The viscosity of the fluid and its influence on the pumping equipment

However, a more viscous liquid affects the pump performance in several ways:

- reduced pump power (kW);

- reduces the pressure and volume of the pumped liquid;

- there are problems with the mechanical seal due to problems moving the shaft and shifting the stuffing box;

- bearings are subjected to a higher radial load when the pump shaft is displaced;

- the pumped liquid will not be able to lubricate the ground surfaces if the thickness of the liquid film is less than 0.000040 microns.

Viscosity is a measure of the density of a liquid. Molasses and engine oil are thick or highly viscous liquids. Gasoline and water are less dense, and liquids with low viscosity. Do not confuse the viscosity with the specific gravity of the same liquid. Specific gravity is a measure of the weight of a liquid compared to an equal volume of fresh water of 20°C.

Engine oil has a low specific gravity (it floats on the surface of water), but has a high viscosity of more than 500 cSt (Centistokes - a unit of kinematic viscosity). Mercury has a high specific gravity (13.7) but a low viscosity of only 0.118 Centistokes.

It is important to note once again that these two properties of the liquid are completely independent of each other.

The viscosity of a liquid may change markedly with changes in liquid temperature, but it rarely changes with changes in pressure. It has long been known that hot oil is more liquid than cold, so before determining the viscosity of the liquid, you should first measure its temperature. If you do not have this information, you may purchase an unsuitable pump.

Temperature is not the only parameter to pay attention to when you test a liquid for viscosity.

There are four classes of liquids that change their viscosity when mixed, and one that does not change its properties:

- linear-viscous (Newtonian) liquids do not depend on the amplitude and type of motion to which they are subjected. Typical liquids for this class are mineral oil and water;

- dilatant (viscosity increases with increasing shear rate) liquids increase the viscosity when mixing, some of these liquids may become almost solid inside the pump or pipeline. Everyone knows that when you mix the cream turns into butter. Confectionary mixes, clay masses, and other highly filled liquids do the same.

- the plastic liquid has a yield point that must be exceeded before the flow begins. From this point on, the viscosity will decrease as you stir. Tomato ketchup is the best example of such a liquid;

- pseudo-plastic liquid shows a decrease in viscosity with increasing mixing, but it does not have a yield point, many emulsions fall into this category;

- thixotrophic liquids exhibit decreasing viscosity with increasing mixing, but the viscosity, at any given speed of movement, may depend on the previous mixing of the liquid. Examples of thixotrophic fluid are: glue, non-drip paint, grease, cellulose compounds, soap, starches, and resin.

There is a dynamic viscosity and a kinematic viscosity.

Consider the dynamic (absolute) viscosity:

- in the international system of units (SI), dynamic viscosity is measured in Pascal seconds (PA * s);

- there are non-si measurement values of dynamic viscosity "poise" (P) and its derivative the "centipoise" (CP);

- water, at a temperature of 20.2°C, has an absolute viscosity of 1 centimetre.;

Consider the kinematic viscosity:

- the main unit of measurement in the system of units (SI) - m2 / s,

- the common unit of measurement in the GHS (centimeter-gram-second) "Stokes" and its derivative "Centistokes" (cSt);

- non-system unit-Engler's degree;

These types of viscosities are related as follows:

Kinematic viscosity = absolute viscosity / specific gravity.

Absolute viscosity = kinematic viscosity * density.

Since the specific gravity of water at 20°C is practically the same, therefore, the kinematic viscosity of water at 20°C for all practical purposes is equal to 1.0 Centistokes.