rotary compressors

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BECHTELMECHANICALMACHINERY

ENGINEERING DESIGN GUIDE FORROTARY TYPE POSITIVE DISPLACEMENT COMPRESSORS3DG M56 002, Rev. 00, 02/93Prepared by: D. Gamlen et al.Approved by: A. J. Reidy

TABLE OF CONTENTS

Page No.

LIST OF TABLES 2

LIST OF FIGURES 3

LIST OF SYMBOLS 4

1.0 PURPOSE 5

2.0 CODES, STANDARDS AND REFERENCE DOCUMENTS 5

3.0 GENERAL 5

4.0 PRINCIPLE OF OPERATION 8

5.0 CONSTRUCTION FEATURES 13

6.0 APPLICATIONS 19

7.0 REFERENCES 26

APPENDIX A LIST OF PREFERRED VENDORS

APPENDIX B TYPICAL ENGINEERING SPECIFICATION


LIST OF TABLES

TABLE 3.1 COMPRESSOR FAMILY TREE

TABLE 4.0 OPERATING CHARACTERISTICS


LIST OF FIGURES

FIGURES 3.1, 3.5, 3.6

COURTESY OF EQUIPMENT DESIGN HANDBOOK FORREFINERIES AND CHEMICAL PLANTS, BY FRANK L. EVANS, JR.(PAGE 42, FIG. 2.5) AND (PAGE 41, FIG. 2.4 AND FIG 2.3)

FIGURE 3.2

COURTESY OF SULZER GROUP, TECHNICAL REVIEW 2/1988.

FIGURES 3.3, 3.4, 3.7

COURTESY OF COMPRESSED AIR-GAS HANDBOOK, 5THEDITION BY COMPRESSED AIR-GAS INSTITUTE, (PAGE 143, FIG.2.49 AND FIG. 2.50) AND (PAGE 164, FIG. 2.64)

FIGURE 5.1 COURTESY OF A-C COMPRESSOR

FIGURE 5.2 COURTESY OF ATLAS COPCO

FIGURE 5.3 COURTESY OF DRESSER

FIGURE 5.4 COURTESY OF A-C COMPRESSOR

FIGURE 5.5 COURTESY OF SIEMENS ENERGY AUTOMATION, INC.

FIGURE 6.1 TYPICAL COMPRESSOR COVERAGE CHART


LIST OF SYMBOLS

ACFM ACTUAL CUBIC FEET PER MINUTE (TAKEN AT ANY LOCATION)

AGMA AMERICAN GEAR MANUFACTURER'S ASSOCIATION

API AMERICAN PETROLEUM INSTITUTE

ICFM INLET CUBIC FEET PER MINUTE (TAKEN AT COMPRESSOR INLETCONDITIONS)

FPS FEET PER SECOND

GHP GROSS HORSEPOWER (OFTEN SEEN AS GAS HORSPOWER)

HG MERCURY

PSIG LBS PER SQUARE INCH GAGE

Ps SUCTION PRESSURE

Pd DISCHARGE PRESSURE

SCFM STANDARD CUBIC FEET PER MINUTE (AT 60°F, 14.7 psia)

ANSI AMERICAN NATIONAL STANDARDS INSTITUTE

ASME AMERICAN SOCIETY OF MECHANICAL ENGINEERS

NEMA NATIONAL ELECTRICAL MANUFACTURERS ASSOCIATION

* FOR ADDITIONAL COMPRESSOR RELATED ABBREVIATIONS,SYMBOLS AND DEFINITIONS, REFER TO "COMPRESSED AIR AND GASDATA" BOOK, 3RD EDITION, 1982.


1.0 PURPOSE

The purpose of this design guide is to provide general information and to assistan equipment engineer to select appropriate equipment types based ondifferent operating parameters such as pressure requirements, gascompositions and mechanical seal requirements etc.

Scope of this design guide is limited to Rotary Positive Displacement blowersand compressors. Other compressor type machines shall be covered in otherdesign guides.

The guide primarily provides an overview of applications and constructionfeatures for different types of rotary blowers/compressors. It provides checklists to assist the equipment engineer for generating Technical Specifications,equipment data sheets and technical notes for Material Requisitions.

2.0 CODES, STANDARDS AND REFERENCE DOCUMENTS

• API Standard 619, Second Edition, May 1985.

- Rotary-Type Positive Displacement Compressors for GeneralRefinery Services.

• Standard Handbook for Mechanical Engineers by Baumeister & Marks,Seventh Edition.

• Compressed Air-Gas Handbook, 5th Edition, by Compressed Air-GasInstitute.

• Equipment Design Handbook for Refineries and Chemical Plants byFrank L. Evans, Jr.

