4 >>> Definition Viscosity is an internal property of a fluid that offers resistance to flow.The resistance of a substance to flow. For example, water has a lower viscosity than molasses and flows more easily. Viscosity is related to the concept of shear forceIt can be understood as the effect of different layers of the fluid exerting shearing force on each other, or on other surfaces, as they move against each other.
5 Dynamic (absolute) Viscosity Types of “Viscosity ”Dynamic (absolute) ViscosityIt is the tangential force per unit area required to move one horizontal plane with respect to the other at unit velocity when maintained a unit distance apart by the fluid.The shearing stress between the layers of non turbulent fluid moving in straight parallel lines can be defined for a Newtonian fluid
6 The dynamic or absolute viscosity can be expressed like τ = μ dc/dy (1)whereτ = shearing stressμ = dynamic viscosityEquation (1) is known as the Newtons Law of Friction.
7 In the SI system the dynamic viscosity units are N s/m2, Pa s or kg/m s where 1 Pa s = 1 N s/m2 = 1 kg/m sThe dynamic viscosity is also often expressed in the metric CGS (centimeter-gram-second) system as g/cm.s, dyne.s/cm2 or poise (p) where1 poise = dyne s/cm2 = g/cm s = 1/10 Pa sFor practical use the Poise is to large and it's usual divided by 100 into the smaller unit called the centiPoise (cP) where1 p = 100 cPWater at 68.4oF (20.2oC) has an absolute viscosity of one centiPoise.
8 Kinematic Viscosityis the ratio of absolute or dynamic viscosity to density - a quantity in which no force is involved. Kinematic viscosity can be obtained by dividing the absolute viscosity of a fluid with it's mass densityν = μ / ρ (2)whereν = kinematic viscosityμ = absolute or dynamic viscosityρ = densityIn the SI-system the theoretical unit is m2/s or commonly used Stoke (St) where1 St = 10-4 m2/s
9 Since the Stoke is an unpractical large unit, it is usual divided by 100 to give the unit called Centistokes (cSt) where1 St = 100 cSt1 cSt = 10-6 m2/sSince the specific gravity of water at 68.4oF (20.2oC) is almost one (1), the kinematic viscosity of water at 68.4oF is for all practical purposes 1.0 cSt.
10 Viscosity and Reference Temperatures The viscosity of a fluid is highly temperature dependent and for either dynamic or kinematic viscosity to be meaningful, the reference temperature must be quoted. In ISO 8217 the reference temperature for a residual fluid is 100oC. For a distillate fluid the reference temperature is 40oC.For a liquid - the kinematic viscosity will decrease with higher temperatureFor a gas - the kinematic viscosity will increase with higher temperature
11 Comparison of temperaturedependent viscositiesGas: oxygenIncrease with temperatureLiquid: anilineDecrease with temperature
12 Other Commonly used Viscosity Units Saybolt Universal Seconds (or SUS, SSU)Saybolt Universal Seconds (or SUS) is used to measure viscosity. The efflux time is Saybolt Universal Seconds (SUS) required for 60 milliliters of a petroleum product to flow through the calibrated orifice of a Saybolt Universal viscometer, under carefully controlled temperature and as prescribed by test method ASTM D 88. This method has largely been replaced by the kinematic viscosity method. Saybolt Universal Seconds is also called the SSU number (Seconds Saybolt Universal) or SSF number (Saybolt Seconds Furol).
13 Kinematic viscosity versus dynamic or absolute viscosity can be expressed as ν = 4.63 μ / SG (3)whereν = kinematic vicosity (SSU)μ = dynamic or absolute viscosity (cP)
14 Degree EnglerDegree Engler is used in Great Britain as a scale to measure kinematic viscosity. Unlike the Saybolt and Redwood scales, the Engler scale is based on comparing a flow of the substance being tested to the flow of another substance - water. Viscosity in Engler degrees is the ratio of the time of a flow of 200 cubic centimetres of the fluid whose viscosity is being measured - to the time of flow of 200 cubic centimeters of water at the same temperature (usually 20oC but sometimes 50oC or 100oC) in a standardized Engler viscosity meter.
15 Example - Converting between Kinematic and Absolute Viscosity for Air Kinematic viscosity of air at 1 bar (105 Pa, N/m2) and 40oC is cSt ( m2/s).The density of air estimated with the Ideal Gas Lawρ = p / R Twhereρ = density (kg/m3)p = absolute pressure (Pa, N/m2)R = individual gas constant (J/kg K)T = absolute temperature (K)ρ = (105 N/m2) / ((287 J/kg/K) (273 oC C) = kg/m3Absolute viscosity can be expressed asμ = (1.113 kg/m3) ( m2/s) = (kg/m s, Ns/m2, P)
17 Different types of Viscosity Shear-thickening liquids -low viscosity when stirred slowly, but viscosity increases when mixed quicklyShear thinning liquids -high viscosity when mixed slowly, easier to mix quicklyBingham liquids -at low stress behaves like a solid, high stress behaves like a liquidGranular materials -like Bingham, but do not have a well-defined viscosity
18 Types of Fluid on Viscosity Basis Newtonian FluidsNon-Newtonian Fluids
20 Newtonian FluidsFluids for which the shearing stress is linearly related to the rate of shearing strain are designated as Newtonian Fluids.Newtonian materials are referred to as true liquids since their viscosity or consistency is not affected by shear such as agitation or pumping at a constant temperature. Fortunately most common fluids, both liquids and gases, are Newtonian. Water and oils are examples of Newtonian liquids.
