Explosion-proof positive pressure cabinets are used in explosive hazardous environments such as petroleum, petrochemical, chemical, brewing, pharmaceutical, paint, textile, printing and dyeing, and military facilities.

Explosion-proof positive pressure cabinets are used in explosive hazardous environments such as petroleum, petrochemical, chemical, brewing, pharmaceutical, paint, textile, printing and dyeing, and military facilities.

With the rapid development of industries such as petrochemicals, metal smelting, and pharmaceuticals, the application of explosion-proof positive pressure cabinets in system control is becoming increasingly widespread. How to accurately use positive pressure explosion-proof controllers in explosive gas hazardous environments is crucial. Because electrical equipment inevitably generates electrical sparks or creates extremely hot surfaces during operation, if these come into contact with explosive gas mixtures in the field, it can lead to an explosion accident, directly endangering the user's property and life safety.

1. When the gas pressure exceeds 1000Pa, the pressure relief valve automatically opens the exhaust valve, automatically adjusting the internal gas pressure to ensure that the product is not deformed or damaged by high-pressure impact.

2. When the air pressure drops below 50Pa, the automatic control system will automatically issue an alarm and output an alarm signal. Internal pressure adjustment is required. Upon successful air replenishment, the system will automatically return to normal. Otherwise, if necessary, the system can be set to automatically shut off power (national standards stipulate that low-pressure alarms in Zone 1 require disconnection of the cabinet power supply, while low-pressure alarms in Zone 2 do not require power disconnection).

3. Only when the cabinet air pressure meets the specified requirements will the positive pressure purging control system effectively ventilate the cabinet (Zone 1 requires 7 times the cabinet's free space, Zone 2 requires 5 times the cabinet's free space). Once effective ventilation is complete, a signal will be sent to power the cabinet.

Application Scope of the Positive Pressure Purge Control System for Explosion-Proof Cabinets

1. Hazardous locations with explosive gas mixtures: Zone 0, Zone 1, Zone 2.

2. Explosive gas mixtures: IIA, IIB, IIC.

3. Temperature groups: T1～T6. 4. Indoor and outdoor (IP55)

5. Combustible dust environments: Zone 20, Zone 21, Zone 22

6. Corrosion resistance rating: WF1

7. Suitable for explosive hazardous environments such as petroleum, petrochemical, chemical, brewing, pharmaceutical, paint, textile, printing and dyeing, and military facilities; Scope and meaning of application groups for positive pressure purging control systems for explosion-proof positive pressure cabinets

1.1 Classification of applications of positive pressure purging control systems for explosion-proof positive pressure cabinets in hazardous areas of explosive gas environments

Different countries and regions classify explosive hazardous areas according to their own standards. In my country, based on the frequency and duration of explosive gas mixtures, explosive gas environments are divided into Zone 0, Zone 1, and Zone 2. (Consistent with the IEC International Electrotechnical Commission)

Zone 0: Environments where explosive gas mixtures are continuously or persistently present.

Zone 1: Environments where explosive gas mixtures may be present during normal operation. Explosion-proof cabinets are typically equipped with X-type series positive pressure purging control systems or Y-type series positive pressure purging control systems.

Zone 2: Environments where explosive gas mixtures are unlikely to occur during normal operation, or if they do occur, they are only present for a short period. Explosion-proof cabinets are typically equipped with Z-series positive pressure purging control systems.

Zone 0 generally exists only in the internal gas spaces of sealed containers, tanks, etc. Zone 1 is rarely found in actual design; most cases fall into Zone 2.

1.2 Explosion-proof electrical equipment is divided into two categories:

Class I: Electrical equipment used in coal mines

Class II: Electrical equipment used in locations other than mines

1.3 Class II electrical equipment is further divided into three categories (IIA, IIB, and IIC) based on its applicable maximum test safe gap or minimum ignition current ratio for explosive gas mixtures; and into six groups (T1-T6) based on its highest surface temperature.

1.4 Explosive gas mixtures are grouped according to their ignition temperature, see Table 1.

1.5 Explosive gas mixtures are classified according to their maximum test safe gap (MESG) or minimum ignition current ratio (MICR), see Table 2.

