Rate of Leakage

Leakage amounts depend upon the amount of misfit between the rings and the piston rod. Leakage can also be the result of disturbances along the faces that must be in contact to form proper seals, as described for the various ring styles. Under the best of circumstances, packings will weep or bleed in small amounts that cannot be detected by ordinary means. Some of this escape is due to separation at the various points of contact as might be caused by vibration or temporary lubrication loss....

Excessive Rod Load

Most major damage to compressors is caused by exceeding recommended rod loads. For this reason, it is important that operators and mechanics understand rod loads before they start a compressor. Figure 2-41. Unloader with Teflon bushings are used with non-lubricated compressors (Source Plant Engineering, October 18,1979).

Piston Rod Column or Frame Loading

Each compressor is subject to a piston rod column or frame load limitation. The size of each compressor establishes a maximum allowable frame or piston rod column load. Keeping within this allowable load ensures that the frame castings and running gear parts, such as crankpins, crosshead pins, main bearings, crossheads, and connecting rods are not subject to loadings or stresses beyond their design points. Piston rod column or frame load is the force that the pressure in a cylinder exerts on...

Air Filtration

Another area requiring special attention for successful trouble-free operation of non-lubricated air compressors is proper filtration of the incoming air. Figure 2-39. Truncated piston for conventional non-lubricated compressor pistons (Source Joy Manufacturing Company, Division of Gardner-Denver, Quincy, Illinois). Figure 2-39. Truncated piston for conventional non-lubricated compressor pistons (Source Joy Manufacturing Company, Division of Gardner-Denver, Quincy, Illinois). Figure 2-40....

Piston Rings

Piston rings made of PTFE will have an end gap clearance of .020 to .024 per inch of piston diameter when fitted into the cylinder bore. Side clearance in the groove should be 0.010-0.020 per inch of width. It should be remembered that Teflon expansion rates are approximately seven times those of cast iron. Values for oil-lubricated compressors with cast iron rings are .0035 per inch of diameter for the ring gap. Figure 2-39 shows a piston for a non-lubricated compressor with a center rider...

Characteristics of Reciprocating Compressors

Reciprocating compressors are the most widely used of all compression equipment and also provide the widest range of sizes and types. Rat- 100 150 200 250 DISCHARGE PRESSURE-PSIG Figure 1-15. Power vs. capacity and pressure for various air compressors (general approximations). 100 150 200 250 DISCHARGE PRESSURE-PSIG Figure 1-15. Power vs. capacity and pressure for various air compressors (general approximations). Figure 1-16. Altitude correction factors for air compressors applied to Bhp 100...

Servicing Pressure Packing

When cleaning packing rings, wire edges may be found around the bore of the rings if considerable wear has occurred. These wire edges must be removed with a file. However, care should be taken to not break the corners where any two surfaces of a packing ring match. If packings are to be paired with old rods, check the rod for size and trueness. If scores or shoulders are present, the rod should be refinished or replaced. All scores and abrasions should be removed by stoning. Packing cups may...

Divider Block Components

These systems are equipped with main line flow switches and mechanical counters for monitoring. The newest type of monitoring and protection device for the lube system is the electronic monitor. These devices give accurate control over the oil-feed rates and protection to the compressor in the event of system or oil supply failure. Figure 3-6 shows a Trabon MV system arrangement and Figure 3-7 a Watchman Lubemation System. Hcwilt tdjuitmoiil toi ttiirnlN Input flow to ayal.m. K. l.t.ra 9 Count....

Maintenance Costs Revealing

Six plants owned by a large corporation and six owned by other firms in North America and Europe were investigated. Not surprisingly, no two locations used identical data-collection and data-reporting methods. Some of the plants monitored repair cost and downtime data whereas, others kept track primarily of spare-parts usage, or perhaps, only the number of outages. However, all of the available statistics did prove relevant. Exceptionally detailed records have been kept for six machines in...

Manufacture of Compressor Piston Rods

Piston rods may be manufactured if proper attention is paid to selection of material, machining accuracies, heat treatment of materials, surface treatments, and surface finishes. Note Do not attempt to manufacture piston rod without first asking questions and consultation on the various aspects of piston rods. Before deciding to manufacture piston rods, the following factors should be considered 1. Determine operating conditions of compressor. 2. Material may be substituted from original design...

Frame Vibration Monitoring

Compressor Unloader Operation

The benefits of frame vibration monitoring are the prevention of catastrophic failure and the reduction of the amount of damage to a machine. This is accomplished by detection of small changes in the case motion. With modern 32-channel reciprocating compressor monitoring systems and associated seismic transducers, companies such as Bently-Nevada offer the user many advantages over the traditional, and often unreliable, g-switch type of detector. Bently-Nevada, one of a number of capable...