3.0 GENERAL

Not too many years ago, it was common practice to use reciprocatingcompressors when high pressures were required. Dynamic type machineswere used only where larger volumes and lower pressures were involved.Dynamic compressors were usually called blowers when air/gas wascompressed to approximately 15-20 40 psig, and the term compressor was


applied to any such machines where air or gas was compressed to a finalpressure about 20 psig.

In recent years, this differentiation has become insignificant. Industry now usesthe term compressor for all type of machines compressing air or gas. Table 3.1shows the compressor family tree.

There are two basic types of compressors: dynamic type and positivedisplacement type. Dynamic compressors are classified as centrifugal, axial ormixed flow machines. Comparison of constant speed characteristics ofdifferent types of compressors is shown in Figure 3.1.

Positive-displacement type compressors are machines in which volumes of airor gas are confined within a closed space. The pressure is increased as thevolume of the closed space is decreased. Four general types of rotary positivedisplacement compressors are available. They are briefly discussed inSections 3.1, 3.2, 3.3, and 3.4 of this guide. Figure 3.2 shows capacity rangesof various compressor types. The centrifugal compressor is essentially avariable-capacity, constant pressure machine; the axial compressor and thepositive-displacement compressors are essentially constant-capacity, variable-pressure machines.

These basic characteristics, however, represent only part of the process ofselecting the type of compressor best suited to a specific application. Equallyimportant is the capacity range that can be built into a single machine. As ageneral rule, positive displacement machines are for small capacities,centrifugals are for medium capacities and axial are for large capacities permachine. As in all other general statements, however, it must be realized thatthere is considerable overlap of capacity range between these different types ofcompressors.

3.1 Rotary Screw Compressors (Figures 3.3, 3.4)

They are further classified as:

a. Oil-injected rotary screw compressorsb. Liquid-injected rotary single screw compressorsc. Oil-free rotary screw compressors.


Rotary screw compressor has relatively high efficiency at low specific speeds,providing small to moderate capacities at high heads. It provides thecharacteristics of reciprocating piston machinery without the problems of apulsating discharge flow, maintenance, space and vibration. It, however,requires four sets of shaft bearings and seals. It also supplements low specificspeeds centrifugal compressors with more favorable efficiency. These areconstant-volume variable-pressure machines.

3.2 Rotary Lobe Compressors (Figure 3.5)

These machines are used extensively for high vacuum and low compression.The slippage or internal recirculation varies directly as the square root of thechange in absolute pressure and inversely as the square root of the absolutetemperature and specific gravity. The rotor tip speed of heavy duty two-lobecompressors is limited to approximately 57 fps, light models range from 25 to30 fps.

3.3 Rotary-Sliding Vane Compressors (Figure 3.6)

The rotor runs eccentrically within the casing. Radial slots in the rotor carrysliding vanes which form a series of longitudinal cells. The cell volumediminishes as the rotor approaches the discharge chamber.

A single stage sliding vane compressor can produce a 28 inch Hg vacuum orcompresses to approximately pump 50 psig. A two stage unit can compress airto approximately 250 psig. This machine is normally furnished as totallyengineered skid mounted packaged unit. Installation is simplified, and itminimizes maintenance and vibration problems and does not require anextensive foundation. Generally, the unit is not suited for handling saturatedand super-saturated vapor, and is particularly inapt for cold jackets. Coldjackets increase the cylinder wall condensation. In some cases the bestsolution is a warm jacket and the use of a lubricant heavily loaded with asolvent resistant inhibitor such as rapeseed oil, lanolin or tallow. However,each application should be looked at on a case by case basis before selectingthis type of machine.

3.4 Rotary Liquid Ring Compressors (Figure 3.7)


In this type of compressor, the vanes, along with the action of centrifugal forcethrow the liquid sealant fluid in a cylindrical form within the casing forming aliquid ring. This liquid ring contains and compresses the gas flow between theentrance and the discharge port.

This compressor is used to handle highly saturated vapor, more corrosive andvolatile gases in chemical plants. It is indispensable for handling exothermicgases like chlorine, oxygen and acetylene. The units are more commonly usedas vacuum pumps with maximum absolute discharge pressure of 2 inch of Hgin single stage and 0.78 inch of Hg in two stages. Compressor efficiency isgenerally less than 50 percent.

Principles of operation, construction features and application of these machinesare discussed in Sections 4.0, 5.0 and 6.0 respectively.

Rotary screw compressors and rotary lobe compressors are specified in APIStandard 619, Second Edition, May 1985. All the "modifications", "additions","deletions", and "decisions" for specific paragraphs of API Standard 619, andall other design requirements for rotary screw and rotary lobe compressors,should be addressed in the Technical Specification.

