Purple K vs Sodium Bicarbonate vs Monoammonium Phosphate
Three agents cover almost every industrial dry chemical decision, and the choice among them turns on two properties: how much Class B fire a pound of powder puts out, and what the residue does to whatever it settles on. Potassium bicarbonate wins the first test by a factor of about two over sodium bicarbonate and leaves a mildly alkaline dust that washes off, which is why Purple K fills most petrochemical, marine and LNG equipment. Sodium bicarbonate trades capacity for price, and it suits a site that wants the same powder in every extinguisher and skid on the property. Monoammonium phosphate is the only one of the three that touches Class A, and it pays for that with an acidic residue that rules it out near switchgear, instrument panels and exposed copper. Potassium chloride and Monnex are the outliers, one now rarely specified, the other bought where a smaller unit justifies the premium.
Class B performance per pound: why potassium beats sodium
All three agents extinguish a hydrocarbon flame the same way, by decomposing in the heat and releasing species that scavenge the free radicals that sustain combustion. The difference is how well the metal ion does that job, and potassium is the better scavenger. UL B ratings on portables hide that difference rather than showing it. Twenty pound cylinders of Purple K, sodium bicarbonate and ABC powder from one maker routinely all carry 120-B:C, because the rating fire measures the square feet a single person can knock down before the cylinder empties, not what a pound of powder is capable of. Move up to a wheeled unit or a skid, where the equipment rather than the person sets the limit, and potassium bicarbonate does roughly twice the Class B work per pound that sodium bicarbonate does. Published claims run from 50 to 100 percent better, and the top of that range is what the industry has settled on for design purposes. Monoammonium phosphate sits between the two.
On a skid or a fixed system the ratio carries through to the design. A 1,000 lb Purple K skid and a 1,000 lb sodium bicarbonate skid discharge at the same flow rate for the same time, but the potassium unit controls roughly twice the fire area in that time, or controls the same fire with agent to spare for the reignition that follows. Put the other way, a hazard that needs 1,000 lb of Purple K would need a tank of about twice that size filled with sodium bicarbonate, and the tank, the nitrogen, the frame and the footprint grow with it. Agent choice is therefore the first decision in sizing, ahead of flow rate and duration, because it sets the tank volume that the rest of the unit is built around.
Two more potassium agents sit alongside Purple K. Potassium chloride (Super K) performs about the same but its residue is more corrosive and it is rarely specified now. Urea potassium bicarbonate (Monnex) is the highest performer of the family, because its particles fracture in the flame and multiply their surface area, and it is priced accordingly. It turns up on high-value hazards, offshore for example, where a smaller unit is worth the premium.
Not sure which agent your hazard calls for? Ask a Salus engineer.
Fire classes: where ABC earns its name and where it costs you
Sodium bicarbonate and Purple K are Class B and C agents. They put out flammable liquids and gases and are non-conductive, so they are safe to use on energized electrical equipment. They have no rating on Class A, ordinary combustibles, because once the flame is knocked off a burning pallet the wood keeps smoldering and reignites as soon as the powder cloud clears.
Monoammonium phosphate adds Class A through a different mechanism. In the flame it breaks down into a metaphosphoric acid glaze that melts onto a burning solid and seals the surface from air, which is enough to secure wood, paper and most plastics. That is why ABC is in nearly every extinguisher in an office, a shop or a vehicle, and it is the right agent for a mixed hazard such as a maintenance workshop with a parts washer, or a warehouse with drums of lubricant on wooden racks. The cost is the residue. The same acidic glaze that seals a pallet attracts moisture and corrodes copper, aluminum and zinc, and on a motor control center, a transformer cabinet, an instrument rack or an engine with exposed aluminum, an ABC discharge turns a small fire into a replacement. The bicarbonate residue is mildly alkaline, brushes or washes off, and does the equipment little harm. ABC is therefore almost never specified on a process-area skid.
None of the three works on Class D. A magnesium, titanium or lithium fire needs a dedicated Class D powder, which is a different product and a different extinguisher, and applying a bicarbonate agent can make the metal fire worse.
Foam compatibility, residue and clean-up
Dry chemical is often used alongside foam, on a twin agent skid or on a site where the brigade follows a knockdown with a foam blanket. Most dry chemicals break down a foam blanket to some degree, because the powder's water-repellent coating and its alkalinity attack the bubble structure. Protein and fluoroprotein foams are the vulnerable ones; the protein-based concentrates that survive contact with powder were formulated for dual-agent use. AFFF tolerates dry chemical better than anything before it, which is why potassium bicarbonate with AFFF became the standard twin agent pairing in aviation, refinery and LNG service. Monoammonium phosphate is the least compatible of the three and is not used in twin agent work. A site running a fluorine-free concentrate should get compatibility data from both the foam supplier and the agent supplier rather than assume the AFFF result carries over.
