The pressure relief vent of a gaseous fire suppression system’s protected zone, often referred to as the relief vent or automatic pressure relief device, is an essential component integrated with the system. It is typically installed on pressure relief openings in the exterior or interior walls of the protected zone.

I. Overview

The pressure relief vent of a gaseous fire suppression system’s protected zone, often referred to as the relief vent or automatic pressure relief device, is an essential component integrated with the system. It is typically installed on pressure relief openings in the exterior or interior walls of the protected zone.

 

  II. The Necessity of Providing Pressure-Relief Openings

According to the requirements of GB 50370-2005, “Code for Design of Gaseous Fire‑Extinguishing Systems,” the design extinguishing concentration for heptafluoropropane fire‑extinguishing systems is typically 8% to 10%. When heptafluoropropane is discharged into a fully enclosed protected zone, under standard atmospheric pressure at 20°C, the release of the propellant gas (nitrogen) and the vaporization of the heptafluoropropane cause the pressure in the zone to rise. Simultaneously, the extinguishing agent absorbs substantial heat, leading to a reduction in the zone’s temperature; however, this results in only a very small decrease in pressure. The increase in pressure is primarily influenced by the degree of enclosure of the protected zone, the design extinguishing concentration, and the sealing integrity of the pressure‑relief openings (automatic pressure‑relief devices). The pressure rise is essentially proportional to the volumetric concentration corresponding to the design extinguishing level, with an increase of 8 to 10 kPa—values that exceed the 1.2 kPa limit for light‑weight, high‑rise, and conventional buildings by a factor of 6 to 8.

In IG‑541 mixed‑gas fire suppression systems, the design extinguishing concentration ranges from 37.5% to 43%; in carbon dioxide gas fire suppression systems, it lies between 34% and 62%. In other words, when these two types of extinguishing agents are discharged into a fully enclosed protected zone, the gas volume within the zone expands rapidly, causing the pressure to exceed the allowable limit of 1.2 kPa by more than 25 times—enough to destroy the entire enclosure structure of the protected area. A certain company conducted an IG‑541 mixed‑gas test in a laboratory measuring 6 m long, 6 m wide, and 4 m high. The protected zone was equipped with a ventilation opening 200 mm in diameter, and the exhaust fan on this opening was operating normally. When seven 70‑liter cylinders of IG‑541 mixed gas were discharged into the laboratory, the laboratory door was blown open, and the exhaust fan was severely deformed.

 

III. Design Basis and Calculations for Relief Vent Area

3.1 Maximum Allowable Pressure of the Enclosure Structure within the Protected Area

The allowable pressure that the glass in doors and windows within the protected zone can withstand shall not be lower than the building’s allowable pressure. The data in Table 4 of the explanatory notes to GB 50370‑2005 are based on those provided in the U.S. NFPA 12B‑1980 standard. At present, domestic design authorities uniformly set the allowable internal pressure that the enclosure structures in the protected zone may endure—at 1.2 kPa—regardless of whether the building is a lightweight or high‑rise structure, a standard‑type building, or an underground facility. This value is established in accordance with Clause 3.2.6 of GB 50370‑2005. If design agencies or end users wish to increase the allowable internal pressure for the enclosure structures in the protected zone, such an increase must be determined through testing conducted by the architectural design department.

Table 4 Allowable Internal Pressure of Buildings

Building Type	Allowable pressure (Pa)
Lightweight and High-Rise Buildings	1200
Standard building	2400
Heavy-duty or underground structures	4800

 

Pressure Relief Area Parameter Table for the Protected Zone

Gas Type		Heptafluoropropane gas fire suppression system		IG541 Mixed Gas Fire Suppression System
Design Concentration (%)		8	10	37.5	42
Spraying time (s)		9	9	55	55
Remaining drug quantity (%)		0.05	0.05	0.05	0.05
Prevention Protection district Capacity Accumulation	100	0.03	0.04	0.04	0.04
	200	0.06	0.08	0.08	0.08
	300	0.10	0.12	0.12	0.13
	400	0.13	0.16	0.15	0.17
	500	0.16	0.20	0.19	0.21
	600	0.19	0.25	0.23	0.25
	700	0.22	0.29	0.27	0.29
	800	0.26	0.33	0.31	0.33
	900	0.29	0.37	0.35	0.38
	1000	0.32	0.41	0.38	0.42

 

Working principle

Pressure relief vents (automatic pressure‑relief devices) are installed in pressure‑relief openings on the exterior or interior walls of the protected zone and remain normally closed. When a fire occurs in the protected zone, prior to the release of the extinguishing agent, the gas‑extinguishing system automatically de‑energizes ventilation equipment and air‑conditioning units to ensure proper agent concentration and impregnation time, thereby maximizing the likelihood of successful suppression. At the same time, ventilation ducts, doors, and windows are sealed to maintain airtightness. Upon activation of the gas‑extinguishing system and the subsequent release of the extinguishing gas, the pressure within the protected zone can rapidly exceed the design‑specified allowable pressure for the building. In such cases, if a non‑powered pressure‑relief vent is installed, once the gas pressure acting on the flap or cover assembly reaches the preset threshold, the pressure‑regulating actuator or weight‑driven mechanism immediately opens the flap or cover to vent excess pressure. Alternatively, if a powered pressure‑relief vent is employed, when the pressure‑sensing device detects the set pressure, it sends an electrical signal to the electric actuator, which swiftly opens the flap or cover to discharge the over‑pressurized gas, thereby preventing damage to the building’s walls, doors, windows, glazing, and other enclosure elements and averting failure of the fire‑suppression effort. Once the pressure within the protected zone falls below the set value, the flaps or covers of both non‑powered and powered pressure‑relief vents automatically close, maintaining the required extinguishing‑agent concentration and ensuring adequate impregnation time to achieve timely fire suppression.

