1. SCOPE ……………………………………………………………………………….2. REFERENCES 3. DEFINITIONS 4. GENERAL …………………………………………………………………………..5. TYPES OF COMPRESSORS 5.1 Dynamic Compressors 5.2 Positive Displacement Compressors ………………………………….6. BASIC PRINCIPLES OF COMPRESSION 6.1 General 6.2 Centrifugal Compressors ………………………………………………..6.3 Reciprocating Compressors
7. SELECTION CONSIDERATIONS 7.1 Safety …………………………………………………………………………………7.2 Process 7.3 Site and Location 8. LUBRICATION SYSTEMS …………………………………………………….8.1 General 8.2 Centrifugal Compressors 8.3 Reciprocating Compressors …………………………………………………..8.4 Rotary Compressors 9. SEAL SYSTEMS 9.1 General ………………………………………………………………………..9.2 Dry Shaft Seals 9.3 Liquid (Oil) Film Seal 9.4 Packing Glands ……………………………………………………………..10. ANCILLARY EQUIPMENT
FIGURE
1 Types of Compressors 2 Typical Operating Envelopes of Compressors (Metric) ………………9 Pressure-Volume Diagram for Compression Processes 10 Typical Performance Map of a Centrifugal Compressor
11 Pressure-Volume Diagram for Reciprocating Compressor ……….12 Ancillary and Supporting Systems for Compressor
Applications
TABLE
I
Summary of Typical Operating Characteristics of
Compressors (Metric) II
Advantages and Disadvantages of Different Compressors ……….
APPENDIX A
Figure 3 Typical Single-Stage Compressor
Figure 4 Typical Multi-Stage Compressor
Figure 5 Typical Axial Compressor …………………………………………………..Figure 6 Typical Piston-Type Reciprocating Positive Displacement
Compressor Figure 7 Typical Diaphragm-Type Reciprocating Positive Displacement
Compressor Figure 8 Typical Flooded Screw-Type Rotary Compressor ………………..
1. Scope
This standard provides guidelines for selecting compressors used in process services.
2. References
Reference is made in this standard to the following documents. The latest issues, amendments, and
supplements to these documents shall apply unless otherwise indicated.
SABIC Engineering Standards (SES)
G03-S01 Lubrication, Shaft Sealing, Control Oil System and Auxiliaries
G04-G01 Guidelines for Centrifugal and Rotary Compressors – Fugitive Emissions
G04-S01 Centrifugal Compressors for Process Services
G04-S02 Packaged, Integrally Geared, Centrifugal Air Compressors
G06-S01 General Purpose Steam Turbines
G07-S02 Special Purpose Gear Units
G08-S01 Rotary Type Positive Displacement Compressors
G12-S01 Special Purpose Couplings
G13-S01 Reciprocating Compressors for Process Services
G19-S01 Machinery Protection Systems
3. Definitions
For the purpose of understanding this standard, the following definitions apply.
Actual Volumetric Flow Rate.
The volume throughput per unit time at the compressor inlet.
Adiabatic Compression.
Compression process where no heat transfer takes place. The process may be
irreversible.
Aftercooling.
Removal of heat from the gas after the final stage of compression.
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 by the piston of reciprocating compressors or by
vanes, screws and lobes of rotary compressors. When used to indicate size or rating, displacement shall
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 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.
Polytropic Compression.
Reversible compression process which follows a path, such that the ratio of
the reversible work input to the enthalpy rise is constant between any two points on the path.
Polytropic Efficiency.
Ratio of the polytropic work to the actual work required for the compression
process.
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.
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.
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.
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
4.1 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 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 shall
represent the best available configuration to meet a prescribed set of requirements.
4.2 Selection requires matching compressor capabilities to process requirements. Mechanical integrity
and process safety are important considerations. The process requirements and criteria shall be
established prior to selecting a compressor system. The compressor system includes lubrication and seal
systems and ancillary equipment, for example drivers, couplings, gearboxes and control systems. The
components of the compressor system determine its capacity, reliability and life-cycle costs. Life-cycle
costs include initial capital, operating and maintenance costs.
