Reciprocating compressors lubrication

This standard could be adapted to the fuel compressor for the natural gas to be brought up to the injection pressure required for the gas turbine. Covers the minimum requirements for reciprocating compressors and their drivers used in petroleum, chemical, and gas industry services for handling process air or gas with either lubricated or nonlubricated cylinders. Compressors covered by this standard are of moderate-to-low speed and in critical services. The nonlubricated cylinder types of compressors are used for injecting fuel in gas turbines at the high pressure needed. Also covered are related lubricating systems, controls, instrumentation, intercoolers, after-coolers, pulsation suppression devices, and other auxiliary equipment. 

Chapter 3, which discussed various reciprocating compressors, stated that riiaii> reciprocators use a pressure-fed lubrication system for the frame bearings. This system is built into the crankcase in many applica tions. I he basics of these systems follow the fundamental criteria which will lie discussed with the fully separate system. The larger reciprocating compressors may use a separate frame lubrication system. 

A basic item on the reciprocating compressor is the lubricated cylinder. The vendor will need guidance from the user advising whether lubrication can be used. 

Several steps can be taken to maximize the run time for the reciprocating compressor. Since wear is a function of rubbing speed, the piston speed can be kept to a minimum. Chapter 3 made recommendations for piston speed. Reliability problems due to valves are reputed to account tor 40% of the maintenance cost of the compressor. Valves are the single largest cause for unplanned shutdowns. Basically, valve life can he increased by keeping the speed of the compressor as low as practical. At 360 rpm, the valves are operated six times a second. At 1,200 rpm, ihc valves operate 20 times a second or 1,728,000 times in a day. It is not difficult to understand why the valves are considered critical. To keep the reliability in mind, valve type, material selection and application considerations such as volume ratio, gas corrosiveness, and gas cleanliness need attention by the experts. One final note is that while lubrication is an asset to the rubbing parts, it is not necessarily good for valve reliability. 

Non-lubricated screw compressors have very close clearances and thus they are designed for limited ranges of discharge temperature, temperature rise, compression ratio, etc., all of which can cause changes in thesc clearances. Lubricated compressors have a somewhat broader tolerance to changes in operating conditions, but they are still more limited than reciprocating compressors. 

Figure 12-5F. Lubricated and nonlubricated balanced opposed process reciprocating compressors, designed to API 618 code. Fixed- and variable-speed drives using gas or diesel engines, steam or gas turbines, or electric motor. Note power drive to connect to right side of cross-head box in center. (Used by permission Bui. PROM 635/115/95-11. Nuovo Pignone S. P. A., Florence, Italy New York Los Angeles and Houston, Texas. All rights reserved.)...

A V-belt-driven, force-fed oil lubrication system is used on water-cooled compressors. Oil goes to both bearings and to several points in the cylinder. Ten times as much oil is recommended to lubricate the rotary cylinder as is required for the cylinder of a corresponding reciprocating compressor. The oil carried over with the gas to the line may be reduced 50 per cent with an oil separator on the discharge. Use of an aftercooler ahead of the separator permits removal of 85 to 90 per cent of the entrained oil. 

Because reciprocating compressors should be supplied with clean gas, inlet filters are recommended in all applications. They cannot satisfactorily handle liquids entrained in the gas, although vapors are no problem if condensation within the cylinders does not take place. Liquids will destroy the lubrication and cause excessive wear. 

The recommendations of the International Standards Organization (ISO) covering mineral-oil lubricants for reciprocating compressors are set out in ISO DP 6521, under the ISO-L-DAA and ISO-L-DAB classifications. These cover applications wherever air-discharge temperature are, respectively, below and above 160°C (329°F). For mineral-oil lubricants used in oil-flooded rotary-screw compressors the classifications ISO-L-DAG and DAH cover applications where temperatures are, respectively, below 100°C (212°F) and in the 100-110°C range. For more severe applications, where synthetic lubricants might be used, the ISO-L-DAC and DAJ specifications cover both reciprocating and oil-flooded rotary-screw requirements. 

Compressor oils This market sector covers a wide range of compressor types used for a number of different gases. Diesters and phthalates have found their major application in air compressor lubricants and are also used in natural gas compressors. In reciprocating compressors, where oils of rather higher viscosity are preferred, trimellitate esters can be used. Diesters and polyol esters may also be blended with PAOs for use in various compressor types. 

Sel iiion of Compressor Equipment, Compressors for such gas processes as liquid-air separation, hydrogenation, and desulfurization must be non-lubricated to avoid possible explosive reaction with lubricating oils. Graphite rings are used in reciprocating compressors, and labyrinth or... 

All the problems associated with chlorine compression apply to diaphragm compressors. They must be protected against combustion of metal parts in dry chlorine. Since the compressor head usually is carbon steel, this means a maximum temperature of about 120°C. There must be no contact between chlorine and a combustible lubricant. Double separation between the two, as was the case with reciprocating compressors, can prevent this contact. Two diaphragms with an inert intermediate fluid are standard. The oil can be a chlorinated fluorocarbon that does not react with chlorine. The space between diaphragms should have a leak detector that sounds an alarm and shuts down the compressor when either diaphragm fails. 

Purchaser should note that a good strainer to remove all dust/liquid entrainment from the suction side should be provided for the reciprocating compressor since the clearances are very small. In case non-lubricated type compressor is required, more than one unit should be procured because these machines have more maintenance problems. 

Reciprocating compressors have a history of chronic failures that include valves, lubrication system, pulsation, and imbalance. Table 8.3 identifies common failure modes and causes for this type of compressor. 

Poor maintenance of lubrication-system components, such as filters and strainers, typically causes premature failure. Such maintenance is crucial to reciprocating compressors because they rely on the lubrication system to provide a uniform oil film between closely fitting parts (e.g., piston rings and the cylinder wall). Partial or complete failure of the lube system results in catastrophic failure of the compressor. 

Reciprocating compressors were formerly lubricated with sulfuric acid, but are now available as dry-ring compressors (no lubrication). They can compress up to 2001 per day. Multistage compressors produce pressures up to 1.6 MPa. The heat of compression of each stage must be removed by heat exchangers or by injection of liquid chlorine (see Fig. 80). Well-purified chlorine gas is essential for trouble-free operation [214]. 

FIG. C-63A Vertical, in-line, reciprocating process compressor, lubricated or oil-free. (Source Sulzer-Burckhardt.)... 

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