Chapter 7—AUXILIARY MACHINERY(MRC) for the proper methods and tools requiredfor removing the heads. The following is generalinformation on removing a piston, fitting newpiston rings, and checking piston end clearancefor a vertical compressor.To remove trunk pistons from vertical com-pressors, it is first necessary to remove the cylinderheads. In the case of three- or four-stage com-pressors, the third and fourth stage cylinders willalso have to be removed. Next, you must turn thecompressor by hand to top center and remove thelower half of the crank pin bearing, or the entirecrank pin bearing box, if so directed. Pull up thepiston and connecting rod through the cylinder.In removing pistons from compressors fitted withcross heads and piston rods, the general procedureis to loosen the piston rod locknuts adjacent tothe cross head, then unscrew the piston rod fromthe cross head and lift the piston and the rod outof the cylinder.If the piston rings are worn or broken andrequire replacement, take accurate measurementsof the cylinder or liner to determine the exactdiameter. Standard size rings may be used in over-size cylinders if the oversize does not exceed 0.003inch per inch of the cylinder diameter. If thecylinder or liner is worn beyond the manufac-turer’s recommended limits, it should be replaced.On compressors not equipped with liners, suchas air-cooled or large pneumatic service com-pressors, one reboring is allowed.When piston rings are being replaced, theyshould first be fitted to the cylinder to check forproper end clearance. If necessary, the endsshould be filed until the manufacturer’s recom-mended clearance is obtained. Rings should thenbe fitted to the piston grooves, making sure thatthe side clearance of each ring is such as to allowit to fall into the groove by its own weight. Thethickness of each ring should be checked to makesure that the groove is deep enough for the ringthickness. In replacing the rings, care must betaken to ensure that they are staggered so that thering splits are not in line. After the piston isassembled, it is advisable to wire the rings tightlywith a soft copper wire so that they will enter thebore without difficulty. As each ring enters thebore, the binding wire can be removed throughthe valve ports. Most cylinder liners are beveledat the top to permit the rings to compress andenter the bore easily. In certain designs ofdifferential pistons, it is necessary to loop a wirearound the top of the lower liner, using valve portsfor access, and squeeze each ring closed as itapproaches the top of the liner. One techniqueis to slip the rings over a thin piece of shim stockwhich, in turn, is slipped over the piston. Thisprevents overstretching and other possible ringdamage.After a piston has been replaced in a com-pressor and the head has been drawn downevenly, it will be necessary for you to check pistonend clearance. NOTE: The piston end clearancemust always be measured and adjusted afterreplacing the pistons, or after any adjustment orreplacement of the main, crank pin, wrist pin, orcross head bearing has been made. This is ac-complished by inserting a lead wire through avalve port or indicator connection, and then jack-ing the compressor over so that when the pistonhas moved to the end of its stroke (top dead center(TDC)), the lead will be flattened to the exactamount of clearance at the end of the stroke. Thelead wire should be long enough so that thereading can be taken at a point near the centerof the piston.In differential piston compressors in whichcompression takes place in two stages during thesame stroke, the clearance must be measured forboth stages. In double-acting compressors, or inthose compressors in which compression in anystage takes place on the reverse stroke, a secondreading of the clearance at the bottom of thestroke (bottom dead center (BDC)) of thatcylinder must be taken.After taking these readings, you can adjust thepiston end clearances. Proper clearances for aspecific compressor are usually found in themanufacturer’s instruction book or on theblueprints. If neither of these documents isavailable, then you should adjust the clearanceso that the TDC stroke is approximately 1/64 inchgreater than the BDC stroke. This is necessary toallow for the expansion of the running parts whenthe compressor is in operation.The method of adjusting the piston clearancevaries with different compressors or designs.Adjustments may be accomplished by one of thefollowing methods: (1) by adding or removingshims in the connecting rod between the lower end7-3
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