Reec001

March 1999

Process Industry Practices Machinery

PIP REEC001 General Guidelines for Compressor Selection

PURPOSE AND USE OF PROCESS INDUSTRY PRACTICES In an effort to minimize the cost of process industry facilities, this Practice has been prepared from the technical requirements in the existing standards of major industrial users, contractors, or standards organizations. By harmonizing these technical requirements into a single set of Practices, administrative, application, and engineering costs to both the purchaser and the manufacturer should be reduced. While this Practice is expected to incorporate the majority of requirements of most users, individual applications may involve requirements that will be appended to and take precedence over this Practice. Determinations concerning fitness for purpose and particular matters or application of the Practice to particular project or engineering situations should not be made solely on information contained in these materials. The use of trade names from time to time should not be viewed as an expression of preference but rather recognized as normal usage in the trade. Other brands having the same specifications are equally correct and may be substituted for those named. All practices or guidelines are intended to be consistent with applicable laws and regulations including OSHA requirements. To the extent these practices or guidelines should conflict with OSHA or other applicable laws or regulations, such laws or regulations must be followed. Consult an appropriate professional before applying or acting on any material contained in or suggested by the Practice.

© Process Industry Practices (PIP), Construction Industry Institute, The University of Texas at Austin, 3208 Red River Street, Suite 300, Austin, Texas 78705. PIP member companies may copy this practice for their internal use.

Not printed with State funds

March 1999

Process Industry Practices Machinery PIP REEC001 General Guidelines for Compressor Selection Table of Contents 1. Introduction ..................................2

7.2 Process .............................................25

1.1 Purpose...............................................2

7.3 Site and Location ..............................26

1.2 Scope..................................................2

7. Lubrication and Seal Systems ..26

1. References....................................2

8.1 Lubrication Systems ..........................26

2.1 Process Industry Practices (PIP) .........2

8.2 Seal Systems ....................................27

2.2 Industry Codes and Standards.............2

8. Ancillary Equipment...................29

2.3 Other References................................3

2. Nomenclature and Definitions ....3 3. General..........................................5 4. Types of Compressors ................6 5.1 Dynamic Compressors ......................12 5.2 Positive Displacement Compressors .16

6. Basic Principles of Compression ..............................18 6.1 General .............................................18 6.2 Centrifugal Compressors...................19 6.3 Reciprocating Compressors...............23

6. Selection Considerations ..........25 7.1 Safety ...............................................25

Process Industry Practices

List of Tables 1a. Summary of Typical Operating Characteristics of Compressors (English)........................................... 7 1b. Summary of Typical Operating Characteristics of Compressors (Metric)............................................. 8 2.

Advantages and Disadvantages of Different Compressors.................... 11

List of Figures 1.

Types of Compressors...................... 6

2a. Typical Operating Envelopes of Compressors (English) ..................... 9 2b. Typical Operating Envelopes of Compressors (Metric) ..................... 10 3 . Typical Single-Stage Compressor... 13 4 . Typical Multi-Stage Compressor..... 14 5 . Typical Axial Compressor............... 15 6 . Typical Piston-Type Reciprocating Positive Displacement Compressor 16 7.

Typical Diaphragm-Type Reciprocating Positive Displacement Compressor.................................... 17

8.

Typical Flooded Screw-Type Rotary Compressor.................................... 18

9.

Pressure-Volume Diagram for Compression Processes ................. 19

10. Typical Performance Map of a Centrifugal Compressor.................. 22 11. Pressure-Volume Diagram for Reciprocating Compressor ............. 23 12. Ancillary and Supporting Systems for Compressor Applications ................ 30

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PIP REEC001 General Guidelines for Compressor Selection

1.

March 1999

Introduction

1.1

Purpose The purpose of this Practice is to provide an initial reference for engineers preparing to deal with compressor vendors, for process engineers, and for project managers.

1.2

Scope This Practice provides overall guidelines for selecting an appropriate compressor for a specific process service. This Practice is neither a procurement document nor a specification.

2.

References The following references are a set of applicable standards and documents that the reader of this Practice can use in selecting, specifying, purchasing, and testing compressors and their auxiliary components for a given application. Short titles will be used herein when appropriate.

2.1

2.2

Process Industry Practices (PIP) –

PIP REIE686 - Recommended Practices for Machinery Installation and Installation Design

–

PIP RESC001 - Specification for Integrally Geared Centrifugal Air Compressor Packages

–

PIP RESE003 - Application of General Purpose Non-Lubricated Flexible Couplings

Industry Codes and Standards American Petroleum Institute (API) –

API 11P - Packaged High Speed Separable Engine-Driven Reciprocating Gas Compressors

–

API 611 - General-Purpose Steam Turbines for Petroleum, Chemical, and Gas Industry Services

–

API 612 - Special-Purpose Steam Turbines for Petroleum, Chemical, and Gas Industry Services

–

API 613 - Special-Purpose Gear Units for Petroleum, Chemical, and Gas Industry Services

–

API 614 - Lubrication, Shaft-Sealing, and Control-Oil Systems for SpecialPurpose Applications

Page 2 of 30

–

API 617 - Centrifugal Compressors for Petroleum, Chemical, and Gas Service Industries

