Dry Chemical Extinguishers Explained: Portable, Wheeled, Cartridge-Operated and Fixed

Dry chemical extinguishers run from a 2.5 lb hand unit on a forklift to a fixed system holding two tons of potassium bicarbonate over a loading rack, and the agent inside is the same powder all the way up. What changes is the quantity, the discharge rate, the reach of the hose, how the unit is pressurized, and which listing standard applies. A portable under UL 299 gives one person 10 to 25 seconds of agent. A wheeled unit gives two people about 45 seconds and a 50 ft hose. A skid gives a crew a minute to several minutes depending on capacity and nozzle, over 100 ft of hose and a UL 1254 listing as a system. A fixed system takes the person out of the loop entirely.

Portable dry chemical extinguishers: the first minute of any fire

Hand portable extinguishers are the units on the wall and on the vehicles. Capacities run from 2.5 to 30 lb of agent, with 10 and 20 lb the most common on an industrial site. Most are stored-pressure: the agent and the nitrogen sit together in one cylinder, and a gauge on the valve shows the charge. Cartridge-operated portables keep the shell unpressurized in storage and are preferred in dirty or high-vibration environments.

The rating on the nameplate is worth reading, and worth not over-reading. A 20 lb Purple K portable commonly carries 120-B:C, the number being the square feet of pan fire a trained operator put out in the UL test. A 20 lb ABC unit from the same maker may carry 10-A:120-B:C, and a 20 lb sodium bicarbonate unit 120-B:C as well. The agents are not equal; the ratings converge because the test is limited by what one person can do in twenty-odd seconds. The gap between potassium and sodium shows up in larger equipment, where the operator is no longer the constraint.

Discharge lasts about 10 to 25 seconds depending on size, and effective range is 15 to 21 ft. The operator can work in bursts, but the agent on board does not change, and a 20 lb unit is empty well inside half a minute of actual flow. NFPA 10 sets the placement, with travel distance to a Class B unit limited to 30 or 50 ft depending on the rating and the hazard level, and requires monthly inspection, annual maintenance, a six year internal examination on stored-pressure units and a 12 year hydrostatic test. Cartridge-operated portables get an internal examination every year rather than every six, which is a real cost difference on a site with a hundred of them.

What a portable is for is the fire that was just discovered. A spill at a pump seal, a fire in a motor, a flash on a hot surface. What it is not for is anything that has grown beyond a few square feet, because the agent runs out before the fire does, and the operator who discovers that at 15 ft has nothing left but a retreat.

Wheeled dry chemical extinguishers: more agent and a real hose

A wheeled extinguisher answers the half-minute problem. Capacities run from 50 to 350 lb, with 150 lb the common size on refineries, airports and loading terminals. Almost all are cartridge-operated or nitrogen cylinder operated: the tank stays unpressurized until the operator opens the cylinder on the frame, which pressurizes the tank and pushes agent up the hose. The hose is 25 to 50 ft and ends in a shut-off nozzle, so the operator can work in bursts, move, and start again. A 150 lb Purple K unit runs roughly 45 seconds on 50 ft of hose, throws a stream 25 to 35 ft and is rated 320-B:C. Seven times the agent of a 20 lb portable buys under three times the rating, which is the clearest illustration there is of how much of a portable's performance is really the operator.

Wheeled units are listed under UL 299 like portables and placed under NFPA 10, which recognizes them where more agent or longer range is needed: flammable liquid storage, aircraft ramps, process areas. In practice they take two people, one to pull and charge the unit and one to run the hose. The hard limits are the wheels and the weight. A 150 lb unit with its frame weighs well over 300 lb, moves on hard surfaces only, and will not cross a curb, a pipe trench or gravel without a fight. A site that relies on them keeps them beside the hazard and keeps the route clear.

Cartridge-operated and nitrogen-expelled skids: the system tier

A dry chemical skid is where the equipment stops being an extinguisher and becomes a system. The agent tank, fabricated to ASME Section VIII, holds 350 to 2,000 lb of powder at atmospheric pressure. Nitrogen sits in separate cylinders at 150 to 200 bar behind a regulator, and on actuation it enters through a fluidizing manifold at the base, lifting the bed into suspension before the discharge valve opens. The hose is 50 to 150 ft on a reel, the nozzle is a shut-off type, and the flow rate is several times a wheeled unit's, which is where the knockdown speed on a pool or a spraying leak comes from.

