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GENERAL INDUSTRY SEGMENT

ENGINEERING MANUALViscous and high density

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What is viscosity?........................................................................................................................................ 2 What is density? .......................................................................................................................................... 3 Effect on centrifugal pumps .................................................................................................................... 4 Choosing a pump ........................................................................................................................................ 5 

General consideration........................................................................................................................... 5 

What Grundfos recommends.................................................................................................................. 5 Shaft seals................................................................................................................................................. 6 

WinCaps / WebCaps................................................................................................................................... 8 List of typical coolants ............................................................................................................................... 9 Diagrams for coolants. .............................................................................................................................. 9 Oils.................................................................................................................................................................40 

Oils for metal cutting ..........................................................................................................................40 Lubricating oils ......................................................................................................................................42 Vegetable oils ........................................................................................................................................46 

Other liquids...............................................................................................................................................47 

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What is viscosity?

Viscosity means resistance to flow within the internal layers of a fluid. The higher theviscosity, the thicker the fluid. Viscosity is a measure of the “thickness” of a liquid. Molasses

and motor oil are thick or high-viscous liquids. Gasoline and water are thin, lowviscosity liquids.The unit of measurement for viscosity is cP (centipoise, normally measured in Pa·s). Inpractice, however, the unit cSt (centistoke, normally measured in mm2/s) is used.Water has a viscosity of 1 cSt at 20°C, while motor oil typically has a viscosity of more than500 cSt at the same temperature.

The conversion factor between cP and cSt is the specific gravity of the liquid, i.e. the viscosityvalue in cP divided by the specific gravity gives the viscosity value in cSt. The table belowshows viscosity values for water and propylene glycol in different units

Example:

If we take the values from the table above, we get the following formula.

smmavitySpecificgr 

cPcSt  /8,68

061,1

73 2===  

cSt is the unit used when sizing a pump in WinCaps/WebCaps, as it influences the pumpcurve.cP relates to the lubricating capability of a liquid. We therefore use cP when discussing amedium’s ability to lubricate the bearings in a pump.

The viscosity of a liquid may change significantly as the temperature of a liquid changes. Weknow that hot oil is “thinner” than cold oil, so we must always know the temperature of aviscous liquid in order to select the right pump.Temperature is not the only variable when considering viscosity. Three classes of fluids existthat change viscosity when agitated and one that does not.

•  Newtonian fluids are unaffected by the magnitude and kind of motion to which theyare subjected. Mineral oil and water are typical of this type of liquid.

•  Dilatant fluids increase their viscosity with agitation. Some of these liquids canbecome almost solid within a pump or pipe line. We all know that with agitation,

cream becomes butter. Candy compounds, clay slurries and similar heavily filledliquids act the same way.

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•  Plastic fluids have a yield value which must be exceeded before they can startflowing. From that point on, viscosity decreases as agitation increases. Tomatoketchup is the best example of this type of fluid.

•  Thixotrophic fluids exhibit a decreasing viscosity with an increase in agitation,although the viscosity at any particular rate of motion may depend upon the previous

agitation of the liquid. Examples are: glues, non-drip paint, greases, cellulosecompounds, soaps, starches, and tar.

1.  The effect of viscosity on centrifugal pump performance

A viscous liquid affects a centrifugal pump in several ways.•  Increased power consumption, i. e. a larger motor is required.•  The head, flow rate and pump efficiency will be reduced.

What is density?

The density is a measure of the weight of the liquid normally expressed in the unit g/cm3 orkg/m3. Water at a temperature of 4°C has a density of 1 g/cm3 or 1000 kg/m3. The term“Specific gravity” is often used. Specific gravity is a measure of the weight of the liquidcompared to an equal volume of 4°C fresh water. Specific gravity therefore has no unit ofmeasurement. Oils have a low specific gravity - lower than that of water while mercury hasa high specific gravity.

1.  The effect of high density on centrifugal pump performance

A high density liquid only affects the power consumption of a centrifugal pump.•  The head, flow rate and pump efficiency will remain unchanged.•  Power consumption increases proportionally as density increases. A liquid with a

specific gravity of 1.2 will thus require a 20% larger power input•  An oversize motor will often be required.

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Effect on centrifugal pumps

In many applications a viscous liquid also has a high density, such as in cooling applicationsusing propylene glycol as an antifreeze. The chart below is based on polypropylene with the

following properties:•  Concentration: 50%•  Temperature: -20°C•  Density: 1061 kg/m3•  Viscosity: 68.8 cSt.

Fig. 4 CR 32-4 pumping 50% poly propylene glycol at -20°C.

The reduction in pumphead is only from theviscosity.

The reduction in pumpflow is both fromdensity and viscosity.

Bigger powerconsumption is both fromdensity and viscosity.

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Choosing a pump

General consideration.

The selection of a pump for liquids with viscosity and density different than water can bedifficult and involve a high degree of uncertainty. It is recommended to use the Grundfossoftware WinCAPS or WebCaps when making these selections.Grundfos pumps are designed for Newtonian liquids (for example water). The selection of apump for non Newtonian liquids involves a high degree of uncertainty, and should thereforebe considered thoroughly.When pumping viscous liquids, it is recommended to select a pump large enough to ensurethat the pump works within the nominal duty point on the Q,H curve. Operation outside thenominal duty point may lead to a wrong duty point.

1.  FlowLiquids with a viscosity lower than 10 cSt. will not affect the flow rate in the pumpsignificantly. Above 10 cSt it is necessary to make calculations for the reduction in flow rate.

2.  HeadConcerning head it will be necessary to make calculations when the liquid exceeds a valueof 5 cSt. The reduction in head is proportional to the flow rate. The pump should thereforenot operate outside its nominal duty point.

