8Centrifugal PumpsA centrifugal pump is one of the simplest types of pump that uses centrifugal force to move fluid into a pipe.The basic purpose of the pump is to convert energy of an electric motor , or engine into velocity or kinetic energy and then into pressure of a fluid that is being pumped.Centrifugal pumps are the most commonly used pump and their operating cost is low compared to other pumps.They are reputed for high reliability and ensure smooth operation.
10Pump TerminologyDuty Point = a certain flow of water at a certain pressure to fulfil the system requirement – constant or varyingPump Efficiency = how effectively the pump converts the input energy into water pressureRotational Speed = Speed at which the Impeller turnsPositive Suction = The water flows into pump on its own e.g. below dam wall
11Pump TerminologyNegative Suction = The pump has to “lift” the water from a source lower than itself e.g. riverNPSH = refers to the maximum “lift” capability a pump has to extract water from a source lower than itselfCavitation = Noise in the Volute Casing of a Pump due to the presence of undesirable air particles imploding
12Pump Identification & Pump Curves Pump Manufacturer – e.g. KSB; Rapid Allweiller; GrundfosPump Type - Single Stage /250; 80/40; 150/315 etcMultistage /40 – 2 stage; WKLN 65/4 etcImpeller Size – e.g. 250mm; 185mmMotor Size – kilowatts or horsepowerSpeed of Motor – 2 / 4 Pole or 29?? / 14?? RPMEfficiency of motor – ?Power factor of motor – cos ?
15What is a Pump Curve?This is the characteristics of the pump when operating at a specific speed and giving the following info:Pressure / Head DevelopedFlow rate of liquidPower Duty point and Non overloadNPSH requiredEfficiency of pump
1665 / 250 Centrifugal Pump 82m Head in Metres Flow rate in m³ per hour 120m^3/hour
1882 meters head71% efficiencyNett pump suction head required is 4,6mMotor size required for non overload is 45 kW120 m^3/hr
19Laws of Affinity They are as follows: The affinity laws express the mathematicalrelationship between the several variablesinvolved in pump performance.They apply to all types of centrifugal andaxial flow pumps.They are as follows:
20Volume CapacityThe volume capacity of a centrifugal pump can be expressed as:q = (n1 / n2) x (d1 / d2) Where:q = volume flow capacity (m³ / hour)n = wheel velocity - revolution per minute - (rpm)d = wheel diameter – (mm)
21Head or PressureThe head or pressure of a centrifugal pump can be expressed as:h = (n1 / n2)² x (d1 / d2)² Where:h = head or pressure (metres)
22Power The power consumption of a centrifugal pump can be expressed as: P = (n1 / n2)³ x (d1 / d2)³ Where:P = power (kilowatt)
23Changing the Impeller Speed If the Impeller diameter is constant - change in Impeller speed can simplify the affinity laws to:Volume Capacityq1 / q2 = (n1 / n2)Head or Pressuredp1 / dp2 = (n1 / n2)²Powerp1 / p2 = (n1 / n2)³
25If the speed of a pump is increased with 10% then: the volume flow increases with 10%the head increases with 21%the power increases with 33 %If we want to increase the volume of an existing system with 10% we have to increase the power by 33%.