Oxidation

Much of the circulating oil in compressor crankcases is broken up into fine spray or mist by splash or oil thrown from rotating parts. Thus, a large surface of oil is exposed to the oxidizing influence of warm air, and oxidation will occur at a rate that depends on the operating temperature and the ability of the oil to resist this chemical change. Oil oxidation is accompanied by a gradual increase in viscosity and, eventually, by the deposition of insoluble products in the form of gum or...

Packing Problems

Some common problems associated with packing repairs in the field are as follows 1. Segment reversal. This term refers to the mismatching of the individual ring segments. Rod packing segments are match-marked at the factory in some manner to assure proper assembly. Improper assembly of ring segments can result in excessive leakage and premature packing failure. 2. Mislocated packing. This term refers to having single-acting packing where double-acting packing should be installed. 3. Wrong side...

Packing Rings

Packing rings are the heart of the packing assembly, sealing along the piston rod and between themselves and the packing case (see Figure 3-29). The tangent ring is cut into three segments so that each cut lies on the side of an equilateral triangle. The cuts of this ring maintain sealing contact regardless of variations of the ring's inside diameter. As wear occurs, the ring segments will close radially to compensate while still maintaining sealing contact at the tangential joints, Figure 3-30...

Packing Types

There are several types of packing available. These include 1. Fully lubricated. Oil is supplied to the packing from the mechanical lubricator. The cases are drilled so that oil will be carried to the oil cups and then to the piston rod through a drilled hole in the oil cups. 2. Mini-lubricated. Same as fully lubricated, except that oil to the packing is decreased approximately 30 percent of normal flow. 3. Micro-flow. There is no oil supplied directly to the packing. The only oil that reaches...

Parallel Flow

Parallel flow is preferred in cases of warm water, or low water pressure, or both. With warm water (approximately 95 F and higher), there is little danger of causing condensation, and the problem is to achieve effective cooling, assuming sufficient water is available. Cylinder cooling is improved slightly because the water is not preheated in the cooler. Also, due to lower pressure drop in parallel circuits, higher flow rates are possible. Higher flow rates may be needed with warm water. Lack...

Personnel Training

Operational and maintenance personnel must be involved in an ongoing training program to achieve the greatest efficiency and reliability of the compressors. All too frequently, the only training that operators and maintenance personnel receive is by word of mouth and by on-the-job training. On-the-job training is excellent as long as what is taught is correct. Unfortunately, improper maintenance techniques and traditions are often passed on by the person in charge. Needless to say, this...

Piston Failures

Figure 4-31 explains typical compressor piston defects and failure locations. A. Piston mating surface fretting and wear on two-piece pistons. maximum Allowable Cylinder Bore Diameter Before Oversize Pistons Are Required Maximum Bore Diameter Increase Allowed Before Oversize Piston is Required Maximum Bore Diameter Increase Allowed Before Oversize Piston is Required maximum Allowable Cylinder Bore Diameter Before Oversize Pistons Are Required All Piston Materials With Non-Metallic Rings &...

Piston Inspection Forms

Inspection forms should be available at the time of inspection so that all measurements and pertinent data may be immediately recorded. This face to be square with piston bore A to within .0005 Figure 4-30. Verifying acceptable piston dimensions. This face to be square with piston bore A to within .0005 Figure 4-30. Verifying acceptable piston dimensions.

Piston Ring Clearances

The suggested minimum side clearances between ring groove (side) and end (gap) clearances for piston rings of various material are given below. Caution should be taken to make sure that each ring has the minimum end clearance. Unless there is sufficient clearance, ring ends may butt together from thermal expansion, causing ring wear, cylinder scoring, and other damage. The end clearances shown in Table 4-4 are for butt cut or step joint rings, as shown in Figure 4-27. For angle cut joints,...

Piston Ring Function

Two principal functions of piston sealing rings are To prevent gas from blowing by the piston. To transfer heat from piston to the cylinder walls and hence to the water jackets. Piston ring groove width variations can have a significant effect on these functions. To ensure proper operation, good contact is required between the ring and the side of the groove. If the grooves become tapered or edges dragged over as shown in Figure 4-22, poor sealing or ring fracture may be expected. If shoulders...