3.  Power consumption

Power consumption will increase noticeably for viscosities above 5 cSt. Therefore a largermotor will often be required. Note that the viscosity may be highest at start-up, for instanceif the system has cooled down during standstill. Be aware that the specific gravity of theliquid also has a large affect on the power consumption.

As written in the 3 bullets above it’s not necessary to think about the viscosity whenchoosing a pump if it’s low enough. But with the features in WebCaps and Wincaps forchanging the viscosity, when selecting a pump, it will be recommendable to always changeit to the right value. See under WinCaps/WebCaps how to do that.

Grundfos recommends

When you need to pump liquids with a density and viscosity different to that of water,Grundfos recommends the following:

1.  High viscosity.When pumping viscous liquids, small pumps with low flow rates are affected more thanlarge pumps with high flow rates. The table below shows the maximum recommendedviscosity for various rated flow rates.

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Note that even though a pump will still be capable of pumping a liquid with a viscosityhigher than that listed in the table, its efficiency will drop off drastically; something whichGrundfos recommends avoiding.

2.  Low viscosity.The minimum permitted viscosity for a pump depends on the pump model. It is thereforerecommended to refer to the relevant pump data booklet for the specific value.

For example, Grundfos CR pumps are capable of pumping liquids with a viscosity down to

0.15 cP while remaining sufficiently lubricated.

Shaft seals

1.  Facts about shaft seals in viscous liquidsThe basic sealing principle of a mechanical shaft seal is that two smooth surfaces are slidingagainst each other at a relatively high surface pressure. The seal faces are typically made ofcarbides (tungsten carbide, silicon carbide) or impregnated carbon. Some shaft seals havecarbide/carbide seal faces, while others have carbide/carbon seal faces. The seal faces arelubricated by the pumped liquid. The liquid enters the seal faces, creates a lubricating liquid

film and finally evaporates outside the mechanical shaft seal. In this way a mechanical shaftseal is not completely leak-free. With the basic principles of a mechanical shaft seal in mindit seems logical that a high viscous liquid creates a thicker liquid film with better lubricatingproperties. But anyway the following shaft seal problems can occur when pumping viscousliquids.

•  If the viscous liquid has a vapour pressure lower than that of water at roomtemperature, it will not evaporate. Consequently it accumulates around the shaftseal. This problem can be solved by using a double shaft seal.

•  If the liquid contains solvents, these will evaporate and leave an even more viscous

liquid in the shaft seal. This high viscous liquid may precipate a coating on the sealfaces causing leakage or abrasive wear. This problem can be solved by using a doubleshaft seal or carbide/carbide seal faces.

•  Coolants e.g. glycols often contain additives (corrosion inhibitors) that may precipateand crystallize on the seal faces causing abrasive wear. Therefore it is recommendedto use carbide/carbide seal faces for all coolants.

2.  Seal face materialsWhen pumping viscous liquids, it is always recommended to use carbide/carbide seal faces.In Grundfos shaft seals carbide seal faces are made of either silicon carbide (code Q) ortungsten carbide (code U).

Seal faces made of impregnated carbon (code A or B) are not recommended due to their lackof abrasion resistance.

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3.  Surface pressureTo minimize precipitation at the seal faces it is important to ensure a high surface pressurebetween the seal faces. This can be achieved by reducing the seal face area (R or G-seals) orby providing the seal with a tight spring. In shaft seals with a high surface pressure the

sliding surfaces have an even smoother surface than normal in ordinary shaft seals.

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WinCaps / WebCaps

In Caps, you can select a pump with the help of the sizing function. If you know the specificpump model you can also select it directly from the catalogue.

Regardless of which function you choose, you will be required to make the followingchoices:

This list includes the mostcommonly-used liquids. If aparticular liquid is missingfrom the list, select ”anyviscous fluid” and enter theviscosity and density values

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If required, contact the manufacturer to obtain the density and viscosity values for aproduct not included in the list.

List of typical coolants

Diagrams for coolants.

The charts on the following pages show viscosity and density for different coolants.Concentrations are generally shown as [mass%]. Concentration in [vol%] will be stated onthe relevant chart.

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Ethylene glycol viscousity

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Ethylene glycol density

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Propylene glycol viscosity

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Propylene glycol density

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Freezium viscosity

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Freezium density

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Hycool viscosity

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Hycool density

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Tyfoxit viscosity

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Tyfoxit density

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Aspen Temper viscosity

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Aspen Temper density

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Pekasol viscosity

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Pekasol density

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Magnesium chloride viscosity

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Magnesium chloride density

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Calcium chloride viscosity

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Calcium chloride density

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Potassium carbonate viscosity

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Potassium carbonate density

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Sodium chloride viscosity

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Sodium chloride density

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Dowtherm viscosity

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Dowtherm density

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Syltherm viscosity

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Syltherm density

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VegoCool viscosity

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VegoCool density

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Ethanol viscosity

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Ethanol density

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Oils.

The word “oil” is used for a wide range of liquids used in numerous applications, fromcrude oil based lubricating oil to the finest vegetabile sweet oil. In this chapter oils formetal cutting, lubricating oils, and vegetable oils are described.

Oils for metal cutting

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Various cutting oils.

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Lubricating oils

The following curves viscosities for various lubricating oils.

Motor oil, semi-synthetic 5W-30

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Motor oil, synthetic 0W-40

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Gear oil 80 W-90

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Gear oil 85W-140

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Vegetable oils

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Other liquidsAmoniak (NH3)

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Potassium carbonate

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Glycerol

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Sodium Chloride

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Viscosity of typical liquids as a function of liquid temperature