4.3 In general, field-proven equipment should be selected. However, where justifiable, newly developed
equipment may be selected with appropriate ongoing technical evaluation. 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. Table I and Figure 2 show typical operating characteristics. Table
II shows the general advantages and disadvantages of the different types of compressors.
Figure 1
Types of Compressors
Table
I
Summary of Typical Operating Characteristics of Compressors (Metric)
Figure 2
Typical Operating Envelopes of Compressors (Metric)
Table
II
Advantages and Disadvantages of Different Compressors
5.1 Dynamic Compressors
5.1.1 General
Dynamic compresssors 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
5.1.2.1 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.
5.1.2.2 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 has to 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:
a. Design or materials of the compressor
b. Allowable temperatures in the process
c. Fouling
d. Polymerization of the process
5.1.2.3 A seal system contains the process gas within the compressor and prevents 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 Appendix A, Figure 3 and 4 respectively. These compressors can be
designed to be non-lubricating if the process gas is required to be oil-free.
5.1.2.4 See SES G04-G01, G04-S01 and G04-S02 for additional information.
5.1.3 Axial Compressors
5.1.3.1 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.
5.1.3.2 Axial compressors require more stages to develop the same pressure rise as centrifugal
compressors, but have a very high efficiency. Intercooling is difficult in axial compressors, and attainable
discharge pressures are usually limited by temperature and bearing span (which limits the number of
blade rows). A typical axial compressor cross-sectional view is shown in Appendix A, Figure 5.
5.2 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:
a. Piston reciprocating in a cylinder
b. Reciprocating flexing of a diaphragm
c. Eccentric rotation of a volume sealed by sliding vanes
d. Matched rotating lobes in a volume cavity
e. Matched male and female helical screws
f. Rotor operating in a sealing liquid in an eccentric casing
5.2.2 Reciprocating Positive Displacement Compressors
5.2.2.1 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. See SES G13-S01 for process type reciprocating compressors.
5.2.2.2 They require special process and piping design considerations due to the pulsating nature of the
flow. Other considerations include foundation design, compressor valve speed, piston rod loads,
intercooling and safety relief valves.
5.2.2.3 Figures 6 and 7 in Appendix A show typical piston-type and diaphragm-type compressors
respectively.
5.2.3 Rotary Compressors
Rotary compressors usually 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 rotary machines are specialized,
with limited applications. A cross-sectional view of typical flooded screw-type rotary compressor is shown
in Appendix A, Figure 8. See SES G08-S01 for process type rotary compressors.
6. Basic Principles of Compression
6.1 General
6.1.1 The basic principles underlying the compression of gases in centrifugal and reciprocating
compressors are discussed briefly below. These two types cover the majority of compressors currently in
operation.
6.1.2 The compression of gases is based upon the following:
a. Boyle’s Law
b. Charles’ Law
c. Avagadro’s Law
d. Ideal Gas Law
e. Bernoulli’s Equation
The thermodynamics of gas compression are derived from these laws. Few gases used in the process
industry follow ideal gas relationships. Therefore, real gas equations of state are used to characterize the
behavior of most process gases.
6.2 Centrifugal Compressors
6.2.1 In a centrifugal compressor, gas compression is most accurately modeled by a polytropic path
defined by the relationship:
PV power raise n= constant
Where:
P = Absolute Pressure
V = Specific Volume
n = Polytropic Exponent
The above relationship is depicted in a pressure-volume diagram shown in Figure 9. An adiabatic process
is also shown on this diagram.
Figure 9
Pressure-Volume Diagram for Compression Processes
6.2.2 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,
to keep its final discharge temperature below the maximum allowable.
6.2.3 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:
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 thermodynamic properties of gas mixtures.
The polytropic exponent is not a constant during the compression cycle, but 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 hp is related to the ratio k of specific heats Cp/Cv and the polytropic efficiency approximately as follows:
6.2.5 The shaft power is the sum of the gas power and frictional losses in bearings, seals and gearing (if
present).