21 Shear-thinning or Pseudoplastic Liquids Shear-thinning or pseudoplastic liquids are those whose apparent viscosity decreases with increasing shear rate. Their structure is time-independent.
23 Thixotropic FluidsThixotropic liquids have a time-dependent structure. The apparent viscosity of a thixotropic liquid decreases with increasing time, at a constant shear rate. Ketchup and mayonnaise are examples of thixotropic materials. They appear thick or viscous but are possible to pump quite easily.
24 Dilatant FluidsShear Thickening Fluids or Dilatant Fluids increase their viscosity with agitation. Some of these liquids can become almost solid within a pump or pipe line. With agitation, cream becomes butter and Candy compounds, clay slurries and similar heavily filled liquids do the same thing.
26 Bingham Plastic Fluids Bingham Plastic Fluids have a yield value which must be exceeded before it will start to flow like a fluid. From that point the viscosity will decrease with increase of agitation. Toothpaste, mayonnaise and tomato catsup are examples of such products.
28 Viscosity and Specific Gravity of some Common Liquids
29 Viscosity and Temperature Kinematic viscosity of liquids like water, mercury, oils SAE 10 and oil no. 3 - and gases like air, hydrogen and helium are indicated below. Note thatfor liquids viscosity decreases with temperaturefor gases viscosity increases with temperature
34 RedwoodA standard British-type viscometer in which the viscosity is determined by the time, in seconds.Required for a certain quantity of liquid to pass out through the orifice under given conditionsIt is used for determining viscosities of petroleum oils.
35 WorkingThe inner cylinder is filled to the marker level and the outer cylinder filled so as to have sufficient height to give good heat transfer.The first reading is taken by removing the stopper from the orifice. The recorded values were the water and test liquid temperatures and the time to collect say 50cc of the liquid being recorded.
36 Conti……………..The collected 50cc of liquid must be put back into the inner cylinder and the used cylinder inverted to empty completely.The level of the sample is checked as, if more of the liquid is needed it would be added. The temperature is then raised and the agitator is used again continuously. This is done until the liquid is brought within 1 degree of the water temperature. The same parameters were again measured. This procedure is repeated for 5 samples in total (at least).
37 Engler ViscometerAn instrument used in the measurement of the degree Engler, a measure of viscosity; the kinematic viscosity ν in stokes for this instrument is obtained from the equation ν = t /t, where t is the efflux time in seconds.
39 Saybolt viscometerIt is Used to determine the viscosity of petroleum products at specified temperatures between 70 to 210 °F.Stainless steel made, the Saybolt viscometer is supplied complete with two interchangeable orifices Furol and Universal, oil bath, electric heater with digital thermoregulator, stirrer, cooling coil, viscosity flask.Thermometers are not included and must be ordered separately
41 Ostwald viscometer Need a small-diameter tube (capillary) Measure time of flow of a specific volume through the capillaryConstant pressure drop across the capillaryUse Poiseuille’s equation to calculate viscosityMust be calibrated with a known material
47 Features Continuous monitoring of viscosity, and temperature Two 4-20mA outputs for remote recordingTwo analog outputs and RS485 communicationCompact, explosion-proof systemtAutomatic self-cleaningReadout in centipoises, cSt, or SSUOptional LCD display
52 Oil and GasDrilling for oil and gas requires sensitive viscosity instrumentation, especially at low-shear rates, to insure that production is optimized.
53 ContinueField viscosity measurements on drilling and fracturing fluids are required to predict flow performance under the extremely low.
54 Industrial Application Viscosity measurement is really important for industry.For correct pump selection careful viscosity measurements are taken
55 ContinueFor the transport and storing facilities for fluids i.e pipes, tanks
56 Why Measuring Viscosity is Important For Many Industries Nearly all fluids (with the notable exception of the liquid form of the helium isotopes helium-3 and helium-4) have some degree of resistance to flow, with the measurement being used to determine the degree of viscosity on a spectrum ranging from no stress resistance (called an ideal liquid) to total resistance.
57 ContinueSince the temperature of a liquid and its viscosity are correlated, a viscosimeter or viscometer must be used in carefully temperature controlled conditions to produce accurate results.
58 ContinueEven a small change in temperature can have enough of an effect to negatively impact the outcome of a quality control test or production process.
59 ContinueTherefore to maintain quality of product, viscosity is measured step to step at industries