Alfa Laval Pump Handbook All You Need To Know - Acuity Process

Terminology and TheoryRelationship Between Absolute and Kinematic ViscosityAbsolute and Kinematic viscosity are related by:n mrwhere r is the fluid density (see 2.1.3).In the cgs system this translates to:Kinematic Viscosity (cSt) Absolute Viscosity (cP)Specific GravityorAbsolute Viscosity (cP) Kinematic Viscosity (cSt) x SGViscosityA viscosity conversion table is included in 14.3.10.TemperatureViscosity Variation with TemperatureTemperature can have a significant effect on viscosity and a viscosityfigure given for pump selection purposes without fluid temperature isoften meaningless - viscosity should always be quoted at thepumping temperature. Generally viscosity falls with increasingtemperature and more significantly, it increases with fallingtemperature. In a pumping system it can be advantageous toincrease the temperature of a highly viscous fluid to ease flow.Fig. 2.1.2a Viscosity variationwith temperatureViscosityNewtonian FluidsIn some fluids the viscosity is constant regardless of the shear forcesapplied to the layers of fluid. These fluids are named Newtonian fluids.At a constant temperature the viscosity is constant with change inshear rate or agitation.Shear rateTypical fluids are: Water Beer Hydrocarbons Milk Mineral Oils Resins SyrupsFig. 2.1.2b Newtonian FluidsAlfa Laval Pump Handbook 9

Terminology and TheoryIt is not always obvious whichtype of viscous behaviour afluid will exhibit andconsideration must be givento the shear rate that will existin the pump under pumpingconditions. It is not unusual tofind the effective viscosity aslittle as 1% of the valuemeasured by standardinstruments.Non-Newtonian FluidsMost empirical and test data for pumps and piping systems has beendeveloped using Newtonian fluids across a wide range of viscosities.However, there are many fluids which do not follow this linear law,these fluids are named Non-Newtonian fluids.When working with Non-Newtonian fluids we use Effective Viscosityto represent the viscous characteristics of the fluid as though it wasnewtonian at that given set of conditions (shear rate, temperature).This effective viscosity is then used in calculations, charts, graphs and‘handbook’ information.Viscosity?Viscosity?Shear rateFig. 2.1.2c Viscosity against shear rateNormalViscometerReadingTypical ShearRate in PumpingSystemShearRateFig. 2.1.2d Viscosity against shear rateTypes of Non-Newtonian FluidsThere are a number of different type of non-newtonian fluids each withdifferent characteristics. Effective viscosity at set conditions will bedifferent depending on the fluid being pumped. This can be betterunderstood by looking at the behaviour of viscous fluids with changesin shear rate as follows.ViscosityPseudoplastic FluidsViscosity decreases as shear rate increases, but initial viscosity maybe so high as to prevent start of flow in a normal pumping system.Typical fluids are: Blood Emulsions Gums Lotions Soap Toothpaste YeastShear rateFig. 2.1.2e Pseudoplastic FluidsDilatant FluidsViscosity increases as shear rate increases.ViscosityTypical fluids are: Clay Slurries Paper CoatingsShear rateFig. 2.1.2f Dilatant Fluids10 Alfa Laval Pump Handbook

Terminology and TheoryViscosityThixotropic FluidsViscosity decreases with time under shear conditions. After shearceases the viscosity will return to its original value - the time forrecovery will vary with different fluids.TimeTypical fluids are: Cosmetic Creams Dairy Creams Greases Stabilised YoghurtFig. 2.1.2g Thixotropic FluidsViscosityAnti-thixotropic FluidsViscosity increases with time under shear conditions. After shearceases the viscosity will return to its original value - the time forrecovery will vary with different fluids. As the name suggestsanti-thixotropic fluids have opposite rheological characteristics tothixotropic fluids.TimeFig. 2.1.2h Anti-thixotropic FluidsTypical fluids are: Vanadium Pentoxide SolutionViscosityRheomalactic FluidsViscosity decreases with time under shear conditions but does notrecover. Fluid structure is irreversibly destroyed.Typical fluids are: Natural Rubber Latex Natural YoghurtTimeFig. 2.1.2i Rheomalactic FluidsStressPlastic FluidsNeed a certain applied force (or yield stress) to overcome ‘solid-likestructure’, before flowing like a fluid.YWhere Y Yield StressTypical fluids are: Barium X-ray Meal Chocolate Tomato KetchupShear rateFig. 2.1.2j Plastic FluidsIt should be noted that somefluids would have boththixotropic and pseudoplasticbehaviour.Alfa Laval Pump Handbook 11

