive voltage electrical equipment.

Such electrical equipment, including protective gas delivery pipelines, should be securely installed in explosive hazardous locations.

(1) Positive pressure protection and automatic safety device

According to the different equipment protection levels ("PB" level or "PC" level) of positive voltage electrical equipment, people can adopt safety measures and safety requirements such as the highest positive pressure detection, protective gas flow detection, minimum positive pressure detection and purge time detection for this explosion-proof type of electrical equipment, and carry out real-time detection and random protection of the positive pressure protection system.

For "PB" level equipment, the protection system should be equipped with 4 automatic safety devices, and these automatic safety devices can automatically detect the protection system in real time at the same time, issue the main circuit start command, and send out alarm signals (sound and light) in time in case of failure, and cut off the power supply of the main circuit at the same time.

For "PC" devices, the protection system can be equipped with only 2 automatic safety devices, which can automatically detect the relevant data in the protection system and send out alarm signals (sound and light) if necessary. The main circuit is controlled automatically or manually based on the detection data of these devices.

(2) Detect the minimum positive pressure and gas flow

A minimum positive pressure must be maintained in the enclosure of the explosion-proof positive pressure cabinet, i.e. the pressure of the protective gas in the enclosure must be separated from the pressure outside the enclosure (which can be atmospheric pressure or a certain pressure). In this way, it is enough to prevent the flammable gas outside the shell from entering the inside of the shell and ensure the explosion-proof safety performance of this explosion-proof type of electrical equipment.

1) "PB" class electrical equipment

In the case of "PB" class electrical equipment, in the positive pressure protection system, the automatic safety device should use a pressure sensor to monitor in real time the minimum positive pressure value that may occur in the operating positive pressure electrical equipment and the corresponding transmission pipeline, and issue commands (output electrical signals, alarms and cut off the power supply) under the specified conditions.

The pressure sensor of the automatic safety device should take the pressure signal directly from the positive pressure housing, and no valves should be set between the sensor and the housing.

In addition, the automatic safety device should also use a gas flow meter to detect the flow of protective gas at the exhaust port of the positive pressure housing. When the equipment is about to start, once the gas flow rate is greater than the specified minimum flow, the flowmeter will send a pre-start command to the timer; Alternatively, while the equipment is operating normally, the flow meter will issue a command as soon as the gas flow rate falls below the specified minimum flow rate. The positive pressure protection system is activated and the main circuit is cut off.

2) "PC" class electrical equipment

For "PC-class electrical equipment", if the positive pressure enclosure is not equipped with an automatic safety device, it should be equipped with a display. The sampling point of the display should be set at the lowest pressure point in the housing under the harshest operating conditions of the equipment, and the pressure value in the housing should be displayed at any time. There should be no valves between the display and the housing.

In addition, in the case of "PE" class electrical equipment, the flowmeter should be installed at the exhaust port of the housing, assuming that a flow meter is used to display the pressure and flow rate in the housing; If the flow meter is used only to indicate the pressure, it can be installed at any of the lowest pressure points in the housing except for the air intake.

In addition, in order to ensure the minimum positive pressure value in the "PC" class positive pressure housing, the designer should also configure an alarm device on the gas supply of the protective gas to monitor the working status (pressure and flow) of the gas supply source in real time; When the gas supply fails, the alarm device will send a warning signal to people, such as an audible and visual alarm signal. Since the "PC" class electrical equipment is not a fully automatic control system, it is very important to monitor the working status of the gas supply in real time; If the gas supply temporarily fails, the positive pressure protection system will fail when the manual monitoring is negligent, which directly affects the safety of the industrial site.

(3) Detect the purge (ventilation) time

Purging (air exchange) time is a very important safety indicator for positive pressure electrical equipment. Purging refers to the process of replacing the flammable gas inside the positive pressure electrical equipment with protective gas into the positive pressure housing before starting. After the purging is completed, the positive pressure housing is filled with protective gas, at which point it is allowed to energize the device and start the equipment.

Purge (air exchange) time is the time required to fill a positive pressure casing with 5 times the volume of protective gas in the casing (including the delivery pipe). Obviously, the purge time is related to the pressure and flow rate of the protective gas charged into the positive pressure housing.

When the positive pressure value in the shell of the explosion-proof positive pressure cabinet is not lower than the minimum positive pressure specified value, and the flow rate of the protective gas is not lower than the minimum flow value of the design, the purge (air exchange) time can be calculated according to the following formula.

Typically, the purge time consists of two parts: the positive pressure electrical equipment itself and the one connected to the protective gas pipeline. The purging time (positive) of the positive pressure electrical equipment itself is calculated and determined by the manufacturer, and the purging time of the transmission pipeline is calculated and determined by the user. In addition, the user adds these two times together as the total purge time of the positive pressure protection system (T=Tl+r2) and marks it in a prominent part of the positive voltage electrical equipment.

For "PB" class electrical equipment, the detection purge time is done by a timer connected to the circuit. When the timer receives the signal from the minimum positive pressure detector and the flow detector, it starts to time automatically. As soon as the timekeeping time reaches the specified value (purge time), the timer sends a command to the main circuit starter to close the main circuit.

For "PC" class electrical equipment, it is allowed to mark the minimum flow rate and purge time of the protective gas on positive pressure electrical equipment. The pressure of the shielding gas, the minimum flow rate and the purge time can be measured to determine when to energize the equipment (automatically or manually) and start the positive pressure electrical equipment.