–

API 618 - Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services

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PIP REEC001 March 1999

General Guidelines for Compressor Selection

–

API 619 - Rotary-Type Positive Displacement for Petroleum, Chemical, and Gas Industry Services

–

API 670 - Vibration, Axial-Position, and Bearing-Temperature Monitoring Systems

–

API 671 - Special-Purpose Couplings for Refinery Service

–

API 672 - Packaged, Integrally Geared Centrifugal Air Compressors for Petroleum, Chemical, and Gas Industry Services

–

API 677 - General-Purpose Gear Units for Petroleum, Chemical, and Gas Industry Services

American Society of Mechanical Engineering (ASME) –

ASME B19.3 - Safety Standard for Compressors for Process Industries

–

ASME PTC-10 - Performance Test Codes - Compressors and Exhausters

Association of German Engineers (VDI) –

VDI 2045 - Acceptance and Performance Test of Dynamic and Positive Displacement Compressors

Gas Processors Suppliers Association – 2.3

3.

Engineering Data Book

Other References –

Compressors: Selection and Sizing, 2nd ed., Gulf Publishing Co., Royce N. Brown

–

Compressor Application Engineering, Volume 1, Compression Equipment, Gulf Publishing Co., Pierre Pichot

–

Compressor Application Engineering, Volume 2, Drivers for Rotating Equipment, Gulf Publishing Co., Pierre Pichot

–

Leak-Free Pumps & Compressors, Elsevier Advanced Technology, Gerhard Vetter

Nomenclature and Definitions Absolute Pressure: Pressure measured from absolute zero (i.e., from an absolute vacuum). Absolute pressure is equal to the algebraic sum of the atmospheric pressure and the gauge pressure. Absolute Temperature: Temperature above absolute zero Actual Volumetric Vlow Rate: The volume throughput per unit time at the compressor inlet

Adiabatic Compression: Compression process where no heat transfer takes place (process may be irreversible) Aftercooling: Removal of heat from the gas after the final stage of compression

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Altitude: Elevation above sea level Capacity: Volume rate of flow of gas compressed and delivered, expressed at inlet conditions Density: Mass of the gas per unit volume Discharge Pressure: Pressure at the discharge flange of the compressor Discharge Temperature: Gas temperature at the discharge flange of the compressor Displacement: Average volume displaced per unit time by the piston of reciprocating compressors or by vanes, screws and lobes of rotary compressors. When used to indicate size or rating, displacement must be related to a specified speed. For multistage machines, it refers to the first stage cylinder(s) only. Inlet (Suction) Pressure: Pressure at or near the inlet flange of the compressor. In an air compressor without inlet pipe or duct, absolute inlet pressure is equal to the atmospheric pressure. Inlet Temperature: Gas temperature at the inlet flange of the compressor Intercooling: Removal of heat from the gas between compressor sections Isentropic Compression: Reversible (constant entropy) compression process where no heat transfer takes place Isentropic Efficiency: Ratio of the isentropic work to the actual work required for the compression process Isentropic Head: Work required to compress a unit mass of gas in an isentropic process from the inlet pressure and temperature to the discharge pressure Isentropic Power: Power required to isentropically compress and deliver the volume rate of flow of gas represented by the capacity from the temperature and pressure at the compressor 2 inlet to the discharge pressure of the compressor with no friction or leakage and with inlet and discharge pressure constant. For a multi-stage compressor, the isentropic power is the sum of the isentropic power of each of the stages. Kinetic Energy (of a Mass): Total work which must be done to bring a mass from a state of rest to a velocity, v. For rectilinear motion, the kinetic energy equals ½ mv . Molecular Weight: Relative weight of a molecule of a substance referred to that of an atom of Carbon-12 as 12.000 Polytropic Compression: Reversible compression process which follows a path such that between any two points on the path, the ratio of the reversible work input to the enthalpy rise is constant Polytropic Efficiency: Ratio of the polytropic work to the actual work required for the compression process

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General Guidelines for Compressor Selection

Polytropic Head: Reversible work required to compress a unit mass of the gas in a polytropic compression process Pressure (Compression) Ratio: Ratio of absolute discharge pressure to the absolute inlet pressure Relative Humidity: Ratio of actual vapor pressure to the pressure of saturated vapor at the prevailing dry-bulb temperature Section (of a Centrifugal Compressor): Group of stages (impellers and associated stationary parts) in series. In other words, the term “section” defines all the compression components between inlet and discharge flanges. A compressor case (body) may contain multiple sections if the gas is removed for cooling and returned for further compression. Shaft Power: Measured power input to the compressor. The term “shaft power” is expected to replace the more commonly used term “brake horsepower.” Specific Gravity: Ratio of the density of the gas at a standard pressure and temperature to the density of dry air at the same pressure and temperature and a molecular weight of 28.970 Specific Volume: Volume occupied by a unit mass of the gas. Stage of Compression (in Centrifugal Compressors): Refers to a single impeller with the associated stationary parts. For axial machines, each set of blades (one rotating row followed by one stationary row) represents a stage of compression. In reciprocating compressors, a cylinder, piston, and associated valves and parts comprise a stage. Compressor stages for other types of compressors can be defined in a similar manner. Standard Conditions: Several definitions depending on the particular engineering society or industry defining it. It is important to define clearly the “standard” that is being used – be it in SI units or English units. Standard Volumetric Flow Rate: Volume throughput per unit time where the pressure and temperature for defining the volume are “standard” values Stonewall (Choke) Point: Occurs when flow in a centrifugal compressor cannot be increased further. In this condition, sonic flow is approached in some part of the gas path within the compressor. Surge Point: Minimum flow point in a centrifugal or axial compressor. When flow is reduced below this point, cyclic variation (and even reversal) of gas flow and discharge pressure occurs. Volumetric Efficiency: Ratio of the capacity of the compressor cylinder to the displacement of the cylinder

4.