Piston Ring Installation

Care should be taken when expanding rings over the piston. To prevent permanent deformation or breakage, they should be spread no further than necessary to clear the outside diameter of the piston. Thin steel strips placed as shown in Figure 4-29 will help prevent damage from overstress or catching in unfilled grooves. Four strips are usually sufficient, one near each ring end and the other two evenly arranged on the opposite side of the piston.

Piston Rod Oil Wiper Packing

The stuffer in the frame head of the cylinder housing or crosshead guide contains a set of piston rod oil scraper rings and a gland (see Figure 3-51) that prevents crankcase oil from being carried out of the crankcase along the rod. A typical gland is designed to allow no more than .005 total side clearance for the scraper rings, generally .002 to .003. If the clearance is insufficient for the rings to float on the rod, they will not function properly. Too much clearance will cause the rings to...

Pistonto CylinderLiner Bore

The piston-to-cylinder or liner bore clearances vary with piston design and piston ring configuration. Oil-Lubricated Cylinders with No Rider Band The clearance between the piston-to-cylinder bore or the liner bore on an oil-lubricated cylinder can be determined as follows Cast iron piston .00125 inch bore diameter Example 20 diameter cylinder 20 x .00125 .025 clearance Aluminum piston .003 inch bore diameter Example 20 diameter cylinder 20 x .003 .060 clearance Oil-Free Cylinder Rider Band See...

Planning for Preventive Maintenance

Preventive maintenance for reciprocating compressors focuses on Restoring and upgrading compressor equipment. Involving operation and maintenance personnel and upgrading their knowledge and skills on compression equipment. Implementing effective maintenance planning and scheduling techniques. Identification of qualified vendors, suppliers, and contractors. Preventive maintenance includes periodic inspection to detect conditions that might cause breakdowns, production stoppages, or detrimental...

Plasma Spray

Development of plasma spray and suitable coatings allow piston rods not only to be reconditioned but to be given a surface treatment superior to that originally on the rod. Overlays of tungsten carbide and chrome oxide can give the piston rod longer wear life. Care must be taken to check the rod for straightness after spraying. Plasma spray has superior bond strength over conventional metal sprays and is less likely to flake off in service. This technique is used to restore piston rods to their...

Ported Plate Valves

Ported plate valves are similar to the concentric ring valves, except the rings are joined together into a single element (Figure 3-17). There are two advantages to this when compared to the concentric plates The number of plate edges available for impact is reduced. Mechanical damping by using damping plates to absorb opening impact is easily introduced. Spring is generally provided by coil springs that directly contact the plates, eliminating the buttons used in concentric ring valves. The...

Predictive Maintenance

The maintenance methodology known as predictive maintenance and condition-based maintenance is attracting attention as a highly reliable substitute for conventional periodic maintenance and overhaul. Where preventive maintenance fails in efficiency, predictive maintenance offers improvement. This method involves the use of real-time or portable instruments such as vibration, infrared, and wear debris monitors to recognize the symptoms of impending machine failure. Predictive maintenance is an...

Preliminary Checks if Runout Exceeds Limits

If the guidelines above have been closely followed and runout cannot be set at the proper limits, the following checks are recommended. Figure 4-10. Allowable piston rod runout vs. cylinder running clearance. .010 .020.030.040 .050.060.070 .080.090.100 .110.120.130.140.150 .010 .020.030.040 .050.060.070 .080.090.100 .110.120.130.140.150 First, ensure that all the housing nuts are torqued properly. Second, ensure that the packing and scraper assembly are not binding the rod. If the proper runout...

Prevent Gas Leakage

A means of preventing gas from leaking into the compressor during overhaul is necessary. Any gas trapped inside the compressor may leak into the room when the equipment is opened up. When opening up the compressor part of the machine, pancake skillet blinds should be placed between flanges or a blind flange installed on existing piping. Those positive shutoffs are easier said than done at most installations. A double block and bleed valve combination is satisfactory if the bleed line (for...

Preventive Maintenance for Reciprocating Compressors

It may be either impossible or utterly uneconomical to design and manufacture machinery for zero maintenance or perfect reliability with infinite life. Realistically, two principal maintenance philosophies are prevalent in industry Do nothing until the equipment breaks down or an emergency occurs. The compressor is repaired as quickly or as cheaply as possible and is returned to service. Inevitably, the next emergency is just around the corner. Maintain the equipment in excellent condition,...

