Proportioner
The foam compound admixer is a faucet for conveying water that is used to generate extinguishing foam . The proportioner sucks in the foam concentrate and mixes it with the extinguishing water in a certain ratio.
The addition - or the achievement of the correct amount of foam concentrate in the foam concentrate / water mixture - is an extremely important component of foam generation. If the percentage of foam concentrate added is too low, the foam concentrate-water mixture is “too thin” and cannot form a stable foam. If the proportioning rate is too high, the foam becomes too stiff and does not flow sufficiently. Overall, various admixing systems and devices are in use for mobile and stationary applications.
Mixer types
Injector mixer
Injector mixers are the most frequently used proportioning devices. They can be roughly divided into:
- portable proportioner
- stationary built-in proportioner
- Foam generator with self-priming
- Pump premixer
function
The mode of operation is that of a jet pump . Here, motive water is accelerated as it passes through a nozzle and injected into a suction chamber. The water swirls in the suction chamber and transfers part of its energy to the medium in the suction chamber (first air, then foam compound), which is accelerated in the direction of flow of the motive water and leaves the mixer. The special construction of the suction chamber ensures the formation of a negative pressure through which the foam concentrate is sucked from the storage container into the proportioner. A non-return valve (ball valve) prevents the water from flowing back from the proportioner into the storage tank when motive water is available, but no foam compound / water mixture is drawn off.
In the case of mobile proportioners, a control valve is usually installed in the foam suction line in order to be able to react to any changes in the proportioning rate that may be necessary. As a rule, stationary extinguishing systems work with fixed proportioning rates that are tailored to the type of extinguishing system, the foam compound and the material being burned. Changes to the proportioning rate are made by replacing the motive water nozzle.
advantages
- Most inexpensive type of proportioner
- Simple application
- Minimal maintenance (only the non-return valve and possibly the control valve are moving parts)
- No separate energy supply necessary (energy for foam concentrate suction is taken from the motive water)
- Universally applicable for all common types of foam concentrate
disadvantage
- Pressure drop of up to approx. 35% of the inlet pressure (energy for foam concentrate suction is taken from the motive water)
- High counter pressure (p> 2 bar, e.g. friction losses in the hose after the proportioner) prevents the foam concentrate from being sucked in
- For technical reasons (combination of motive water nozzle / suction chamber) only designed for a specified flow rate
Portable proportioner
When the firefighters portable proportioner according to DIN 14 384 are as a rule for the mobile handling in use.
These proportioners are used in three sizes:
| Type | Fixed couplings | Nominal flow rate |
|---|---|---|
| Z 2 | C. | 200 l / min |
| Z 4 | B (formerly: C) | 400 l / min |
| Z 8 | B. | 800 l / min |
Mixers consist of two C or B fixed couplings in the direction of flow (depending on the type) and a D fixed coupling perpendicular to the direction of flow. In addition, the proportioner has a metering device that is permanently set (2, 3, 4, 5 or 6%) or, as in most cases, a control valve that is continuously adjustable using a handwheel . To ensure a firm stand, the proportioner has a foot and a stand.
Retrofittable attachments (with Storz D coupling) also allow dosing in the range of adjustable mixing rates of 0.1%, 0.5%, 1% and 1–6%. This is necessary for special foam concentrates.
It must always be installed in the correct direction (marked by an arrow) and matched to the size of the foam nozzle. At least one length of hose must be connected to the outlet of the proportioner so that the water and foam concentrate can mix sufficiently. The proportioner must be rinsed thoroughly after each use, otherwise the foam concentrate will stick to the ball valve and the proportioner will no longer function.
Stationary installed proportioner
Stationary built-in injector mixers are used in extinguishing systems in which a fixed amount of foam compound / water mixture must be generated. This is e.g. B. the case with tank fire protection (foaming of tank cups or floating roof tanks) when extinguishing water and foam compound can be conveyed separately through separate lines to the point of fire, mixed on site and thus the counter pressure is kept low. Outputs of up to several thousand liters of foam concentrate / water mixture per minute are possible here (e.g. MI-150 F mixer from Skum: 6500 l / min).