General Compressors are the prime movers of gas and air in process industries. They are used to

increase the static pressure of the gas and deliver it at the specified pressure and flow rate in a

Process Industry Practices

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PIP REEC001 General Guidelines for Compressor Selection

March 1999

process application. Part of the increase in static pressure is required to overcome frictional resistance in the process. Compressors are available in a variety of types, models and sizes, each of which fulfills a given need. The selection should represent the best available configuration to meet a prescribed set of requirements. Selection requires matching compressor capabilities to process requirements. Mechanical integrity and process safety are very important considerations. The process requirements and criteria must be established prior to selecting a compressor system. The compressor system includes lubrication and seal systems and ancillary equipment such as drivers, couplings, gearboxes and control systems. All components of the compressor system determine its capacity, reliability and life-cycle costs. Life-cycle costs include initial capital, operating and maintenance costs. In general, field-proven equipment should be selected. However, where justifiable, newly developed equipment may be selected with appropriate technical scrutiny and follow-up. In some cases, more than one type or size of compressor may initially appear suitable for a given application. In that case, a more detailed analysis may be justified to make a final, appropriate selection.

5.

Types of Compressors The two basic categories of compressors are dynamic (centrifugal and axial) and positive displacement (reciprocating and rotary types). Figure 1 shows the various types of compressors that fall into these two categories. Tables 1a and 1b and Figures 2a and 2b show Compressors typical operating characteristics. Table 2 shows the general advantages and disadvantages of the different types of compressors.

Positive Displacement

Dynamic

Centrifugal

Reciprocating

Rotary

Axial Piston

Screw (Wet/Dry)

Diaphragm

Lobe (Straight/Helical)

Thermal/Jet Liquid Ring Sliding Vane

Figure 1. Types of Compressors

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Table 1a. Summary of Typical Operating Characteristics of Compressors (English)

Dynamic Compressors Centrifugal Axial Positive

Inlet

Maximum

Capacity

Discharge

Efficiency

(acfm)

Pressure

(%)

Operating

Maximum

Speed

Power

(rpm)

(HP)

Application

(psig) 100 -

10,000

70 - 85

1,800 -

50,000+

Process gas & air

200,000 30,000 -

250

85 - 90+

50,000 2,000 -

100,000

Mainly air

Air & process gas

500,000

10,000

Displacement Compressors Reciprocating (Piston)

10 - 20,000

60,000

80 - 90

200 - 900

20,000

Diaphragm

0.5 - 150

20,000

60 - 70

300 - 500

200

Corrosive & hazardous process

Rotary Screw (Wet)

50 - 7,000

350

65 - 70

1,500 - 3,600

2000

gas Air, refrigeration & process gas

Rotary Screw (Dry)

300 - 25,000

15 - 600

55 - 65

1,000 -

7000

Air & dirty process gas

Rotary Lobe

15 - 30,000

5 - 25

55 - 65

15,000 300 - 4,000

500

Pneumatic conveying, process

Sliding Vane

10 - 3,000

150

40 - 70

400 - 1,800

450

gas & vacuum Vacuum service & process gas

Liquid Ring

5 - 10,000

80 - 150

25 - 50

200 - 3,600

400

Vacuum service & corrosive process gas

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PIP REEC001 General Guidelines for Compressor Selection

Table 1b. Summary of Typical Operating Characteristics of Compressors (Metric) 3

Maximum Inlet Capacity

Discharge

(m /h)

Pressure

Dynamic Compressors Centrifugal

Adiabatic

Operating

Maximum

Efficiency

Speed

Power

(%)

(rpm)

(MW)

Application

(bar) 170 - 340,000

690

70 - 80

1,800 - 50,000

38

Process gas & air

50,000 -

14

85 - 90+

2,000 - 10,000

75

Mainly air

20 - 34,000

4,150

80 - 90

200 - 900

15

Air & process gas

0 - 250

1,400

60 - 70

300 - 500

0.15

Corrosive & hazardous process

Rotary Screw (Wet)

100 - 12,000

24

65 - 70

1,500 - 3,600

0.15

gas Air, refrigeration & process gas

Rotary Screw (Dry)

