The humble, simple and inexpensive line proportioner is the most abused and misused foam proportioning device. Yet it is not that hard to design and install them correctly and then they will give decades of trouble free performance when used properly.
There are a few applications where they are a great option, but they must be designed and installed correctly. We have seen far too many of photos of incorrectly installed line proportioners, these days, most proportioners seem to be incorrectly installed.
The first important design rule is the 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 50omm of 100mm pipe on the inlet side.
Line proportioners must run at the design pressure plus or minus no more than 100 kPa. They are single flow, fixed pressure devices and the foam concentrate flow rate is constant (for all practical inlet pressures) so the proportioning rate changes when you change the inlet pressure (since the water flow changes). A pressure regulating deluge valve is a good method of dealing with this issue.
You must design the hydraulics with a minimum 30-35% (depending on the proportioner – check with the manufacturer) pressure differential across the proportioner. The proportioner is the flow controller in the system, not the downstream nozzles. This pressure drop is needed to power the foam induction process.
Line proportioners should not be used for sprinkler deluge systems. The chance of blocked nozzles is high and a few blocked nozzles will shut down the line proportioner since the additional restriction at the nozzles will reduce the pressure drop across the proportioner below the necessary 35%. Line proportioners are good for foam systems with large nozzles such as foam chambers on tanks, where the risk of blocked nozzles is much lower and they are often used foam hose reels.
There is just 100 kPa available to drive the foam concentrate flow into the proportioner, so the suction lift must be no more than about 1.5 meters when the foam tank is empty and concentrate piping must be short. Careful concentrate supply piping design is critical for high viscosity concentrates.
Mount the proportioner on top of the foam tank or the concentrate will drain into the proportioner.
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.
Since the line proportioner uses a vacuum to draw in the foam concentrate, there is less than 100 kPa available for all processes. This means piping must be short, not restricted and the total suction height should be kept to 1.5 meters vertical. Elevation and friction losses should be less than 25 kPa.
Proportioners should always be mounted above the foam tank. Mounting them below the level of the foam concentrate will be expensive.