Primary Forces

Primary forces are composed of rotating and reciprocating masses and occur at crankshaft speed. In effect, the primary force can be likened to a piece of string weighted at one end, with the opposite end tied to a ring that rotates about a fixed steel rod (see Figure 2-53). Figure 2-53. Unbalanced and balanced forces acting on a shaft system. Figure 2-53. Unbalanced and balanced forces acting on a shaft system. As long as the weight (mass) revolves about the rod, the string will be in tension,...

Principle of Operation

An unloading device (see Figure 3-61), is fitted to each suction valve. At partial loads, this device does not allow the suction valve to close when the piston is in its bottom dead-center position, but delays this closing in a controlled way. Thus, a certain amount of gas, which is stepless-ly adjustable, is allowed to return towards the suction manifold before the compression starts. Figure 3-61. Reverse flow unloading device, schematic view (Source Hoerbiger Corporation of America, Pompano...

Procedures and Specifications

The following questions can be used to evaluate the vendor's procedures. 1. Does the vendor have a written quality control manual Are its procedures and work processes adhered to by shop employees, or is the manual just for show 2. Does the vendor have written specifications and procedures for reconditioning or manufacture of compressor components for which they are quoting If the vendor has the specifications, have they been made a part of his quotation without being requested Are the...

Procedures for Inspection of Compressor Piston Rods

Use the following procedures to inspect compressor piston rods 1. Remove piston from piston rod. 2. Place piston between centers (be sure existing centers are in good condition) or in V-blocks (see Figure 4-40). 3. With dial indicator, indicate over length of rod. Rod should not run out more than .001. 4. Measure diameter to be sure rod is round within .001 and not tapered more than .001 over its length, or worn undersize. If rod is found to be worn not more than .005 undersize and still round...

Program

Although monitoring of reciprocating compressors is not as simple and definitive as monitoring other rotating equipment, there are some things that can be and should be monitored. The first step in the program is to decide what is to be monitored. An effective predictive maintenance program should include the following 1. Daily operating reports and logs. These are used to observe operating parameters, pressures, temperature, flows, etc. These are often overlooked and recorded as part of the...

Pulsation Form

Pulsation is caused by the periodic action of compressors propelling gas through a pipe. The piston-crank-valve mechanism generates a variable pressure which, over time, creates a pressure wave in the suction and discharge piping that resembles a composite wave as shown in Figure 2-56. This composite wave is made up of a number of waves that are multiples of the fundamental sine wave. The multiples make up the composite wave form. Therefore, multiple frequencies of pulsation exist that can...

Pumps for Divider Block Systems

There are three basic types of pumps used for these systems. All pressurized inlet pumps must have an adequate supply of pressure and volume to handle the required flow rates. The typical range of inlet pressure varies from mere inches of water column to 30 psi. The first type of pump is the box lubricator converted to pressurized inlet. The sight glass and drip tube are replaced by a cap with inlet fittings for the supply lines. The suction tube is removed or plugged. Typically, two pumps are...

Pumps With Pressurized Supplies

Rotation of the lubricator cam actuates the pump rocker arm assembly to operate the pump piston. On the piston downstroke, spring pressure is exerted on the piston causing it to follow the cam. As it moves down, a pressure reduction (vacuum) is created between the piston and the discharge check valve, and the valve closes. This allows the pressurized supply to unseat the supply inlet shutoff ball and pressurize the piston bore with lubricant. On the piston upstroke, the piston forces the supply...

Pumps With Sight Glass

Rotation of the lubricator cam actuates the pump rocker arm assembly to operate the pump piston. On the piston downstroke, spring pressure is exerted on the piston causing it to follow the cam. As it moves down, a pressure reduction is created between the piston and the check valve and Figure 3-3. Force-feed box lubricator with sight glass (Source Lubriquip, Inc., Cleveland, Ohio). Figure 3-3. Force-feed box lubricator with sight glass (Source Lubriquip, Inc., Cleveland, Ohio). the valve...

