Centrifugal pumps


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Centrifugal pumps

  2. 2. CENTRIFUGAL PUMPSQ. Give a number of examples for reducing of axial pressure/force in thecentrifugal pumps.A.There are three types of axial pressure (force balancing): a) The sealing device 2and1, equalizing hole 3 –connects suction and discharge space. b) Balanced impeller (used in multistage pumps). c) Simple balancing disk 1 and chamber 5. d)Q. What is the purpose of the shafts “PROTECTING SLEEVE”?A. Pump shafts are usually protected fromerosion, corrosion, and wear at the seal chambers, leakage joints,internal bearings, and in the waterways by renewable sleeves. Unlessotherwise specified, a shaft sleeve of wear, corrosion, and erosion-resistant material shall be provided to protect the shaft. The sleeveshall be sealed at one end. The shaft sleeve assembly shall extendbeyond the outer face of the seal gland plate. (Leakage between theshaft and the sleeve should not be confused with leakage through themechanical seal). Q. What is the function of a “WEAR RING”?A. The function of the wear ring is: Wear ring provides an easily andeconomically renewableleakage joint between the impeller and the casing.For efficient operation it is important to ensure that leakage from the highto low-pressure side is kept to a minimum. This is achieved by the use of 2
  3. 3. wearing rings. Traditionally these are fitted to the casing, to increase thelongevity of the impeller wear ring tires may be fitted.If the clearance becomes too large the pump efficiency will be lowered causing heatand vibration problems. Most manufacturers require that you disassemble the pump tocheck the wear ring clearance and replace the rings when this clearance doubles. Q. What is the function of a “DIFUSER”? A. The function of a “ DIFUSER” is included in the casing of pump.Circular casing have stationary diffusion vanes surrounding the impellerperiphery that convert velocity energy to pressure energy. Conventionally, thediffusers are applied to multi-stage pumps.The casings can be designed either as solid casings or split casings.Solid casing implies a design in which the entire casing including thedischarge nozzle is all contained in one casting or fabricated piece. A split casing implies two or more parts are fastened together. When thecasing parts are divided by horizontal plane, the casing is described ashorizontally split or axially split casing. When the split is in a verticalplane perpendicular to the rotation axis, the casing is described asVertically split or radically split casing. Casing Wear rings act as the sealbetween the casing and the impeller.Q. What are the parameters that are responsible for the capacity and pressure in acentrifugal pump?A. CapacityCapacity means the flow rate with which liquid is moved or pushed by the pumpto the desired point in the process. It is commonly measured in either gallons perminute (GPM) or cubic meters per hour (m 3 /hr). The capacity usually changes withthe changes in operation of the process. For example, a boiler feed pump is anapplication that needs a constant pressure with varying capacities to meet achanging steam demand.The capacity depends on a number of factors like:Process liquid characteristics i.e. density, viscositySize of the pump and its inlet and outlet sectionsImpeller size 3
  4. 4. Impeller rotational speed RPMSize and shape of cavities between the vanesPump suction and discharge temperature and pressure conditionsFor a pump with a particular impeller running at a certain speed in a liquid, theonly items on the list above that can change the amount flowing through the pumpare the pressures at the pump inlet and outlet. The effect on the flow through apump by changing the outlet pressures is graphed on a pump curve.As liquids are essentially incompressible, the capacity is directly related with thevelocity of flow in the suction pipe. D B RPM Q. Why are centrifugal pumps not provided with safety valve? A. A centrifugal pump does note create pressure, it only provides flow. Pressure is just an indication of the amount of resistance to flow. Q. How can we regulate the PRESSURE and CAPASITY of a Centrifugal pump? A. There are a few methods to change pressure and capacity of Centrifugal pump: a) By closing the flap or valve on discharge side. b) Relieving part of the liquid flowing from discharge space to the suction space. c) Changing RPM of the motor (if it possible). 4
  5. 5. Q. a) From what material is the WEAR RING made from? b) What is the allowable clearance for the WEAR RING? c) Sketch at least three wear ring types? A. A) WEAR RING MATERIAL: Aluminum, Bronze, Brass, Carbon- fiber, Vein forced thermoplasticmaterial Stainless alloys. b) The clearance between the impeller and the casing Wearing ring must be a maintained as directed by the manufacturer. When clearances exceed the specified amount, the casing-wearing ring must be replaced. The clearance given for wear rings is often a source of contention especially when dealing with on-ship made rings. A clearance of 1/1000 of the diameter of the bore is often quoted although this may be very difficult to achieve in practice 2-STAGE SELF PRIMING P/P Symbols Clearance in MM On the Drawing Manufac Overhaul Max. ture Clearanc available size e. clearanc new Intermed e. cond. iate repair. 1-STAGE NON SELF PRIMING P/P 5
  6. 6. 1-STAGE NON-SELF PRIMING P/P Symbol Clearance in MM Q On the drawingm³/h MANUFACTURE Max. Max. SIZE availabl availabl NEW COND. e e clearanc clearanc e. e. 6
  7. 7. Q. Wear ring types:A. There are various types of wearing-ring designs, and selection of the most desirable type depends on the liquid being handled, the pressure differential across the leakage joint, the surface speed, and the particular pump design. In general, centrifugal-pump designers use the ring construction they have found suitable for each particular pump service. The most common ring constructions are the flat-type, the L- type and U- type. Q. What will happen when a centrifugal pump will turn in opposite direction? Will the pump build up pressure? And what will the power gauge show? A. When the centrifugal pump turns in the opposite direction of rotation, it can cause the following: a) Reduction in capacity b) Pressure droop c) Increased load – high current Q. If the clearance of the wear ring increases, will the load increase, or decreases? A. If the clearance becomes too large, the pump efficiency will be reduced, causing heat and vibration problems. When the clearance between the wear ring increases, it will cause part of the liquids volume to pass over from the discharge space to the suction space. It will create an internal resistance. The pump must to wear down (overcome) the resistance of its own transferring liquid and also resistance of the leakages. The pump consumes more energy. The load will be increased. It will be indicate on the Ampere Meter. 7