6.2.6 Typically, the adiabatic process is suitable for defining the compression of gases that exhibit ideal
gas behavior, for example air. In all other cases, the polytropic process shall be used. It should be noted
that:
a. Adiabatic Head < Polytropic Head
b. Adiabatic Efficiency < Polytropic Efficiency
6.2.7 The performance of centrifugal compressors is usually represented by head-flow curves and
efficiency-flow curves. A typical example is shown in Figure 10. 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 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 with operating speed and gas composition (for example density and molecular
weight).
6.2.8 A gas being compressed in a centrifugal compressor approximately follows the Fan Law which
states that:
a. Volumetric flow rate is approximately proportional to the impeller rotating speed
b. Head is approximately proportional to speed squared
c. Shaft power is approximately proportional to speed cubed
The above relationships govern the behavior of centrifugal compressors as operating parameters change.
However, deviation from the Fan Law increases as the number of stages increases.
Figure 10
Typical Performance Map of a Centrifugal Compressor
6.3 Reciprocating Compressors
Compression in a reciprocating compressor can be depicted on a pressure volume diagram, as 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:
PVk = constant
Figure 11
Pressure-Volume Diagram for Reciprocating Compressor
The adiabatic process is also depicted in Figure 9.
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 have to be provided with discharge
relief valves to protect against exceeding the safety parameter. Compression stage ratios are typically 3:1.
Also reciprocating compressors do not have the surge and choke limitations which are associated with
centrifugal compressors.
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:
The volumetric efficiency Ev
is defined as the ratio of the gas handled at inlet conditions to the theoretical
volume displaced by the compressor, expressed as a percent.
c = Clearance Volume (see Figure 11)
L = Effects from, for examples internal leakage, gas friction and pressure drop through valves
(approximately 5 percent 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. The volumetric efficiency of a given cylinder configuration
(fixed diameter, stroke, speed, internal clearance volume) is not a constant parameter, decreases with
increasing pressure ratio across the cylinder. Additionally, volumetric efficiency decreases with increasing
internal cylinder clearance volume. Thus, the volumetric efficiency, and 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:
Note: Temperatures are in absolute units. The equipment manufacturer should be able to provide
estimates of predicted discharge temperature and shaft power for given operating conditions.
7. Selection Considerations
The following major areas shall be considered when establishing compressor system selection
requirements.
7.1 Safety
7.1.1 Safety attributes include:
a. Limiting gas properties, for example decomposition, flammability and toxicity. Normal
compressor operation shall not violate these limiting gas properties
b. Compatibility of process gas with materials of construction, for example H2S ethylene oxide with brass
c. Containment, collection and disposal of seal and vent gases
d. Over-pressure protection
7.1.2 Process and economic issues listed in 7.2 shall also be evaluated for safety implications.
7.2 Process
7.2.1 Key process variables which shall be considered include:
a. Mass flow rate
b. Suction pressure and temperature
c. Discharge pressure and temperature
d. Gas physical properties, for example composition, molecular weight, ratio of specific heats and
compressibility with copper, ande. Effects of process gas on the compressor system, for example corrosion, erosion, fouling,
chemical reaction, coking, polymerization, condensation and liquid removal
f. Machinery interaction with the process gas, for example lubricants, buffer fluids, seal media
g. Startup and shutdown process/mechanical conditions. These are sometimes very different from
normal operating conditions and may significantly influence the selection process
h. Preferred and acceptable methods of capacity controls
7.2.2 A normal operating point is usually specified in the selection process, which is the point at which the
process is expected to operate. It is common to expect variations in operating conditions, however, so it is
important to establish expected ranges of the above variables for normal operating, startup and shutdown
conditions.