Quantity of Material to be Removed When Reboring

The maximum diameter to which a compressor cylinder can be rebored is usually limited by the counter bore. Generally, the counter bores are V to V larger than the cylinder bores. While a cylinder may have sufficient strength to be bored larger than the counter bore diameter, it may be impractical to do so because there would be no way to center the heads without major redesign (see Figure 4-16). Figure 4-16. Cylinder bore and counter bore configurations. Figure 4-16. Cylinder bore and counter...

Rebuilding Compressor Pistons

There are several types of repair and rebuilding needed with worn or scuffed compressor pistons. These include 1. Remachine piston ring lands to correct worn grooves, then install wider piston rings. 2. Turn OD of piston to remove any scoring or scuffing. Build up OD with metal spray of proper material and remachine OD to original diameter. 3. Repair cracks in ribs or hubs in cast iron pistons by appropriate grinding, cleaning, and welding. Piston must be remachined after welding on OD, hub...

Reciprocating Compressors

This book is dedicated to the mechanics, technicians, engineers, and managers whose quest for excellence leads them to be resourceful and who recognize that resourcefulness includes reaching for the written word. Copyright 1996 by Butterworth-Heinemann. All rights reserved. Printed in the United States of America. This book, or parts thereof, may not be reproduced in any form without permission of the publisher. Originally published by Gulf Publishing Company, Houston, TX. For information,...

Reconditioning and Rebuilding

The valves removed from the compressor should be sent to the valve manufacturer's shop for cleaning, reconditioning, installation of all new components (sealing elements, springs, etc.), and testing. Operation and Maintenance of Reciprocating Compressors 1 63 ICOQK MANLEY HSRI Operation and Maintenance of Reciprocating Compressors 1 63 ICOQK MANLEY HSRI Figure 3-22B. High-security valve restraint (Source Cook Manley, Houston, Texas). Figure 3-22B. High-security valve restraint (Source Cook...

Record Keeping

Experience has also shown that a systematic study of valve problems will provide the best and often the quickest solutions to valve problems. Record keeping is necessary to successfully evaluate and solve valve problems. It is obvious that records should be sufficiently detailed to show which parts are involved and where these parts were located in the compressor. A picture of some sort is helpful in spotting a pattern of valve failure. A typical valve location chart is shown in Figure 3-20....

Regular Inspection and Maintenance

Compressor installations should be inspected regularly at intervals that will have to be determined by the severity of operating conditions. Necessary maintenance work such as replacing worn or broken parts installing new packing changing crankcase oil cleaning the crankcase, the force-feed lubricator and the air filter and removing deposits from valves, discharge passages, coolers, and water jackets should be carried out promptly. Following an overhaul, the entire system should be thoroughly...

Rider Rings

Rider rings or bands are designed to support the weight of the piston and piston rod assembly. These are available in a number of configurations that include the following Cut-type rider rings are snapped over the piston with the same care as piston rings. Cut-type rider rings are supplied either as butt cut or as angle cut, similar to piston rings. Both types require side relief grooves to relieve the gas pressure from behind the ring so that they will not act as a piston ring. Rider rings...

Rod Position Monitoring

This monitor enables the user to detect when the rider ring bands have worn away enough to require shutdown before the piston contacts the cylinder. A proximity probe is mounted vertically next to the cylinder to observe the piston rod position. The new monitor provides two modes of operation average and instantaneous rod position. The instantaneous position measurement uses a Keyphasor transducer to provide a once-per-turn crankshaft revolution reference signal for sampling the rod drop probe....

Rod Under Tension

As the piston discharges toward the crank end on the return stroke, as seen in Figure 2-43, the net force of the suction and discharge pressures results in a tension load on the rod. This is called rod load tension. The operator can damage the compressor by decreasing the suction or increasing the discharge pressure too far above the design pressures. Figure 2-43. Piston rod stretch during compression in crank end portion of cylinder. Figure 2-43. Piston rod stretch during compression in crank...

Rule of Thumb for General Running Clearances

Before an overhaul is attempted, it is important to read and understand the instruction manual supplied by the manufacturer of the compressor. It is particularly important to establish the recommended clearances given for the various components of the compressor. If the manufacturer's data for running clearances are not available, the following may be used as guides and good rules of thumb. It must be remembered that repair and overhaul procedures may not lead to restoration of the original...