The listing is the part that matters to the AHJ and the insurer. A skid is listed to UL 1254 as a pre-engineered dry chemical extinguishing system unit, which means the tank, nitrogen supply, hose, nozzle and agent have been fire tested and discharge tested together, and the unit is designed and maintained under NFPA 17 rather than NFPA 10. A Salus Global skid comes in 350, 1,000 and 2,000 lb capacities, and is filled with the agent the hazard calls for, Purple K as the usual choice for petrochemical and LNG. Moving up from wheeled units buys four things: far more agent, a hose that reaches around a pump row instead of to the end of it, a discharge long enough to hold the fire down while a valve is closed, and a package that can be forklifted onto a jetty or a drilling pad and left there.

The trade-offs belong to the agent rather than the tier: no cooling, residue on everything, and wind taking a share on an open deck. A skid still needs a trained operator and still is not automatic, so where nobody will be within reach in the first minute it is the wrong choice. Sizing works from the hazard outward: the area and the fuel set the flow rate and the discharge time, and those set the agent quantity, so a unit picked by tank size alone is usually the wrong unit.

Fixed dry chemical systems: nobody has to be there

A fixed system replaces the hose with piping and nozzles aimed at the hazard, and replaces the operator with detection. Flame detectors, heat detectors or a fusible link line trip the nitrogen release, the tank pressurizes, and every nozzle discharges at once for the listed minimum time, which for most systems is measured in tens of seconds. Capacities run from a few hundred pounds over a dip tank to engineered systems of roughly 4,000 lb over an LNG impoundment or a long loading rack, and Salus supplies large dry chemical powder systems in that range. Engineered layouts are proven by discharge test at commissioning, because powder separates in piping with too many bends.

The case for a fixed system is the hazard that is unattended, unreachable or too fast for a person: a compressor in an enclosure, a transformer at the back of a substation, a hot oil pump under a pipe rack, a fueling point on an unmanned platform. The case against is cost and rigidity, since the system covers the area its nozzles were aimed at and nothing else. Many sites pair a fixed system over the one hazard that justifies it with a skid at the end of the row for everything else.

The four tiers side by side

	Portable	Wheeled	Skid	Fixed system
Agent capacity	2.5 to 30 lb	50 to 350 lb	350 to 2,000 lb	Hundreds of lb to roughly 4,000 lb
Pressurization	Stored pressure or cartridge	Cartridge or nitrogen cylinder	Separate nitrogen cylinders, 150 to 200 bar	Separate nitrogen cylinders
Delivery	Short hose or horn, 15 to 21 ft range	25 to 50 ft hose, 25 to 35 ft range	50 to 150 ft hose on a reel	Fixed piping and nozzles
Discharge time	10 to 25 seconds	About 45 seconds	One to several minutes, set by capacity and nozzle flow rate	Listed minimum, typically tens of seconds
Operators	One	Two in practice	One or two, trained	None, automatic
Listing and standard	UL 299, NFPA 10	UL 299, NFPA 10	UL 1254, NFPA 17	UL 1254 or engineered, NFPA 17
Typical hazard	Incipient fire anywhere	Flammable liquid storage, ramps	Pump rows, racks, jetties, wellsites, LNG	Unattended or enclosed process hazards

Deciding which tier a hazard needs

First, size the fire the equipment has to handle, not the fire anyone expects. A pump seal leak that is caught at once is a portable's job. The same leak found after the pump has sprayed diesel over a 200 square foot pad for a minute is beyond any portable, at the limit of a wheeled unit, and inside a skid's capacity. Second, measure the route. A wheeled unit that has to cross gravel and a pipe trench to reach the rack is, for practical purposes, not there. Third, look at who is around. A manned loading rack during the day shift can rely on a skid. The same rack at 3 a.m. with one operator in the control room needs a fixed system or detection that brings the brigade.

NFPA 10 and NFPA 17 are not in competition. Portables stay on the wall because OSHA's 29 CFR 1910.157 and most insurers require them, and because most fires start small enough to be worth one. Skids and fixed systems are for the hazards a site has decided it cannot afford to lose, and the upgrade from wheeled to skid is usually made after a fire rather than before one. On cost, the useful comparison is not skid against wheeled unit but skid against the loss of one pump, one transformer or one loading bay for the weeks it takes to replace it.

Salus Global dry chemical skids are UL listed and assembled, filled and inspected at our facility in Texas. Call +1 360 901 9828 or email info@salusglobal.co.