An example of large vertical pump installation in an industrial plant
Optimization and Ease of Maintenance
When a system is packaged, the arrangement should allow for system optimization and minimized maintenance. All auxiliary pumps (including lubrication pumps) should be easy to align. Attention should be given to the lubrication system. Oil suction pipes should be arranged to provide a positive suction head on oil pumps, with a slope toward the pump. All the pumps’ materials of construction should be chosen carefully. Cast iron casing is not desirable for any auxiliary system equipment/machinery in a packaged system. Because cast iron is brittle, it can fail quickly in emergency situations, particularly in the case of fire.
Generally, an auxiliary skid system should be balanced among performance, reliability, easy access, maintenance requirements and a compact design. The Tubular Exchanger Manufacturers Association C shell & tube heat exchangers (with removable bundles) are often used in auxiliary systems (including lubrication oil skids). The water is on the tube side, and the lubrication oil on the shell side. The oil pressure should be more than the cooling water pressure to avoid water leaking in the oil if an unexpected problem occurs. Plate type heat exchangers are not popular (except for revamp projects with limited available footprints). Tubular heat exchangers are often used for small packages.
Maintaining the lubrication system in packaged systems is critical for successful operation. Pump oils are subjected to a wide range of harsh conditions, such as:
- Extreme temperatures (in hot or cold applications)
- Inadvertent mixing with other substances
The effects of these harsh conditions can degrade the integrity of oil base stock and deplete the additive chemistries, causing irreversible molecular changes. Most centrifugal pump trains use a relatively low-viscosity oil compared to gear units and reciprocating pumps (for example, ISO VG 32 or ISO VG 46). This reduces the power waste for the operation of centrifugal pumps. The typical expectation is relatively low makeup oil for centrifugal pump lubrication oil (below 4 percent per year). This factor encourages high-quality, long-life lubricant applications for a modern pump.
If the oil in a pump train (without a gear unit) is selected properly and maintained correctly, it usually does not need to be drained and replaced frequently and could last for a relatively long time. Pump oils should be well-maintained to extend their service life and simultaneously provide the maximum pump train performance.
For any equipment that runs at high operating temperatures (such as high-temperature pumps), the oxidation of oil could be an important issue. Heat can also reduce oil life (the main mechanism again could be oxidation). For these high temperature applications, the oxidation rate could be doubled for every 10 C oil temperature increase.
Pump oils can also fail because of contamination. Oil additives could effectively minimize this contamination to a certain level, but a better solution is a sealing system that keeps the pumped liquid from leaking into the lubrication oil system. Additives for pump oils should be verified and tested. The oil and the additives should be carefully formulated and blended in a tightly controlled process.
The key to excellent oil is property retention. Successful (long-term satisfactory operation) references are important. Lubrication oil in some high-temperature pumps could contact metal surfaces above 140 C. These high temperatures and the possibility of cyclical operation can result in significant thermal and oxidative effects on lubrication oil. For high-temperature pump applications, sophisticated synthetic oils are good options.
Instead of degradation occurring in a linear and predictable fashion, many modern pump oils could fail rapidly. Some standard oil analysis tests offer little indication of when a pump lubricant begins to degrade. Again, a reference check for exactly the same pump application is vital.
Oil Analysis to Determine Wear
Ferrography is a technique that provides valuable information about the wear of machinery through the analysis of a representative lubrication oil sample. The analysis can be important in maintaining and troubleshooting large packaged pump systems.
In analytical ferrography, the solid debris suspended in a lubricant sample is separated. Solids are then passed across a bipolar magnetic field. A solvent wash cycle removes any lubricant remaining on the substrate, resulting in a ferrogram in which the particles are arranged by size and permanently attached to the slide for optical analysis using a biochromatic microscope. The particles are then examined and classified by size, shape, concentration and metallurgy. The information carried by the wear particles is valuable to help identify the wear mode and mechanism.
The analytical ferrography can be particularly effective in the detection of soft contaminants and the identification of their nature. It can be a powerful technique to identify machinery oil-related issues, root cause analysis, the morphology and the characteristics of insoluble particles, and the progressive mechanism of varnish formation. The ferrography test procedure is lengthy and requires a highly skilled analyst. However, the benefits can outweigh the costs. This analysis is the most recommended test if abnormal wear is observed.