500 - 42,000

1 – 41

55 - 65

1,000 - 15,000

5

Rotary Lobe

25 - 50,000

0.3 - 1.7

55 - 65

300 - 4,000

0.4

Pneumatic conveying, process

Sliding Vane

15 - 5,000

10

40 - 70

400 - 1,800

0.35

gas & vacuum Vacuum service & process gas

Liquid Ring

10 - 17,000

5.5 - 10.5

25 - 50

200 - 3,600

0.3

Vacuum service & corrosive

Axial Positive

850,000

Displacement Compressors Reciprocating (Piston) Diaphragm

Air & dirty process gas

process gas

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General Guidelines for Compressor Selection

100000 Reciprocating Compressor

10000 Centrifugal Compressor

Rotary Screw

1000

Compressor (Dry)

100 Liquid Ring Compressor

10

Rotary Lobe Compressor

1

Rotary Screw Compressor (Wet)

0

0

1

10

100

1000

10000

100000

1000000

I n l e t

pacity (ACFM)

Figure C 2a. Typical Operating Envelopes of Compressors (English)

Axial Compressor

Compressor

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Sliding Vane

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Diaphragm Compressor

Discharge Pressure (psi)

a

March 1999

PIP REEC001 General Guidelines for Compressor Selection

100000

10000

1000

Liquid Ring Page 10 of 30

Compressor

100

10

3

1

0

pressor

Rotary Lobe Compressor Rotary Screw

Compressor

1

10

100 1000 10000 C e Inlet Capacity (m /h) n t r i f Figure 2b. Typical Operating Envelopes of Compressors (Metric) u g a l C o m

Axial Compressor

Compressor (Dry)

Compressor (Wet)

Sliding Vane

0

Rotary Screw

C o m p r e s s o r

Diaphragm Compressor

Discharge Pressure (bar)

R e c i p r o c a t i n g

100000

1000000

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PIP REEC001 March 1999

General Guidelines for Compressor Selection

Table 2. Advantages and Disadvantages of Different Compressors

Advantages

Disadvantages

Dynamic Compressors Centrifugal

Wide operating range

Instability at reduced flow

High reliability and low maintenance Axial

High capacity for a given size and high efficiency

Sensitive to changes in gas composition Susceptible to rotor-dynamics problems Low compression ratios

Thermal/Jet

Heavy duty and low maintenance No moving parts and low maintenance

Limited turndown Very low efficiency

High pressure ratio

Narrow range of application

Reciprocating Compressors

Wide pressure ratios

(Piston)

High efficiency

Heavy foundations required due to unbalanced forces Flow pulsation can cause vibration and structural problems

Diaphragm

Very high pressure

High maintenance compared to dynamic compressors Limited capacity range

Available in special materials

Periodic replacement of diaphragms required Flow pulsation problems Noisy

Lobe

No moving seals Wide range of applications Low flow Wet screw has high efficiency and high pressure ratio Dry screw insensitive to changes in gas composition and can handle dirty gases Simple in design and construction

Sliding Vane

Low cost Simple in design

Capacity control limited to suction throttling Generally unsuitable for process gases

Liquid Ring

High single-stage pressure ratio High vacuum capability

Low reliability Sealing liquid/process gas compatibility required Sealing liquid separation equipment required

Positive Displacement

Screw

High single-stage pressure ratio High reliability

Wet screw not suitable for corrosive or dirty gases Limited operating range and pressure ratio

Limited suction pressure

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PIP REEC001 General Guidelines for Compressor Selection

5.1

March 1999

Dynamic Compressors 5.1.1 General Dynamic compressors function by imparting velocity to a gas, and subsequently converting the kinetic energy of the moving gas to an increase in static pressure. Two primary types of dynamic compressors are centrifugal and axial. A combination of the two types may be used to satisfy a given set of process requirements. 5.1.2 Centrifugal Compressors Centrifugal compressors impart velocity to the gas through rotating impellers. The gas is introduced at the eye of the impeller and discharged radially at the outer circumference (impeller tip) at a higher velocity and kinetic energy. The gas then passes through a stationary diffuser where its velocity is reduced, and its kinetic energy is converted to static pressure. Part of the static pressure rise occurs in the impeller and part in the diffuser. Impeller diameter and width, rotational speed and the angle of the vanes that make up the impeller are important parameters in the design of centrifugal compressors. One or more impellers (stages) may be required to obtain the desired discharge pressure. The temperature of the gas increases as it is being compressed. If the discharge temperature is likely to increase beyond the maximum allowable temperature before the desired discharge pressure is achieved, the gas must be cooled before it can be compressed further. Heat exchangers, called intercoolers, may be required to cool the gas. Typically, these intercoolers are installed between compressor sections (one or more stages typically make up a compressor section). Maximum allowable temperature is determined by: 1 . Design or materials of the compressor 2 . Avoiding excessive temperatures in the process 3 . Fouling 4 . Polymerization of the process A seal system contains the process gas within the compressor as well as prevents any contamination of the gas by bearing lubricants. Cross-sectional views of a typical single-stage compressor and a typical multi-stage compressor are shown in Figures 3 and 4 respectively. These compressors can be designed to be non-lubricating if the process gas is required to be oil-free.

