1. The problem statement, all variables and given/known data I have a task right now that requires me to calculate the energy savings that would be made by using a variable speed drive. Question: The factory process water distribution system consists of a 150mm cast iron ring main at a pressure of 5 bar (50m). The local water is pumped into the factory water ring main from a storage tank. Water ring main pressure and flow rates produced by this pump are mechanically controlled by a pressure relief valve. This method of control causes the motor to run near the top end of the power range on a continuous basis while the plant is in operation. The pressure in the water ring main is presently controlled by the pressure relief valve which diverts excess water back into the storage tank. This system causes the water pump to be fully loaded at all times. The factory works a 46 week year and runs continuously seven days per week as shown on the process bar chart. Assume the cost per killowatt hour to be £0.08 when calculating the annual savings in energy costs 3. The attempt at a solution There's 4 different flow rates: 50m3/h 80m3/h 100m3/h 170m3/h From the pump chacateristics graph, you can see that the power required for each flow rate is (going along from 50m head) 50m3/h = 13kW 80m3/h = 16kW 100m3/h = 18.5kW 170m3/h = 33kW The motors efficiency is 90% so the power into motor is 50m3/h = 13kW / 0.9 = 14.44kW 80m3/h = 16kW / 0.9 = 17.78kW 100m3/h = 18.5kW / 0.9 = 20.6kW 170m3/h = 33kW / 0.9 = 36.7kW I'm stuck from here. I need to calculate what the price of energy per year is without the variable speed drive, and then calculate what the price is with the variable speed drive, and then subtract them to find the savings achieved per year. On the pump characteristics graph, what is the thick black line referring to? Is the powers I calculated with or without the variable speed drive? Do I just add up all the powers and then calculate how much it costs to run it for 46 weeks? I don't understand.