4.0 PRINCIPLE OF OPERATION

Positive displacement compression occurs when successive volumes of gasare confined within a closed space and elevated to a higher pressure. Theincrease in pressure can come from either trapping the gas in an enclosure,then reducing the volume and thereby increasing the pressure, or the trappedgas may be carried in the enclosure without a change in volume to thedischarge opening, thus allowing the backflow from the discharge system to dothe compression.

Positive displacement rotary compressors types include screw, lobe, slidingvane, and liquid piston. Typical operating characteristics are:

TABLE 4

OPERATING CHARACTERISTICS

InletMaximumDischarge Adiabatic Operating Maximum


Capacity(ACFM)

Pressure(PSIG)

Efficiency(%)

Speed(RPM)

Horsepower(GHP)

Screw (Wet) 50-3,500 300 60-70 3600 400

Screw (Dry) 50-25,000 15-400 55-65 2500-5,000 800

Rotary Lobe 15-30,000 15-30 55-65 300-4,000 200

Sliding Vane 10-3,000 130 40-70 400-1,500 450

Liquid Ring 5-2,500 80-150 25-50 200-3,600 300

4.1 Screw Compressor (Helical or Spiral-Lobe)

This compressor is a rotary positive displacement machine in which twointermeshing rotors, each with a helical form, compress and displace the gas.

The basic element is the housing with its enclosed rotor assemblies. The lobesof the two rotors are not identical. The male, or driven, rotor has a form that fitsinto the pocket of the female, or gate, rotor. About 85 to 90 percent of thepower is used by the main rotor, the gate requiring only 10 to 15% of the totalpower at the most.

There are two types, usually considered "wet" or "dry" screws. The dry screwcompressor uses timing gears to properly phase the two rotors at all times.The rotors do not come into contact with each other and sealing is by closetolerances, so no lubrication is necessary. The wet screw compressor uses amist of oil through the machine to lubricate and seal and to cool thecompressed gas. In this style, the timing gears may be omitted and the secondrotor is driven from the first. Wet screws have capacities from 50 to 3500 CFM;dry screw capacities range from 50 to 25,000 CFM.

These units have internal compression. The built-in or design compressionratio is predetermined by the location of the opening edges of the dischargeport and the wrap angle of the lobes. There are no valves. The maximumefficiency is obtained when the discharge pressure corresponds to themaximum pressure developed within the compressor. If the dischargepressure is higher than the maximum pressure developed by the compressor,gas will flow back into the compressor momentarily when the rotor lobes openthe discharge port. If the discharge pressure is lower than the maximumpressure developed within the compressor, the compressed gas will expand


inefficiently into the discharge line when the discharge port is initially exposed.These two conditions decrease the efficiency of the compressor and createhigh frequency pressure waves in the discharge line resulting in excessivenoise.

The rotors may or may not have the same number of lobes. Usually the mainrotor has fewer than the gate and therefore operates at a higher speed.Designs vary in the helix angle and the contour of the lobes. Cross sectiondiagrams for single stage and double stage compressors are shown in figures3.3 and 3.44.

4.2 Rotary Lobe Compressor (Two Impeller Straight Lobe)

This compressor is a rotary positive displacement machine in which twostraight mating lobed impellers trap gas and carry it from intake to discharge.There is no internal compression.

A two-impeller straight lobe positive displacement compressor elementconsists of a casing containing duplicate symmetrical rotors or impellers usuallyhaving a figure eight cross section. Some have three lobes. These intermesh,are kept in phase by timing gears, and rotate in opposite directions. The term"cycloidal" occasionally is used for this type even though the impellers mayhave other than cycloidal form.

There is no compression or reduction of gas volume during the turning of therotors. The rotors merely move the gas from the inlet to the discharge.Compression is by backflow into the casing from the discharge line at the timethe discharge port is uncovered. Displacement of the compressed gas into thedischarge system then takes place. There are no valves. The operation canbe visualized from figure 3.5.

There is no contact between the impellers or between the impellers and thecasing. Sealing is by close clearances and lubrication is not required within thegas chamber. One impeller is driven directly while the other is driven throughphasing gears. Both impellers do the same amount of work.

4.3 Sliding Vane Compressor


This is a rotary positive displacement machine in which vanes slide radially in arotor eccentrically mounted in a cylindrical casing. The vanes are free to moveradially within slots and maintain contact with the cylindrical casing by thecentrifugal force generated as the rotor turns. Gas trapped between vanes iscompressed and displaced, (see figure 3.6). This compressor has a rathernarrow range of capacity and pressure because of inherent limits imposed byvane length, rubbing speed on the cylinder wall, and the bending forces actingon the vane when in an extended position.

