Reciprocating Compressors – Frame Lubrication System
The frame lubrication system circulates oil to the frame bearings, connecting rod bearings, crosshead shoes, and can also supply oil to the packing and cylinder lubrication system. Splash lubrication systems are the least expensive and are used in small air compressors. Forced-feed systems are used for almost all oilfield gas compression applications.
Figure 11-22 shows a splash lubrication system where an oil ring rides loosely and freely on the rotating shaft, dipping into the oil sump as it rotates. The ring rotates because of its contact with the shaft, but at a slower speed. The oil adheres to the ring until it reaches the top of the journal when it flows onto the shaft.
lo a forced-feed lubrication system, a pump circulates lubricating oil through a cooler and filter to a distribution system that directs the oil to all the bearings and crosshead shoes. Figure 11 -23 is a schematic of a typical system. The details of any one system will vary greatly. Major components and considerations of a forced feed lubrication system are as follows:
* Main oil pump
- Driven from crankshaft.
- Should be sized to deliver 110% of the maximum anticipated flow rate.
* Auxiliary pump
- Backup for the main oil pump.
- Electric motor driven.
- Should start automatically when supply pressure falls below a certain level.
• Pre-lube pump
- Manual or automatic.
- Prevents running bearings dry at start.
• Oil cooler
- Keeps oil temperature below 165°F.
- Can use shell-and-tube exchanger with jacket cooling water or aircooled exchanger.
- Sized for 110% of the maximum anticipated duty.
• Oil filter
- Dual, full flow, with isolation valves arranged so switching can occur without causing a low-pressure shutdown.
- Size should be determined by vendor; in lieu of other information use API 618 requirements.
• Overhead day tank
- Sized to handle one month of oil consumption.
- Should be equipped with a level indicator,
- Stainless steel downstream of filters.
- No galvanizing.
- No socket welding or other pockets that can accumulate dirt downstream of filter.
- Carbon steel lines should be pickled, passivated, and coated with rust inhibitor.
- Lube oil system from pump discharge to the distribution system should be flushed with lube oil at 160°F-180°R Oil should flow across a 200 mesh screen and flushing should cease when no more
dirt or grit is found on the screen.
Packing/cylinder lubrication can be provided from a forced feed compressor lube oil system. For very cold installations, immersion heaters and special lube oils must be considered. If the lube oil temperature gets too cold, the oil becomes too viscous and does not flow and lubricate properly.