Rod Under Compression

Consider first a double acting cylinder, as shown in Figure 2-42. As the piston pumps toward the head end, the discharge pressure (P9) on the piston tends to compress and buckle the piston rod. At the same time, gas is entering the cylinder behind the piston, exerting suction pressure force (Pj) on the back side of the piston. Two forces are opposite in direction, but since the discharge pressure is greater, the net result is a force com pressing the rod. This is called rod load compression. It...

Types of Compressors

The principal types of compressors are shown in Figure 1-5 and are defined below. Cam, diaphragm, and diffusion compressors are not shown because of their specialized applications and relatively small size. Compressors Dynamic I Displacement Figure 1 -5. Principal compressor types used in industry. Positive displacement units are those in which successive volumes of gas are confined within a closed space and elevated to a higher pressure. Rotary positive displacement compressors are machines in...

Effectiveness of Preventive Maintenance

Preventive maintenance, when used correctly, has shown to produce maintenance savings in excess of 25 , but beyond this, its benefit quickly approaches a point of diminishing return. It has been estimated that one out of every three dollars spent on preventive maintenance is wasted. A major overhaul facility reports that 60 of the hydraulic pumps sent in for rebuild had nothing wrong with them. This is a prime example of the disadvantage of performing maintenance to a schedule as opposed to the...

Piston Ring Sealing

For a piston ring to seal properly, it must have contact with the cylinder wall and also the side of the groove. Piston rings are made with a free diameter larger than the cylinder so that, when installed, they exert outward initial unit pressure against the cylinder wall, as shown in Figure 4-25. Figure 4-25. Free gap must exceed operating gap of piston rings. The initial pressure varies from about zero in plastic or segmental rings to possibly several hundred psi in some situations (with...

Valve Lift

Valve lift is defined as the distance the valve sealing elements move from closed to fully open. The life of any valve depends upon the amount of lift, which varies according to compressor speed, valve diameter, pressures, and molecular weight of the gas. One authority stated that the first question to be answered when designing or selecting a valve for a given set of operating conditions is What is the proper valve lift In any valve application to a compressor, lift affects both efficiency and...

What is a Compressor

Before discussing the types of compressors and how they work, it will be helpful to consider some of the basic gas laws and the manner in which they affect compressors. By definition, a gas is a fluid having neither independent shape nor form, which tends to expand indefinitely. Gases may be composed of only one specific gas maintaining its own identity in the gas mixture. Air, for example, is a mixture of several gases, primarily nitrogen (78 by volume), oxygen (21 ), argon (about 1 ), and...

Crosshead Guide Repair

Crosshead guides are either built integral with the compressor frame or are separate and bolted to the frame. The guides form slides for the crossheads they must have a good surface finish and must be in line with the centerline of the cylinder. Crosshead guides can be rebored in place or in the service center. This is usually a field job because it is much cheaper to machine a guide in place than it is to remove the guide from the foundation. Removable guides, such as tail rod crosshead...

Safe Maintenance Procedures Restated

Prior to doing any inspection, servicing or disassembly, take the following precautions 2. Cut off the electrical supply from the motor by opening a manual disconnect switch in the power line to the motor, and tag the disconnect switch so it will not be closed accidentally. 3. Lock the belt wheel or block crankshaft to prevent rotation. 4. When servicing the cylinder end, drain the water from cylinder jackets. 5. Never install a shut-off valve between the compressor and air receiver or...

Rod Load Assignment

The rod can easily absorb most excessive tension and compressive forces, but other parts, such as pistons, connecting rods and bolts, crossheads and shoes, bushings and bearings, are also stressed. The most highly stressed part, therefore, determines the rod load assigned by the compressor builder. This value is different for each model of compressor, but generally crosshead pin bushing pressures are limiting. Rod loads can be calculated by simple arithmetic, but in operation, suction and...

Drain Water

Water should be drained from the compressor cylinder jacket when servicing the cylinder end. If the water is drained from the engine radiator during a compressor shutdown because of repair work, fill the radiator as soon as repairs are completed. If it isn't, the deposits inside the radiator will dry. Then, when the engine is put back into service, deposits will circulate through the system. A filter ahead of the water pump will catch those materials. Outwardly the water pump will appear to be...

