To fight small and medium size fires, it is necessary to direct water from a safe distance onto the fire area, in the required form (large/ small droplets). Pressurized water can be conveyed to the affected area using fire fighting hoses. However, to project it to the fire area, it is required to convert the pressure energy of the water into velocity energy. At the same time, it is necessary to project the water in the desired form i.e. jet, spray or fog, to ensure maximum effectiveness depending upon the situation. Handline firefighting branches and nozzles do the job of converting the pressure energy to velocity energy and projecting the water in the required form.
There is no standard classification of handline nozzles, as they can be sub-divided into different types depending upon on pattern, flow quantity, etc. Normally they are classified as follows –
- Solid bore nozzles (which discharge water in form of a solid jet only)
- Combination or Spray nozzles (or Jet/Spray type nozzles) which discharge in form of a hollow jet, narrow fog or wide fog. These can be further sub-divided into –
- Constant flow nozzles (those discharging same flow at a given pressure)
- Variable flow nozzles (flow can be varied manually for a given pressure)
- Automatic nozzles (flow varies depending upon pressure)
- Multipurpose nozzles (those discharging water in form of a solid jet as well as spray, even simultaneously).
In short branch with nozzle, the pattern of flow is only in form of a solid jet, which means that this branch is unable to project water in form of fog, or spray for protection. At the same time, due to the absence of a valve in the nozzle, it could not be shut off by the nozzle man, which means that he has no control over the branch.
The main advantage of the combination or spray nozzle is that it is provided with a pattern changeover mechanism as well as a shut off arrangement. The pattern changeover mechanism allows changing the pattern from jet to spray or vice-versa, while the shut-off mechanism allows the operator to shut down the flow, when required.
In the basic spray nozzle, the flow varies as the pattern changes from the hollow jet to the spray pattern (normally the flow in jet pattern is lesser and increases as the pattern changes to fog)
In certain combination nozzles, the flow changes with pattern. However, in Constant flow nozzles (also known as Single gallonage nozzles), the flow remains constant throughout the different range of discharge patterns i.e the flow will remain same whether discharging in form of a jet or spray.
Selectable flow nozzles incorporate an arrangement to vary the outlet orifice size, hence the flow can be varied by changing this orifice size. The operator can set the flow without shutting down the line, depending upon the pressure available, and other operational considerations. This means that the nozzle can be used effectively even in varying pressure conditions by the operator himself, without any dependence on the pump operator.
An automatic nozzle maintains a relatively constant pressure over a wide range of flows. Such nozzles have an arrangement in the nozzle which controls the flow by restricting the orifice outlet to maintain pressure.
If the pressure is relatively low, the nozzle orifice will automatically restrict the flow, causing pressure buildup at the outlet, and consequently resulting in a good quality discharge pattern. Similarly, if the pressure increases, the orifice opens to allow more flow, again adjusting to a constant pressure at the outlet of the nozzle.
Providing teeth on the nozzle outlet serves two purposes; first, in the narrow fog position, it deflects part of the water to the centre of the spray (which is otherwise hollow). Secondly, in the wide fog position, it caused the water curtain to break up into streams which then impinged on one another to create a better quality fog. A variety of different designs of teeth (fixed/ spinning/ flexible) can be used to create this fog.
There is no clear research data to prove superiority of spinning teeth over solid teeth. Certain trials show that spinning teeth rotating in the fog position, cause the break up of the water curtain into fine droplets, which are ideal for fire gas cooling operations. It may be noted, however, that there is no conclusive evidence on the superiority of spinning teeth.
For a long time, manufacturers have been using the ball valve for controlling the flow in the nozzle, as it is a proven design, operates well at the pressures used, and is relatively maintenance free. The ball can be of metal, or polymer based materials.
Another type of valve used nowadays is the slide valve, in which a hollow cylinder moves back and forth along the central axis, and seals against a polymer seat. Due to the movement of the cylinder, it causes a variation in the orifice gap between the cylinder and the seat and hence controls the flow.