This is possibly the single most misinformed subject that we come across in our business(apart from the one where a smaller pipe gives more pressure). So often we have customers coming to us with the intent of buying a twin impeller firefighting pump. When we ask why, the answer is always "Because you get more water don't you."
Don't you?
Well, maybe you do. And maybe you don't. Ask yourself this: Is a 4WD faster than a 2WD? It certainly is up a muddy steep climb or through a boghole. But probably not for a trip to town.
So let's take a closer look. What is an impeller? What does the twin impeller do?
Impellers are the spinny thing at the heart of your pump. As it spins it draws water into its centre and throws it out through its vanes, creating both pressure and flow. They come in lots of different diameters and widths and can spin at different rates. The diameter of an impeller will affect the pressure while the thickness will affect the flow rate. Extra impellers, in series, add pressure to each other, but do not increase the flow rate (in fact the flow rate decreases). So a twin impeller pump will deliver higher pressure but lower flow.
So, which pump do you need?
Take a look at the pump curves below. The single impeller pump delivers better flow rates below 40m of total dynamic head, while the twin impeller delivers more above 40m. So for general purpose pumping a single impeller pump will generally suffice. However for high pressure situations the twin impeller pump will be the best choice. Don't forget the pipe friction. In some situations the pipe friction will add up so that the total dynamic head is greater than 40m.