Proximity Probe Method of Monitoring

A more efficient method of measuring rider band wear is to use a proximity probe, mounted vertically to the packing case, to measure the position of the piston rod. Probe gap voltage can then be read with an electronic rod drop readout device. Monitors such as Bently-Nevada's Six-Channel Rod Drop Monitor use a Keyphasor , probe to provide a once-per-revolution crankshaft pulse. This pulse is used as a reference, so an instantaneous rod position can be displayed. There are three main advantages...

Compressor Pistons

Figure A Conventional small diameter piston without rider rings. Material Iron, steel or aluminum. Single bolt piston to rod attachment. Figure B Two piece small diameter piston with single rider ring. Material Iron, steel, aluminum or combinations thereof. Single bolt piston to rod attachment. Figure C Three piece piston with two rider rings. This design utilizes two rider ring carriers and one piston ring carrier. Materials as In B. Single bolt piston to rod attachment. Figure A Conventional...

Valve Temperature Monitoring

A modern 32-channel temperature monitor provides increased productivity and efficiency. The monitor detects changes in valve condition by using differential measurement of valve temperatures. This allows you to make shutdown decisions based on real machine conditions rather than preset maintenance schedules. Reduced machine maintenance cost is also a benefit of this system. Temperature trends can be used to schedule maintenance to minimize downtime and to provide information to help diagnose...

Compressor Piston Rod Packing

Piston rod packing is almost always the full floating type. It consists of several annular cups, segmental rings, and a flange-like gland held in the cylinder head stuffing box as a complete assembly by properly proportioned studs and nuts that secure it against a sealing gasket. The segmental packing (sealing rings) is contained in the cups and is held together as an assembly by garter springs that hold the rings firmly on the piston rod. These rings are free to float in the cups. This packing...

Reapplication of Babbitt Sprayed Rider Bands

The following procedures should be followed when reapplying babbitt-sprayed rider bands. 1. Clean all old babbitt from the rider ring grooves by machining to base metal. Take extreme care at this point to assure that all babbitt is removed, while keeping the depth of the groove to a minimum. If machining is not necessary, sandblast the rider groove to bare metal. 2. Thread rider ring groove in accordance with instructions given in Figure 4-34. 3. Keep the cleaned area free of all oil and spray...

Procedure for Inspection of Compressor Pistons

The following procedures should be followed when inspecting compressor pistons. 1. Check piston in lathe or place in V-blocks, then indicate inside diameter of piston boss. 2. Indicate counterbore or face of piston that bears against piston rod collar or shoulder. This must be perpendicular with inside diameter of the boss within .0005. If indication exceeds this amount, machine counterbore or face until it indicates within .0005. 3. Indicate counterbore or face of piston where piston nut makes...

Disc Valves

The geometry varies from a single small disc in small valves to multiple discs in large valves. They can be found in applications ranging from a few hundred psi, as in poppet valves for gas transmission compressors, to over 50,000 psi in poppet valves for hyper compressors. The design characteristics of poppet valves for gas transmission service are multiple nonmetallic and streamlined sealing elements in single-or double-deck valves. (See Figure 3-18)....

Foreign Materials

Foreign materials may be listed in the following categories Liquid carryover from a process or from interstage coolers is apt to cause premature failures, particularly on intake valves. A slug of liquid is particularly hard on valves and may even break a seat. Liquid also destroys lubrication, thus, accelerating wear. Carryover can result from poor separation of condensate ahead of the compressor, or improperly designed piping where low spots permit liquid accumulation. A sudden flow change can...

Predictive Maintenance vs Periodic Inspection Maintenance

Predictive maintenance evolved from several basic facts Certain vital parts last longer and operate better if not frequently taken apart, but operation until complete destruction is not only foolish, but unsafe, and costly. 99 of all failures are preceded by signs, conditions, or indications of failure. Periodic inspection maintenance could disturb good parts. Assembly errors are possible whenever machines are taken apart. Running fit and finish are sometimes jeopardized during internal...