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General Guidelines for Compressor Selection

Figure 3. Typical Single-Stage Compressor

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Figure 4. Typical Multi-Stage Compressor

5.1.3

Axial Compressors Gas moves axially along the compressor shaft (parallel to the machine axis) through alternating rows of rotating and stationary blades. Each set of blades (one rotating row followed by one stationary row) represents a stage of compression. The rotating blades impart velocity to the gas; the stationary blades slow the gas and direct it into the next row of rotating blades. As the gas passes through each stage, its pressure and temperature are further increased until it finally exits the compressor at the required pressure and associated temperature. Axial compressors require more stages to develop the same pressure rise as centrifugal compressors, but can handle very high flows at very high efficiency. Axial compressor design does not lend itself to intercooling, and attainable discharge pressures are generally limited by temperature and bearing span (which limits the number of blade rows). A typical axial compressor cross-sectional view is shown in Figure 5.

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General Guidelines for Compressor Selection

Figure 5. Typical Axial Compressor

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PIP REEC001 General Guidelines for Compressor Selection

5.2

March 1999

Positive Displacement Compressors 5.2.1 General Positive displacement compressors include reciprocating (piston and diaphragm) and rotary (screw, lobe, sliding vane and liquid ring) compressors. This category of compressor functions by enclosing an initial volume of gas and reducing that volume mechanically thereby increasing its pressure. Volume reduction is accomplished by one of the following: Piston reciprocating in a cylinder Reciprocating flexing of a diaphragm Eccentric rotation of a volume sealed by sliding vanes Matched rotating lobes in a volume cavity Matched male and female helical screws Rotor operating in a sealing liquid in an eccentric casing 5.2.2 Reciprocating Positive Displacement Reciprocating positive displacement compressors have a piston in a cylinder or a diaphragm operating in a shaped cavity to compress the gas. These compressors have suction and discharge valves that control the gas flow. They require special process and piping design considerations because of the pulsating nature of the flow. Other considerations may include foundation design, compressor valve speed, piston rod loads, intercooling and safety relief valves. Figure 6 shows a piston-type compressor and Figure 7 shows a diaphragm-type compressor.

Figure 6. Typical Piston-Type Reciprocating Positive Displacement Compressor

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General Guidelines for Compressor Selection

Figure 7. Typical Diaphragm-Type Reciprocating Positive Displacement Compressor

5.2.3 Rotary Rotary compressors generally have no suction or discharge valves, and use suction and discharge ports that are alternately exposed and covered by the rotating elements or sliding elements. Screw, rotary lobe, liquid-ring and sliding vane-type compressors fall into this category. Most of the rotary machines are specialized with limited applications. A typical flooded screwtype rotary compressor, cross-sectional view is shown in Figure 8.

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PIP REEC001 General Guidelines for Compressor Selection

Figure 8. Typical Flooded Screw-Type Rotary Compressor

6.

Basic Principles of Compression

6.1

General

March 1999

The basic principles underlying the compression of gases in centrifugal and reciprocating compressors are discussed briefly below. These two types cover the

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General Guidelines for Compressor Selection

majority of compressors currently in operation. Discussion about other types of compressors can be found in the references listed in Section 2. In general, the compression of gases is based upon the following: Boyle’s Law Charles’ Law Avagadro’s Law Ideal Gas Law Bernoulli’s Equation The thermodynamics of gas compression is derived from these laws. Few gases used in the process industry, such as air, follow ideal gas relationships. Therefore, real gas equations of state are used to characterize the behavior of most process gases. Centrifugal Compressors In a centrifugal compressor, gas compression is most accurately modeled by a polytropic path defined by the relationship: n

PV = constant Where: P

= Absolute Pressure

V = Specific Volume n

= Polytropic Exponent

The above relationship is depicted on a pressure-volume diagram shown in Figure 9. An adiabatic process is also shown on this diagram. Absolute Pressure, P

6.2

Discharge

Polytropic Path

Adiabatic Path Suction

Specific Volume, V

Figure 9. Pressure-Volume Diagram for Compression Processes

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The compression of gas from suction to discharge conditions may be accomplished using a single impeller or multiple impellers depending upon the pressure rise required, the need to improve efficiency and to limit pressure and temperature differentials. In either case, the polytropic curve in the Figure represents the pressure-volume path that the gas follows as it flows through the compressor. A section of a compressor may consist of multiple impellers. As the gas is compressed, its temperature increases. The gas may have to be cooled (using gas intercoolers) between compressor stages or compressor sections in order to keep its final discharge temperature below the maximum allowable temperature. The efficiency of compression depends on the aerodynamic design and operating condition of the impellers and the stationary gas passages and elements. This efficiency can be defined as follows: Reversible Work Input Polytropic Efficiency, h p =

Enthalpy Rise

The enthalpy rise of a gas can be obtained from a Mollier chart of the gas being compressed if it is a single component. Frequently, however, process gases are a mixture of several hydrocarbon gas elements. Computer programs are available to calculate required thermodynamic properties of gas mixtures. The polytropic exponent is not a constant during the compression cycle. However, for most practical applications, it can be assumed to be a constant. Therefore, along a polytropic path, the ratio of the reversible work input to the enthalpy rise is constant. The exponent n is related to the ratio k of specific heats Cp/Cv and the polytropic efficiency p approximately as follows: n -1

k -1

1

n

k

hp

The work input per unit mass is typically referred to as polytropic head and can be evaluated as follows: n 1