The rotary sliding vane compressor has as its basic element the cylindricalcasing with its heads and rotor assembly. There are no valves; inlet anddischarge is determined by the ports over which the vanes pass. Gas entersthrough the inlet port and is trapped between vanes between the rotor and thecasing. The inlet port is normally wide and is designed to admit gas up to thepoint when the pocket between the two vanes is the largest. The pocket isclosed when the following vane of each pocket passes over the edge of theinlet port. As the rotor turns, vanes are pressed inward toward the rotor by theeccentrical casing, decreasing the volume and increasing the pressure of thegas. The compressed gas then exits the discharge port when the leading edgeof the vane passes over the port.

Due to the fixed location of the inlet and discharge ports in the compressioncycle, discharge pressure always occurs at the design point, regardless of thepressure of the receiver into which it is discharging. If the discharge pressure islower than the receiver, then gas will momentarily reverse flow into the pocketbefore the gas is discharged. If the discharge pressure is higher than thereceiver, gas will expand inefficiently into the receiver. Either of these two off-design conditions lowers the adiabatic efficiency and can contribute toexcessive noise. Flows range from 50 to 3,000 SCFM.

4.4 Liquid Ring (Liquid Piston) Compressor

This is a rotary positive displacement machine in which a ring of liquid (usuallywater) forms a variable volume chamber to compress and displace the gashandled, (see Figure 3.7).

A rotary liquid piston or liquid ring compressor uses an eccentrically mountedrotor with multiple fixed forward turned blades turning about a central cone (acharacteristic of Nash compressors) containing inlet and discharge ports, the


blades driving a captive ring of liquid around the inside of an elliptical or circularcasing. The basic element of the liquid piston compressor are the casing,heads, and rotor assembly.

Whirling liquid partially fills the casing and is trapped between adjacent blades.As the rotor turns, the liquid face follows the circumferential contour of thecasing and forces the liquid to enter and recede from the motor chambers.This creates, in effect, a liquid piston. Gas is admitted to the rotor through andremoved through ports within the ends of the casing. The liquid orients itself ina ring like manner as the rotor turns. The center of the ring forms a gas pocketaround the admitted gas, and as the rotor turns it carries the gas from themaximum clearance side to the minimum clearance. The ring seals off the inletport, the compressed gas is discharged. Porting in the central cone is built-inand fixed; there are no valves.

In those liquid ring compressors with elliptical casings, two eccentric sweepsusually are provided to form the elliptical casing. These are opposeddiametrically and thus balance out radial thrust loads. For every revolution, twocompression cycles are completed in each rotor chamber.

There is compression within the pockets or chambers between the bladesbefore the discharge port is uncovered. Since the port location must bedesigned and built for a specific compression ratio, the discharge will always beat the design point, regardless of the pressure of the receiver.

The cooling of a liquid ring compressor is direct rather than through the walls ofa casing. The required additional cooling liquid is fed into the casing where itcomes into direct contact with the gas being compressed. The excess liquid isdischarged with the gas.

The discharged mixture is passed through a conventional baffle or centrifugaltype separator to remove the free liquid. Because of the intimate contact of gasand liquid, the final discharge temperature can be held close to the temperatureof the inlet cooling water. However, the discharge gas is saturated at thedischarge temperature of the compressing liquid.

The amount of liquid that may be passed through the compressor is not criticaland can be varied to obtain the desired results. The unit can handle saturatedvapors, entrained liquid and occasional foreign matter. A unique characteristic


of this type of compressor, is that the unit will not be damaged if a large quantityof liquid enters its suction.

Lubrication is required only in the bearings which are generally located externalto the casing. The gas or air being compressed is therefore oil free. The liquiditself acts as a lubricant, sealing medium and coolant for the stuffing boxes.

5.0 CONSTRUCTION FEATURES

5.1 Oil Free Screw Compressor

5.1.1 Materials

5.1.1.1 The major components of an oil free rotary screw compressor are the case,male and female rotors, radial bearings, thrust bearings, and timing gear.Refer to figure 5-1 for a typical assembly view of the compressor. To determinespecific materials recommended for each major component, refer to APIStandard 619, Second Edition, May 1985, Appendix B.

5.1.1.2 The compressor case is generally a casting and can be made of ductilesteel. The compressor case can either be vertical or horizontal split dependingupon the manufacturer's design.

5.1.1.3 The male and female rotors are generally made from a forging, either madeof carbon steel, alloy steel, or stainless steel. For special application services,the rotor can be plated with special materials such as nickel.