Safety Precautions Read Carefully Before Installing the Compressor

Where lubricating oil is present in the compressor discharge, an aftercooler should be installed in the final compressor discharge line. It should be mounted as close as possible to the compressor. When installing a new compressor, it is essential to review the total plant air system. The use of plastic or carbonic bowls on line filters without metal guards can be hazardous. A pressure relief valve must be installed in the discharge piping between the compressor and any possible restriction,...

Sample Calculation A New Machine

Process gas molecular weight 17.76 k-value 1.26 Suction temperature 60 F Suction pressure 150 psia Compression ratio 6.67 overall, or V6.67 2.58 per stage From Figure A-l, we obtain a discharge temperature of 308 F. Specific gravity of gas 17.76 28.97 0.613 Compression efficiency from Figure A-7, ead 80 * Compressibility factor Zs is obtained from Figure A-8. General HP formula Figure A-7. Estimated efficiency as a function of compression ratio reciprocating compressors. Figure A-7. Estimated...

Sealing Elements

Materials used for sealing elements fall into two classes high strength steels and plastic composites. Feather valve strips, valve channels, and springs are made of steel, usually 410 stainless that is stress relieved, hardened, and tempered for maximum wear resistance. For more corrosive applications, channel may be made from 316 stainless. Feather valve strips may be made of Inconel 17-4 PH materials for greater corrosion resistance. As indicated in Table 3-6, valve discs or plates may be...

Secondary Forces

The secondary forces are the result of reciprocating masses only, and because of acceleration and deceleration of these masses, occur at twice crankshaft speed. They act only in the plane of the cylinder axis, and can only be balanced by counterweights rotating at twice crankshaft speed, and in opposite directions. Such an arrangement requires a separate counterweighting system, which is gear or chain-driven. Because of costs, space requirements, and design difficulties, balancing of single...

Selection of a Contractor

Because the contract service business requires low capital investment, it occasionally attracts participants who have neither the knowledge nor the technology to properly service compressors. For this reason, it is imperative that the capabilities of every contractor be thoroughly examined. To work effectively with a contractor, the client must have confidence that the firm will be able to do the job properly and on time. The points outlined for the selection of a vendor also apply in the...

Service Condition Changes

A change in suction or discharge pressure may cause problems. Observing these pressures should be part of any troubleshooting effort. A plant operator advised that he had purchased a 12 x 9 single-stage horizontal compressor and that he was using it for 100 psi pressure. He inquired about the working conditions for which this machine was suit able and, upon investigation, found that the compressor was designed and built for a maximum air pressure of 40 psi. The operator was, accordingly,...

Setting Piston End Clearance

Whenever the piston rod has been removed or even turned in or out of the crosshead, it is necessary to check the piston end clearance and make any adjustments necessary to restore it to the proper amount. If the piston has not been removed from the rod, the original clearance can easily be maintained by center punching both the rod and the crosshead and making a suitable scribing tool, as shown in Figure 4-39, before removing the rod from the crosshead. Note The length of the scribing tool will...

Spare Parts

A very important part of compressor rebuilding, continuing reliability, and a factor in the preventive maintenance program is having an adequate inventory of parts for each compressor. Maintenance needs an effective parts supply. A good store is essential. You can't keep equipment running without parts. It is well known that the compressor manufacturer cannot be depended upon to furnish critical parts on a short-time basis. In order to properly overhaul and maintain the compressor, each user...

Tapered Piston Rods

Because lubricating oil films may tend to fill up some small passages, packing can usually function in the presence of slight piston rod taper. Generally, tapered conditions are found at either end of the stroke, while the in-between portion of the rod may remain unaffected. Excessive amounts of taper will destroy the seal in the following manner. When the rings are riding on the tapered part (see Figure 3-45), one edge of the ring bore is in contact, while the other edge is away from the...

Testing

Even if valve maintenance has been given a high priority and all parts have been reconditioned as carefully as possible, one more check should still be made on every reconditioned valve the so called kerosene test. Actually, kerosene is rarely used a commercial solvent substitute is more common. Here, the objective is to set an assembled valve over a pail with the seat in the top position. The valve is then filled with solvent and left for a few minutes (see Figure 3-25). No valve is free of...