Residue is the practical difference a maintenance manager sees the day after the fire. The bicarbonates leave a white or violet dust that is swept up and washed down. ABC leaves a yellow, sticky deposit that sets as it cools and has to be scraped from hot metal, and its acidity means anything with a circuit board or a bare aluminum surface should be cleaned within hours. On a skid discharge in a process area, clean-up is a nuisance with any agent and a repair bill only with ABC.
Cost and availability
Sodium bicarbonate is the cheapest agent per pound, Purple K costs more, and Monnex costs a good deal more again. ABC is priced close to sodium bicarbonate because of the volume produced for portable extinguishers. Agent is a small share of what a skid costs, which is dominated by the tank, the nitrogen cylinders, the hose and reel, the frame materials and the coating. The choice still drives the rest of the bill, because a sodium bicarbonate unit sized to match a Purple K unit is bigger in every dimension.
All three agents are widely available, and a skid can be recharged with agent from a different manufacturer provided it is the same chemical type and listed for the unit. What cannot be done is mixing them. Bicarbonate and phosphate agents react when combined in a moist tank and generate carbon dioxide, which can pressurize a sealed container, and a unit being converted from one agent to another has to be emptied and cleaned first.
Selection table
| Sodium bicarbonate | Purple K (potassium bicarbonate) | Monoammonium phosphate (ABC) | |
|---|---|---|---|
| Fire classes | B, C | B, C | A, B, C |
| Class B performance per pound | Baseline | About twice baseline | Between the two |
| Residue | Mildly alkaline, washes off | Mildly alkaline, violet stain until rinsed | Acidic, sticky, corrodes copper and aluminum |
| Use on switchgear and instruments | Yes | Yes | Avoid |
| Foam compatibility | Confirm the pairing; protein foams degrade under any powder | Standard twin agent pairing with AFFF; ask for data on fluorine-free | Poor, not used in twin agent service |
| Agent cost | Lowest | Moderate | Low |
| Typical specification | Budget Class B protection, kitchens before wet chemical, general extinguishers | Petrochemical, LNG, refinery, marine, aviation, most industrial skids | Offices, warehouses, workshops, vehicles, mixed hazards |
Choosing for a specific hazard
A refinery pump row, a truck loading rack, a jetty or an LNG transfer area gets Purple K. The hazard is Class B, the equipment around it is worth protecting from residue, and the performance per pound keeps the skid small enough to place where it is needed. Salus Global fills its 350, 1,000 and 2,000 lb skids with Purple K for petrochemical and LNG work and supplies sodium bicarbonate where a site standard calls for it. A remote tank battery on a tight budget with nothing delicate nearby can make a case for sodium bicarbonate, particularly if every portable on site already holds it. A fabrication shop, a vehicle bay or a warehouse with pallets and drums in the same aisle gets ABC, in portables and wheeled units rather than a skid, with bicarbonate units kept for the electrical room door.
Where the hazard is a large pool that needs to be secured after knockdown, the agent choice is only half the answer, and a twin agent skid that carries Purple K and a compatible foam on one frame is the configuration to look at.
Frequently asked questions
Is Purple K the same as potassium bicarbonate?
Yes. Purple K is the trade name that has become the generic term for potassium bicarbonate dry chemical, dyed violet to distinguish it from sodium bicarbonate. Any potassium bicarbonate agent listed for the unit can be used.
Can a sodium bicarbonate skid be recharged with Purple K?
Only if the tank is emptied and cleaned first and the unit is listed for potassium bicarbonate. The two bicarbonates do not react with each other, but residual sodium bicarbonate dilutes the Purple K fill, and the unit's listing and nameplate specify the agent. Converting from ABC to either bicarbonate is a more careful job, because the phosphate and bicarbonate react in the presence of moisture.
Why not use ABC everywhere, since it covers more classes?
Because the Class A rating costs corrosion. On a process unit, a compressor station or an offshore deck there is no Class A hazard worth the damage ABC residue does to instruments, switchgear and aluminum, and Purple K puts out more Class B fire per pound anyway. ABC is the right agent for mixed hazards with ordinary combustibles, and the wrong one around equipment that has to run after the fire.
Choosing between agents
Tell us the fuel, the hazard and what the residue would land on, and we will recommend the agent and the capacity.
