Reciprocating Compressors Lubrication
In these compressors, the simplest arrangement is splash lubrication, shown in Figure 1, whereby a projection on the connecting rod dips into oil held in the crankcase, which acts as an oil reservoir and must be closed with the exception of a small breather hole. Lubricant is dispersed, mainly in the form of an oil mist reaching the bearings, sliding surfaces and cylinders. For best control the splash oil should be taken from a trough, or failing that some provision must be made to keep the crankcase oil at a constant level. Water which collects at the bottom of the crankcase should be drawn off at the intervals specified by the manufacturer, either manually or automatically.
Splash lubrication is largely limited to small single-acting reciprocating compressors. It has a number of disadvantages:
• the lubricant cannot be filtered and its life is limited,
• lubrication at start up may be marginal,
• the bearing film is thin and is not supplied under pressure,
• the level of oil cannot be guaranteed unless strict maintenance schedules are observed and the compressor kept level.
Flood lubrication is widely used on horizontal compressors for lubricating the bearings and crosshead. The oil flow is under the pressure of gravity after first being elevated by a disc, oil ring, chain or collar on the crankshaft which lifts it to a trough with distribution tubes or to the upper part of the journals (see Figure 2). This provides better control of the oil flow but the pressure is low and not generally considered sufficient for modern high speed compressors.
Force feed lubrication utilizing a circulatory system is normally to be preferred; this is similar to automobile engine lubrication. Feed is taken from an oil reservoir by a plunger or gear pump. Actual circulation may be by gravity when the oil is pumped to an overhead reservoir, or may be fed directly from the pump under pressure through pipes to the various lubrication points. Oil pressure is maintained constant by a spring loaded overflow valve. For cylinder lubrication of crosshead compressors, piston type lubricators are normally used.
Sight-feed drip oilers are occasionally used to lubricate small parts. These are similar to cylinder lubricators and can be regulated within close limits provided they are kept in good condition. Siphon wick oilers may be used when it is important that there should be no undue variation in oil level in the container. Grease lubrication may be provided at certain points (eg to lubricate rolling bearings). In this case the lubricating points are either fitted with grease nipples or screw-down oil cups.
In the case of cylinders, the ideal oil feed is that which will provide efficient and effective lubrication with a minimum amount of oil and at the same time maintain a good piston seal – an excess of oil is undesirable because it then has to be removed. A practical check is to service the discharge valves periodically and examine their appearance. They should not be dry nor show signs of rust (insufficient oil) or be excessively wet (excess of oil). They should have a wet appearance and be oily to the touch. Excess of oil can also be noted by the appearance of pools of oil lying in the cylinders or surplus oil on the connecting rods.
Figure 2 shows the various lubrication flows for a typical reciprocating compressor. This is for a compressor supplying industrial air in which a small amount of oil is acceptable, so the cylinders are lubricated. If oil-free air were required, the cylinder lubrication would be omitted, but crankcase lubrication would remain. It is customary to incorporate an externally mounted gear pump with a pressure relief valve which controls the pressure to the moving parts. Oil is drawn into the pump through a submerged oil strainer and from the pump passes through a pressure filter with a disposable element. Oil is fed to the main bearings, through the connecting rods to the small-end bearings and crosshead and drains back into the crankcase.
In this design, the cylinders are lubricated by a crankshaft-driven force feed lubricator drawing oil from a separate reservoir. The valves seldom require separate lubrication.