Watermist Systems Classifications
When selecting a watermist system special care must be taken to ensure your chosen system is fit for purpose. Watermist systems are often proprietary designs made by the manufacturers who designed them, they are not commoditised, one-size-fits-all solutions. This is why watermist is able to perform so well in so many distinct applications: because the properties of watermist are customised to the application, resulting in a variety of nozzles, positions and activation methods. They are developed and validated by their individual manufacturers. The list of classifications below is a glossary of the many different watermist configurations:
2.1.1 Local Application
A local application (or object protection) system is used to protect stand-alone equipment only, such as a diesel engine generator or a deep fat fryer, by discharging water directly onto the fire risk. Their use is advantageous when a specific risk is disproportionately higher than the risk in the rest of the space.
Local application solutions typically use open nozzles within a ‘dry system’ activated by a fire detection system, or automatic nozzles with thermal fuses. The nozzles may have different spray angles, positions and flow rates to maximise coverage and protection of the equipment in the event of a fire.
2.1.2 Volume Protection
Volume protection systems are used to discharge watermist into a volume to enable it to be entrained into the fire without relying on targeting. They may use automatic nozzles where the fire triggers the water discharge only in the vicinity of the fire or in higher hazard or sensitivity environments the nozzles may be zoned to allow for the quick or more strictly controlled discharge of one or multiple nozzles at once. Typical applications include areas such as machinery spaces, domestic and hotel bedrooms.
2.2 Operating Pressure
• low pressure is defined as less than 12.5 bar.
• medium pressure is defined as between 12.5 and 35 bar.
• high pressure ranges from 35 to 200 bar.
2.2.1 Low Pressure Watermist Systems
Despite their low pressure, similar to traditional sprinklers, these systems use different nozzle designs to create the small droplets.
Low pressure open nozzle systems are typically used for the protection of hazards involving flammable liquids such as gas turbines. The addition of aqueous film-forming foam (AFFF) will maximise extinguishing performance against these types of fires anywhere in the protected space. They are also well suited for the protection of rooms, corridors and spaces where light-to-moderate fire loads of ordinary combustibles exist.
2.2.2 Medium Pressure Watermist Systems
These systems were primarily used for local applications as good, low-cost alternatives to the more efficient high pressure systems. With a minimum operating pressure of 35 bar they were typically used for the protection of diesel engine generator sets, boiler fronts and oil separators etc.
2.2.3 High Pressure Watermist Systems
High pressure watermist is the most common implementation of watermist, using pressure to create the atomisation of the spray pattern.
Oxygen depletion is significantly increased by high pressure watermist at the flame front in both object protection and total flooding applications within completely enclosed spaces, particularly where the fire is large in relation to the space. Wetting of surfaces is also an important effect where control of deep-seated fires is a prerequisite. A further highly beneficial effect of high pressure watermist is the dramatic reduction of harmful products of combustion caused by the washing out of smoke particles from the fire. The components are frequently manufactured from stainless steel due to the required high pressures of operation.
2.3 In Nozzle Type
2.3.1 Open Nozzles
Low pressure watermist open nozzles are primarily used to protect flammable liquid fire risks such as gas turbines. Closed, ‘heat sensitive’ nozzles have been designed primarily for area protection (total flooding) applications.
2.3.2 Automatic Nozzles (mechanical)
The heat sensitive bulb is designed to break when it reaches a pre-determined temperature allowing the nozzle to discharge. This temperature is a function of the occupancy (residential rooms at 57C and saunas at 95C). It also requires the nozzle to be placed on the highest point in the volume as the bulb needs to be exposed to the hot smoke layer which is floating towards the ceiling.
Because only the nozzles exposed to the heat will activate, not all nozzles in a compartment will necessarily activate, only those in the vicinity of the fire. However, an assumed number of nozzles must be considered to activate as a function of the occupancy/risk and size of the area protected. This is because the nozzle is automatic but its activation cannot be controlled because it is mechanical. This is the Assumed Maximum Area of Operation (AMAO) and is a key performance requirement for the correct operation of a system by ensuring there will be enough water supply for the “number of nozzles activated” in a worst case scenario.
2.3.3 Electronic Nozzles
Although not very common yet, there are a few implementations of watermist where a controlled, limited nozzle, discharge is desirable. This can be done by electronically controlling the opening of the nozzle instead of making it an open nozzle with a section valve upstream linking to several nozzles. The elimination of
the uncertainty associated with uncontrolled and potentially unintended thermo-mechanical sprinkler has the potential to substantially reduce the water demand for the system, increasing reliability and reducing the cost of water infrastructure.
2.2.4 Watermist Pipes and Fittings
The material used for the pipework and fittings for watermist systems is governed by the nature of the estimated fire/fuel load and the water pressure required to protect the perceived risk. High pressure systems normally require the use of stainless steel pipework and fittings. Low pressure system pipes and fittings can be of zinc-plated carbon steel or copper or cpvc depending on the type of application and risk involved.
2.3 In Water Supply Methods
Watermist can be delivered either through a pre-pressurised system or a flow generating device (a pump) through the system’s distribution pipework to the discharge nozzles, using either a single or twin fluid delivery system.
2.3.1 Single Fluid Pumped
A single fluid system generates watermist by delivering water through the nozzle under pressure from either a low or high pressure pump system. It is important to note that safety concerns of pressurised systems (such as the EU pressure directive) do not apply as water is incompressible: there is not expansion with the change in pressure from high to low with an incompressible medium.