5.1.1.4 The timing gears are generally made of chrome molybdenum steel and heattreated for added strength. The gears are pressed onto the rotors and doweledin place to obtain proper rotor-to-rotor clearance and timing gear backlash.Since the rotors do not touch the case or each other, there is no need forlubrication in the compression chamber. Lubrication is only required for thebearings and timing gears.

5.1.2 Bearings

5.1.2.1 The radial bearings are a sleeve type and are pressure lubricated to supportthe radial loads imposed upon the rotors. The bearings are located outboardthe compressor chamber to prevent oil entrainment into the process stream.


5.1.2.2 The thrust bearings are generally a tilting pad type design to carry the axialloads imposed by the compression gas loads and the thrust reaction of thesynchronizing gear teeth upon the rotors. A tapered-land or fixed wedge typethrust plate should be provided to reduce the reverse thrust loads during start-up or shutdown.

5.1.3 Mechanical Seals

5.1.3.1 The main types of mechanical seals available are mechanical contact type,restrictive-ring type, and labyrinth type. Each seal has its own uniqueapplication and should be reviewed carefully.

5.1.3.2 The restrictive-ring type seal is generally used for air and non-hazardous ornon-toxic gas applications within the maximum pressure limits of thecompressor. It is a close clearance type seal easily replaceable as a unit formaintenance service. The seal can be designed with options for eductorsystem or with a combination clean gas buffer and eductor system for use inhazardous or toxic gas services.

5.1.3.3 The labyrinth seal has the advantage of being able to handle particulatecontained in the process gas. It typically has a higher leakage rate than therestrictive-ring type seal. The seal can be designed with options for eductorsystem or with a combination clean gas buffer and eductor system for use inhazardous or toxic gas services.

5.1.3.4 The mechanical contact seal is used for applications where toxic orhazardous gases are not permitted to leak into the atmosphere or into thecompressor. A positive pressure oil film is provided between the shoulderwhich rotates with the shaft and the stationary pressure balanced face seal ring.A clean buffer gas can be added to fill the seal chamber and separate theleakage oil from the process gas. Dry running gas-buffered mechanical sealsare now available where no oil film is required to operate the seal. These typesof seals are considered special types and additional engineering is required foreach particular application.

5.2 Oil Flooded Screw Compressor

5.2.1 Materials


5.2.1.1 The oil flooded screw compressor is generally designed the same as an oilfree screw compressor with exception, there typically is not any timing gearsrequired to maintain tolerances between the male and female rotor. The oilflooded screw compressor typically has drive gears which compensate the axialthrust loads during the compression cycle. The oil provides a film and preventsany contact between the rotors. Refer to Figure 5-2 for a typical assembly viewof the compressor. To determine specific materials recommended for eachmajor component, refer to API Standard 619 Second Edition, May 1985,Appendix B.

5.2.1.2 The compressor case is generally a casting and can be made of ductile ironor cast iron.

5.2.1.3 The male and female rotors are generally made from a forging either madeof carbon steel or alloy steel.

5.2.2 Bearings

5.2.2.1 Each rotor and shaft is supported by antifriction type bearings located nearthe ends of the rotor body. The bearings at one end, usually the discharge end,take the rotor axial thrust and support the radial loads.


5.2.3.1 The typical mechanical seal supplied for an oil flooded screw compressor isa restrictive-ring type seal. This is a general purpose seal used for air or non-hazardous gas applications.

5.3 Lobe Compressor

5.3.1 Materials

5.3.1.1 The major components of a lobe compressor are the case, shaft, lobe,bearings, and timing gear. Refer to Figure 5-3 for a typical assembly view ofthe compressor. To determine specific materials recommended for each majorcomponent, refer to API Standard 618 Second Edition, May 1985, Appendix B.

5.3.1.2 The compressor case is generally a one piece cast construction withseparate head plates. The typical metallurgy of the compressor case with head


plates is ductile iron. The castings can either be air cooled or internallyjacketed and water cooled. Proper cooling of the air end is important to assuredimensional stability and to allow operation over a wide range of temperatures.

5.3.1.3 The compressor shafts are generally made form alloy steel forging.

5.3.1.4 The lobes are made from ductile iron, stainless steel, or coated carbon steel.The profiles have computer-generated shapes that optimize the compressionand minimize the discharge port lobes. The profile consists of either one or twolobes extending form a center hub section. Since the profile is a plane figure,no inherent axial thrust loads are created by the rotor as in rotary-screwdesigns. The compressor has a constant slippage or recirculation rate for afixed set of clearances, pressure, temperature and gas molecular weight.These clearances are located between the housing and facing, between theintermeshing rotor lobes, and between the rotor outside diameter and thecompressor housing.