The difference between suction and discharge events

The basic cycle repeats itself over and over again and is representative for every valve of every design and configuration. The differences between suction and discharge events are the piston velocities at which the suction versus the discharge valves open and close, the time the total cycle consumes, and the pressure and temperature at which the gas is being processed. These criteria are important to remember because they lead to the design differences between suction and discharge valves.

The Effect of Unbalance

Compressor inertia forces may have two effects. One is a force in the direction of the piston movement, and the other is a couple or movement that is developed when there is an offset between the axis of two or more pistons on a common crankshaft. The interrelation and magnitude of these forces will depend upon such factors as number of cranks, their longitudinal and angular arrangements, cylinder arrangement, and extent of counterbalancing possibility. Two significant vibration (movement)...

The flow of the gas

The flow of gas through the seat keeps the sealing element open. As the flow diminishes due to decreasing piston speed, the springs or other cushioning elements found in most valves force the sealing element to return to the seat lands and close the valve. Preferably, the valve is completely closed when the piston is near dead center. Figure 3-12. Valve seat land and lift representation.

Thermostatic and Thermosyphon Systems

These systems may be used where a small amount of cooling is advisable but the expense of a system involving a pump does not seem justified. They make use of the natural thermal circulation that results when the cooling water is heated in the cylinder. In these systems, the jackets are filled with water, oil, or ethylene glycol, and the heat of the cylinder is distributed by convection currents. The thermosyphon system is used with gases having a k value of 1.26 and below and with cylinder...

Thin Film Lubrication

During operation, fluid oil films exist between bearing surfaces. However, while stopped, adequate fluid films can no longer be maintained, and some metal-to-metal contacts may occur. Under these conditions, and regardless of bearing type, the film strength property of oil is of increasing importance. An oil having adequate film strength will resist rupture and, thus, minimize metal-to-metal contact, friction, and wear. Diester and polyalpha-olefin (PAO)-based synthesized hydrocarbon oils are...

Troubleshooting Valve Problems

Insufficient or improper lubrication 4. Pulsations on both suction and discharge 2. Cylinder jacket water temperature is lower than 3. Gas may be wet and washing inlet lubrication 1. If failures occur in winter months only, liquid is the probable cause and piping should be insulated or steam traced and or knockout pots placed close to the cylinder tightening or being knocked loose due to liquid 1. Valve loose by improper tightening 2. Valve tightened against broken pieces of 3. Improper...

Types of Cylinder Bore Wear

Cylinder bores usually wear in the following ways. 1. Compressor cylinder bores generally wear in a barrel-shape manner, that is, they wear more in the center than at either end of the compressor stroke (see Figure 4-15). 2. Bores may also wear out of round, that is, larger in the vertical than horizontal direction, or the reverse. 3. Taper wear may also be evident, that is, larger at the head end than crank end, or the reverse.

Undersized Piston Rods

If a piston rod is undersized but concentric and there is no taper, the packing can form an effective seal within reasonable limits because bore contact will be in the center of each segment and the cut in each ring is overlapped by its mate. Nominally sized rings can be used if the piston rod undersize dimension does not exceed .002. Whenever certain under-size conditions exist, wear-in time is required before the packing will properly seal. If the piston rod is greatly undersized, leakage...

Valve Analysis

If a suction valve does not close and seal completely, the following occurs During the compression stroke, some of the normally displaced gas will leak or blow by to the inlet side of the valve. The design volume of actual displaced gas will decrease. As the compressed hot gas passes over the valve in the wrong direction, the valve heats up. The inlet temperature of the gas on the suction side of the valve goes up, thus reducing the volumetric efficiency of the machine, that is, hotter than...

Valve Failure Record

How long has valve been in service (hours) Is lubrication on cyl. bore and valve adequate (lubricated units only) Figure 3-21. Typical valve failure record (Source Dresser-Rand, Painted Post, New York). 1. Install new valve seat gaskets of the proper diameter, thickness, and material. 2. Place valve assembly on seat, taking care that the gasket stays in place and is not pinched. 3. Place yoke against valve and tighten set screws to hold assembly in position (lower valves). Extreme care must be...

