The Nuts and Bolts of Pump Selection
Earlier we discussed that pressure is a form of energy and the centrifugal pump converts the input energy to pressure, arming the fluid with this energy to overcome the pipeline resistance and move from point A to point B. If we pick a point on the pump curve it has a corresponding flow rate (Q) on the X-axis and head (H) on the Y-axis. It tells us that this specific pump can arm the flow rate of “Q” with the pressure energy of “H”.
Head is expressed in a unit of length (meters or feet) and represents the equivalent pressure. To better understand, imagine a pump with a frictionless vertical discharge piping. The water comes in through the input side of the pump and is pumped out the discharge end with high-pressure. Head is the measurement of how high the column of water in the pipe would be.
A head of 10.2 meters on a pump curve represents one bar or 14.5 psi of pressure. Did you notice that water was underlined? It is because the mentioned pressures correspond to water density, if we have oil with lighter density then the pressure would be proportionally less or vise versa.
For example, a KSB Omega 300-560 water pump with an impeller diameter of 553 mm turning at 1790 rpm can arm a flow rate of 600 l/s with 142 m of head (around 14 bar or 202 psi pressure) or 420 l/s @ 160 m head. When you look at this Omega pump curve (below) you find a corresponding head for any flow rate between zero to 820 l/s. Yes, we see that, but it doesn’t mean that it is safe to use this pump for all points on the curve. First of all, we stay away from all points to the left of Qmin. Operating points in the “preferred operating range” (POR), which is somewhere in the middle of the curve are best, but we will address this topic later.
Now one may ask: If I have a pipeline from point A to point B and I need to pump 600 l/s then is this Omega 300-560 a suitable pump? The answer is only if your pipe resistance and carrying flow rate are as given, 142 m and 600 l/s respectively. If your resistance and carrying flow rate is different, then the operating point would be different. When resistance is less we see an increased flow, and when resistance is more we see a decreased flow, or we could be totally out of the operating range of the pump and should look at another size.