Polytropic Head, H

= p

Where:

Z avg R Ts

Pd

MW(n - 1) / n

Ps

n

- 1

Zavg = Average Compressibility Factor of the gas R = Universal Gas Constant (English: 1545 ft-lbf/lbm-°R; SI: 8314 J/kg-°K) MW = Gas Molecular Weight T = Absolute Gas Temperature P = Absolute Gas Pressure Note: Subscripts s and d refer to suction and discharge conditions

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General Guidelines for Compressor Selection

The gas power can be calculated using the following relationship: p Gas Power = C wH

hp Where: C = a constant (English: 1/33000; SI: 1/3600) depending on units used w = Mass Flow Rate The shaft power is the sum of the gas power and frictional losses in bearings, seals and gearing (if present). Typically, the adiabatic process is suitable to define the compression of gases such as air that exhibit ideal gas behavior. In all other cases, the polytropic process should be used. It is of interest to note that: Adiabatic Head < Polytropic Head Adiabatic Efficiency < Polytropic Efficiency The performance of centrifugal compressors is typically represented by “head-flow” curves and “efficiency-flow” curves. A typical example is shown in Figure 9. Some manufacturers also provide curves of pressure ratio and shaft power versus volumetric flow rate. The end points of the curves represent two important limits of centrifugal compressors – “surge” and “stonewall” or “choke point.” Surge is characterized by cyclic variation (and even reversal) of gas flow and discharge pressure, and it occurs when the flow is reduced below the surge point. It is usually accompanied by abnormal noise and vibration and it can lead to significant damage if the compressor’s operating condition is not changed quickly to increase the flow. A “stonewall” or “choke” condition is encountered when the gas flow reaches sonic conditions somewhere in the compressor passages. In this condition, flow through the compressor cannot be increased further. These performance curves vary typically with operating speed and gas composition (density, molecular weight, etc.). A gas being compressed in a centrifugal compressor approximately follows the Fan Law which states that: Volumetric flow rate is approximately proportional to the impeller rotating speed Head is approximately proportional to speed squared Shaft power is approximately proportional to speed cubed These relationships govern the behavior of centrifugal compressors as operating parameters change. However, deviation from the Fan Law increases as the number of stages increases.

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PIP REEC001 March 1999

Efficiency, %

General Guidelines for Compressor Selection

Polytropic Head

Volumetric Flow Rate, acfm

Increasing Speed

Surge Point

Choke Point

Shaft Power

Volumetric Flow Rate, acfm

Volumetric Flow Rate

Figure 10. Typical Performance Map of a Centrifugal Compressor

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6.3

General Guidelines for Compressor Selection

Reciprocating Compressors Compression in a reciprocating compressor can be depicted on a pressure volume diagram shown in Figure 11. For the most part, the process can be represented as an adiabatic process using the adiabatic exponent k. In this process: k

PV = constant

D

C

Pd

B

A Ps

Actual Capacity

Piston Displacement

Cylinder Volume, V

Figure 11. Pressure-Volume Diagram for Reciprocating Compressor

The adiabatic process was also depicted in the pressure-volume diagram shown in the previous section. Typically, reciprocating compressors do not have a performance curve like the one for centrifugal units. These compressors can deliver whatever pressure is required to overcome the discharge back pressure unless some material or design limit is reached. Therefore, these machines must be provided with discharge relief valves to protect

against exceeding safety parameter. Compression stage ratios are typically 3:1. Reciprocating compressors also do not have surge and choke limitations which are associated with centrifugal compressors.

Clearance Volume

Cylinder Pressure, P

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The work required for adiabatic compression is obtained in a manner similar to the polytropic process. However, the exponent n is replaced with the ratio of specific heats, k. Therefore: k-1

Adiabatic Head, H

= ad

Z avg R Ts

Pd

MW k 1 k

Ps

k

- 1

The volumetric efficiency E v is defined as the ratio of the gas handled at inlet conditions to the theoretical volume displaced by the compressor expressed as a percent. Actual Capacity Ev

=

Piston Displacement

=

100 - {(Zs/Zd) r

1/k

- 1}c - L

p

Where: rp = Pressure Ratio c = Clearance Volume (see Figure 11) L = Effects from internal leakage, gas friction, pressure drop through valves, etc. (approximately 5% for lubricated compressors) The volumetric efficiency defines the volume flow capability of a cylinder in terms of inlet volume flow rate under a specific set of operating conditions. It is important to recognize that the volumetric efficiency of a given cylinder configuration (fixed diameter, stroke, speed, internal clearance volume) is not a constant parameter. Volumetric efficiency decreases with increasing pressure ratio across the cylinder. Additionally, volumetric efficiency decreases with increasing internal cylinder clearance volume. Thus, the volumetric efficiency (hence, capacity) of a given cylinder configuration can be altered by varying the internal clearance within the cylinder control volume. This clearance can be varied through the use of fixed or variable volume clearance pockets which add internal clearance volume to the cylinder when manually or automatically actuated for capacity control. The theoretical discharge temperature is given by: k -1 Td = Ts Pd

k

1

Ps

Note: Temperatures are in absolute units. The equipment manufacturers should be

able to provide estimates of predicted discharge temperature and shaft power for given operating conditions.

