Turbine proportioners use a water turbine, often a positive displacement water turbine, that is direct coupled to a positive displacement foam pump that is sized to provide the required foam concentrate mixing rate.
There are a range of different types of turbine proportioners on the market utilising different technologies. There are at least three different water turbine types and two different foam pump types in use. Making the right choices for the turbine and the foam pump can be very important for your application.
While it might be nice to think of turbine proportioner systems are simple to design and free of the design restriction of other foam proportioning systems, this is not the case. Turbine proportioners have their own set of design issues. Since these proportioners are relativity new technology, there may be things we have yet to learn also.
Turbine choice. The three types of turbine commonly used are, vane turbines, basically a vane pump used as a turbine, lobe turbines, a lobe pump used as a turbine and axial flow turbines related to axial flow pumps (which are not positive displacement).
The vane turbines as relatively simple to design and build but potentially relatively fragile (the vanes being the weakest point). There appear to be some issues with vane turbines and water hammer and in deluge systems with large empty pipe volumes where the turbine can over speed resulting in damage.
Lobe turbines are substantially more robust but also more difficult to manufacture. They probably have higher internal friction and may not be as susceptible to over speed and will be much more resistant to water hammer.
Axial flow turbines are probably the lowest cost turbine and are robust, however, since they are not positive displacement turbines the flow range over which they can accurately proportion is relatively narrow. We have not heard of any problems with these units with water hammer or over speed.
Foam pump choice. The common foam pumps used are piston pumps and gear pumps, though other types of positive displacement pump could be used.
Piston pumps seem to be popular as metering pumps due to their very accurate metering (low slippage). However, they also have some limitations – higher maintenance requirements, lower suction lift capability and potential problems with pumping high viscosity foam concentrates. The suction lift available is about half that for a gear pump, maybe 3 meters head maximum, and there maybe additional slippage in the check valves required to make piston pumps work when using high viscosity foam concentrates. There is quite a bit of anecdotal evidence that piston pumps have problems with high viscosity foam concentrates. piston pumps are likely to have higher maintenance costs.
Gear pumps are the most common choice of pump for conventional foam pumps but are somewhat less favoured for turbine proportioners due to less accuracy than piston pumps. The headline performance for a gear pump-based turbine proportioner will be not a good as for a piston pump-based unit, however, this is a long way short of the full story. Gear pumps can work up to around 5 meters suction lift, providing more flexibility for suction piping design and they actually work more efficiently with high viscosity concentrates making them ideal for this application. Quite often the wide range is not necessary for the design and very low flows can often be designed out by over protecting a small risk with very little cost impact but opening up more proportioner options.
As for any proportioning system, a deeper look at the application is needed before making a turbine proportioner choice. Some examples are:- If water hammer or large dry pipe volumes for deluge systems might be involved then a vane turbine-based unit may not be the best choice. If high viscosity foam concentrate is used, or might be needed in the future, a gear pump-based unit might be the best choice. If your operating flow range requirement is relatively narrow, an axial flow turbine unit might offer substantial cost savings.
There are big differences between proportioners in terms of their ability to be tested without actually mixing foam. This will be very important in the future.
It should be noted that a single central turbine proportioner does not replace, with the same level of reliability, a dual foam pump balance pressure proportioning system.
High viscosity foam concentrates represent a substantial challenge for the foam system designer. There are already too many cases where this has gone wrong for turbine proportioner systems.