Hydraulics Laboratory Experiment Report Name: Ahmed Essam Mansour Section: Title: "1", Monday 2-5 pm Centrifugal Pumps Date: 27 November, 2006
Objectives: To study the performance of a centrifugal pumps by plotting the performance diagram To study the performance of pumps in series. Theory: Pumps are generally used to provide certain head at certain flow rate to create a flow from low pressure to high pressure. Centrifugal pump are one type of pumps that converts the energy of prime mover (electric motor) into velocity (kinetic energy) by centrifugal force provided by the rotation of the vanes, and then the kinetic energy is transformed into pressure energy of the fluid that flows in the pump. This conversion process of energy into pressure is done by two main parts: 1. Impeller: rotating part that converts the driver energy into kinetic energy. 2. Volute (diffuser): The stationary part that converts the energy into pressure. The amount of energy given to fluid is direct proportional to the velocity of the flow at the edge of the vanes of the impeller; the faster the impeller is or the larger it was (v = ω * r), the larger kinetic energy is transformed to the fluid. This kinetic energy of the fluid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to Bernoulli s principle;
Centrifugal pump performance curve: Every pump's performance is represented by the pump performance curve which is a plot of the developed head against the flow rate, also the curve shows the efficiency of the pump, the speed of the impeller and its size, these curves are generated according to tests performed by the manufacturer. Efficiency of a pump: The efficiency is an important factor in selecting a pump for a certain system, this quantity represents the ratio between energy input (from motor) to energy output (to the flow) of the pump; Pumps in Series and pumps in parallel: Some times the required head for a system can't match any single pump performance curve or the suitable pump is not in stock, for those purposes two or more pumps can be connect in series to increase the head coming out of pumps; Pumps are connected in series by attaching on of the pumps discharge to the other ones suction. When larger flow rate is required and no single pump is available for use, then two or more pumps can be connected in parallel; Pumps are connected in parallel when their discharge is connected to a common pipe.
Apparatus: 1. Centrifugal pumps 2. Flow meter 3. Digital tachometers read the rotation speed in rpm Procedures: 1. The motors connected to the pumps were started, and the valves were opened in a manner that each pump discharges alone. 2. Rotation speed of each pump was set to a different value. 3. Flow was allowed to occur. 4. Readings of the pressure gauges before and after the pumps and the flow rate were taken. 5. The flow rate was decreased at each reading of pressures 6. Then the valves were opened in a manner that the two pumps are connected in series, and the readings were taken again in the same manner.
Conclusions: 1. Generally in pumps increasing the flow rate will decrease the provided head as shown in the plots of pump 1 and pump 2, a point of equilibrium between the both criterions needed for the systems shall be found. 2. Some experimental errors regarding the first point were found in pump 2 performance curve and in the experimental curve for pumps in series. 3. Pump 1 (N = 3000 rpm) provided a performance curve larger than that of pump 2 (N = 3500 rpm), which is only possible if pump 1 (smaller speed) has larger impeller size than pump 2 (larger speed). 4. The experimental curve of the pumps in series showed smaller maximum head (Q = 0) than the theoretical one calculated from plots of pump 1 and pump 2, so when selecting the pumps in series to be used for a system I think we should test them experimentally rather than calculating them theoretically so that we get the exact amount of head, selecting the pumps in series using theoretical analysis would result a failure in achieving the required head in an actual system. 5. The efficiency of the pumps was not calculated due to lack of givens (input power) References: http://www.cheresources.com/centrifugalpumps1.shtml