...an oversized pump will tend to operate on the right hand side of the performance curve at lower efficiencies and with a higher NPSH required. Cost per cubic meter of liquid pumped (specific energy) and, in all likelihood, pump maintenance costs will increase significantly and plant reliability will decrease.
How many pumping systems are designed on a single duty point? If many of the authoritative statistics that come out of the market are to be believed, a lot. One figure bandied about is that more than half of the pumps in Europe are too big for the application! This in a so called first world environment. What would it be in the less developed countries? It can only be speculated that it is considerably worse. How does this happen and where does it start?
Experience in the pump manufacturing and marketing sector saw many clients phone up and ask for a pump set to do nice round duty points such as 30l/s against a Total Dynamic Head of 50m. Without getting too smart for one's own good, the question that begs answering is why not 31,6l/s at a TDH of 49,2m. Possibly the answer lies in rounding up or down coupled with allowances made for future expansion, inexperience of junior engineers or differing views as to what the real duty is. One instance of total overshoot in sizing was a platinum mine that doubled the duty point of their slurry circuit pumps as it was planned to double production after 5 years. The result was 3 x 300kW pumps in series doing the work of 3 x 110kW pumpsets. A Life Cycle Cost exercise showed a payback period of less than a year if the larger pumps were replaced with the smaller units. To rub salt in the wound, the worldwide economic meltdown occurred exactly on the (five year) date set for the doubling of production just in time for a massive slump in demand for platinum. 5 years of inflated energy consumption, poor power factors and sky high maintenance for no gain!
What are the lessons from this and too many other case studies? Simply put, life is all about variations and it is how these variations are handled that is the key to squeezing costs down as far as they can possibly go. Pump selections are about a range of duty points. These duty points start from a maximum and progress to a minimum. The trick is to bracket the Best Efficiency Point, or BEP with these two points.
More about all this later.