Terminology and TheoryDensity in gases variesconsiderably with pressureand temperature but can beregarded as constant in fluids.2.1.3 DensityThe density of a fluid is its mass per unit of volume, usually expressedas kilograms per cubic metre (kg/m3) or pounds per cubic foot (lb/ft3).Density is usually designated by the symbol r.1 m³ of ethyl alcohol has a mass of 789 kg.i.e. Density 0.789 kg/m3.1 ft³ of ethyl alcohol has a mass of 49.2 lb.i.e. Density 49.2 lb/ft3.m1 m1Mass ofethyl alcohol789 kg1 ft1 m1ft1 ftMass ofethyl alcohol49.2lbFig. 2.1.3a Density2.1.4 Specific WeightThe specific weight of a fluid is its weight per unit volume and isusually designated by the symbol g. It is related to density as follows:g rxgwhere g is gravity.The units of weight per unit volume are N/m3 or lbf/ft3.Standard gravity is as follows:g 9.807 m/s2g 32.174 ft/s2The specific weight of water at 20oC (68oF) and 1 atmosphere is asfollows:g 9790 N/m3 62.4 lbf/ft3Note! - Mass should not be confused with weight. Weight is theforce produced from gravity acting on the mass.12 Alfa Laval Pump Handbook

Terminology and Theory2.1.5 Specific Gravitym1 mMass ofethyl alcohol789 kg11 m1 m1m1 mMass ofwater1000 kgThe specific gravity of a fluid is the ratio of its density to the density ofwater. As this is a ratio, it does not have any units of measure.1 m³ of ethyl alcohol has a mass of 789 kg - its density is 789 kg/m³.1 m³ of water has a mass of 1000 kg - its density is 1000 kg/m³.Fig. 2.1.5a Specific gravitySpecific Gravity of ethyl alcohol is:789 kg/m³1000 kg/m³ 0.789or1 ft³ of ethyl alcohol has a mass of 49.2 lb - its density is 49.2 lb/ft³.1 ft³ of water has a mass of 62.4 lb - its density is 62.4 lb/ft³.Specific Gravity of ethyl alcohol is:49.2 lb/ft³62.4 lb/ft³ 0.789This resultant figure is dimensionless so the Specific Gravity (or SG) is0.789.Temperature is a measure ofthe internal energy level in afluid, usually expressed inunits of degrees Centigrade( C) or degrees Fahrenheit ( F).2.1.6 TemperatureThe temperature of the fluid at the pump inlet is usually of mostconcern as vapour pressure can have a significant effect on pumpperformance (see 2.1.8). Other fluid properties such as viscosity anddensity can also be affected by temperature changes. Thus a coolingof the product in the discharge line could have a significant effect onthe pumping of a fluid.The temperature of a fluid can also have a significant affect on theselection of any elastomeric materials used.A temperature conversion table is given in section 14.3.11.2.1.7 Flow CharacteristicsWhen considering a fluid flowing in a pipework system it is importantto be able to determine the type of flow. The connection between thevelocity and the capacity of a fluid (similar to water) in different tubesizes is shown in table 14.6.Under some conditions the fluid will appear to flow as layers in asmooth and regular manner. This can be illustrated by opening awater tap slowly until the flow is smooth and steady. This type of flowis called laminar flow. If the water tap is opened wider, allowing thevelocity of flow to increase, a point will be reached whereby theAlfa Laval Pump Handbook 13

Terminology and Theorystream of water is no longer smooth and regular, but appears to bemoving in a chaotic manner. This type of flow is called turbulent flow.The type of flow is indicated by the Reynolds number.VelocityVelocity is the distance a fluid moves per unit of time and is given byequation as follows:In dimensionally consistent SI unitsVelocity V QAwhere V fluid velocity (m/s)Q capacity (m³/s)A tube cross sectional area (m²)Other convenient forms of this equation are:Velocity V Q x 353.6D²where V fluid velocity (m/s)Q capacity (m³/h)D tube diameter (mm) Q x 0.409D²where V fluid velocity (ft/s)Q capacity (US gall/min)D tube diameter (in) Q x 0.489D²where V fluid velocity (ft/s)Q capacity (UK gall/min)D tube diameter (in)orVelocity VorVelocity VFluid velocity can be of great importance especially when pumpingslurries and fluids containing solids. In these instances, a certainvelocity may be required to prevent solids from settling in thepipework, which could result in blockages and changes in systempressure as the actual internal diameter of the pipe is effectivelydecreased, which could impact on pump performance.14 Alfa Laval Pump Handbook

