Viscous liquids

There is no doubt about it; water is the most common liquid that pumps handle. However, in a number of applications, pumps have to handle other types of liquids, e.g. oil, propylene glycol, gasoline. Compared to water, these types of liquids have different density and viscosity.

Viscosity is a measure of the thickness of the liquid.

The higher the viscosity, the thicker the liquid. Propylene glycol and motor oil are examples of thick or high viscous liquids. Gasoline and water are examples of thin, low viscous liquids.

Two kinds of viscosity exist:

The dynamic viscosity which is normally measured in Pa*s or Poise. (1 Poise = 0.1 Pa*s)

The kinematic viscosity (v), which is normally measured in centiStokes or m²/s (1 cSt = 10^-6 m²/s)

The relation between the dynamic viscosity () and the kinematic viscosity (v) is shown in the formula on your right hand side.

On the following pages, we will only focus on kinematic viscosity (v).

The viscosity of a liquid changes considerably with the change in temperature; hot oil is thinner than cold oil. As you can tell from figure 1.5.1, a 50% propylene glycol liquid increases its viscosity 10 times when the temperature changes from +20 to -20 °C.

For more information concerning liquid viscosity, go to appendix L.

Non-Newtonian liquids

The liquids discussed so far are referred to as Newtonian fluids. The viscosity of Newtonian liquids is not affected by the magnitude and the motion that they are exposed to. Mineral oil and water are typical examples of this type of liquid. On the other hand, the viscosity of non-Newtonian liquids does change when agitated.

This calls for a few examples:

• Dilatant liquids like cream - the viscosity increases when agitated

• Plastic fluids like catsup - have a yield value, which has to be exceeded before flow starts. From that point on, the viscosity decreases with an increase in agitation

• Thixotrophic liquids like non-drip paint - exhibit a decreasing viscosity with an increase in agitation

The non-Newtonian liquids are not covered by the viscosity formula described earlier in this section.

The impact of viscous liquids on the performance of a centrifugal pump

Viscous liquids, that is liquids with higher viscosity and/ or higher density than water, affect the performance of centrifugal pumps in different ways:

• Power consumption increases, i.e. a larger motor may be required to perform the same task

• Head, flow rate and pump efficiency are reduced

Let us have a look at an example. A pump is used for pumping a liquid in a cooling system with a liquid temperature below 0°C. To avoid that the liquid freezes, an antifreeze agent like propylene glycol is added to the water. When glycol or a similar antifreeze agent is added to the pumped liquid, the liquid obtains properties, different from those of water. The liquid will have:

• Lower freezing point, t_f [°C]

• Lower specific heat, c_p [kJ/kgK]

• Lower thermal conductivity, X [W/mK]

• Higher boiling point, t_b [°C]

• Higher coefficient of expansion, p [m/°C]

• Higher density, p [kg/m³]

• Higher kinematic viscosity, v [cSt]

These properties have to be kept in mind when designing a system and selecting pumps. As mentioned earlier, the higher density requires increased motor power and the higher viscosity reduces pump head, flow rate and efficiency resulting in a need for increased motor power, see figure 1.5.2.

Selecting the right pump for a liquid with antifreeze

Pump characteristics are usually based on water at around 20°C, i.e. a kinematic viscosity of approximately 1 cSt and a density of approximately 1,000 kg/m³. When pumps are used for liquids containing antifreeze below 0°C, it is necessary to examine whether the pump can supply the required performance or whether a larger motor is required. The following section presents a simplified method used to determine pump curve corrections for pumps in systems that have to handle a viscosity between 5 - 100 cSt and a density of maximum 1,300 kg/m³. Please notice that this method is not as precise as the computer aided method described later in this section.

1.10

1.05

1.00

Pump curve corrections for pumps handling high viscous liquid

Based on knowledge about required duty point, Q_s, H_s, and kinematic viscosity of the pumped liquid, the correction factors of H and P₂ can be found, see figure 1.5.3.

Figure 1.5.3 is read in the following way:

When k_H and k_p2 are found in the figure, the equivalent head for clean water H_w and the corrected actual shaft power P_2s can be calculated by the following formula

where

H_w: is the equivalent head of the pump if the pumped liquid is "clean" water

P_2W: is the shaft power at the duty point (Q_S,H_W) when the pumped liquid is water

H_s: is the desired head of the pumped liquid (with agents)

Р_2s : is the shaft power at the duty point (Q_s,H_s) when the pumped liquid is water (with agents)

p_s: is the density of the pumped liquid

p : is the density of water = 998 kg/m³

The pump selection is based on the normal data sheets/ curves applying to water. The pump should cover the duty point Q, H = Q_S, H_W, and the motor should be powerful enough to handle P_2S on the shaft.