8. Safety measures and safety requirements for positive voltage electrical equipment with a release source

In modern chemical enterprises and other production environments where flammable gases and/or flammable liquids exist, due to the needs of the process, there are often some process pipelines containing flammable gases and/or flammable liquids that need to pass through some electrical devices (equipment). These process pipes can be a source of internal release of so-called flammable gases and/or flammable liquids in positive pressure electrical equipment.

Typically, the part of the process piping in a positive pressure electrical device that is capable of forming an internal release source is referred to as a so-called "built-in system".

(1) Built-in system and its release condition

According to the reliability of the built-in system structure and the setting state of the control system, the built-in system can be divided into trouble-free built-in system and faulty built-in system. Therefore, the internal release in the positive pressure housing is also divided into non-release condition and limited release condition.

1) Trouble-free built-in system and its non-release working condition

When the built-in system is a trouble-free built-in system, the built-in system does not release flammable substances into the positive pressure housing.

In order to prevent the built-in system from releasing flammable substances, it is necessary to ensure that the built-in system does not fail under any circumstances, and second, the built-in system is equipped with automatic safety devices to protect the system.

(1) For the former, because the structure of the built-in system is very reliable, it is unlikely to cause any release and leakage of flammable substances in the built-in system.

(2) For the latter, when the minimum positive pressure in the positive pressure housing of the equipment under the specified extreme temperature and normal operating conditions is 50Pa higher than the maximum pressure of the flammable gas (flammable liquid or vapor) in the built-in system, it can be considered that no release and leakage will occur in such a built-in system. When the pressure difference between the positive pressure housing and the built-in system is less than 50Pa, the automatic safety device should immediately and reliably alarm and cut off the corresponding power supply.

It should also be noted that when the concentration of a flammable gas mixture passing through a built-in system is consistently below its lower explosion limit, such a built-in system is also considered to be a trouble-free built-in system and will not be subject to any release and leakage, since it is impossible for the mixture to burn and explode in any case.

2) Faulty built-in system and its limited release condition

When the built-in system is faulty, the built-in system will release a certain amount of flammable material into the positive pressure housing.

This limited release condition of the built-in system is further divided into limited release of flammable gases and limited release of flammable liquids.

(1) For the limited release of flammable gases, the flow rate of flammable gases released from the built-in system under all possible fault states is predictable. In the worst-case scenario, this traffic should be the traffic that goes into the built-in system.

(2) For the limited release of flammable liquids, as well as the limited release of flammable gases, the flow of liquid in the positive pressure housing during release can also be predicted. However, one should take into account the serious consequences that these liquids released into the positive pressure explosion vent enclosure can cause when they accumulate inside the positive pressure enclosure. Also, one should also anticipate the maximum flow of oxygen if these liquids are likely to release oxygen.

After analyzing the release conditions of the built-in system, people can take corresponding technical measures to ensure the operational safety of positive voltage electrical equipment with internal release sources.

(2) The particularity of positive voltage protection technology for positive voltage electrical equipment with a release source

For such positive voltage electrical equipment with an internal release source, the designer should use continuous dilution positive pressure protection technology as much as possible, and in special cases, leakage-compensated positive pressure protection technology.

Due to the different characteristics of flammable substances released by internal release sources, these positive pressure protection technologies also have certain particularities.

1) When serial dilution positive pressure protection is adopted

In the case of serial dilution positive pressure protection, regardless of whether the internal release source is normal or abnormal under limited release conditions, when the upper limit of the explosion limit of the released gas is less than 80%, the protective gas can be either air or inert gas; When the upper limit of the explosion limit of the released gas is greater than 80%, the protective gas can only be air.

In the case of limited flammable gas (or vapor) release, in the event of a serious failure of the built-in system, the flow of protective gas during serial dilution should still dilute the released flammable gas to the following states:

(1) When the protective gas is air, the concentration of the flammable gas released is diluted to no more than 25% (volume fraction) of the lower limit of its explosion limit.

(2) When the protective gas is an inert gas, the flammable gas released is diluted to its oxygen content not exceeding 2% (volume fraction).

(3) When the upper limit of the explosion limit of the flammable gas released is greater than 80% (volume fraction), one should use air to dilute it to no more than 25% of the lower limit of its explosion limit (volume fraction).

(4) In the case of limited flammable liquid release, the protective gas should be inert gas. The vapor of the flammable liquid released is continuously diluted with an inert gas to its oxygen content not exceeding 2% (volume fraction).

2) When leakage compensation positive pressure protection is adopted

In the case of leakage-compensated positive pressure protection, the protective gas should be an inert gas, regardless of whether the internal release source is normal or an abnormal limited release condition.

Leakage-compensated positive pressure protection is only available for flammable gases released by internal release sources with an explosion limit of up to 80% (volume fraction), and the oxygen content of the mixture in the enclosure should not be greater than 2% (volume fraction) after positive pressure compensation.

This type of positive pressure protection is not suitable for released substances with an upper limit of more than 80% (volume fraction) of the explosion limit. Because when the upper limit of the explosion limit of a flammable gas is greater than 80% (volume fraction), it can be burned with no more than 4% (volume fraction) of oxygen under the action of an ignition source with sufficient energy. This is a very dangerous flammable substance.

The oxygen concentration defined here is no more than 2% (volume fraction), and this is the reason.

Whether it is continuous dilution positive pressure protection or leakage compensation positive pressure protection, the reason why inert gas can be used as a protective gas is because under certain conditions, inert gas can dilute the oxygen content in the flammable gas air mixture. Because in the case of insufficient oxygen content, in any case, the explosion will not happen.