The service life of a firefighting vehicle varies between 15 and 25 years. That is a long time, with regard to the technological innovations in vehicle construction and the ever-increasing and changing demands on fire departments. Many wish lists for new fire and tanker trucks are therefore made according to the Jack-of-all-trades principle: one vehicle for all situations. In most cases, however, this is not so easy to achieve, and this approach has more cons than pros. Let’s look at what really matters.
When planning and procuring a new fire truck, it is important to think about the vehicle’s jobs that need to be done. You could also say: ‘What is the core business of my fire department?’ With this approach, a useful vehicle can be designed for almost all operational scenarios.
Next, the specifications must be prepared. One item in the specifications will be the extinguishing technology. The performance characteristics of the fire pump, the number of pressure outlets and the size of the water tank are specified by the standard. However, when it comes to foam and its proportioning, the fire department is free to choose. While the minimum quantity of foam concentrate loaded is specified by the standard, the choice of proportioning technology is not restricted, so that Z (venturi) proportioners, pressure proportioners, CAFS, etc. are available for selection.
The foam agent also determines the proportioning technology
Whereas in the past, foam agents with a proportioning rate of at least 3% or 6% were commonplace, today’s modern foam agents are mainly based on synthetic ingredients and are mixed into the extinguishing water at 1% or even less. At the same time, the range of applications for foam agents has expanded.
Twenty years ago, foam agents were mainly used to fight liquid fires, but nowadays, they are also used for fires involving solids (fire class A). If you want to use the new foam agents in a variety of ways now, the classical Z (venturi) proportioner is out of the question. Although it is small, lightweight and inexpensive to purchase, it has too many disadvantages.
The classical proportioner
A Z4R proportioner is designed for an operating point of 400 l/min. If this flow rate is undershot or exceeded, foam does come out of the foam pipe but it will be of inferior quality as the foam agent is underdosed. This has a direct effect on the extinguishing performance.
The same applies to hose lines longer than 40m (2 B lengths) or, for example, for a foam attack via the turntable ladder. The back pressure is too high and the pressure difference is no longer sufficient to draw in enough foam agent using the Venturi principle. Another disadvantage is the high-pressure loss of at least 30%. This has an impact on the throw distance, or the fire pump has to work harder to reach the minimum pressure of 5 bar at the ejector. The foam quality is influenced by this as well as by the proportioning rate.
Around-the-pump foam systems
‘Around-the-pump’ foam systems are ruled out for standard fire trucks when looked at more closely. In this system, the foam agent is injected into the fire pump and all connected pressure outlets are supplied with premix. This type of proportioning is often found in airfield fire trucks, which perform a foam attack using one or more firefighting monitors when needed. The proportioner only works when using a water tank or water supply via a hydrant. After use, the entire system, from the fire pump to the discharge device, must be flushed with clean water.
Compressed-air foam systems (CAFS)
Compressed-air foam systems (CAFS) certainly have their right to exist. The compressed air foam is already generated in the fire truck and conveyed as a finished air-foam mixture into the hose line to the jet pipe. This makes the hoses lighter than they would be if filled with water, and the compressed air bound in the hose or foam increases the throwing distance. Unfortunately, the CAFS system has a disadvantage compared to other systems. It consists of at least one foam proportioning unit and a pressure chamber with a compressor for the foam generation. This has an impact on weight and installation space in the vehicle. The electronic complexity of this system and the associated spare parts supply should neither be underestimated.
Pressure proportioners
Electronic and mechanical foam proportioning systems, the so-called pressure proportioning systems (PPS), are much more suitable here. They have a working range, i.e. they are not restricted to a specific water flow or output within their technical capacity. This way, a DZA 800 PPS is able to mix the foam agent correctly at a water output of 200–800 l/min. The pressure loss compared to Z (venturi) proportioners is relatively low and far below 30%. Since this proportioning technology works with piston pumps, so-called positive displacement pumps, even long hose lines or larger differences in height are no problem for proportioning. But there are also differences among the PPS.
After turning away from, or even prohibiting of, fluorine-containing foaming agents, such as AFFF, the products following are no longer liquid like water but behave more like honey or ketchup from a bottle. Special foam agents for alcohol fires have a very high viscosity. The piston pumps available on the market from the chemical industry can suck in these viscous liquids only very poorly, if at all. Here, a precise analysis and calculation by the system supplier of the PPS is necessary. In addition to the viscosity, this is also due to the decisive interaction of other factors, such as the suction height or the diameter of the suction line.
Electronic pressure proportioning systems Another point that may cause problems occurs when generating wetting agent by use of electronic pressure proportioning systems. These systems are driven via the vehicle’s on-board 24V electrical system. The 24V drive is not capable of driving the piston pump at low proportioning rates and flow rates below 200 l/min. This becomes very clear in the working diagrams of the pressure proportioners. Due to this, the electronic PPS are not able to generate wetting agent at low flow rates.
Mechanical pressure proportioners
An exception is made here by purely mechanically driven pressure proportioners. Depending on their size, they can produce wetting agent reliably far below 100 l/min. This is related to the way how they are driven.
Mechanical pressure proportioners, such as the FireDos FZ1000, require no external energy. They are driven by the extinguishing water flow, which comes from the fire pump and is conveyed to the jet or foam pipe. The piston pump for the foam agent is driven by a water motor, a kind of turbine, using only the energy of the water flow. Thus, the water motor also serves as a flow meter at the same time. There is no need for an electronic measuring section. When the jet pipe is opened, the water flows through the water motor and drives the foam agent pump. When the pipe is closed again, the water motor also stops immediately and the proportioning is interrupted.
As there is a proportional dependency between the quantity discharged (the flow rate) and the foam agent proportioning, over- or underdosing is impossible. Depending on the quantity discharged through the jet pipes, the water motor or foam agent pump works faster or slower and always conveys the correct quantity of foam agent. This is a great advantage, especially in the case of low flow rates and/or the pulsating/intermittent extinguishing method.
Conclusion
In order to select the right proportioning system when planning and purchasing a new fire truck, a few basic considerations must be made in advance. It is important to be clear what the focus of your own fire department is on
- A factory fire department will probably focus more on foam and high flow/discharge rates. When planning crew-cab fire trucks, the focus is more on the demand for wetting agent and low flow rates.
- Today, installation size and weight play a major role in vehicle procurement with subsidies from the state. If two B pressure outlets are to be supplied with premix, the choice is a DZA 16 (PPS), which is relatively large and heavy and also has a high start-up limit for the proportioning. A DZA 8 (PPS), on the other hand, can only supply one B pressure outlet but is smaller, lighter and better suited for wetting agent.
However, the foam agent is the most important factor in the selection of the proportioner. The maximum proportioning rate of modern foam agents today is 1% or 3%. A legitimate question is whether it is still appropriate for proportioners to be designed for 6% as in the past because this may have some disadvantages. The foam agent pumps for 6% are, of course, larger and have an impact on the start-up limit and suction line dimensioning. Foam agent logistics should also be kept in mind when proportioning at 6%. At a flow rate of 400 l/min and 3% proportioning, a 120-litre foam agent tank is enough for a 10-minute firefighting operation. At 6% proportioning, it lasts only 5 minutes.
Making such considerations in advance pays off when planning a fire truck, as you will not be overwhelmed by the issue afterwards. This applies in full to the selection of proportioning technology: You must make one choice, with all the consequences.
About the Author
Tristan Reitz Dipl.-Ing. (FH) is Market Manager Fire Service and Mobile Fire Protection at FireDos.