5.3.1.5 The timing gears are generally made from a forging and manufactured toAGMA standards. Depending on the manufacturer, these gears can operateeither independent from the drive gears or perform a dual function or timing anddrive.


5.3.3.1 There are various type of seals which are available for use in compressors toprovide oil-free process systems. These include labyrinth seals, restrictive-ringtype seal, and mechanical contact seal.

5.3.3.2 To prevent lubrication from migrating along the rotor shaft and into the rotorchamber, the same principle can be applied as noted for the screw compressorwhere a buffer or inert gas can be injected into the seal.

5.4 SLIDING VANE COMPRESSOR

5.4.1 Materials

5.4.1.1 The major component of a sliding vane compressor are the cylinder, heads,rotor and shaft, blades, and bearings. Refer to figure 5-4 for a typical assemblyview of the compressor. To determine specific materials recommended for


each major component, refer to API Standard 619 Second Edition, May 1985,Appendix B.

5.4.1.2 The cylinder and heads are generally made from a casting either of ductileiron or gray iron materials.

5.4.1.3 The rotor and shaft assembly are generally machined from a single forging orbar stock made typically either of carbon steel or low alloy steel. The radialslots for the blades must be machined for the full length of the rotor.

5.4.1.4 The blades are made of a laminated cloth impregnated with a blendedphenolic resin. Each blade in heat treated and then machined tomanufacturer's tolerances to allow for thermal growth. After machining andheat treating, each blade is checked for warpage and then impregnated withhot oil to maintain manufacture's tolerances. The blades also are available in akevlar material impregnated with a phenolic resin composite.

5.4.2 Bearings

5.4.2.1 The rotor and shaft is generally supported at each end by anti-friction typebearings.


5.4.3.1 The typical mechanical seal applied for the sliding vane compressor is aliquid film single mechanical contact type. A seal ring is installed between thebearing and the compression chamber and oil is injected which acts as a bufferto minimize gas leakage to the bearings. Depending on the manufacturer, theseal faces can either be oil mist or force lubricated. A liquid film doublemechanical contact type seal is also available for applications requiring greatercontrol of toxic or hazardous gases.

5.5 Liquid Ring Compressor

5.5.1 Materials

5.5.1.1 The major components of a liquid ring compressor are the case, rotor, shaft,port piece(s), heads, and bearings. Refer to figure 5-5 for a typical assemblyview of the compressor. To determine specific materials recommended for


each major component, refer to API Standard 619 Second Edition, May 1985,Appendix B as a guide only as API Standard 619 does not include liquid ringcompressors.

5.5.1.2 Depending on the manufacturer and the particular size of the liquid ringcompressor, the case can either be cast or fabricated in carbon steel, low alloysteel, or stainless steel.

5.5.1.3 The shaft is generally machined from a single forging or bar stock madetypically either of carbon steel, low alloy steel, or stainless steel.

5.5.1.4 The rotor assembly is typically cast and is made of either of carbon steel, lowalloy steel, or stainless steel. The rotor is pressed fit onto the shaft and is notkeyed.

5.5.1.5 The port pieces and heads are cast and is made of either of carbon steel, lowalloy steel, or stainless steel.

5.5.2 Bearings

5.5.2.1 The rotor and shaft is generally supported at each end by anti-friction typebearings. The bearings can either be oil mist or grease lubricated dependingon the application requirements.


5.5.3.1 The typical seals supplied on the liquid ring compressor are single, double, ortandem acting mechanical seals. The seals can be designed with varioustypes of flush plans to detect the failure of a seal face.

6.0 APPLICATIONS

Rotary positive displacement compressors have a wide range of gascompression applications. These machines combine the benefits of positivedisplacement with simplified rotary motion mechanics. This Section willdiscuss the most common applications of the following types of compressors:

1. ROTARY SCREW (Wet)2. ROTARY SCREW (Dry)


3. ROTARY LOBE4. SLIDING VANE5. LIQUID RING

Variations of the above types of compressors, with custom designed features,may be desirable in some applications. However, this Section will discuss thestandard configurations available from established manufacturers.

6.1 Application Parameters

The typical operating range of rotary positive displacement compressors isshown in Figure 6-1, Typical Compressor Coverage Chart. This chart is basedon the normal range of operation of commercially available machines. Inaddition to this chart the following application parameters should beconsidered:

1. Inlet Capacity (5 - 30,000 cfm)2. Discharge Pressure (15 - 400 psig)3. Head (100 - 10,000 ft gas)4. Efficiency (25 - 70 %)5. Operating Speed (200 - 5,000 rpm)6. Horsepower (1 - 800 hp)

The limiting values (min/max) for rotary positive displacement compressors areindicated in parentheses. If the normal operating conditions fall outside theabove limits, rotary positive displacement compressors may not be suitable oravailable.

