As seen from above, pump performance and efficiency are central to efficient transmission of power in a hydraulic system. It therefore becomes necessary to carry out periodic maintenance activities from time to time though it is an established fact that pumps like gear pumps are generally maintenance free within themselves. Let us now discuss in detail, the various factors affecting pump performance, considerations involved in pump selection and also find out how periodic and timely maintenance action can help overcome problems that adversely affect pump performance.
Factors affecting pump performance
Following are some of the external factors affecting the performance of pumps:
• Presence of foreign particles
• Foams and bubbles
• Overheating of oil
• Wrong selection of oil.
Presence of foreign particles
Fluids are contaminated by gritty or metal particles, which come in during the course of extraction, transportation, loading and unloading. These particles get carried in the fluid, and cause damage to the internal surfaces of a pump. They can also accelerate the process of wear and tear of running parts. For example, in gear pumps, the most susceptible components are the side plates and bushings, followed by gear teeth and bearings.
All pumping systems necessarily have a good means of filtration. The filter element provided to ensure clean oil should be periodically replaced. The oil should also be replaced periodically at specified intervals, as it tends to lose its viscosity with prolonged use. The dust in oil promotes oxidation and the oxidized oil in turn adversely affects the hydraulic system.
Foam and bubbles
A noisy pump is often the result of use of foamy oil. Foam and bubbles generate a loud noise when they mesh at the gear teeth. An air bubble between the meshing gear teeth is like a hard rock and can destroy the molecular surface structure of the metal and produce a cavity, often referred to as pitting.
The oil in service might generate foam within itself, particularly when the pressure is
low, as in the case of a beer bottle whose cap is prized off the bottle. It entrains external air, with which it may come into contact during re-circulation through the system. Foamy oil is a poor lubricant which reduces pump capacity and promotes oxidation or rusting of metals within the system.
Usually foaming of oil is due to one or more of the following reasons:
• Use of the incorrect type of oil
• A high or low oil level
• Air entering the suction side of pump, which could be due to loose pump bolts or damaged gaskets
• Water mixing with oil.
Overheating of oil
Hot oil is a poor lubricant and increases the internal leakage, thereby reducing pump capacity. It also combines readily with oxygen to accelerate its oxidizing tendency. The temperature limit as regards the phenomenon of overheating is around 90 °C. Sufficient care should be taken to see that the pump is not run at temperatures exceeding 90 °C. High oil temperatures also reduce the life of seals.
Wrong selection of oil
It is important to normally select the oil in accordance with the ambient temperature. This is done in order to enhance the life of the pump components. Furthermore, careful attention is required at the time of adding large quantity of make-up oil. If the condition of the oil in the system is bad, the newly added oil will also get wasted. If the oil in the tank contains soluble sludge, the addition of a large quantity of make-up oil may result in rapid precipitation of this sludge. The normal unwritten rule is that not more than 10% of make-up oil should be added to old oil.
3.8.2 Comparison of various pump performance factors
In general, although gear pumps are the least expensive, they also provide the lowest level of performance. These pumps are simple in design and compact in size. The volumetric efficiency of gear pumps is greatly affected by the leakages that in turn are a result of their constant wear and tear.
Vane pump efficiencies and costs fall between gear and piston pumps. They also last for a longer period.
Although piston pumps are the most expensive of the lot, they provide the highest level of performance. They cannot only be driven at speeds up to 5000 rpm but also operated at very high pressures. Additionally, they are also long lasting, with life expectancy levels of at least seven years. However one major disadvantage with piston pumps is that they cannot be normally repaired in the field on account of their complex design.
The chart below shows a comparison of various performance factors for hydraulic pumps.