  8. 8. Q. What types of bearings are used on a CENTRIFUGAL PUMP?A. ; Journal Thrust Support Support Thrust bearing: bearing. bearing bearing Bearing Made of Dbl. Row Single Single row- Dbl. row bronze or Roller row-ball roller Ball bearing rubber bearing bearing bearing Heavy- Deep duty Groove angular Ball contact bearing ball bearing 8
  9. 9. Q. Give a number of examples for centrifugal pumps that are used on the vessel with a NON-RETURN VALVE on the discharge line. A. 1) Sea water cooling pump 2) F.W. central cooling pump 3) Fire Fighting and General service pump 4) Boiler feed and circulating pump 5) HT cooling pump Q. What is the significance of checking “PRESSURE TEST”, on? A centrifugal pump? A. Pressure test =Head test it is measuring of KINETIC ENERGY. Significance of using the “head” term instead of the “pressure” term The pressure at any point in a liquid can be thought of as being caused by a Vertical column of the liquid due to its weight. The height of this column is called the static head and is expressed in terms of feet of liquid. The same head term is used to measure the kinetic energy created by the pump. In other words, head is a measurement of the height of a liquid column that the pump could create from the kinetic energy imparted to the liquid. Imagine a pipe shooting a jet of water straight up into the air, the height the water goes up would be the head. The head is not equivalent to pressure. Head is a term that has units of a Length or feet and pressure has units of force per unit area or pound per Square meter. The main reason for using head instead of pressure to measure a centrifugal pumps energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not change. Pressure to Head Conversion formula The static head corresponding to any specific pressure is dependent upon the weight of the liquid according to the following formula: Head= Pressure (PSI) ×2.31 Specific GravityQ. How do we check CAPASITY of the centrifugal pump? 9
  10. 10. A. There are two methods to carry out a capacity test for a centrifugal pump. a) Calculation by means of formula: Q (m³/h) Q=m×v×A m- mass of fluid in (kg) v- velocity of flow in (m/sec) A-area of the pipe in(m²) OR: Q (feeet³/h) Q=449×v×A v- velocity of flow in (feet/sec.) A-area of the pipe in(feet²) b) In order to measure the capacity, we should fill the specific volume (tank), which is calibrated by sounding table. Time of filling must be noted and regarded to this volume.Q. In what way can we cool a mechanical seal?A. Where mechanical shaft seals are used, the design ensures that positive liquid pressure is supplied to the seal faces under all conditions of operation and that there is adequate circulation of the liquid at the seal faces to minimize the deposit of foreign matter on the seal parts. a) Cooling water is supplied from the discharge side of the pump. b) Cooling liquid is supplied from the external source:(central cooling water system or M.E. Jacket cooling system). Those pump usually are designed for the high temperature liquid, like Exhaust Boiler Circulating Pump . c) The high pressure and high temperature liquid, while it passes through the minimum clearance between the throttle bush and shaft sleeve, increases the pressure reduction. The 3-gland packing is sufficient in order to prevent the liquid from leaking out.Since the tempo the leaking water is 80º-95ºC considerations is given to the extensionof service life by providing a cooling water jacket around the gland. Q. What is the difference between parallel and series pumping in a 10
  11. 11. Centrifugal pump? Give a few examples. A. Series Pumping .Pumps in series double the head at the same flow condition point One pump discharge is piped into the suction of the second pump producing twice the head capability of each pump separately. The second pump however must ,be capable of operating at the higher suction pressurewhich is produced by number one. This mode of operation?is a very cost effective way of overcoming high discharge .heads when the flow requirement remains the same Parallel Pumping Identical PumpsOperating two identical pumps in parallel the assumption is made that theflow will double. This however is not the case. In order to calculate the additional flow realized byrunning two identical pumps in parallel the following calculations must be made. ONE P/P RUNNIG TWO P/P RUNNING TDH FLOW(gpm) TDH FLOW(gpm) 130 100 130 200 120 190 120 380 110 280 110 560 100 360 100 720 0perating point A 0perating point B 85 470 105 660First lets look at the pump curve and system head curve with one pump running.Second lets look at the pump curve and system head curve with two pumpsrunning.The addition of the second identical pumps produces and extra 190 GPM and doesnot double the flow. Point C is the flow produced by each pump when both pumpsare running.There are some systems, which designed to be operated with two ore more parallelconsumers, for example coolers. Entering in parallel operation with an additionalconsumer (cooler), which will be accompanied by additional cooling pump. 11
  12. 12. For example changing over from the harbor system to sea system.Series operation of the centrifugal pump usually applied to MULTISTAGECENTRIFUGAL PUMPS, like boiler feed pump, hydrafores pump andSome fire fighting high-pressure pumps.Q. What forces act on the shaft of a CENTRIFUGAL PUMP?A. R-Radial thrust which is developed in high-pressure centrifugal pump. A-Axial force. Without careful design an axial force is created by the action of the impeller. This is due to the low pressure acting on the suction eye whilst the rest of the impeller is a subjected to discharge pressure. RQ. How it is possible to change the relative location of the impeller relative to thecasing in a centrifugal pump?A. There are tree possibilities that depend on the pumps construction:1) We can move the impeller closer to the casing by adding a distance washer under thethrust bearing.2) We can add packing, which will produce an additional distance between the bottomscasing and the top casing, that holds the mechanical seal and bearing.3) We can change the relative location of the impeller on the shaft (by adding a distance piece between the shaft shoulder and sleeve). 12
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