Foam generator with self-priming
Self-priming foam nozzles have a built-in device for drawing in foam concentrate. These are primarily used with foam / water cannons, especially on trailers. The suction takes place through an injector mixer, which is not built up separately, but is part of the foam generator. The foam concentrate is stored in the immediate vicinity of the foam generator and sucked in directly through hoses. The advantages are the inexpensive design, the simple operation and maintenance, as well as the exact admixture, since the flow rate and back pressure are known. In addition, only the foam generator is contaminated with foam concentrate. A disadvantage is the lack of mobility due to the fact that the foam compound container is carried along. In addition, the tracking of the foam concentrate to the point of use is problematic due to the large amount of consumption, since the foam concentrate usually has to be brought to the point of use by hand in containers. The foam gun is a combination of a proportioner and a foam nozzle .
Pump premixer
Pump premixers are installed in fire trucks and extinguishing systems, among other things. In terms of their mode of operation, they are injector mixers. However, they are not integrated into the delivery line; instead, part of the extinguishing water is branched off at the pump outlet and flows through a bypass to the mixer. Here, by creating the negative pressure, foam compound is sucked in and mixed with the motive water in a concentration of up to 40%. The outlet of the proportioner is connected to the pump suction line so that the enriched foam concentrate / water mixture is fed to the sucked in water. The proportioning rate can be set via a control valve in the foam concentrate suction line to the proportioner. The low cost of the system as well as the simple maintenance and operation are advantageous. There is no pressure drop due to the admixture and an additional power supply is not necessary, as the energy is taken from the motive water. The extraction of the motive water has a disadvantageous effect on the available delivery rate of the pump, since the removed motive water and circulated is part of the total delivery rate (up to 10%) and thus reduces the amount of water available for extinguishing purposes. This must be taken into account when designing the pump. In addition, the pump inlet pressure must be below a certain value (max. 0.5 bar), as otherwise the pressure gradient between the inlet and outlet of the proportioner is insufficient to ensure that the proportioner functions correctly. In addition, the entire system is exposed to a foam compound / water mixture and must then be cleaned.
Pump admixture
In extinguishing areas in which fixed volume flows and pressure ratios are specified and pressure losses that result from the use of an injector mixer are not acceptable, it can be useful to add the foam concentrate directly to the water flow. Since this admixing does not require a regulating device, the correct foam agent / water ratio must be achieved through the precise design of the water / foam agent pump combination. However, this is very difficult since the delivery rate of the foam concentrate pump depends on the temperature (and thus the viscosity), the specific weight of the foam concentrate and the pressure difference between the water and the foam concentrate. The advantage here is that the amount of foam concentrate / water mixture is not subject to any upper or lower limit. All volume flows can be displayed from the smallest to the largest amount.
The further development of this type of mixer is the controllable pump mixer. The volume flow of the extinguishing water is recorded by measuring devices and the foam concentrate volume is adjusted by regulating the foam concentrate pump accordingly. This is usually done by changing the speed.
A special form of pump admixing is the use of mechanical metering systems, in which a water motor is driven by the extinguishing water flow, which in turn drives a foam concentrate pump. Exact mixing is achieved by adjusting the ratio of the volume of the water motor to the volume of the foam concentrate pump. Here, however, there is a pressure loss (energy to drive the foam concentrate pump) which, however, is in the range of 0.5 bar to 2 bar (depending on the flow). It is important here, however, that these admixing systems are insensitive to back pressure, that temperature, viscosity and specific weight of the foam concentrate are irrelevant and that they have a wide range of uses (200 l / min to 6000 l / min water flow and 0 - 6% admixture depending on the device). These systems are available as turbine mixers (V ~ 400 l / min) or as systems with positive displacement motors and pumps.
Differential pressure mixer
Differential pressure mixers are usually used in stationary extinguishing systems, less often in mobile systems, in which the pressure conditions or flow rates change during operation.
Depending on the foam concentrate delivery, a distinction is made between bladder tank mixers and differential pressure mixers.