Valve Materials

Valve components are made of various materials depending on the operating pressures and type of gas to be handled. Materials used for valve seats and guards include Cast iron most common for low and medium pressures Ductile iron for medium pressures Cast steel for high pressures Steel (bar stock) for high pressures Depending on the pressures, combinations of the above are commonly used, that is, cast iron guards with the seat made of ductile iron or steel. For highly corrosive gases, various...

Valve Types Used in Reciprocating Compressors

Prior to the development of automatic valves, compressors used mechanically operated suction and heavy poppet-type discharge valves. These valve types were efficient and satisfactory at relatively low rotative speeds, but limited the speeds at which the compressors could operate. The lightweight, fully automatic valve allowed for higher rotative speeds, greater output, and increased efficiency from the compressor. There are many different valve designs available for modern reciprocating...

Vibration

Observe forces that influence compressor operation. Forces in the machine go in many directions. These can cause damaging vibration plus excessive deflection of parts of the equipment. For that reason, grout is added between the compressor and its foundation. Oil, vibration, and heavy stress loading frequently cause grout to fail or break up. Epoxy grouts are popular, having replaced cement grouts both at the initial setting of the machinery and when there is no recourse but to regrout. Be...

Vibration vs Crankangle Patterns

The use of vibration and ultrasonic patterns in combination with cylinder pV patterns will aid in diagnosing the condition of valves, rings, and packing. When valves open and close, they produce vibrations that are detected with an accelerometer. Figure 5-5 shows a compressor pressure vs crankangle trace. The vibration and ultrasonic patterns are also shown. These indicate valve timing and normal valve action. Figure 5-5. Pressure-time trace (pressure angle) with vibration and ultrasonic traces...

Water

Although water may enter compressor crankcases by condensation from the atmosphere during idle periods or possibly from leaking jackets, there is generally little water present because of the continuous venting of water vapor at crankcase temperatures. Normally, therefore, there is little opportunity for the formation of troublesome emulsions, which could combine with dust and other contaminants to form sludges that would restrict the flow of oil to lubricated surfaces. A good compressor...

Water Jackets

After years of operation, water jackets become dirty and fouled, particularly if jacket water quality is not good and the jacket cooling effect may have deteriorated considerably. The inside of the jacket walls may be covered with sand, lime, or magnesium, which interferes with normal heat transfer and cooling. The solution to this problem is to clean not only the cylinder jackets but also the water cooling passages in the front and rear heads. The only way to effectively do this is by...

Wear and Fatigue

Wear, as such, cannot be completely eliminated. It can be minimized by proper lubrication, design, and selection of materials. Most valves require some means of guiding. Wear at the guides, if severe, can result in sloppy action, cocking, poor seating, and failure. Wear must never be permitted to develop to this degree. Fatigue is the result of repeated cyclic stress. The stress level, as well as the stress range, must be considered in any design. However, barring abnormal action, well-designed...

Weld Repair Procedure

It is important that the proper weld procedures be followed when making weld repairs on crankshafts. An approved procedure will go a long way towards maintaining the integrity of the crankshaft. 1. Cracks in the crankshaft shall be removed by grinding. 2. Crankshaft shall be inspected with magnetic particle method to ensure that the cracks are removed. 3. Crankshaft shall be preheated to a temperature between 350 F and 800 F. This preheat temperature shall also be the minimum interpass...

What We Know

The few fires we know of and have investigated were caused by the following 1. Dirty, scaled-up cylinder water jackets. 2. Oil collecting in low points in discharge piping. 3. Overheating due to compressor valve failure. 4. Overheating due to cooling water failure. However, it is not possible to attribute a definite cause to all fire events. How Fires Start The exact cause of a fire or explosion in a compressor discharge system is seldom, if ever, known. However, excessive deposits and high...

Torque Values For Crosshead Nuts

On those piston rods that use a nut tightened to the face of the crosshead, it is important that the proper torque value be achieved to prevent loosening or overstressing the rod. Table 4-9 gives torque values for various size piston rod thread sizes at the crosshead end. Note It is very important to properly block the crosshead to prevent it from cocking when tightening the nut. Failure to take this precaution can damage the crosshead pin bushing as well as the connecting rod bearing.

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...