Terminology and TheoryV velocityumax maximum velocityLaminar FlowThis is sometimes known as streamline, viscous or steady flow. Thefluid moves through the pipe in concentric layers with the maximumvelocity in the centre of the pipe, decreasing to zero at the pipe wall.The velocity profile is parabolic, the gradient of which depends uponthe viscosity of the fluid for a set flow-rate.Fig. 2.1.7a Laminar flowTurbulent FlowThis is sometimes known as unsteady flow with considerable mixingtaking place across the pipe cross section. The velocity profile ismore flattened than in laminar flow but remains fairly constant acrossthe section as shown in fig. 2.1.7b. Turbulent flow generally appearsat relatively high velocities and/or relatively low viscosities.V velocityumax maximum velocityFig. 2.1.7b Turbulent flowTransitional FlowBetween laminar and turbulent flow there is an area referred to astransitional flow where conditions are unstable and have a blend ofeach characteristic.This is a ratio of inertia forcesto viscous forces, and as such,a useful value for determiningwhether flow will be laminar orturbulent.Reynolds Number (Re)Reynolds number for pipe flow is given by equation as follows:In dimensionally consistent SI unitsRe DxVxrmwhere D tube diameter (m)V fluid velocity (m/s)r density (kg/m³)m absolute viscosity (Pa.s)Alfa Laval Pump Handbook 15

Terminology and TheoryOther convenient forms of this equation are:Re DxVxrmwhere D tube diameter (mm)V fluid velocity (m/s)r density (kg/m³)m absolute viscosity (cP) 21230 x QDxmwhere D tube diameter (mm)Q capacity (l/min)m absolute viscosity (cP) 3162 x QDxnwhere D tube diameter (in)Q capacity (US gall/min)n kinematic viscosity (cSt) 3800 x QDxnwhere D tube diameter (in)Q capacity (UK gall/min)n kinematic viscosity (cSt)orReorReorReSince Reynolds number is a ratio of two forces, it has no units. For agiven set of flow conditions, the Reynolds number will not vary whenusing different units. It is important to use the same set of units, suchas above, when calculating Reynolds numbers.Re less than 2300-Re in range 2300 to 4000-Re greater than 4000-Laminar Flow(Viscous force dominates - highsystem losses)Transitional Flow(Critically balanced forces)Turbulent Flow(Inertia force dominates - lowsystem losses)Where transitional flow occurs, frictional loss calculations should becarried out for both laminar and turbulent conditions, and the highestresulting loss used in subsequent system calculations.16 Alfa Laval Pump Handbook

Terminology and Theory2.1.8 Vapour PressureFluid (liquid form)Pvp Vapour pressure (externalpressure required to maintain as a fluid)Fluids will evaporate unless prevented from doing so by externalpressure. The vapour pressure of a fluid is the pressure (at a giventemperature) at which a fluid will change to a vapour and is expressedas absolute pressure (bar a or psia) - see 2.2.2. Each fluid has its ownvapour pressure/temperature relationship. In pump sizing, vapourpressure can be a key factor in checking the Net Positive SuctionHead (NPSH) available from the system (see 2.2.4).Fig. 2.1.8a Vapour pressureTemperatureVapour pressure (bar)0 C (32 F)0.006 bar a (0.087 psia)20o C (68o F)0.023 bar a (0.334 psia)100o C (212o F)1.013 bar a (14.7 psia)ooWater will boil (vaporise) at a temperature of:0 C (32o F) if Pvp 0.006 bar a (0.087 psia).20 C (68o F) if Pvp 0.023 bar a (0.334 psia).100 C (212o F) if Pvp 1.013 bar a (14.7 psia)(atmospheric conditions at sea level).In general terms Pvp:Is dependent upon the type of fluid.Increases at higher temperature.Is of great importance to pump inlet conditions.Should be determined from relevant tables.The Pvp for water at various temperatures is shown in section 14.4.2.1.9 Fluids Containing SolidsIt is important to know if a fluid contains any particulate matter and ifso, the size and concentration. Special attention should be givenregarding any abrasive solids with respect to pump type andconstruction, operating speed and shaft seals.Size of solids is also important, as when pumping large particles thepump inlet should be large enough for solids to enter the pumpwithout ‘bridging’ the pump inlet. Also the pump should be sized sothe cavity created in the pump chamber by the pump elements is ofsufficient size to allow satisfactory pump operation.Concentration is normally expressed as a percentage by weight(W/W) or volume (V/V) or a combination of both weight and volume(W/V).Alfa Laval Pump Handbook 17