Typical Compressor Problems

For possible solutions to typical problems, check Troubleshooting Chart, Table 5-1. Always be sure to study and consult the compressor instruction book. And DO NOT FORGET TO CHECK THE PRESSURE GAUGES. Air discharge temperature above normal Piston ring, piston, cylinder wear excessive Piston rod or packing wear excessive Intake pipe restricted, too small, too long Table 5-1 (Continued) Troubleshooting Chart Table 5-1 (Continued) Troubleshooting Chart Air discharge temperature above normal Piston...

Evaluating Inspection Data

Recording inspection data serves several purposes 1. To establish the exact condition of all wearing parts. 2. To establish the wear rate of parts, which, if promptly replaced, will not deteriorate to such a degree that associated parts will be damaged and will also require replacement. 3. To determine which parts require reconditioning and which parts can be reconditioned to like-new condition. Compressor-Piston Rod Inspection Form U-2.995 0-2.998 T-2.998 Note U Undersize 0 0ut Round T Taper...

Major Sources of Packing Problems

There are several indicators of troubles with metallic packing that are frequently misunderstood and in reality are a manifestation of trouble originating elsewhere. Excessive or rapid wear is an example of this. Barring improper selection of mating materials or of materials and gas, excessive wear can be caused by several factors. To fully understand and diagnose abnormal wear, normal wear should be understood. Figure 3-43. Pressure balance set with seal ring fastened directly to the packing...

Design and Materials for Reciprocating Compressor Components

Today's modern, heavy-duty, continuous-service reciprocating compressor cylinders can be attached to a single horizontal or vertical frame. When more than one cylinder is used, the various configurations are almost endless. As previously seen, these machines range from simple one-cylinder, single-stage compressors for air service to multi-stage, multi-service process gas compressors. Regardless of the service, from the smallest to the largest, reciprocating compressors share both the same...

Procedure for Torquing Piston Nuts

The following procedure has been developed for the torquing of piston nuts when installing cast iron, aluminum, and steel pistons, either two pieces or one piece, with straight bores. It must not be used for pistons that use piston rods with taper fit to the piston. 1. Clamp piston rod behind the collar in a suitable fixture similar to the one shown in Figure 4-36. Care must be taken not to clamp on any coated surface if the rod has been coated, because cracking or breaking the bond will...

Design Overview of Labyrinth Piston Compressors

Earlier in this chapter, we alerted the reader to a special non-lubricated reciprocating compressor configuration, the labyrinth piston, or Laby machine. Although available since the mid-1940s, labyrinth compressors are not as well known in the United States as they are in other parts of the world. In labyrinth piston compressors, an extremely large number of throttling points provide the sealing effect around pistons and piston rods. No contact seals are used. Whereas plastic sealing rings...

Calculations

Reciprocating Compressor Curves

Solving compressor problems by the use of the ideal gas laws has been reduced to a relatively simple sequence of applying a few basic formulae and obtaining values from some basic curves. In order to help in the understanding of these terms and equations, the first chapter of this text covers some basic thermodynamics. Also, following the sequence of events as they occur in a compressor cylinder may help develop an understanding of the terms encountered in most reciprocating compressor problems...

Theory of Reciprocating Compressors

Reciprocating compressors are the best known and most widely used compressors of the positive displacement type. They operate on the same principle as the old, familiar bicycle pump, that is, by means of a piston in a cylinder. As the piston moves forward in the cylinder, it compresses the air or gas into a smaller space, thus raising its pressure. The basic reciprocating compression element is a single cylinder compressing on one side of the piston (single-acting). A unit compressing on both...

Excessive Oil Feed

All oil fed to the cylinders is subjected to oxidizing conditions. Under prolonged heating, even the best quality compressor oils will oxidize to some extent. Therefore, feeding more oil than is actually needed results in increasing the amount of oxidation products formed. Because the highest temperatures are encountered on discharge valves and in discharge passages, and most of the oil fed to the cylinders eventually leaves through the discharge valves, it is here that deposits tend to...

Compressor Cylinder Alignment

The relation of the compressor cylinders to the frame or crankcase should be examined with laser-optical instruments or by the tight wire method. This checks the relationship of the compressor cylinder bore, packing case bore, housing bore, and crosshead guide bore to the crankshaft. Refer to Figure 4-9 for reference points for wire alignment, as given by Dresser-Rand for the Model HHE Compressor. If it is found that the alignment of the compressor cylinder is not correct or that the crankshaft...

Materials of Construction

A knowledge of the materials used in the construction of the major components of the heavy-duty water-cooled compressor will permit the proper maintenance and repair of those components. Frame or crankcase is a high-grade cast-iron casting similar to ASTM class 40 or 50 it is designed with suitable supports or ribbing to mount the compressor cylinders, crankshaft, and other running gear parts and hold them in accurate alignment under the stresses imposed during operation. Crankshafts are made...