What To Watch

The following checkpoints should be watched An example of the seriousness of these fires and explosions can be seen with the case of a soot-blowing compressor. This was a 200 HP, three-stage machine with a 500 psi discharge. It experienced an explosion in the discharge line between the compressor discharge and the aftercooler. The explosion ruptured a 4 pipeline. In another case, a 2000 HP air compressor in an automobile plant had an explosion in the air discharge of the high pressure...

How Compressor Packing Works

As previously described, a packing set consists of a series of sealing units. Compressor packing is not bottle-tight, but the amount of leakage is extremely small, a fraction of one percent of the capacity of the machine and usually within tolerable limits. In case of toxicity of the gas or danger of explosion or corrosion, leakage may be vented to a safe place. Because the individual rings will leak slightly, it is through the series of rings that the pressure is broken down from discharge to...

Disassembly Assembly and Inspection Procedures Disassembly

Refer to Figure 3-22A for a typical valve assembly with conventional restraint. 1. Remove valve screw cap and back off valve jam screw a minimum of four complete turns. 2. Remove valve cover nuts and remove valve cover. 3. Loosen set screws holding valve yoke and remove yoke. 4. Remove cover and valve seat gaskets and discard. 5. Carefully clean gasket seating surfaces on valve covers and valve seats. 6. Visually inspect valve seats for any signs of damage such as burring, wire drawing, and...

Splash and Flood Lubrication

Many horizontal compressors use the flood system for bearing and crosshead lubrication. Oil is elevated from the reservoir by discs on the crankshaft and is removed from the discs by scrapers. Some oil is diverted to the main and crankpin bearings, and the remainder is led to a large pocket over the crosshead from which a stream of oil flows to the crosshead pin and crosshead bearing surfaces. In some designs, the main bearings are lubricated from this pocket and the oil flows through external...

Troubleshooting Lubrication Systems

The following lists will help identify the cause of some of the more common compressor lubricating oil systems problems. 3. Defective pump or relief valve 4. Relief valve on header faulty 5. Worn bearings or excessive clearance 1. Relief valve in header faulty 5. Pressure regulating valve set too high Excessive Oil Consumption 1. Oil level too high in crankcase 3. The oil pressure may be too high 4. Piston rings and cylinder are worn Force-Feed Lubricator Incorrect Delivery of Lubricator 3....

High Pressure Cylinders

High-pressure cylinders, as shown in Figure 2-9, are made from a steel forging, with only nominal water cooling as compared to the other cylinders. Because the smallest possible number of openings in the forging is desired, capacity control mechanisms are not normally provided. Tie bolts may be installed perpendicular to the bore to pre-stress the forging, decreasing the maximum tensile stress induced by the gas pressure. Valve ports are finished to a highly polished surface, which reduces the...

Reconditioning of Compressor Piston Rods

If a rod is scratched, nicked, worn, or a slight shoulder exists, it is possible to recondition. If previous inspection has revealed any indication of cracks, inclusions, or imperfections in material, the rod must be scrapped. Note Do not attempt to weld piston rods. Regrinding Piston rods may be ground undersize, and the standard packing rings can be used. Generally .002-.003 per inch of piston rod diameter can be ground for medium pressures, but as the operating pressure increases above 1000...

Case Histories From Service Technician Reports

Service technicians compiled examples of problems that could have been prevented by more-detailed or careful examination of early symptoms. The following are some of the typical problems they described. Symptom Compressor would not unload, pumping until safety valve blew. Excessive blow-by existed at the piston rod packing. Findings The unloader piston was found to have rusted in its cylinder due to excessive condensation. Jacket water temperature at 50 F was increased to 100 F outlet. Unloader...

Compressor Component Repair and Rebuilding

In the overhaul and maintenance of the reciprocating compressor, many repairs can and are made to the major components. When making repairs, several things must be considered to prevent jeopardizing the integrity of the part and to ensure that it performs as good or better than the original installed in the compressor by the manufacturer. Here are questions to ask Is the repair made with sound engineering judgment Is the repair done with care and proper manufacturing techniques, such as,...

