Transcription

TUTORIALCENTRIFUGAL PUMP SYSTEMSby Jacques Chaurette p. eng. copyright 2005Fluide Design Inc., 5764 Monkland avenue, Suite 311, Montreal, Quebec, Canada H4A 1E9Tel: 514.484.PUMP (7867) E-mail: [email protected] Web site: www.fluidedesign.com

page .2Table of contents1. Different types of pump systems2. Three important characteristics of a pump system: pressure, friction and flow3. What is friction in a pump system4. Energy and head in pump systems5. Static head6. Flow rate depends on elevation difference or static head7. Flow rate depends on friction8. How does a centrifugal pump produce pressure9. What is total head10 What is the relationship between head and total head11. How to determine friction head12. The performance or characteristic curve of the pump13. How to select a centrifugal pumpExamples of total head calculations - sizing a pump for a home owner application14. Examples of common residential water systems15. Calculate the pump discharge pressure from the pump total head Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .3Appendix AFlow rate and friction loss for different pipe sizes based at different velocitiesAppendix BFormulas and an example of how to do pipe friction calculationsAppendix CFormulas and an example of how to do pipe fittings friction calculationsAppendix DFormula and an example of how to do velocity calculation for fluid flow in a pipeAppendix EThe relationship between pressure head and pressure Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .4ForewordThis tutorial is intended for anyone that has an interest in centrifugal pumps. There is nomath, just simple explanations of how pump systems work and how to select acentrifugal pump. For those who want to do detail calculations, some examples havebeen included in the appendices.This tutorial answers the following questions:-What are the important characteristics of a pump system?-What is head and how is it used in a pump system to make calculations easier?-What is static head and friction head and how do they affect the flow rate in apump system?-How does a centrifugal pump produce pressure?-Why is total head and flow the two most important characteristics of a centrifugalpump?-What is meant by the pump rating? And what is the optimal operating point of acentrifugal pump?-How to do details calculations that will allow you to size and select a centrifugalpump?-How to verify that your centrifugal pump is providing the rated pressure or head?-What is density and specific gravity and how do they relate to pressure andhead? Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .51. Different types of pump systemsThere are many types of centrifugal pump systems. Figure 1 shows a typical industrialpump system. There are many variations on this including all kinds of equipment thatcan be hooked up to these systems that are not shown. A pump after all is only a singlecomponent of a process although an important and vital one. The pumps’ role is toprovide sufficient pressure to move the fluid through the system at the desired flow rate.Figure 1 Typical industrial pump system.Back in the old days domestic water supply was simpler.aaah the good old days.Goodnight John boy.Figure 1a The old days. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .6Domestic water systems take their water from various sources at different levelsdepending on the water table and terrain contours.Figure 1b Water supply sources (source: The Ground Water Atlas ofColorado).The system in Figure 2 is a typical domestic water supply system that takes it's waterfrom a shallow well (25 feet down max.) using an end suction centrifugal pump. A jetpump works well in this application (see http://www.watertanks.com/category/43/) .Figure 2 Typical residential pump system.The system in Figure 3 is another typical domestic water supply system that takes it'swater from a deep well (200-300 feet) and uses a multi-stage submersible pump oftencalled a turbine pump (http://www.webtrol.com/domestic pumps/8in turbine.htm). Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .7Figure 2a Typical jet pump.igure 3 Typical residential deep well pumpsystem. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .8Figure 3a Typical residential deep well pump system(source: The Ground Water Atlas of Colorado).Figure 3b Another representation of a typical residentialdeep well pump system. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .9Figure 3c Typical deep well residential submersible pump.2. Three important characteristics of pump systems: pressure, friction and flowFigure 4 Three important characteristics of pump systems.Pressure, friction and flow are three important characteristics of a pump system.Pressure is the driving force responsible for the movement of the fluid. Friction is theforce that slows down fluid particles. Flow rate is the amount of volume that is displacedper unit time. The unit of flow in North America, at least in the pump industry, is the USgallon per minute, USgpm. From now on I will just use gallons per minute or gpm. In themetric system, flow is in liters per second (L/s) or meters cube per hour (m3/h). Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .10Pressure is often expressed in pounds per square inch (psi) in the Imperial system andkiloPascals (kPa) in the metric system. In the Imperial system of measurement, the unitpsig or pounds per square inch gauge is used, it means that the pressure measurementis relative to the local atmospheric pressure, so that 5 psig is 5 psi above the localatmospheric pressure. The kPa unit scale is intended to be a scale of absolute pressuremeasurement and there is no kPag, but many people use the kPa as a relativemeasurement to the local atmosphere and don't bother to specify this. This is not a faultof the metric system but the way people use it. The term pressure loss or pressure dropis often used, this refers to the decrease in pressure in the system due to friction. In apipe or tube that is at the same level, your garden hose for example, the pressure is highat the tap and zero at the hose outlet, this decrease in pressure is due to friction and isthe pressure loss.As an example of the use of pressure and flow units, the pressure available to domesticwater systems varies greatly depending on your location with respect to the watertreatment plant. It can vary between 30 and 70 psi or more. The following table gives theexpected flow rate that you would obtain for different pipe sizes assuming the pipe ortube is kept at the same level as the connection to the main water pressure supply andhas a 100 feet of length (see Figure 4a).Table 1 Expected flow rates for 100 feet of pipe of various diameters based on availablepressure. