MEng Civil Engineering Level II Hydraulics Laboratory Centrifugal Pump Test
In order to construct the characteristic performance curves for the centrifugal pump, 3 tests were carried out within laboratory conditions. This is achieved by determining the capacity and efficiency of a centrifugal pump when operating under given conditions.
Centrifugal pumps are commonly used to move liquids through a piping system. Water enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radically outward into a diffuser, from where it exits into the downstream piping system. Centrifugal pumps are used for large discharge through smaller heads.
The work performance the pump is based on the function of the total head and the weight of the liquid pumped in a given time period. The design of the impeller and the casing is important for efficient pump performance.
For the experiment, the centrifugal pump is driven by an AC electric motor. On the side, a motor control panel is attached to the pump to control motor speed, and to provide measurements of electric current (amps) and total input electric power PE (kw). Motor speed n is shown in Hz, which translates to the pump speed N (rpm) by N = 58n.
There are two pressure gauges, #1 and #2 to show the inlet and outlet pressure in metres. For the head of water, the datum of the gauges have a difference of Z2 - Z1 = 0.15 m. To measure the discharge, water level scale on the tank and a stopwatch has been used.
To start the procedure the control valve at the manifold must be closed before switching on the pump system. The speed indicated on the motor control panel has to be set on zero. To operate the pump the RUN key is pressed and the maximum speed 50 Hz is selected. The flow control valve is gradually opened allow the pump and pipe work to prime.
This experiment included three different pump speeds (N = 1750, 2000 & 2250 rpm). To begin the first test, control valve is fully closed and pump speed is set on the given speed. Reading from pressure gauge #3 is taken. This shows the maximum possible reading on this gauge for this speed. Moreover a set of reading is also taken at pressure gauge #1, #2 and the electric power. The initial set of readings corresponds to the zero discharge.
The control valve is slowly opened so the reading at pressure gauge #3 is reduced approximately by a tenth of H3max. A set of readings is taken, followed by discharge volume and time. This procedure is repeated until 10 set of readings have been taken, and after that this test is repeated for the remaining pump speeds.
To maintain the accuracy communication within the group is necessary so people on the other side are constantly checking with the people who are controlling the flow of the water.
By having the data from three tests, it is recommended to write them on an excel sheet. It is very easy to convert values or draw graphs on excel. The total head generated by the pump is given by: , where Houtlet & HInlet are outlet and inlet pressure readings.
The discharge is calculated by converting discharge volume from litres to cubic meters and then divided by Time in seconds. Hydraulics power is given by and Electric power was taken from the motor control panel. The efficiency of the pump is determined by this equation: .
By obtaining the required data, Hm, P and are plotted against the discharge (Q). The efficiency of the pump is determined by drawing constant efficiency lines on the ?-Q graph and plotting the corresponding discharge values on to the Hm-Q graph.
In order to make a meaningful comparison between the performance data for the three speeds, it is necessary to reduce the figures to on common speed. In this case a base speed of 2000 rpm is used and all the data are multiplied as follows:
As seen on the graph, the lines obtained for H2000 are fairly close to each other, this indicates that, the closer the line are together, the better the pump experiment has been carried out.
A centrifugal pump is considered to be relatively simple and can be used for a wide variety of applications. It converts energy of a prime mover, in other words the electric motor, first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. At two main parts of the pump (the impeller and the volute or diffuser) are where the change of energy occurs. The rotating part known as the impeller converts driver energy into kinematic energy.
The volute or diffuser of the pump is the part where the kinetic energy is converted into pressure energy.centrifugalpumps8.gif (11596 bytes) The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller rotates or the bigger the impeller is, the greater the energy is exposed to the liquid.
The head, the efficiency and the power consumption change as the flow rate of the pump changes. By plotting these values against the flow rate a pump characteristics graph is obtained. The best efficiency point is when a certain flow rate the pump efficiency had a maximum reading.
When choosing centrifugal pumps, the design has a major effect on the pumps efficiency and it has to be taken into consideration. The efficiency of centrifugal pumps is given as a percentage and represents a unit of measure describing the change of centrifugal force into pressure energy. The best efficiency point is the area on the curve where the change of velocity energy into pressure energy at a given discharge is optimum; in other words, the point where the pump is most efficient.
In the results obtained from the experiment, there are some possibilities of human and technical error. Valves at the pipe must be very accurately adjusted ensuring that the readings obtained from the pressure gauges are as accurate as possible. When measuring the time from the stop watch, human reaction on how long it takes to react and stop the stop watch can also create some errors in the reading.
Communication within the group is very important to keep people with different tasks of the experiment up to date. When recording the results it is better to have more, wider range of results so when it is plotted on the diagram it is more accurate than having fewer results. It has been recommended to repeat the experiments for each pipe to ensure that the numbers archived are adequate for the calculations and the graph.