7.3 Pumping oil
Pumping oil with different properties
Pumping oil with different properties require knowledge of their viscosity and density which varies with the temperature. Oils are classified according to their origins into mineral oils, animal and vegetable oils, but in the context of pumps they can all be treated the same. Along with mineral oils, we can also reckon such petroleum products as solvents, petrol, kerosene and the like, which should be considered when pumping oil stocks. When pumping oil, the upper and lower operating temperature limits, viscosity, cloud point, lowest flow (pour point), solidifying temperature and the vapour pressure should all be established.
The flow capability of oil follows Newton’s law. In common with water, they have constant viscosity independent of shear rate and time. The viscosity is temperature dependent, oil are flowing more easily when heated. The viscosity falls as the temperature rises. In order to assess the needs of a pump installation, the viscosity-temperature relationship must be known and the way the oil behaves with variations in operational temperatures must also be clarified.
Cloud point, lowest flow temperature (pour point) and solidifying temperatures
Mineral oils transform gradually from the liquid to the solid state, as opposed to other liquids (water, for example, which has an exact freezing point). When the oil is chilled, it goes cloudy at a certain temperature by reason of the precipitation of paraffin crystals, i.e. generation of wax. This temperature is called the cloud point (cold filter plugging point). The pour point is reached if the temperature is
further reduced. A few degrees below this temperature, the oil changes to a completely solid form, the solidifying point.
Because of waxing, it is considered that mineral oils can only be handled by pumping at a temperature of at least 10°C above the pour point. When considering pumping sluggish, high-viscosity oils with solidifying temperatures near to or above the ambient temperature, (for outdoor installations for example), notice must be taken of the pour point and the installation must be designed so that the pipes and pump can be heated. Pipes which are not heated must be able to be emptied in order to prevent stoppages building up if pumping is interrupted in ambient temperatures below the lowest flow temperature. Low-sulphurous heating oils have higher pour point temperatures than high-sulphurous ones.
Complex petroleum products, petrol for example, have a vapor pressure range which is dependent upon the most easily flowing components. This circumstance affects the calculation of the NPSHr (NPSH required) for the pump.
Viscosity
The scales in the viscosity diagrams are themselves not linear but are adjusted so that the viscosity relationships are linear. The diagram may be used to construct the viscosity relationship for other oils if the associated viscosity temperature values are known for two points. These relationships will also be linear.
Burner oils
Class E-H are residual blended oils for atomizing burners and normally require pre-heating before atomisation. The four lines on the chart show average viscosity/temperature relationships for fuel of Classes E to H at the maximum viscosity allowed by the specifications. The approximate viscosity and temperature relationship for any petroleum fuel within these classes, for which viscosity at one temperature is known, can be determined by drawing through the known viscosity / temperature intersection a line parallel to those shown. From this line can be read the temperature required for any desired viscosity e.g. that specified by burner manufacturers for proper atomisation.
Figure 7.3a Kinematic viscosity temperature chart for burner oil
Class of fuel | Min. temp. for storage | Min. temp. for outflow from |
E | 7°C | 7°C |
F | 20°C | 27°C |
G | 32°C | 38°C |
H | Special purpose fuel | Special purpose fuel |
Table for Kinematic viscosity / temperature chart
As the temperature is lowered towards the pour point (lowest flow temperature) there is an increasing upward deviation from the viscosity indicated on the chart. This deviation is of such a magnitude that in no case shall the chart be used within 15°C of the pour point, information on which can be obtained from the suppliers.
Engine oil
The SAE system was devised in America and laid down by the Society for Automotive Engineers (SAE) in 1926. Lower numbers in the SAE series mean thinner oils and the letter W after the number indicates that the oil is suitable for use in Winter. The SAE system is accepted and used internationally.
Table 7.3b Kinematic viscosity for engine oils SAE 5W – SAE 50.
Below the cloud point (cold filter plugging point) temperature, there is no rectilinear relationship because of waxing.
Pour point, -20°C to -30°C (Iower for certain special qualities).
Lowest recommended handling temperature: 10°C – 15°C above pour point.
Pumping gearbox oil
Below the cloud point (cold filter plugging point) temperature, there is no rectilinear relationship, because of waxing.
Pour point is -20°C to 30°C (lower for certain special qualities).
Lowest recommended handling temperature: 10°C to 15°C above pour point.
Figure 7.3c Kinematic viscosity for gearbox oils, SAE 75W – SAE 140.
Industrial oil
The ISO (International Organisation for Standardisation) has developed a system of viscosity classification for lubricants for industrial use which came into use effectively from 1977. The system consists of 18 viscosity categories stated in cSt at 400C. Each class of viscosity is identified by an ISO VG (viscosity grade) number which in general coincides with the mean value in accordance with figure 7.3d.
Figure 7.3d Viscosity classes in accordance with ISO 3448
The classification system advantages:
- the ISO VG number gives information on the viscosity of the oil.
- ISO 3448 is fully supported by the leading national standardization organisations which makes it easy to compare with oils specified by industries and machine manufacturers
Pour point 20-50°C (lower values for certain special qualities).
Lowest recommended handling temperature: 10°C -15°C above pour point.
Figure 7.3e Kinematic viscosity for industrial oils in accordance with ISO 3448.
Kinematic viscosity mm²/s (cSt), density, vapour pressure and other properties for various oils, Aviation fuel, Castor Oil, Corn Oil, Groundnut Oil, Kerosene, Linseed Oil, Olive Oil, Petrol, Soya Oil, Turpentine, Whale Oil / Whale Train (Valtran), at a given temperature in figure 7.3f and 7.3g.
