1. Basic Concept

Dry powder fire extinguishing systems are fire suppression systems that use dry powder as the extinguishing agent and discharge it in a predetermined manner onto the protected object or into the hazard area to extinguish fire, control fire development, or limit fire spread. These systems may be actuated automatically, manually, or by a combination of automatic and manual means.

Use of dry powder systems is most common in hazards involving flammable liquids, flammable gases, and energized electrical equipment. Typical applications include petroleum, petrochemical, chemical processing, oil and gas storage and handling, and other industrial occupancies with elevated fire risk. In rapidly developing, high-intensity fire scenarios, particularly liquid fires, gas fires, and jet fires, dry powder systems are widely recognized for their rapid knockdown capability and high extinguishing effectiveness.

2. Dry Powder Extinguishing Agents and Extinguishing Mechanisms

Dry powder extinguishing agents are mainly classified into ABC dry powder agents and BC dry powder agents. In addition, for combustible metal fires, there are also Class D special dry powder agents.

In international standards, the terminology used for these agents varies by region. In Europe, the term powder is more commonly used, as in ABC powder and BC powder, whereas in the United States the term dry chemical is more commonly used, as in ABC dry chemical and BC dry chemical.

2.1 ABC Dry Powder Agents

ABC dry powder agents typically use ammonium phosphate salts as their main active ingredients. A common ingredient is ammonium dihydrogen phosphate (NH₄H₂PO₄), commonly referred to in English as monoammonium phosphate (MAP).

The extinguishing action of ABC dry powder agents has two main aspects. First, they suppress the flame by interfering with the combustion chain reaction. Second, when used on fires involving ordinary combustible solids, the heated agent can form a coating layer on the surface of the burning material, thereby isolating oxygen and helping to prevent re-ignition.

Accordingly, ABC dry powder agents are generally suitable for fires involving ordinary combustible solids, flammable liquids, flammable gases, and fire risks associated with energized equipment. As multipurpose dry powders, their main advantage lies in their broad applicability: they provide chemical flame inhibition while also forming a protective layer on the surface of Class A combustibles.

2.2 BC Dry Powder Agents

BC dry powder agents typically use sodium bicarbonate, potassium bicarbonate, or other bicarbonate- and potassium-salt-based formulations as their main active ingredients.

Their primary extinguishing mechanism is rapid flame suppression through interference with the free-radical chain reactions in the flame, together with certain secondary effects such as heat absorption, dilution, and localized surface coverage.

Compared with ABC dry powder agents, BC dry powder agents generally do not rely on forming a molten coating layer on the surface of solid combustibles. They are therefore better suited to fire scenarios where flame knockdown is the primary objective, especially fires involving flammable liquids and flammable gases.

In petroleum, petrochemical, chemical, and oil and gas facilities where liquid fires, jet fires, and gas fires are the main hazards, BC dry powder agents are generally of high practical value. In some applications dominated by Class B and gas fire hazards, their suitability and extinguishing effectiveness may be superior to those of ABC dry powder agents. In particular, potassium-salt-based BC dry powder agents continue to offer clear advantages in industrial occupancies where Class B liquid fire hazards predominate.

2.3 Class D Special Dry Powder Agents

Class D special dry powder agents are intended specifically for combustible metal fires, and their extinguishing mechanisms differ significantly from those of ABC and BC dry powder agents.

Class D dry powder agents are generally not universal in application. Different combustible metal fires, such as those involving magnesium, sodium, lithium, or titanium, often require different specialized extinguishing agents. Therefore, the selection of a Class D dry powder agent must be matched to the specific metal involved and its combustion characteristics.

3. Main Structural Configurations of Dry Powder Fire Extinguishing Systems

From the standpoint of system configuration, dry powder fire extinguishing systems are generally divided into pre-engineered systems and piped systems.

3.1 Pre-engineered Dry Powder Fire Extinguishing Systems

In a pre-engineered dry powder fire extinguishing system, the dry powder container, actuation arrangement, discharge components, and other essential parts are preassembled into a relatively self-contained extinguishing unit. In the event of a fire, the unit discharges dry powder directly onto the protected object or into the protected area.

Such systems are typically compact, easy to install, and relatively independent in operation. They are commonly used where the protection area is limited, where hazards are dispersed, or where individual extinguishing units are required. In practice, suspended units, cabinet-type units, and other modular dry powder extinguishing units generally fall into this category.

3.2 Piped Dry Powder Fire Extinguishing Systems

Within a piped dry powder fire extinguishing system, the dry powder storage, expellant gas supply, actuation and control components, piping, and discharge nozzles are arranged as a complete system. When actuated, the dry powder is conveyed through the piping network to the protected area or protected object and discharged through nozzles for fire suppression.

These systems are generally suited to applications involving larger hazards, multiple discharge points, or situations where the extinguishing agent storage and control equipment are intended to be centrally arranged. Compared with pre-engineered systems, piped dry powder systems can usually protect larger areas and provide greater flexibility in system layout, making them more suitable for fixed and centralized protection schemes.

It should be noted that, although piped dry powder fire extinguishing systems are clearly recognized in relevant standards, they are relatively uncommon in engineering practice and are usually found only in certain industrial applications with specific protection needs. By contrast, pre-engineered systems remain the most widely used configuration.

4. International Terminology and Fire Classification Notes

It should be noted that the meanings of fire classification letters are not completely the same in different countries and regions. In the current Chinese system, fires are commonly classified as A, B, C, D, E, and F, where Class E refers to fires involving energized electrical equipment. In the European system, fires are commonly classified as A, B, C, D, and F, and Class E is generally not listed as a separate category for energized electrical fires. In the U.S. NFPA system, the common classifications are Class A, Class B, Class C, Class D, and Class K.

In the Chinese and European systems, Class C generally refers to flammable gas fires. In the U.S. NFPA system, however, Class B primarily covers flammable liquid fire hazards and, in engineering practice, also includes related hazards involving flammable gases, while Class C refers to fires involving energized electrical equipment. Class K refers to cooking media fires and is broadly equivalent in meaning to Class F in the Chinese and European systems.

Therefore, U.S. ABC dry chemical and BC dry chemical generally correspond, in terms of agent type, to what are commonly referred to in China and Europe as ABC powder and BC powder. However, the specific meanings of Class B and Class C in the U.S. NFPA system must be understood separately and should not be interpreted solely on the basis of the classification letters.