Terminology and Theory2.2 Performance Data2.2.1 Capacity (Flow Rate)The capacity (or flow rate) is the volume of fluid or mass that passes acertain area per time unit. This is usually a known value dependent onthe actual process. For fluids the most common units of capacity arelitres per hour (l/h), cubic metres per hour (m³/h) and UK or US gallonsper minute (gall/min). For mass the most common units of capacityare kilogram per hour (kg/h), tonne per hour (t/h) and pounds per hour(lb/h).2.2.2 PressureF ForcePressure is defined as force per unit area:P FAwhere F is the force perpendicular to a surface and A is the area of thesurface.A11Fig. 2.2.2a PressureIn the SI system the standard unit of force is the Newton (N) and areais given in square metres (m²). Pressure is expressed in units ofNewtons per square metre (N/m²). This derived unit is called thePascal (Pa). In practice Pascals are rarely used and the mostcommon units of force are bar, pounds per square inch (lb/in²) or psi,and kilogram per square centimetre (kg/cm²).Conversion factors between units of pressure are given in section14.3.5.Different Types of PressureFor calculations involving fluid pressures, the measurements must berelative to some reference pressure. Normally the reference is that ofthe atmosphere and the resulting measured pressure is called gaugepressure. Pressure measured relative to a perfect vacuum is called‘absolute pressure’.Atmospheric PressureThe actual magnitude of the atmospheric pressure varies withlocation and with climatic conditions. The range of normal variation ofatmospheric pressure near the earth’s surface is approximately 0.95to 1.05 bar absolute (bar a) or 13.96 to 15.43 psi gauge (psig). Atsea level the standard atmospheric pressure is 1.013 bar a or 14.7psi absolute (bar a or psia).18 Alfa Laval Pump Handbook

Terminology and TheoryGauge PressureUsing atmospheric pressure as a zero reference, gauge pressure isthe pressure within the gauge that exceeds the surroundingatmospheric pressure. It is a measure of the force per unit areaexerted by a fluid, commonly indicated in units of barg (bar gauge) orpsig (psi gauge).Absolute PressureIs the total pressure exerted by a fluid. It equals atmosphericpressure plus gauge pressure, indicated in units of bar a (barabsolute) or psia (psi absolute).Absolute Pressure Gauge Pressure Atmospheric PressureVacuumThis is a commonly used term to describe pressure in a pumpingsystem below normal atmospheric pressure. This is a measure of thedifference between the measured pressure and atmospheric pressureexpressed in units of mercury (Hg) or units of psia.0 psia 760 mm Hg (29.9 in Hg).14.7 psia 0 mm Hg (0 in Hg).Inlet (Suction) PressureThis is the pressure at which the fluid is entering the pump. Thereading should be taken whilst the pump is running and as close tothe pump inlet as possible. This is expressed in units of absolute bar a(psia) or gauge bar g (psig) depending upon the inlet conditions.Outlet (Discharge) PressureThis is the pressure at which the fluid leaves the pump. Again thisreading should be taken whilst the pump is running and as close tothe pump outlet as possible. The reading is expressed in units ofgauge bar (psig).Differential PressureThis is the difference between the inlet and outlet pressures. For inletpressures above atmospheric pressure the differential pressure isobtained by subtracting the inlet pressure from the outlet pressure.For inlet pressures below atmospheric pressure the differentialpressure is obtained by adding the inlet pressure to the outletpressure. It is therefore the total pressure reading and is the pressureagainst which the pump will have to operate. Power requirements areto be calculated on the basis of differential pressure.Alfa Laval Pump Handbook 19

Terminology and TheoryExample: Inlet Pressure above Atmospheric PressureOutletInletDifferential5.013 bar a(72.7 psi a)4 bar g(58 psi g)1.5 bar g(21.75 psi g)1

Alfa Laval in brief Alfa Laval is a leading global provider of specialized products and engineering solutions. Our equipment, systems and . Section 14: Technical Data Summary of the nomenclature and formulas used in this handbook. Various conversion tables and charts are also shown. 213 14.1 Nomenclature 213