Table 1. Group Ignition Temperature t (°C)

T1 450 < t

T2 300 < t ≤ 450

T3 200 < t ≤ 300

T4 135 < t ≤ 200

T5 100 < t ≤ 135

T6 85 < t ≤ 100

Table 2. Category Maximum Test Safe Gap (MESG) (mm) Minimum Ignition Current Ratio

ⅡA ≥ 0.9 > 0.8

ⅡB 0.5 < MESG < 0.9 0.45 ≤ MICR ≤ 0.8

ⅡC ≤ 0.5 < 0.45

Property: Under specified test conditions, the ratio of the minimum ignition current of a gaseous or vaporous explosive mixture to the minimum ignition current of a methane explosive mixture. China classifies hazardous gaseous or vaporous substances into classes IIA, IIB, and IIC based on their minimum ignition current ratio. The minimum ignition current ratio is ≤1.0, >0.8 for Class IIA; ≤0.8, >0.45 for Class IIB; and ≤0.45 for Class IIC. For example, ethane, ethylene, and acetylene are Class IIA, IIB, and IIC hazardous substances, respectively.

The function of the positive pressure purging control system for explosion-proof positive pressure cabinets and the structural characteristics of the cabinet body:

1.

Its explosion-proof principle mainly adopts explosion-proof measures by isolating the ignition source with a medium and forming positive pressure protection to achieve the purpose of electrical explosion protection;

2.

Structurally, it can be customized into box-type, console-type, and cabinet-type structures according to user requirements. Cabinet-type structures are further divided into top-bottom and left-right structures. Box-type cabinets are generally made of welded steel plates with a powder-coated surface and front-opening operation; console-type and cabinet-type cabinets use standard profiles welded for the frame, with a powder-coated surface, and all joint surfaces undergo special sealing treatment; front-opening operation and rear-opening maintenance. 3. The steel plate material is generally selected from GB710-88 cold-rolled steel plate.

According to the air intake method, it is divided into continuous purging type and leakage compensation type. The continuous purging type provides uninterrupted air supply, and the working heat of the components in the positive pressure chamber can be carried away with the air path, providing excellent heat dissipation. The leakage compensation type provides intermittent air supply, and the positive pressure chamber has good sealing performance and low air consumption.

4. The positive pressure purging control system has complete protection devices. It will automatically supply power only after the specified air exchange time has been reached, and has automatic audible and visual alarms for high and low pressure, as well as safety interlock protection such as pressure lower limit, automatic power-off when the door is opened, and emergency manual power-off.

5. Reliable sealing performance. The shell adopts multiple sealing protections, with a long pressure holding time, saving operating costs.

6. This cabinet adopts cable trench mounting. Users need to provide a protective gas source. 7.

The product can internally house detection instruments, analytical instruments, display instruments, low-voltage electrical appliances, frequency converters, soft starters, or computer control systems, serving as a central signal processing system and central control system. During manufacturing, the user must provide a complete electrical system diagram and a list of materials for the control system components.

8.

Multiple units can be paralleled.

9.

Production can be based on the electrical schematic provided by the user.

10.

The product casing is constructed from high-quality steel plate or stainless steel, with a surface treated by high-speed shot blasting followed by electrostatic powder coating, resulting in an aesthetically pleasing appearance.

11.

The explosion-proof positive pressure cabinet consists of two parts: the cabinet body and the positive pressure purging control system. The air supply to the explosion-proof positive pressure cabinet enters the cabinet through the positive pressure purging control device and purifies it to the required safe pressure. When a low-pressure alarm occurs, the positive pressure purging control device will output an alarm signal and, if necessary, cut off the power supply to the positive pressure cabinet.

12.

Under the control of the positive pressure control system, the positive pressure cabinet is vented with protective gas. The pressure inside the positive pressure chamber is higher than the pressure outside, creating a slightly positive pressure safe microenvironment. This prevents flammable and dangerous gases from entering the positive pressure chamber, ensuring the safe operation of the internal instruments and electrical components. The automatic control system mainly performs advanced functions such as ventilation, gas replenishment, automatic pressure relief for over-pressure applications, low-pressure alarms, and low-pressure interlock power-off.