The capabilities of standard equipment based on these parameters is shown inTable 4.0, Operating Characteristics.

6.2 Selection Criteria

Many factors influence the selection of a particular type of rotary positivedisplacement compressor. These factors involve both process/operationcompatibility and economics. The following checklists may be a useful guide tobegin the selection process.

6.2.1 Process/Operation Compatibility Factors


1. Mass Flow Rate2. Suction Pressure/Temperature3. Discharge Pressure4. Gas Physical Properties5. Process Gas Interaction with Machinery (Corrosion, Erosion, Fouling)6. Machinery Interaction with Process Gas (Lubricants, Buffer Fluids,

Sealants)7. Upset or Off Design Operating Conditions8. Start-Up and Shut-Down Process Conditions9. Desirability of positive displacement characteristics in the system

application.

6.2.2 Economic Factors

1. Safety Standards2. Capital Costs vs Energy Costs3. Maintenance Costs vs Capital/Energy Costs4. Machine Reliability vs Replacement Availability vs spared units5. Space and Utility Requirements6. Spare Parts Availability7. Auxiliary and Accessory Equipment Requirements

6.2.3 These checklists should provide a useful guideline. Any checklist that isdeveloped should be continually updated in order to improve the decision-making selection process.

6.3 Advantages/Disadvantages

6.3.1 The general advantages and disadvantages of rotary positive displacementcompressors are listed below:

ADVANTAGES

1. Combines positive displacement characteristics with simplified rotarymotion mechanics.

2. Provides a broad "surge free" range of operation for gases with varyingmolecular weights. (Some types i.e. screw types are inefficient in lowmolecular-weight applications)


3. Less potential erosion damage due to moderate rotative tip speeds.4. Very suitable for direct drive applications.5. Smooth operation with no unbalanced forces and no special foundation

requirements.

DISADVANTAGES

1. Capacity limited to about 30,000 cfm.2. Compression ratio limited to about 6:1.3. Auxiliary skid-mounted accessory equipment may be required.4. Multistage (3 or more) configurations are not standard, but available.

A discussion of the relative advantages and disadvantages of each type ofcompressor follows.

6.3.2 Rotary Screw (Wet)

ADVANTAGES

1. Wide range of operating conditions.2. Wide range of applications.3. Excellent efficiency.4. High single stage compression ratios.5. High capacity-to-size ratio.6. Low parts wear due to lubricating design.

DISADVANTAGES

1. Noisy, may require noise hood and line silencers.2. Accessory oil separation equipment required.3. Four shaft seals required.

6.3.3 Rotary Screw (Dry)

ADVANTAGES

1. Gases are not contaminated with oil.2. Widest capacity-head range of application.3. Good efficiency.


4. Multistage configurations are available.5. Minimum amount of auxiliary and accessory equipment is required.

DISADVANTAGES

1.. Noisy, may require noise hood and line silencers.2. Four shaft seals and timing gears required.3. Efficiency and noise level are adversely affected if the machine and

process requirements are not precisely matched.4. Precision machining tolerances are required to obtain maximum

efficiency and avoid mechanical failure.

6.3.4 Rotary Lobe

ADVANTAGES

1. Simple construction features.2. High capacity range (up to 30,000 cfm)3. Excellent choice for air blower low pressure ratio applications.

DISADVANTAGES

1. Limited discharge pressure and compression ratio.2. Used primarily for air service with very limited process applications.3. Four shaft seals and timing gears required.

6.3.5 Sliding Vane

ADVANTAGES

1. Very simple construction features.2. Very reliable with high efficiency operation if properly matched with

process application.3. Very suitable for direct drive with common industrial drivers.4. Wide range of process applications.

DISADVANTAGES

1. Accessory oil separation equipment required.


2. Limited range of application with respect to capacity (up to 3000 cfm) anddischarge pressure (up to 150 psig for small units).

3. Sensitive to particulate materials in the gas resulting in excessive wearand/or potential jamming of the vanes.

6.3.6 Liquid Ring

ADVANTAGES

1. Very simple construction features.2. High reliability with relatively low maintenance requirements.3. Wide range of process applications.4. Liquid compressant can act as a coolant to control discharge

temperature.5. Smooth operation virtually free of measurable pulsation.6. Corrosive gases may be handled with standard materials.

