Tank design for foam concentrates is a technical field. It might seem simple but it isn’t.
The goals for your tank depend on the type of foam concentrate:-
To maximise the life of your foam some understanding of the design issues is required. Tank materials are a contentious issue.
Here are some pointers.
Plastic Tanks: For long term use, polyethylene tanks need to be made from cross linked high density polyethylene (HDXLPE). To my knowledge there are no Australian manufacturers of HDXLPE tanks. If someone can prove they have HDXLPE tanks, we will advertise their tanks here for free. All tanks that we know of are linear low density polyethylene. You can expect 2-4 years service life before they fail. Be careful of imported tanks, some we have tried have cracked also. The failure mechanism is environmental stress cracking (ESC), induced by the foam concentrate, read more here… HDXPLE polyethylene is the only way to avoid this. Exposure to a surfactant is an accelerated test for ESC problems. LDPE is not compatible with surfactants (so not compatible with foam concentrates). If you think about it, there is substantial risk for the buyer when you select a poly tank, there is no easy way to verify that the material is HDXLPE, and if it is not, tank failure is guaranteed. Is it worth the risk?
Given the more strict environmental policing of foam concentrate spills (and the cost of the concentrate) taking a risk on plastic tanks probably doesn’t make economic sense. Plastic tanks are high risk.
Protein based foam concentrates have high levels of chlorides, often considerably higher than sea water levels. Despite the foam manufacturer’s recommending 316SS tanks, they fail, particularly in warmer climates. Chlorides attack the welded areas unless they are properly passivated. Even very careful passivation won’t protect your tank in hot climates.
316SS tanks are good for synthetic foam concentrates.
GRP tanks (isophthalic chemical barrier) are a good general purpose tank for all foam concentrates.
Pressure vacuum (PV) vents are essential for tanks. Open breathers are a bad idea, particularly for protein and high viscosity foams. The main decay mechanism for protein based foams is by oxidation, so minimising free air flow is essential. Similarly, the PV vent is required for high viscosity foams to avoid evaporation. For AFFF’s the PV vent prevents contamination and evaporation. The operating pressure for PV Vents needs to be as low as practical, in the order of 5 kPa.
For small applications single IBCs with can be used but the IBC needs a pressure vacuum vent fitted and UV protection. Multiple IBC tanks are a very bad idea read more here…
For protein and high viscosity foams, an expansion dome of 5% of the tank volume is highly recommended. The tank should be filled to about half way up the expansion dome to minimise the air contact area.
High viscosity foams have a unique challenge. Due to their high viscosity they have effectively no convection currents to remix the foam concentrate. If you have an open breather on the tank the top layer dries out and will not redissolve. A hard crust develops which will block proportioning equipment and there is no way to recover the foam concentrate. In closed containers the foam concentrate might develop a thin water layer on top. This is not product separation, instead some water evaporates and condenses but then does not remix into the concentrate. The process should stop with a thin layer of moisture on top of the concentrate.