- Uses and advantages of the Volumetric dosing systems
The mixer unit is proposed as alternative to the traditional mixing systems, such as Venturi systems, membrane tanks, variable flow systems.
The possibility of drawing up foaming liquid from any type of atmospheric stockage, without interruptions or any particular operations of re-filling during the emergency phase.
The machine is in fact able to draw up the foaming liquid even in negative inlet pressure.
Therefore pressure tanks, necessary in membrane type units, and also the pressurisation systems with pumps and distribution networks, necessary in pressure control units, are eliminated.
The advantage is evident in terms of cost, space and, above all, in the operability of the system.
With this equipment it is just necessary to install a minimum amount of foam liquid stock on site for the initial intervention and then refill the tank by means of connection with mobile vehicles, drums or containers.
There is a drastic reduction in the possibility of breakdown and maintenance; the total absence of a membrane for the separation of water/foaming liquid, calibrated diaphragms or regulating valves, together with the constructive simplicity and the choice of materials, reduce to the minimum the possibility of breakdown and maintenance work.
The membrane proportioner needs complicated maintenance operations (use of crane for lifting etc.) for the membrane removal. All these operations are no longer needed, saving money and time.
Simplicity in operation: the unit does not need any activation operation apart from the opening of the delivery valve of the foaming solution to the various utilities.
The system can adjust itself automatically to the required flow-rate.
Since the versatility in the injection capacity is independent to the up- and downstream pressure capacity of the unit, the proper modifications to get really needed output can be easily made.
It is possible to develop a foam mixing distribution network connecting several parallel mixing units; in this way, each unit’s flow-rate can be added to the others’ until reaching the required total flow-rate.
This type of application is ideal for fires in large scale plants due to the fact that foam mixture is available in all zones of the plant, and, as it is possible to activate
only the premixing units further away from the fire front, a continuous re-filling of foam solution by vehicles which can work at safe distances, is guaranteed.
Easy execution of periodical tests, since it is possible to re-cycle the foaming liquid in the tank without mistakes in handling and avoiding waste of product.
Versatility in mixing any kind of foam liquid, since the machine is calibrated for a mixing percentage fixed beforehand.
- Comparative table for premixing units
BLADDER TANK PREMIXER
VARIABLE FLOW RATE SYSTEM
1) Working Principles
Venturimeter + Calibrated orifice disk
Dosage according to pressure & flow rate (Venturi)
Volumetric ratio water/foaming liquid
2) External energy sources
Water under pressure
Water under pressure Foam under pressure
Water under pressure
3) External elements required for operation
Foam pumping units (electrical and/or diesel pumps) and distribution network
4) Type of foam stock
Local in pressure tanks with separator membrane
Local in any atmospheric container
5) Required conditions for mixing
Pressurisation of foaming liquid storage tank
Foam feeding network pressurisation
None. Complete autonomy in operation
6) Intervention autonomy
Limited by the capacity of the storage tank and drainage operation complexity
Limited by the capacity of centralised storage tank, which can anyway be refilled by vehicles
Unlimited. It can draw up foam in any conditions, also by negative inlet pressure
7) Reintegration of foaming liquid
Possible by doubling the storage tank in pressure. Otherwise it is necessary to stop foam output.
Complex operation requiring personnel in critical areas
Possible by transferring foaming liquid from vehicles to storage tank
Unlimited until foaming liquid stock is finished.
No interruption in foam output.
Foam reintegration by gravity
8) Foam concentrate refilling
Very complicated: one of the 2 tanks must be isolated, and water drained by removing it with the same amount of foaming liquid.
Operations made in critical phases.
Simple: Normal operations of foaming liquid transfer in security zones.
Simple: Normal operations of liquid transfer carried out at safety distance by means of mobile connections.
9) Variations field of foam distributions flow-rate
Generally, the calibration curves are not available
Generally, calibration curves are not available
From a minimum of 50 m^3/h to maximum 1000 m ^3/h obtained with single units
10) Mixing % shifting from nominal value upon variation of flow-rate
Operative tests have shown high shifts from nominal values. Foam gives worse performance and by a greater consumption intervention time is shorter
Differences in the nominal values are negligible and mixing is constant.
Calibration curves are available
11) Total pressure loss
Not always calibration curves referring to variation of flow rate (min./max.) are available.
Working tests have shown high pressure loss values during the max. flow rate
There are not always calibration curves
Pressure loss is very low.
Accurate calibration curves are available
12) Variation field of % nominal value
Complex. Only possible by using a throttle device on feed-line. Effect only theoretic as precision is very low
Only possible with throttle on feed-line effect theoretical as precision is low
Effective control of % as function is based on ratio of volumes
13) Starting-up operations
Complex. Controls and membrane venting operation necessary
Complex. Filling lines, venting, calibrating recycle valves, startup pumps
No particular operations
14) Maintenance operations
Complex. Removal of membrane needs several people and cranes.
Water & foaming liquid must be drained
Membrane substitution needs specialised workers & accurate calibration of dosage valve
No maintenance or calibration. Materials used & construction exclude need for maintenance
15) Movement error
Possible during refilling operations due to the number of valves and to
the movement sequence
Possible in pumping station
Practically none due to autonomy of machine
16) Hot areas: Countries or regions where summer temperatures exceed 40ºC
Temperature could melt compound protecting membrane joints;
foaming liquid is therefore going inside the membrane itself