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .11Figure 4a A typical garden hose connection,see Table 1 for flow rate vs. pressure. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .12The unit of friction is.Sorry, I think I need to wait 'til we get closer to the end to explainthe reasoning behind this unit.Figure 5 A typical pump system.The pump provides the energy necessary to drive the fluid through the system andovercome friction and any elevation difference.Pressure is increased when fluid particles are forced closer together. For example, in afire extinguisher work or energy has been spent to pressurize the liquid chemical within,that energy can be stored and used later.Is it possible to pressurize a liquid within a container that is open? Yes. A good exampleis a syringe, as you push down on the plunger the pressure increases, and the harderyou have to push. There is enough friction as the fluid moves through the needle toproduce a great deal of pressure in the body of the syringe.Figure 6 Pressure produced by fluid friction in a syringe.If we apply this idea to the pump system of Figure 5, even though the discharge pipeend is open, it is possible to have pressure at the pump discharge because there issufficient friction in the system and elevation difference. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .133. What is friction in a pump systemFriction is always present, even in fluids, it is the force that resists the movement ofobjects.Figure 7 Friction, the force that resist movement.When you move a solid on a hard surface, there is friction between the object and thesurface. If you put wheels on it, there will be less friction. In the case of moving fluidssuch as water, there is even less friction but it can become significant for long pipes.Friction can be also be high for short pipes which have a high flow rate and smalldiameter as in the syringe example.In fluids, friction occurs between fluid layers that are traveling at different velocities withinthe pipe (see Figure 8). There is a natural tendency for the fluid velocity to be higher inthe center of the pipe than near the wall of the pipe. Friction will also be high for viscousfluids and fluids with suspended particles.Figure 8 Friction between layers of fluid and the Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .14pipe wall.Another cause of friction is the interaction of the fluid with the pipe wall, the rougher thepipe, the higher the friction.Friction depends on:-average velocity of the fluid within the pipeviscositypipe surface roughnessAn increase in any one of these parameters will increase friction.The amount of energy required to overcome the total friction loss within the system hasto be supplied by the pump if you want to achieve the required flow rate. In industrialsystems, friction is not normally a large part of a pump's energy output. For typicalsystems, it is around 25% of the total. If it becomes much higher then you shouldexamine the system to see if the pipes are too small. However all pump systems aredifferent, in some systems the friction energy may represent 100% of the pump's energy,This is what makes pump systems interesting, there is a million and one applications forthem. In household systems, friction can be a greater proportion of the pump energyoutput, maybe up to 50% of the total, this is because small pipes produce higher frictionthan larger pipes for the same average fluid velocity in the pipe (see the friction chartlater in this tutorial).Another cause of friction are the fittings (elbows, tees, y's, etc) required to get the fluidfrom point A to B. Each one has a particular effect on the fluid streamlines. For examplein the case of the elbow (see Figure 9), the fluid streamlines that are closest to the tightinner radius of the elbow lift off from the pipe surface forming small vortexes thatconsume energy. This energy loss is small for one elbow but if you have several elbowsand other fittings it can become significant. Generally speaking they rarely representmore than 30% of the total friction due to the overall pipe length.Figure 9 Streamline flow patterns for typical fittings such an elbowand a tee.4. Energy and head in pump systemsEnergy and head are two terms that are often used in pump systems. We use energy todescribe the movement of liquids in pump systems because it is easier than any other Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .15method. There are four forms of energy in pump systems: pressure, elevation, frictionand velocity.Pressure is produced at the bottom of the reservoir because the liquid fills up thecontainer completely and its weight produces a force that is distributed over a surfacewhich is pressure. This type of pressure is called static pressure. Pressure energy is theenergy that builds up when liquid or gas particles are moved slightly closer to eachother. A good example is a fire extinguisher, work was done to get the liquid into thecontainer and then to pressurize it. Once the container is closed the pressure energy isavailable for later use.Any time you have liquid in a container, even one that is not pressurized, you will havepressure at the bottom due to the liquid’s weight, this is known as static pressure.Elevation energy is the energy that is available to a liquid when it is at a certain height. Ifyou let it discharge it can drive something useful like a turbine producing electricity.Friction energy is the energy that is lost to the environment due to the movement of theliquid through pipes and fittings in the system.Velocity energy is the energy that moving objects have. When a pitcher throws abaseball he gives it velocity energy. When water comes out of a garden hose, it hasvelocity energy.Figure 10 The relationship between height, pressure and velocity.In the figure above we see a tank full of water, a tube full of water and a cyclist at the topof a hill. The tank produces pressure at the bottom and so does the tube. The cyclist haselevation energy that he will be using as soon as he moves. