Side Cartridge vs. Stored Pressure Fire Extinguishers
1. WHAT IS THE DIFFERENCE BETWEEN STORED PRESSURE FIRE EXTINGUISHERS AND SIDE CARTRIDGE OPERATED SUPPRESSION SYSTEM?
Side cartridge operated units store the dry chemical in a non-pressurized container and the compressed nitrogen gas in a separate cartridge. Only at the time of actuation is the agent tank under pressure, and then only briefly.
Stored pressure extinguishers have the compressed nitrogen gas that is used as the propellant for the dry chemical agent stored in the same canister as the dry chemical.
2. DOES THE A:B:C DRY CHEMICAL COMPACT IN EITHER OF THEM?
Due to the vibration experienced on heavy equipment, settling and compaction are common issues for any vehicle fire suppression system. To counteract this, side cartridge units are designed to break up and fluff the powder before discharge.
3. DOESN’T A STORED PRESSURE EXTINGUISHERS KEEP THE DRY CHEMICAL IN SUSPENSION?
No. The specific gravity of nitrogen is .97 and the specific gravity of A:B:C powder is 1.8. In an extinguisher, these materials will settle according to their specific gravity, with the heaviest settling to the bottom. Since the A:B:C powder is heavier than the nitrogen gas it will settle over time.
4. IF THE POWDER COMPACTS, HOW DOES THIS AFFECT THE DISCHARGE OF THE EXTINGUISHER?
In side cartridge operated systems, the nitrogen gas is introduced to the agent tank through a tube designed to break up and fluidize the dry chemical before it is forced out the bottom of the tank.
Stored pressure units depend upon the downward pressure of the nitrogen gas to force the compacted powder back up a siphon tube and out the extinguisher.
5. HOW ARE THESE EXTINGUISHERS INSPECTED? DOES THE INSPECTION REQUIRE SPECIAL EQUIPMENT?
The National Fire Protection Association, in its Standard No. 17, the Standard for Dry Chemical Extinguishing Systems, outlines the monthly and semi-annual inspection requirements for both stored pressure and side cartridge operated systems.
In a side cartridge system, the dry chemical is inspected every six months to insure it is not contaminated or clumped. These inspections are performed on site and take just a few minutes per agent tank. The chemical does not need to be replaced unless it shows signs of moisture.
Stored pressure extinguishers must be de-pressurized, the valve assemblies rebuilt and the dry chemical removed and replaced every six years. Because special equipment is required to perform this service, the extinguisher must be removed from the installation and taken to a service facility. No periodic inspections are performed to determine if the dry chemical has been contaminated, is clumped, or if the siphon tube has been damaged.
6. HOW IMPORTANT IS IT TO HAVE A GAUGE ON THE EXTINGUISHER? ARE THEY RELIABLE?
Pressure gauges are used on stored pressure units to indicate if they are properly charged. The accuracy of these gauges is reflective of their cost - less than $1 each. They often experience issues with sticking needles and improper readings. Relying on these gauges to indicate the readiness of the system, or even just the extinguisher, would be risky and necessary maintenance can be overlooked due to a faulty gauge. There is no substitute for a thorough examination of the system on a regular periodic basis for assessing its readiness.
7. HOW LIKELY IS IT THAT THE NITROGEN WILL LEAK OUT OF EACH TYPE OF UNIT?
Side cartridge units are sealed with a bronze disc that does not deform. The cartridges are weighed at each inspection to insure they are properly sealed.
Stored pressure extinguishers depend upon rubber “O” rings on the valve stem and in the valve assembly to seal the compressed nitrogen gas in the extinguisher. These “O” rings can be deformed by vibration or improper installation, allowing the pressure to leak. A vent is present on the control head to bleed off pressure so it does not pre-maturely discharge the agent cylinder. A pressure gauge must be used to monitor the amount that has leaked.
8. HOW DO I DECIDE WHICH IS BETTER FOR MY APPLICATION?
The difference between side cartridge and stored pressure extinguishers is found in more than just their design. Each type will do well when used in the right application and in the right environment. The secret is knowing when and where to use each type.
Side cartridge systems are preferred industry wide in the mobile heavy equipment market for the following reasons:
- heavy duty construction
- simple design eliminates failure points
- easily serviced on-site
- low maintenance
- no special tools required
- over 40 years of fire protection
Stored pressure units are better suited as handheld portable extinguishers and in stationary situations such as protecting restaurant range hoods and electrical switching stations.