Rotating equipment typically requires forced lubrication for gears and other mechanical components. Pumps and engineered systems that supply lubricating and cooling oil generally operate 24/7, making reliability a mission-critical issue. In addition, high-pressure oil may be required for generator journal bearings, reduction gears and accessory drives.
In power plant applications, systems for lift oil, jacking oil, cooling oil and general lubrication typically are installed with the pumping systems mounted in a vertical skid configuration. A power generation system typically includes a primary‚ standby and emergency backup system. A seal oil pump also may be included for hydrogen-cooled generators. In some other cases, the lubrication system is configured with the control oil system.
Both centrifugal and positive displacement pumps are used in lube oil systems (LOS). Centrifugal pumps use an impeller to move fluid through the process. The velocity of the rotating impeller imparts energy on the liquid and causes a rise in pressure (or head) that is proportional to the fluid’s velocity.
Rotary positive displacement pumps do not develop pressure. They work on the principle of moving fluid from one chamber to the next and are generally insensitive to pressure changes. Pressure is generated by resistance to the liquid’s movement downstream of the pump. Which technology is most suitable to deliver lubrication and cooling benefits at the optimal total cost of ownership will depend on a number of factors.
Hydraulic Factors to Consider
Hydraulic characteristics of the two pump types are key considerations. Life-cycle costs can be managed by operating a pump closest to its most efficient range for the applicable conditions. When the system pressure is subject to change, positive displacement pumps will remain efficient even when operating at varying pressures. A best efficiency point (BEP) does not govern a positive displacement pump’s performance—its reliability is not compromised by pressure variations.
While centrifugal pumps provide efficiency within a relatively limited range of head, when the system has varying pressure requirements or the head is too low, it will be forced to operate outside its BEP. This mode of operation deflects the shaft and causes vibration, leading to reduced performance and accelerated wear.
Positive displacement pumps often are the most economical choice when liquid viscosity changes but flow volume is constant. When handling high-viscosity liquids (above 20 centistokes), two- or three-screw rotary positive displacement pumps operate more efficiently than centrifugal pumps. Centrifugal pumps can incur higher costs for spare parts, maintenance and downtime if viscosity and pressure are not constant.
Required Flow Rates
Required flow rates depend on rotating equipment size. In steam and gas turbines above 50 megawatts (MW) with flow rates up to 2‚500 gallons per minute (gpm), a centrifugal pump is optimal for supplying lube oil to turbines, generators, large compressors and heavy gear sets. Below this output (less than 500 gpm), turbines, diesel engines and compressor units can be supported with three-screw or gear pumps.
High-performance three-screw pumps can operate with system pressures up to 4,500 pounds per square inch (psi) (310 bar) and flows to 3,300 gpm (750 cubic meters per hour [m3/h]) with long-term reliability and excellent efficiency. Two-screw pumps are available for pressures to 1,450 psi (100 bar) and flow rates to 18,000 gpm (4,000 m3/h), and can efficiently handle corrosive or easily stained materials.
Other Efficiency Considerations
Over-engineering equipment should be avoided because it can create a system that is too large, performs at less than optimum efficiency, consumes excessive energy, and adds maintenance and service costs. Conversely, a pump that is undersized may cost less at the outset, but the resources to rework or replace the system to meet expectations can outweigh the initial savings.
It is best to size for worst-case flow and power requirements. To help ensure the system delivers adequate flow, especially in liquid fuel injection applications, sizing the pump for the highest temperature and lowest viscosity conditions is ideal.
In contrast, during cold startup conditions, the fluid will have a higher viscosity with a higher power requirement. To overcome this, screw pumps offer excellent suction lift. They provide an operation margin during startup or when there is an extended distance to the supply reservoir. This margin allows the use of smaller supply piping, reducing component costs and allowing for a simpler design. Motor sizing is also important to deliver required power.
Promoting uptime with preventive maintenance is vital in a power plant. Scheduled preventive maintenance is critical for the system’s design life. Pumps in auxiliary systems for lubrication will, by nature of their clean working conditions, require less preventive maintenance than pumps used for primary applications.
In both positive displacement and centrifugal pumps, the following components should be inspected, serviced and replaced when necessary:
- Radial bearings: If not sealed, these will need periodic lubrication and may need to be replaced if vibration or excessive heat is generated.
- Mechanical seals: Check for leakage and replace if necessary. Note that a mechanical shaft seal is not a zero-leak device.
- O-rings and gaskets: Check for deterioration and replace if necessary.
Pump manufacturers often provide these parts in a “minor kit,” allowing the user to easily order what they need. Centrifugal pumps also require inspection of the impeller. For positive displacement pumps, virtually every wearing part can be replaced in the form of a “major kit.” Repairing a pump with a major kit essentially yields a new pump at a cost lower than replacing the entire pump.
Maintaining quality when servicing pumps and fluid-handling systems is best achieved using replacement parts manufactured or certified by the pump manufacturer. In some cases, not using original equipment manufacturer parts will void warranties. For many end users, using manufacturer-approved parts could result in a longer service life, extended intervals between maintenance, a more predictable process and increased system efficiency—and peace of mind.