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .16As we open the valve at the tank bottom the fluid leaves the tank with a certain velocity,in this case pressure energy is converted to velocity energy. The same thing happenswith the tube. In the case of the cyclist, the elevation energy is gradually converted tovelocity energy.The three forms of energy: elevation, pressure and velocity interact with each other inliquids. For solid objects there is no pressure energy because they don’t extendoutwards like liquids filling up all the available space and therefore they are not subjectto the same kind of pressure changes.The energy that the pump must supply is the friction energy plus the difference in heightthat the liquid must be raised to which is the elevation energy.PUMP ENERGY FRICTION ENERGY ELEVATION ENERGYFigure 11 Pump energy equals elevationenergy plus friction energy.You are probably thinking where is the velocity energy in all this. Well if the liquid comesout of the system at high velocity then we would have to consider it but this is not atypical situation and we can neglect this for the systems discussed in this article. Thelast word on this topic, it is actually the velocity energy difference that we would need toconsider. In Figure 11 the velocities at point 1 and point 2 are the result of the position ofthe fluid particles at points 1 and 2 and the action of the pump. The difference betweenthese two velocity energies is an energy deficiency that the pump must supply but asyou can see the velocities of these two points will be quite small.Now what about head? Head is actually a way to simplify the use of energy. To useenergy we need to know the weight of the object displaced.Elevation energy E.E. is the weight of the object W times the distance d:EE W x d Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .17Friction energy FE is the force of friction F times the distance the liquid is displaced orthe pipe length l:FE F x lHead is defined as energy divided by weight or the amount of energy used to displace aobject divided by its weight. For elevation energy, the elevation head EH is:EH W x d / W dFor friction energy, the friction head FH is the friction energy divided by the weight ofliquid displaced:FH FE/W F x l/WThe friction force F is in pounds and W the weight is also in pounds so that the unit offriction head is feet. This represents the amount of energy that the pump has to provideto overcome friction.I know you are thinking: “ this doesn’t make sense”, how can feet represent energy?If I attach a tube to the discharge side of a pump, the liquid will rise in the tube to aheight that exactly balances the pressure at the pump discharge. Part of the height ofliquid in the tube is due to the elevation height required (elevation head) and the other isthe friction head and as you can see both can be expressed in feet and this is how youcan measure them.Figure 12 Measuring elevation head andfriction head. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .185. Static headWebster’s dictionary definition of head is: “a body of water kept in reserve at a height”.Figure 13 The definition of head.It is expressed in terms of feet in the Imperial system and meters in the metric system.Because of its height and weight the fluid produces pressure at the low point and thehigher the reservoir the higher the pressure (see Figure 13).Figure 14 Pressure depends on the height of theliquid surface.The amount of pressure at the bottom of a reservoir is independent of its shape, for thesame liquid level, the pressure at the bottom will be the same. This is important since incomplex piping systems it will always be possible to know the pressure at the bottom ifwe know the height (see Figure 15). To find out how to calculate pressure from height goto section 14. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .19Figure 15 The pressure level at the bottom of a tank dependson the liquid surface height.When a pump is used to displace a liquid to a higher level it is usually located at the lowpoint or close to it. The head of the reservoir, which is called static head, will producepressure on the pump that will have to be overcome once the pump is started.To distinguish between the pressure energy produced by the discharge tank and suctiontank, the head on the discharge side is called the discharge static head and on thesuction side the suction static head (see Figure 16).Figure 16 The static head on the pump when theUsually the liquid isdisplacedsuction tank is full.from a suction tankto adischarge tank. The suction tank fluid provides pressure energy to the pump that helpsthe pump. We want to know how much pressure energy the pump itself must supply sotherefore we subtract the pressure energy provided by the suction tank. The static headis then the difference in height of the discharge tank fluid surface minus the suction tankfluid surface. Static head is sometimes called total static head to indicate that thepressure energy available on both sides of the pump has been considered (see Figure16). Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .20Since there is a difference in height between the suction and discharge flanges orconnections of a pump by convention it was decided that the static head would bemeasured with respect to the suction flange elevation (see Figure 17).Figure 17 The static head on the pumpis measured with respect to the pumpsuction.end is open to atmosphere then the static head is measured with respect to the pipe end(see Figure 18).Figure 18 The static head on the pump with the discharge pipe endopen to atmosphere. Copyright . 2005----www.lightmypump.com---------- Revised October 9, 2007

page .21Sometimes the discharge pipe end is submerged such as in Figure 19, then the statichead will be the difference in elevation between the discharge tank fluid surface andsuction tank fluid surface. Since the fluid in the system is a continuous medium and allfluid particles are connected via pressure, the fluid particles that are located at thesurface of the discharge tank will contribute to the pressure built up at the pumpdischarge. Therefore the discharge surface elevation is the height that must beconsidered for static head. Avoid the mistake of using the discharge pipe end as theelevation for calculating static head if the pipe end is submerged (see Figure 20).Note: if the discharge pipe end is submerged then a check valve on the pump dischargeis required to avoid backflow when the pump is stopped.Figur