Web Deflection Measurements

Web deflection measurements are thus required for determining deviations in the crankshafts of large industrial marine engines and reciprocating compressors. These measurements and their interpretation are vital in disclosing conditions that can lead to disastrous and expensive crankshaft failures. Accurate periodic measurements, combined with careful analysis, will permit such conditions to be corrected before they cause problems. Web deflection is defined as any movement of the crank webs...

Valve Cover Bolting

Compressor valve covers should be checked to make sure they are fully bolted rather than short bolted, meaning they have only some of the threads of the nut in contact with the stud bolt. Full-thread contact is necessary to reduce the risk of the compressor valve cover being blown off. There is a remote possibility that a longer stud could have been substituted, which gives the appearance of short bolting. But this is unlikely. Figure 7-t. Noninterchangeable valves. Figure 7-t....

Routine Maintenance is Sufficient in Fouling Service

The excellent performance of the labyrinth-piston compressors in fouling service at Plant A can be attributed to the routine maintenance the units receive. Six four-throw, two-stage machines are given routine overhauls after every 8,000-10,000 operating hours. Three of the six machines are always in service, and there are times when all six machines are online. Overhauls are generally completed in three to four working days, with three or four machinists working 12-hour shifts. Typical tasks...

Equipment Improvement

Breakdown and other losses cannot be eliminated unless basic equipment conditions are maintained and possible deterioration is checked for on a daily basis. Operators play a role in this effort by cleaning, lubricating, tightening bolts, and learning to conduct routine inspection. Before an effective program can be developed, the compression equipment must be inspected to determine 2. What is needed to bring it up to like-new condition or standard 3. New component parts required 4. What...

Repairing Foundation Cracks

The notch provided in the top of a foundation for the oil pan creates a perfect location for stress risers. A moment is created by lateral dynamic forces multiplied by the distance between the machine base and transverse reinforcing steel in the foundation below. The possibility of foundation cracking at this location increases as the depth of the notch increases. The farther the distance between the horizontal forces and transverse reinforcing steel, the greater the moment. Figure 4-11...

Pressure Volume Patterns

Inspection of the pressure-volume patterns displayed on an instrument such as the Beta 250 Engine Compressor Analyzer* will identify cylinder problems that cause loss of efficiency. These include Figure 5-3. Ultrasonic detection in progress. (Source UE Systems Company, Elmsford, New York). Figure 5-3. Ultrasonic detection in progress. (Source UE Systems Company, Elmsford, New York). Leaking rings, suction valves, or discharge valves Suction or discharge valve springs too stiff Figure 5-4...

Inspection of Cylinders

Cylinders are inspected either visually or, with greater precision, by measuring bore size. Visual inspection should be done at every shutdown, for example on the occasion of a valve change-out. This can be accomplished by looking through the valve ports and inspecting the bore for any sign of scuffing or scoring. Also, the amount of cylinder oiling should be determined, that is, determine whether the cylinder is being over-oiled or not enough oil is being fed to the cylinder. Adjustment to...

Weld Repair

Crankshaft journals, mains as well as throws, can be repaired by welding and grinding back to standard. However, weld repairs are feasible only with steel shafts. This technique requires real weld repair expertise. Because of the carbon content of these shafts, the material is difficult to weld without inducing surface embrittlement and cracking. These cracks may be microscopic and usually cannot be detected except by magnetic particle inspection or metal etch testing. If cracks are induced...

Cylinder Bore Measurements

Compressor cylinder bore measurements are taken in the vertical and horizontal directions and in at least three locations along the bore, each end of ring travel, and in the center (see Figure 4-14). All measurements are taken with a micrometer and should be recorded on appropriate inspection report forms. One form should be used for each compressor cylinder and all pertinent data entered. Figure 4-14. Measurement locations along cylinder bore. Figure 4-14. Measurement locations along cylinder...

Vacuum

A gas is said to be under vacuum when its pressure is below atmospheric. There are two methods of stating this pressure, only one of which is accurate in itself. Vacuum is usually measured by a differential gauge that shows the difference in pressure between that of the system and atmospheric pressure. This measurement is expressed, for example, as Millimeters of Hg vacuum (mm Hg Vac) Inches of Hg vacuum (in. Hg Vac) Inches of water vacuum (in. H20 Vac) Unless the...