Bladder tank mixer
The proportioning unit is divided into two parts: the bladder tank and the proportioner. The bladder tank is a pressure vessel that contains a rubber bladder filled with foam compound. There is a screen in the mixer in which a pressure gradient builds up when the extinguishing water flows through. A part of the extinguishing water is removed in front of the screen and directed into the space between the container wall and the rubber bladder de bladder tank. As a result, a higher pressure is built up on the foam compound in relation to the pressure behind the screen of the proportioner. This pressure difference causes the foaming agent to be conveyed to the proportioner and into the extinguishing water behind the cover. The volume is regulated by the variable pressure difference at different volumes (low flow rate = low pressure difference = low flow rate, large flow rate = large pressure difference = large flow rate) (see also measuring orifice ).
advantages
- Few moving parts
- Low pressure drop (> 1 bar)
- Wide range of applications (volume flow up to 30,000 l / min)
- All types of foam can be used
- There is no external energy supply for foam concentrate delivery
disadvantage
- Refilling and especially refilling during operation difficult or impossible
- The rubber bladder is sensitive to the long-term effects of the foam concentrate and to mechanical stresses
- The tank must be one (or more) pressure vessels that can withstand the operating pressure of the extinguishing system.
Differential pressure mixer
Unlike the bladder tank, this proportioning unit is divided into three parts: foam concentrate tank, foam concentrate pump and proportioner. The foam concentrate is fed to the admixer by the foam concentrate pump at a higher pressure than the operating pressure of the extinguishing system. There is also a screen in this proportioner in which a pressure gradient builds up when the extinguishing water flows through. The pressure of the water in front of the screen regulates the foam concentrate pressure (to approx. 1 bar above the water pressure in front of the screen). This creates a constant pressure ratio between extinguishing water and foam compound. If the flow rate increases, the pressure ratio of foam compound / water in front of the orifice remains the same, while the water pressure behind the orifice decreases. As a result of the then greater foam compound overpressure compared to the water pressure behind the screen, more foam compound is pumped into the extinguishing water through the constant foam compound nozzle.
advantages
- Precise mixing regardless of pressure and flow
- Easy construction
- Refilling of the foam concentrate supply possible during operation
- Low pressure drop
- Tank only needs to be resistant to the foam compound (no pressure vessel)
- Wide range of applications (volume flow up to 30,000 l / min)
- All types of foam can be used
disadvantage
- External power supply for the foam concentrate pump
- Under certain circumstances higher pressure drop with large flow fluctuations
Multi-range bladder tank mixer / multi-range active pressure mixer
The multi-range bladder tank mixers and multi-range active pressure mixers are a further development of the bladder tank mixers and differential pressure mixers. By applying a different working principle, the working area of the proportioner is expanded, especially in the lower area. (Differential pressure mixer NW 200: 3000 - 13000 l / min, multi-range active pressure mixer: 150 - 11000 l / min). Instead of a fixed screen, the opening of a variable foam concentrate nozzle is adjusted according to the flow rate via a throttle valve. Since the pressure conditions upstream and downstream of the throttle valve remain largely the same, the foaming agent quantity corresponding to the water flow rate is added to the extinguishing water at a constant foam compound overpressure through the variable foam compound nozzle.
Advantages disadvantages
- see bladder tank mixers and differential pressure mixers
Basic rules for the selection of suitable admixing systems
- Small risks, mobile devices without complex technology, no water supply, only use of foam: premix systems
- Fixed flow rate, fixed pressure ratios, larger pressure drops tolerable, easy handling: injector mixer
- Variable flow rate, large pressure drop unacceptable and only foam compound / water mixture at the pump outlet: pump premixer
- Fixed flow rate, large pressure drop unacceptable and only foam compound / water mixture at the pump outlet: direct pump admixture
- Variable flow rate, variable pressure ratios: differential pressure mixer / bladder tank mixer
- Variable flow rate, variable pressure ratios, variable proportioning rates: multi-range effective pressure mixer / multi-range bladder tank mixer
literature
- Lutz Rieck: Die Rote Hefte, booklet 06 - fire extinguishing fittings . Kohlhammer, Stuttgart 2000, ISBN 978-3-17-015171-0 , pp. 91-94 .
- Holger de Vries (ed.): Fire fighting with water and foam. 3rd, revised edition. ecomed-Verlag, 2008, ISBN 978-3-609-68742-1 .
- Josef Helpenstein, Silvio Faulstich: Fire fighting with foam. (= Fire brigade magazine. Special issue 1/2010). 2nd updated edition. 2010, DNB 1028550774 .
- W. Hamilton (abbreviated): Handbook for the fire service. 21st edition. Boorberg Verlag, 2012, ISBN 978-3-415-04560-6 .