Operational Differences

A basic operational difference between the lubricated and non-lubricated cylinder should be explained. The piston works against pressure and should form a sliding seal so that it can compress the gas without leakage. So perhaps, in the lubricated cylinder, the simplest piston would be a plug piston with a very close fit to the cylinder bore. But, because of temperatures and other engineering and economic reasons, this is not practical. Piston rings are therefore used for sealing. These piston...

Suction Valve Failures

As has been pointed out, capacity loss can most often be attributed to problems with valves. If a suction valve does not close and seal completely, the following occurs 1. During the compression (discharge) stroke, some of the normally displaced gas will leak or blow by the suction valve to the suction side of the valve. 2. The volume of gas actually displaced will decrease. 3. As the compressed hot gas passes over the valve in the wrong direction, the valve heats up. 4. The inlet temperature...

Pistonto Piston Rod Connections

There are several methods commonly used to fasten the piston to the piston rod. Taper Fit. The end of the piston rod is machined with a tapered bore in the piston. The end of the piston rod is threaded, and a nut draws the piston to the rod and holds it securely (Figure 2-17). Interference Fit. The piston bore is machined to accept the piston rod with an interference. No threaded connection or nut is used to hold the piston to the rod, and the piston is peened at the rod end (Figure 2-18)....

Rod Load Calculations

Two methods of calculation are used, an average method and an exact method, which requires calculation of compression and tension loads. F P x A (Pd - Ps) x net cylinder area Net cylinder area cylinder area - lA rod area Exact or Compression and Tension Method Piston moving toward head end or rod in compression (a) Fc (Pd x Ahe) - (Ps x Ace) Piston moving toward crank end or rod in tension Where Ps suction pressure, psia Pd discharge pressure, psia Ahe cylinder head end area, sq. in. Ace...

Packing Ring Types

There are several types of packing rings available. This is a true sealing ring, made up of a pair of rings, the first of which is radially cut with considerable clearance at each end. The second ring is tangentially cut forming an overlapping seal joint that prevents gas passage from the outside periphery toward the rod bore. The inner radial cuts of this tangent ring are arranged so that they occur approximately between the radial cuts of the first ring in the pair. In this manner, gas...

Interpreting Web Deflection Readings

In order to interpret the readings, some experience and sound thinking are required, but the effort will be worth it. Because problems with the lower end of the machine are never the same, the best way to deal with instructions on analyzing the data is to examine several hypothetical cases. The reader can better follow the examples and subsequent problems by using a model crankshaft made from wire or a paper clip. The readings listed in the table of Figure 4-7 will be used as Case 1 and were...

Installation of Rider Bands

Solid PTFE rider bands are machined with an interference fit so that, once expanded over the piston end, the rings will contract like a rubber band to provide a snug fit on the piston. Unlike piston rings that must move freely in their grooves to seal, rider rings are literally stretched in order to slide them over the end of the piston and into the ring groove. This can be accomplished in either one of two ways, depending upon whether the rings are pre-stretched or not. Pre-stretched rider...

Foundations For Reciprocating Compressor

The reciprocating compressor, because of the alternating movement of its pistons and various others parts, develops a shaking or inertia force that Figure 2-54. Balanced-opposed cylinder arrangement with closely spaced cranks minimizes moment unbalance (Source Dresser-Rand, Painted Post, New York). Figure 2-54. Balanced-opposed cylinder arrangement with closely spaced cranks minimizes moment unbalance (Source Dresser-Rand, Painted Post, New York). alternates in direction. The net result of this...

Piston Rod Runout

Piston rod runout is a measure of how well the cylinder is lined up with the frame and, also, how well the internals of the compressor piston, rod, crosshead are lined up with each other and the frame and cylinder combination. This runout is checked and adjusted with all parts cold the ambient temperature should be in the 60 -68 F range. Prior to performing runout measurements, cylinder and support, cylinder frame head, and doghouse nuts should be properly set or torqued. Oil scraper and...

Check Crankshaft for Deflection and Runout

Deflection readings give some indication of whether the crankshaft is straight, properly supported and running in correct alignment. Deflection is normally tested with a deflection gauge. Deflection gauges measure changes in the spread between adjacent crank webs while the crank is rotated see Figure 4-4 and 4-6 . Whenever a main bearing fails or any time bearings are replaced, it is vitally important to check both runout and deflection. Reading deflection alone does not give the full picture...

Piston Rod Materials

Compressor piston rods are made from various types of steel depending on stress levels and the composition of the gas handled. Low carbon steels, such as AISI 1037, and low alloy steels, such as AISI 4140, are commonly used. For certain corrosive gases, 410 stainless, K-Monel, or Hastalloy steels may be used. The normal frictional forces acting between the piston rod packing rings and the piston rod increase with pressure. Experience has shown that piston rod packing wear becomes excessive with...