There is a high potential to reduce pumping-energy-consumption cost through properly designing and selecting the pumping system and controls. Electrical motor-driven pump systems account for nearly 20 percent of the world energy use and 25-50 percent of the total electrical energy use in certain industrial facilities. Pump energy consumption is often one of the larger cost elements and may dominate pump life-cycle cost, especially if pumps run more than 2,000 hours per year. Pumping applications with a variable-flow requirement typically use a throttling valve, recirculation line, or a variable-motor speed to deliver the desired process flow rate. Flow control by valve throttling wastes energy by diverting excess flow through a bypass or by restricting the pump discharge. Additionally, valves can be a source of emissions and suffer from corrosion, erosion, plugging, sticking, cavitation, and leakage.
The most efficient mean of flow manipulation is pump-speed adjustment. Since per the “affinity law” brake horsepower varies with the cube of centrifugal pump speed, reducing pump speed reduces pressure imparted to the fluid and in return reduces centrifugal power consumption. In addition to energy conservation, there are also a number of operational benefits, such as improved reliability, process performance, reduced life-cycle cost, and decreased fugitive emissions by eliminating the control valve and the associated piping.
An electric motor-driven centrifugal pump is a process device that consists of a set of rotating vanes (impeller or rotor) enclosed within a casing that continuously imparts kinetic energy to a fluid. The head developed by the pump is entirely the result of the velocity imparted to the pump fluid by the impeller. The pressure increase across the pump is the result of converting the velocity head to pressure in the pump casing.
Selecting centrifugal pumps for an application requires evaluating pump performance characteristics against the process requirement or system curve. Pump characteristics are typically delivered by pump manufacturers in a graphical format called characteristic curves. These curves provide information about pump performance in terms of total dynamic head, brake horsepower, net positive suction head required, and efficiency for the capacity range of the pump.
Before proceeding to pump selection, the head-flow curve for the piping system that is being pumped into must be defined. This type of curve is called a system curve and typically represents the sum of the static head and the dynamic head that you need to pump against. The static head is a function of the elevation difference between the suction and the discharge or back-pressure that the pump is operating against. The dynamic head represents the friction losses from the fluid that results from the piping system. Plotting the pump curve and system curve will result in an intersection point, which is called the operating point. This point indicates the head and the capacity at which the pump operates in the system and gives the maximum flow required by the system.
Most often this intersection point does not fulfill all the plant process requirements downstream of the pump. In some cases, the process requires varying flow rates and head conditions. Hence, some sort of control will need to be applied either in the pump discharge or in the pump prime mover. The most common method of flow control is to throttle the flow on the discharge side of the pump. A more efficient option is to regulate pump flow by varying pump speed of the motor in the prime mover.