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PIP REEC001

March 1999

7.

General Guidelines for Compressor Selection

Selection Considerations The following major areas should be considered when establishing compressor system selection requirements:

7.1

Safety Safety attributes include: Limiting gas properties (e.g., decomposition, flammability, toxicity) Note: Normal compressor operation should not violate these limiting gas properties. Compatibility of process gas with materials of construction (e.g., H2S with copper, ethylene oxide with brass, oxygen) Containment, collection and disposal of seal and vent gases Over-pressure protection Process and economic issues listed below should also be evaluated for safety implications

7.2

Process Key process variables which must be considered include: Mass flow rate Suction pressure and temperature Discharge pressure and temperature Gas physical properties (e.g., composition, molecular weight, ratio of specific heats, compressibility) Effects of process gas on the compressor system (e.g., corrosion, erosion, fouling, chemical reaction, coking, polymerization, condensation and liquid removal) Machinery interaction with the process gas (e.g., lubricants, buffer fluids, seal media) Startup and shutdown process/mechanical conditions Note: These conditions are sometimes very different than normal operating conditions and may significantly influence the selection process. Preferred and acceptable methods of capacity controls Typically, a normal operating point is specified in the selection process. This is the point at which the process is expected to operate. However, it is common to expect variations in operating conditions. It is, therefore, important to establish an expected range of the above variables during normal operating, startup and shutdown conditions.

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PIP REEC001 General Guidelines for Compressor Selection

March 1999

Economic issues include: Life-cycle cost (trade-off between capital, operating cost and maintenance costs over life of equipment) User and vendor capabilities and facilities for maintaining the equipment Expected equipment reliability Level of sparing dictated by production needs (e.g., warehoused or installed) Standardization of equipment and lubricants Project life expectancy 7.3

Site and Location Available utilities Accessibility to services and support Environmental conditions The above represent the most common, but important, attributes. Depending on the application, other variables may have to be considered as well. The relative importance of these variables should also be established to ensure that the best type and size of compressor is selected.

8.

Lubrication and Seal Systems

8.1

Lubrication Systems 8.1.1 General Compressors usually require oil for bearing lubrication and bearing cooling. Oil is supplied by the lubrication system. Components in a lubrication system include an oil reservoir or tank, lube oil pump or pumps, oil filters, oil coolers, and associated piping and instrumentation. The cleanliness of the lubrication system is critical to long term reliability of the compressor system. Lubrication systems should be constructed of stainless steel. If carbon steel components are used, then provision for cleaning and passivating should be included. Flushing and cleaning of the lubrication system constitute major commissioning activities. 8.1.2 Centrifugal Compressors Lubrication oil is required for the journal and thrust bearings. Lubrication oil is supplied from a reservoir by a lube oil pump. This pump may be compressor shaft-driven or independent. An independent main oil pump and an auxiliary oil pump are preferred for centrifugal compressors.

8.1.3 Reciprocating Compressors Reciprocating compressor cylinders may be lubricated or non-lubricated. Lubricated reciprocating compressor cylinders utilize forced feed mechanical

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General Guidelines for Compressor Selection

lubricators. Cylinders should be pre-lubricated prior to starting the compressor. If contamination of compressed gas by lubrication cannot be tolerated, non-lubricated cylinder designs are available that use piston rings and rod packing which do not require cylinder lubrication. To prevent carryover of oil from frame crankcase to cylinder, a distance piece should be used between the frame and cylinder. The compressor crankcase also contains the oil for the frame lubrication. A wiper packing is installed between the crankcase and the distance piece to wipe excess crankcase oil from the piston rod. 8.1.4 Rotary Compressors Rotary compressors may be lubricated or non-lubricated. Flooded screw compressors use common oil systems for lubricating the rotors, sealing clearances, removing heat of compression and lubricating bearings. A large amount of oil is circulated through the compressor. This oil is in contact with process gas and must be separated from the gas and returned to the lubricating points as well as to the compressor inlet. Vane compressors inject a minimum amount of oil to lubricate the sliding vanes in the cylinder. Straight lobe and dry screw compressors do not use liquid for sealing the rotor clearances and driving the non-coupled rotor. Lubrication is required for bearings and gears. The compressors are furnished with seals which separate the lubrication from the process. The size and complexity of the lubrication and seal oil systems vary considerably with compressor size, type and application. The rotor-to-rotor relationship is maintained by timing gears on each rotor. 8.2

Seal Systems 8.2.1 General Compressor seals are furnished to restrict or prevent process gas leaks to the atmosphere or seal fluid leaks into the process stream over a specified operating range, including start-up and shut-down conditions. Depending on the application, the seals may be of a dry or liquid lubricated type. In applications where leakage can be tolerated, a labyrinth seal may be used. Seals may require a buffer gas, seal oil, or both. Manufacturers provide a variety of seals and sealing systems to meet the intended service. If seals require higher pressure oil than required by bearings, seal oil requirements may be met by booster pumps or a separate system. Seal selection also depends on whether the shaft is rotating or reciprocating. If completely separate seal oil and lubrication systems are required, major items such as coolers, filters, reservoirs and pumps must be furnished for each of the seal oil and lubrication systems. If gas handled by compressor causes contamination, chemical reaction or any other deterioration of oil, sealing systems must completely isolate gas from lubrication.