2.3.2 Single Fluid Pressurised
A single fluid system generates watermist by delivering water through the nozzle under pressure from pressurised cylinders. These do not mix with the water being pressurised, it is only used for flow generating purposes. Still, care with cylinder storage and condition is important because of the pressurised gas present.
2.3.3 Twin Fluid
A twin fluid system generates watermist by mixing an inert gas fed from separate pipework to the water supply pipework, that mixture being delivered through the nozzle under pressure. The inert gas helps the fire suppression effect by helping remove the oxygen from the fire environment, but it also creates a more hazardous atmosphere to occupants (if present) which is why this is typically used in places where occupant evacuation is not a concern.
2.4 In System Operation
2.4.1 Wet Pipe System (automatic nozzles)
This watermist system uses automatic, ‘heat sensitive’ type nozzles fitted into distribution pipework that is permanently pressurised with water. In the event of a fire, wet pipe systems are the most common water-based suppression configuration and are typically used to protect areas where temperatures are above 40C or are unlikely to fall below freezing point, for example: machinery spaces or in rooms, corridors and spaces where light-to-moderate fire loads of ordinary combustibles exist, such as offices and residential.
2.4.2 Dry pipe system (automatic nozzles)
This watermist system again uses automatic, nozzles fitted into distribution pipework that is permanently charged with air, nitrogen or pressurised gas. In the event of a fire, when a nozzle operates, the pressure drop within the distribution pipework actives the system control unit to release water into the pipework where it is discharged through the nozzle.
These are typically used in the same applications as wet pipe systems where the key constraint is the need to avoid freezing of water in the pipes. There is naturally a delay in flow of water because of the time it takes to fill the distribution network with water, which increases significantly with the size of the area being protected.
2.4.3 Deluge system (open nozzle)
A deluge system is designed to bring (one or more) open nozzles into action (simultaneously) in the event of a fire without the use of an automatic nozzle. This is achieved when a pilot line, fire detection and alarm or manual over-ride is activated. This in turn operates the main control unit to release the water through all the open watermist nozzles with an open path to the pump to effect rapid control and/or extinguishment of the fire.
Because the nozzles are open, the water distribution system is kept dry and unpressurised (no leaks, freezing risk) but as a result, there is also a delay while the water is being driven through the distribution system. However, in many cases, such as in local applications, the activation of the fire detection system (where flame, IR or UV detectors can be employed) is so much more sensitive than the frangible bulb (which has thermal inertia) that the overall time for discharge may be much smaller than from an automatic nozzle setup. This depends on the type of detection and distribution piping. Additionally, this setup predetermines the number of nozzles that will be discharged, so the area of operation is fixed, bringing certainty to the water supply requirements.
A deluge system may also have sections where only the nozzles in that zone where the fire was detected to activate. This allows for several areas to be protected but only one area (zone) discharges.
These systems are most frequently used in high fire load applications, such as recycling plants, where a delay in water flow through progressive nozzle activations could result in too little water discharge, too late, to successfully control the fire. There are also domestic systems which utilise of this principle to activate sooner than an automatic nozzle, thus requiring less water flow to address the fire.
2.4.4 Pre-action system (automatic nozzle) (or double interlock)
Such systems comprise a watermist installation plus an independent system of heat or smoke detectors installed in the same areas as the automatic nozzles. On receipt of a signal from two or more detectors, the main control panel automatically opens the control valves, allowing water to flow into the distribution pipework in readiness for the first watermist nozzle to operate due to the breakage of the frangible bulb.
These systems are commonly used for the protection of high value, ‘mission-critical’ areas such as data centres, server rooms or communications switches or where sensitive electrical equipment and goods are stored. They provide prior warning of system discharge (through the sounding of the alarm system) and prevent system discharge caused by accidental damage to a watermist nozzle’s thermal element or system pipework (by having no pressurised water in the distribution system).
2.5 In Design Method
Some watermist systems are modular in nature and therefore can have interchangeable parts as these extend into larger or smaller areas. By allowing interchangeability, there is the need to ensure that the chosen combination will work: that the intended amount of water will come out of the nozzles.
Engineered systems are those who require a designer to specify the different nozzle spacings, pipe diameters and pump specifications tailored for a specific project. This is to ensure that a) the correct density of watermist will be applied in the right location b) the water supply will provide the required water flow through the piping (pipe lengths and diameters, pump and tank sizing, etc). These allow for more flexibility in the areas of coverage but also require additional documentation in the form of hydraulic calculations to demonstrate that the system has been designed to work as intended. These hydraulic calculations will be a function of the AMAO, flow from each nozzle, nozzle type, hazard type etc. These are common in building or storage protection systems where the area of protection can vary significantly (how big is that warehouse again?).
These systems have predetermined flow rates, nozzle pressures, and water quantities regardless of the installation. This results in very little work to be done by the designer as there are usually no variations in pump sizes (or quantities) pipe diameters, or nozzle types. These systems have tighter specifications and variations in the area they can cover because of their pre-engineered nature. They tend to be designed to protect a specific piece of equipment, such as a turbine or engine, where the area of coverage is consistent, but it is also found on some domestic systems (designed only for domestic and with a limited area of coverage, so not residential).