IV. Pressure Relief Area

At present, the specifications of pressure relief vents manufactured by various domestic producers are not standardized; consequently, the vent’s relief area, external dimensions, and orifice size vary among manufacturers. Typically, the relief area ranges from 0.04 to 0.24 m². ² Between them, the total relief area—calculated as the sum of the areas of all pressure‑relief openings or the number of installed relief vents in each protected zone—shall not be less than the relief area determined by the design institute. The relief area of each vent shall be proportionate to the area of its respective protected zone. For protected zones served by IG‑541 mixed‑gas and carbon‑dioxide fire‑extinguishing systems, at least two relief vents shall be provided; alternatively, a single vent with a large relief area may be replaced by two or more vents with smaller relief areas. Failure to comply with these requirements may compromise the safety of the enclosure structure and result in damage.

V. Installation and Use of Pressure Relief Vents

Taking an opening size of 810 mm × 410 mm as an example, the key installation points are outlined as follows:

1. As shown in Figure 3, an opening is provided in the wall, with dimensions of 810 mm × 410 mm.

2. According to the position shown in Figure 3, drill circular holes with a diameter of 8 mm and a depth of 40 mm in the exterior wall and on the inner surface of the opening.

3. Install the pressure relief device and louvers using M5×40 expansion bolts as shown in Figures 1 and 2.

VI. Precautions for Pressure Relief Vents

The XK0.12–0.25 series pressure relief devices are easy and convenient to install; however, during actual installation, the following points should also be observed:

1. The wall thickness must be greater than or equal to 130 mm.

2. Openings in walls shall have smooth interior surfaces to prevent damage to pressure-relief devices.

3. The side wall of the pressure relief device must not be subjected to compression to ensure proper operation.

4. After installation is complete, verify that the window sashes open and close properly.

 
  VII. Location and Method for Installing Pressure-Relief Openings

According to the requirements of the code for the design of gaseous fire‑extinguishing systems, pressure relief vents should be installed on exterior walls, with their elevation exceeding two-thirds of the net height of the protected zone. This is a relatively general regulatory provision; however, it should be applied flexibly in specific project cases.

The following introduces several application examples:

7.1 Example As shown in Figure 1, this example represents the most commonly used configuration in gas fire‑extinguishing systems. A single room serves as a gas‑protected zone and is enclosed by exterior walls. When pressure relief openings are required for such a zone, they need only be provided in the exterior wall.

7.2 Example 2 As shown in Figures 2 and 3, two adjacent rooms, Room A and Room B, are designated as a single gas‑protection zone, with both rooms having exterior walls. There are two methods for arranging the pressure‑relief openings. Method 1, illustrated in Figure 2, provides separate pressure‑relief openings for Rooms A and B: the opening in Room A is sized to meet the required relief area for that room, and the opening in Room B is sized to meet the required relief area for that room. Method 2, shown in Figure 3, installs Pressure‑Relief Opening A in the shared wall between Rooms A and B, while Pressure‑Relief Opening B is provided on the exterior wall of Room B. The size of Opening A corresponds to the relief area required for Room A, and the size of Opening B corresponds to the total relief area required for both Rooms A and B.

If multiple rooms are designed as a single gas‑protected zone, the method for installing pressure relief vents may be applied accordingly.

 

7.3 Example 3: As shown in Figure 4, the protected area has no exterior walls. The pressure‑relief openings are arranged as follows: an exhaust vent is provided at the upper portion of the protected area, with an area equal to that of the pressure‑relief opening; a pressure‑relief opening is also installed at an appropriate location on the exterior wall; and a dedicated relief pipe connects the exhaust vent to the pressure‑relief opening. This configuration allows excess pressurized gas from the protected area to be discharged directly outside the building, thereby fully complying with fire‑safety code requirements.

During the actual fire‑extinguishing process of a gaseous fire‑suppression system, the vented gas is often at high temperature or contains substantial dense smoke. Therefore, it is imperative not to simply locate the pressure‑relief opening on an interior wall and discharge the gas directly into the corridor or into the suspended ceiling outside the protected area; otherwise, evacuation may be impeded, and the fire could even spread.

 

8. Conclusion

This paper briefly analyzes the importance and applications of pressure relief vents in gaseous fire‑extinguishing systems, aiming to underscore the need for firefighters to recognize their critical role. Designers should comprehensively account for all system‑related factors and must never arbitrarily overlook auxiliary elements such as pressure relief vents or enclosure structures, thereby ensuring the reliability and safety of gaseous fire‑extinguishing systems.