DISADVANTAGES

1. Sealing liquid separation equipment required.2. Process gas / Sealing liquid compatibility is critical to application.3. Suction pressure is limited to vapor pressure of sealing liquid.4. Relatively low efficiency.5. Relatively low capacity range.

6.4 Typical Applications

Rotary positive displacement compressors have a wide range of industrialapplications from simple air blower service to handling corrosive anddangerous gases. Some of the more typical applications are listed below foreach type of compressor.

6.4.1 Rotary Screw

1. Petrochemical and Refinery Process Gases2. Refrigeration Packages3. Gas Gathering4. Landfill Gas5. Fuel Gas Boosting


6. Inert Gas Boosting7. Sewage Gas Compression8. Vapor Recovery

6.4.2 Rotary Lobe

1. Air Handling2. Vacuum Service3. Pneumatic Service

6.4.3 Sliding Vane

1. Some Process Gases2. Refrigeration Packages3. Gas Gathering4. Landfill Gas5. Sewage Gas Compression6. Vapor Recovery7. Blast Hole Drilling8. Vacuum Service

6.4.4 Liquid Ring

1. Corrosive/Explosive Gas Handling2. Fuel Gas Boosting3. Air Handling4. Digester Gas Circulation5. Furnace Flue Gas Compression6. Vapor Recovery

7.0 REFERENCES

7.1 BOOKS

1. Baumeister and Avallone MARKS' STANDARD HANDBOOK FORMECHANICAL ENGINEERS, 8th Ed., McGraw-Hill, New York, 1978.

2. Perry's CHEMICAL ENGINEERS' HANDBOOK, 6th Ed., McGraw-Hill,New York, 1984.


3.Loomis, A. W., COMPRESSED AIR AND GAS DATA, 3RD Ed. IngersollRand Company Washington, N.J. 1982.

7.2 TECHNICAL PAPERS AND ARTICLES

1. Example of Rotary Type Possitive Displacement Air CompressorSpecification with ten page Compressor Data Sheets (attached).

7.3 PROFESSIONAL ORGANIZATION PUBLICATIONS

1. API STANDARD 619, Rotary-Type Positive Displacement Compressorsfor General Refinery Services, 2nd Ed., May 1985, American PetroleumInstitute, Washington, D.C.

7.4 VENDOR DOCUMENTATION

1. A-C COMPRESSOR CORPORATION, COMPRESSOR CAPABILITIESCatalog.

2. DRESSER INDUSTRIES,INC., LeROI DIVISION, LeROI ROTARYSCREW GAS COMPRESSORS Catalog.


TABLE 3.1

COMPRESSOR FAMILY TREE

INTERMITTENT FLOW CONTINUOUS FLOW

POSITIVE DISPLACEMENT DYNAMIC TYPE

RECIPROCATING ROTARY TYPE

RADIAL AXIAL MIXEDFLOW FLOW

CYLINDER LUBRICATED NONLUBRICATED CENTRIFUGAL AXIAL MIXEDCYLINDER TYPE FLOW

CRANKCASE TYPE CROSSHEAD TYPE

CROSSHEAD DIAPHRAGMCYLINDER TYPE HYDRAULIC TYPE

CYLINDER LUBRICATED NONLUBRICATED CYLINDER LIQUID PISTON

OIL FLOODEDSINGLE SLIDING OIL FLOODED OIL FLOODED STRAIGHT LOBE HELICAL LOBE HELICAL LOBE SPIRAXIALECCENTRIC VANE HELICAL LOBE VANE ROOTS TYPE LYSHOLM 1.25 TO 2.0 1.5 TO 3.0LOBE AND 1.6 TO 4.5 Pd/Pg Pd/Pg

SLIDING VANE Pd/Pg1

125 PSIG SMALL PLANT 125 PSIG SMALL PLANTAIR AND CONSTRUCTION AIR AND CONSTRUCTION

AIR PORTABLE AIR PORTABLE


APPENDIX A

LIST OF PREFERRED VENDORS

Typical manufacturers include:

Roots Division of Dresser IndustriesGardner-DenverSutorbilt of CooperM-D Pneumatics of Tuthill Corp.SpencerIngersoll RandAtlas-CopcoA-C CompressorsGraham Manufacturing CompanyMycomCarrierSullairHowdenDresser RandKobelco (Turbomachinery Industries)Siemens Energy and AutomationNashSihi


APPENDIX B

Typical Engineering Specification:

Specification: CH04035-56-16-501Rotary Type Positive Displacement Air Compressor Package for Spent CausticTreatment System (oil flood screw compressor)

rotary compressors

Documents

winword3dgm5602

simple construction

typical assembly

general refinery

low alloy

require noise

construction

maximum pressure