Simply described, if the water and foam concentrate are supplied to a venturi proportioner at the same pressure then accurate proportioning is possible over a reasonably wide range of flows. There are two basic versions of this type of system, pump systems and bladder tank systems.
5 Diameter Rule. This applies to all venturi proportioners and to high back pressure foam makers as well – you must have five (5) diameters of pipe connected to the inlet and outlet of the proportioner, with no diameter change, and no valves or fittings within 5 diameters of the proportioner. If it is a 100mm proportioner, you will need 500mm of 100mm pipe from the end of the proportioner before any valve or fitting and 500mm of 100mm pipe on the inlet side.
For any venturi type proportioner (and high back pressure foam makers) it is impossible to get a good pressure reading closer than 5 pipe diameters to the outlet of the proportioner.
Pump systems are relatively expensive but can be very economic and reliable for large centralised facilities. They are usually dual pump, so that one pump can be taken out of service for maintenance without compromising the fire protection, thus providing a very high level of up-time. Foam pumps are most commonly gear pumps, though vane pumps are also sometimes used on lower pressure systems. The proportioners are a venture device, so the five-pipe diameter rule applies to installations (there are 1 or 2 newly developed devices where less straight pipe is needed, but if in doubt, use the 5 diameter rule).
Careful hydraulic design is important for high viscosity foam concentrates on the pump suction side and for long pipe runs on the pump discharge side.
Maintenance costs are high for pump systems.
This type of system probably has the highest reliability rating.
Bladder tanks use a pressure vessel with internal bladder and the tank is pressurised from the fire water system, thus meeting the balanced pressure requirement. They are much lower maintenance than foam pump system but, due to lack of training, there are frequent operational problems that lead to damaged bladders. Tank design has improved to reduce this risk in recent years.
The five-pipe diameter rule generally applies, with rare exceptions.
Pipe runs for water into the tank and concentrate out must be kept short to avoid breaking the pressure balancing rule. Friction losses should be less than 35 kPa in total (water inlet & foam concentrate outlet)
The proportioner must be above the tank or must be installed with a hydraulic concentrate valve.
The biggest problems with bladder tanks occur when there is air in the tank and the system is operated or when someone inadvertently opens a valve they shouldn’t touch. These problems have given this type of proportioning a bad reputation. In recent years bladder tank manufacturers have incorporated features to reduce the risk of bladder rupture.
Bladder tanks are hard to fill, so they can’t be topped up during fire fighting operations.