Use Water Monitor When

Class A fires only (solids: wood, rubber, plastic) · Equipment cooling · Structural steel protection

Use Foam-Water Monitor When

Class B fires (petroleum, solvents, aviation fuel) · Tank farms · Loading racks · Anywhere flammable liquids can spill and burn

The Hard Rule

Water applied to a burning liquid surface makes it worse. If there is any possibility of a burning liquid fuel pool — specify foam-water

Step 1

Upstream Proportioning

Foam concentrate is injected into the water supply line upstream of the monitor at the correct ratio (3% or 6%) by a proportioner unit. The water and concentrate mix forms a foam solution.

Step 2

Air Aspiration at Nozzle

The foam solution passes through the monitor body and enters the air-aspirating nozzle. The nozzle draws in air through side openings — the venturi effect of the flowing solution creates suction that pulls ambient air into the stream.

Step 3

Foam Formation

The air-solution mixture passes through a screen or mesh that breaks it into millions of uniform small bubbles. This is the finished low-expansion foam — a stable, air-filled aqueous blanket ready for discharge.

Step 4

Blanket Application

Foam lands on the burning liquid surface and spreads horizontally, forming a continuous blanket that floats on the fuel. This blanket simultaneously excludes oxygen from the fuel surface and prevents fuel vapour from reaching the flame — starving the fire of both fuel and oxidant.

Foam Concentrate Storage Tank

A tank containing sufficient foam concentrate to supply the monitor system for the required discharge duration. Per NFPA 11 §11.6, fixed foam-water monitor systems at flammable liquid storage facilities must provide a minimum of 65 minutes of foam concentrate supply. For a single PL10/64 monitor at 64 L/s with 3% AFFF, the concentrate consumption is 64 × 0.03 = 1.92 L/s = 115 L/min — 65 minutes requires approximately 7,500 litres of concentrate tank capacity for that single monitor. For a multi-monitor system, the tank must supply all simultaneously operating monitors for the full duration.

Foam Proportioner

The proportioner injects concentrate into the water supply at the correct ratio (3% or 6%) to form the foam solution delivered to the monitor. Three types are commonly used:

Foam Solution Piping and Isolation Valves

The piping from the proportioner to each foam-water monitor must be designed to deliver the foam solution (water + concentrate mix) without demixing or degradation. Foam solution should not be stored in the pipe network for extended periods between activations — stagnant foam solution can deteriorate in the pipe. The system design should ensure that the monitor discharges either water-only (pre-flush) or foam solution, and that the pipe between the proportioner and monitor is flushed after each test or actual use to remove residual concentrate.

Can I use a water monitor for cooling at a petroleum tank farm and a separate foam-water monitor for suppression?

Yes — this is a common arrangement at petroleum tank farms. Water monitors (PS Series) are used for equipment cooling at positions where their specific function is to keep adjacent tank shells and structures cool during a fire involving another tank. Foam-water monitors (PL Series) are used at positions intended to suppress the burning liquid surface in the bund. The two monitor types operate from the same fire water ring main but have different upstream connections — the foam-water monitors connect to the foam proportioner system; the water monitors connect directly to the ring main without foam injection. Both types can be designed into the same system provided the hydraulics account for simultaneous operation of all monitors at their respective positions per NFPA 11.

Does a foam-water monitor have a shorter throw range than a water monitor?

Slightly — foam has more air resistance than water and the foam bubble structure is less aerodynamically efficient than a solid water stream. At the same flow rate and pressure, a foam-water monitor typically achieves a throw range 5–10% shorter than an equivalent water-only monitor. The PL Series foam-water monitors are rated at ≥60 m at their respective flow rates, compared to ≥60–85 m for the PS Series water monitors at equivalent flows. In practice this difference is rarely design-critical, as both exceed the typical monitor-to-target distances at most industrial facilities. Always verify the required throw range against the monitor’s rated range (with wind safety factor) for the specific installation geometry — see the fire fighting water monitor buyer’s guide for the sizing method.

What happens if you operate a foam-water monitor with water only — without foam concentrate upstream?

The monitor will discharge water with slightly increased turbulence due to the air-aspirating nozzle design, but will not produce foam. This is useful for system testing and for situations where the target hazard is Class A only and foam concentrate use would be wasteful. The air-aspirating nozzle has slightly lower throw range efficiency when operating on water-only compared to a streamlined PS Series water nozzle. For planned water-only operation, a PS Series monitor is the better choice — the PL foam-water monitor’s air-aspirating nozzle is optimised for foam production, not for maximum water-only range.

How often does foam concentrate need to be replaced?

Foam concentrate shelf life depends on the concentrate type and storage conditions. AFFF and FFFP concentrates typically have a stated shelf life of 10–25 years in sealed, undisturbed original containers stored at appropriate temperatures. However, the concentrate must be tested periodically (every 3 years per NFPA 11 §12.8) to verify it retains its proportioning ratio and foam quality performance. Any concentrate used in an actual fire or a full-flow test must be replenished immediately — concentrations discharged cannot be reused. Concentrate in partially used tanks must be tested before returning the system to service.

Our facility stores Class A combustibles but is adjacent to a fuel storage area. Which monitor type should we specify?

Specify foam-water monitors. The determining factor is whether burning fuel from the adjacent Class B area could spill or spread into your protected area — if yes, your monitors must be capable of foam attack. It is better to specify foam-water monitors and occasionally operate them on water-only mode than to specify water monitors and be unable to suppress a Class B fire that spreads from an adjacent area. The additional cost of foam capability (monitor nozzle upgrade + upstream proportioner + concentrate tank) is modest compared to the cost of a facility fire that water-only monitors cannot suppress. See also the advice in the fire monitor for petrochemical plants guide.