Process Industry Practices

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PIP REEC001 General Guidelines for Compressor Selection

March 1999

8.2.2 Dry Shaft Seals Dry shaft seals are available in five general types: Labyrinth Bushing (Carbon Ring) Film-Riding Face Seal Contacting Face Seal Circumferential Seal Sometimes a combination of two or more types is used to achieve the required result. Labyrinth seals are used where high leakage rate can be managed or tolerated. Labyrinth seals have no pressure or speed limits and may incorporate buffer gas. Bushing seals function by inhibiting flow through a close clearance around the shaft. The leakage rate is relatively high. They may be used as a single bushing or a series of bushings and may incorporate buffer gas and vents. Film-riding face seals (also known as mechanical seals) can seal high pressures and allow high shaft speeds. They are used in single, tandem, or double arrangement and require clean process, flush, or buffer gas. Double seals with buffer gas pressure higher than process gas pressure prevent any leakage of process gas past the seal. Contacting face seals and circumferential seals are less commonly used in compressors. They have limited shaft speed capability and shorter life. 8.2.3 Liquid Film Seal (Oil Film Seal) Liquid film seals are available in eight general types: Labyrinth Bushing (Carbon Ring) Windback (Reversed Helical Groove Bushing) Restricted Bushing (Trapped Bushing) Film-Riding Face Seal Contacting Face Seal Circumferential Seal Lip Seal The liquid film seal or oil film seal is particularly applicable to high speed machines. The actual seal is accomplished by a thin oil film supplied by the seal oil pump to a space between the rotating and stationary seal elements. This oil contacts process gas and must be degassed before return to the oil reservoir. Contaminated oil must be reconditioned or discarded. When handling hazardous, toxic, or emission-regulated gases, the seal also must prevent gas leakage to the atmosphere after the compressor has tripped due to seal oil system failure. Various devices within the seal support system

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Process Industry Practices

PIP REEC001 March 1999

General Guidelines for Compressor Selection

are available to assure that the compressor seal contains the gas at a standstill, even if no seal oil is being pumped to the seal. Elevated seal oil tanks can provide for the necessary static differential pressure of the fluid above the sealing pressure for a sufficient time to allow the compressor to be depressurized before the elevated tank oil supply is depleted. 8.2.4 Packing Glands Packing glands are another type of sealing system and are used in reciprocating compressors to control gas leakage from the cylinders. The packing gland contains a series of segmented packing rings around the piston rod. A purge gas, such as nitrogen, may be used to provide a positive seal from the atmosphere and improve the venting of the process gas from the sealing area. The leakage is usually vented to a low pressure vent system or to a low pressure flare header. Care should be exercised to ensure that the packing gland pressure is compatible with the flare header pressure.

9.

Ancillary Equipment Ancillary equipment and supporting systems that are part of the selection and operation of compressors include the following: Drivers Couplings Gearboxes Cooling Systems (inter- and aftercoolers) Pulsation Suppressors Separators (process gas, seal and lube oil) Control Systems including Anti-Surge Systems Monitoring Systems (performance and vibration) Piping Systems including Safety Valves Foundation, Grouting and Mounting Plates Suction Strainers and Filters Silencers Discussion of the above topics is beyond the intended scope of this Practice. However, all of them play a very important role in the selection, design, installation and reliable operation of compression systems. Figure 12 summarizes the important sub-elements of these items. Details may be obtained through the practices and standards listed in Section 2.

Process Industry Practices

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March 1999

PIP REEC001 General Guidelines for Compressor Selection

Gear Box Driver

• Turbine • Motor

Lubrication System • Engine •• Bearing Lubrication/Cooling

Coupling

(If Required)

Coupling

Compressor

• Reducing/Increasing • Transmission • Clutch

• Dynamic

Bearings

• Positive Displacement ••Shaft Bearings ••(Journal, Pressure Dam,

• Lube Oil Pumps ••Oil Reservoir/Filtration

Monitoring System

• Lube Oil Pumps

• Vibration Condition

••Piping/Instrumentation

• Process Control

Cooling System

Seal System

• Driver Load

• Compressor Cylinder • Compressor Cylinder

••Seal ••Sealing Fluid Supply System •• Seal Leakage Control System ••

Atmospheric Drain System

• Acoustical Control • Acoustical Control

(For Positive Displacement

••Thrust Bearings

••Lube/Seal Oil

Operational and Control Systems ••Operating Limits (Surge, Choke)

SuctionVariance Throttling, Discharge Throttling) ••Capacity (Variable Speed, Inlet Guide Vanes,

Pulsation Suppressors

Tilt-Pad)

Suction Throttling, Discharge Throttling) ••Emergency Shutdown (Overspeed Trip, High Thrust,

High Vibration)

•• Interstage/Aftercooler ••Driver

Coupling ••Lubricated ••Non-Lubricated (Flexible

Element, Disc, Diaphragm) Compressors only) ••Special

Figure 12. Ancillary and Supporting Systems for Compressor Applications

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Process Industry Practices