Rider Ring Clearances

The suggested minimum side and end clearances for rider rings of different materials are shown below. Minimum Side and End Clearances for Rider Rings of Different Materials Minimum Side and End Clearances for Rider Rings of Different Materials End Clearance (Per inch of cylinder diameter) The clearances shown in Table 4-5 are for butt cut or step joint rings as shown in Figure 4-27. For angle cut joints multiply value in table by the amounts shown in Figure 4-28. The side clearance for Teflon...

Inspection and Reconditioning Piston Rods

Before installing the piston piston rod assembly in the compressor cylinder or attempting to recondition a piston rod, inspect and measure it thoroughly. An important part of this inspection is checking the piston rod and the piston to be sure the assembly is able to run true to the centerline of the cylinder bore. Inspection is done by the following method 1. Visually inspect for signs of scuffing, longitudinal scratches, damaged or pulled threads. 2. Measure all diameters and lengths to...

Maintenance Costs

Perhaps the most frequent problem maintenance departments face is rising cost and diminishing resources. While being constantly confronted with budget questions and the pressure to reduce costs, maintenance managers are still expected to raise the level of service in the plant. Maintenance often falls victim to budget cuts because management mistakenly believes maintenance can be deferred. It is thus important to wisely and effectively allocate and spend maintenance money. Inspection records...

Reverse Flow Control

The reverse flow regulation used by Hoerbiger is a stepless capacity control system that can be used on reciprocating compressors. It allows close adjustment of the output of a piston compressor to the demands of a process. It can follow variations of gas demand, gas composition, and Table 3-1 1 power Consumption of Compressor at Different Load Conditions Table 3-1 1 power Consumption of Compressor at Different Load Conditions *Kilowatts required bhp x. 746 motor efficiency *Kilowatts required...

Emergency Repairs Should Be Minimized

Plant systems must be maintained at their maximum level of performance. To assist in achieving this goal, maintenance should include regular inspection, cleaning, adjustment, and repair of equipment and systems. On the other hand, performing unnecessary maintenance and repair should be avoided. Breakdowns occur because of improper equipment operation or failure to perform basic preventive functions. Overhauling equipment periodically when it is not required is a costly luxury. At the same time,...

Method of Application

Oil is fed directly to the cylinder walls at one or more points by means of a mechanical force-feed lubricator or a centralized lubricator system. The oldest and most basic lube system is the box lubricator. Here, an individual pump feeds each point, each with an adjustable stroke and some sight glass to view the adjusted feed rate. The suction stroke of the pump pulls oil from the reservoir and discharges it down the line. Box lubricators are driven either from the crankshaft, another moving...

Operational Problems and Maintenance of Compressor Valves

In a reciprocating compressor cylinder, the valves, without a doubt, have the greatest effect on the operating performance of the machine, both from an efficiency standpoint and from a mechanical reliability standpoint. The valves are installed in the compressor cylinder directly in the air or gas stream, and are subjected to considerable abuse. It is surprising that we generally have as little valve trouble as we do. Stop and consider for a moment just what is expected of a compressor valve....

Remanufacturing of Shell Type Bearings

Remanufacturing or rebabbitting of shell-type compressor bearings should be done only to specific styles. The old bearing shells with thick babbitt, usually .032 to .064, can be rebabbitted by a shop experienced in this type of work. Centrifugal casting or spraying of the babbitt has been found superior to pouring babbitt in these shells. The newer shells with reduced babbitt thickness (.007-.015), can be rebabbitted by centrifugal casting if the backing is made of cast iron or steel. Bearings...

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...

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

Recip Compressor Crosshead

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...

Synthetic Compressor Lubricants

Synthetic oils have become more widely used and accepted as compressor lubricants because of their higher auto-ignition characteristics and ability to prevent carbon buildup on valves and piston rings. The use of synthetic oils may allow the reduction of feed rates to the cylinder by approximately one third. This reduction in feed rates will result in less oil in the downstream piping system. Reduced oil accumulation and the fire resistant characteristics help to prevent fires in the discharge...

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...