7.2.3 Economic issues include:
a. Life-cycle cost, which is a trade-off between capital, operating cost and maintenance costs over
the life of the equipment
b. User and vendor capabilities and facilities for maintaining the equipment
c. Expected equipment reliability
d. Level of sparing dictated by production needs, for example whether the spares are warehoused
or installed
e. Standardization of equipment and lubricants
f. Project life expectancy
7.3 Site and Location
Depending on the application, other variables may also have to be considered. The relative importance of
these variables shall be established, to ensure that the best type and size of compressor is selected.
a. Available utilities
b. Accessibility to services and support
c. Environmental conditions
8. Lubrication Systems
8.1 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 shall be included. Flushing and cleaning of the lubrication system constitute major
commissioning activities. See SES G03-S01 for additional information.
8.2 Centrifugal Compressors
Lubrication oil is required for the journal and thrust bearings, and 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.
8.3 Reciprocating Compressors
Reciprocating compressor cylinders may be lubricated or non-lubricated. Lubricated reciprocating
compressor cylinders utilize forced feed mechanical lubricators. Cylinders shall 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 carry-over of oil from frame crankcase to cylinder, a distance piece is 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.
8.4 Rotary Compressors
8.4.1 Rotary compressors may be lubricated or non-lubricated. Flooded screw compressors use a
common oil system for lubricating the rotors and bearings, sealing clearances, removing heat of
compression. A large amount of oil is circulated through the compressor. This oil is in contact with process
gas and has to be separated from the gas prior to returning to the lubricating points and compressor inlet.
8.4.2 Vane compressors inject a minimal amount of oil to lubricate the sliding vanes in the cylinder.
8.4.3 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.
9. Seal Systems
9.1 General
9.1.1 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 for different services. If
seals require higher oil pressure 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.
9.1.2 If completely separate seal oil and lubrication systems are required, major items, for example
coolers, filters, reservoirs and pumps shall be furnished for each of the seal oil and lubrication systems. If
the gas handled by the compressor causes contamination, chemical reaction or any other deterioration of
oil, sealing systems shall completely isolate gas from lubrication.
9.1.3 See SES G03-S01 for additional information.
9.2 Dry Shaft Seals
9.2.1 Dry shaft seals are available in five general types, although a combination of two or more types is
sometimes used to achieve the required result:
a. Labyrinth. Labyrinth seals are used where high leakage rate can be managed or tolerated. They
have no pressure or speed limits and may incorporate buffer gas.
b. Bushing (carbon ring). 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.
c. Film-riding face seal. 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.
d. Contacting face and Circumferential seals. These seals are less commonly used in
compressors. They have limited shaft speed capability and shorter life.
9.3 Liquid (Oil) Film Seal
9.3.1 Liquid film seals are available in eight general types:
a. Labyrinth
b. Bushing (carbon ring)
c. Windback (reversed helical groove bushing)
d. Restricted bushing (trapped bushing)
e. Film-riding face seal
f. Contacting face seal
g. Circumferential seal
h. Lip seal
9.3.2 The liquid film seal or oil film seal is particularly applicable to high speed machines. The 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 shall be degassed before return to the oil
reservoir. Contaminated oil shall be reconditioned or discarded.
9.3.3 When handling hazardous, toxic, or emission-regulated gases, the seal shall also prevent gas
leakage to the atmosphere if the compressor trips due to seal oil system failure. Various devices within the
seal support system are available to ensure 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 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.
9.4 Packing Glands
Packing glands 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, for
example 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. The packing gland pressure shall be compatible with the flare header pressure.
10. Ancillary Equipment
10.1 Ancillary equipment and supporting systems that are part of the selection and operation of
compressors include the following:
a. Drivers
b. Couplings
c. Gearboxes
d. Cooling systems (inter-and aftercoolers)
e. Pulsation suppressors
f. Separators (process gas, seal and lube oil)
g. Control systems including anti-surge systems
h. Monitoring systems (performance and vibration)
i. Piping systems including safety valves
j. Foundation, grouting and mounting plates
k. Suction strainers and filters
l. Silencers
10.2 Discussion of the above topics is beyond the scope of this standard. All of them play a very important
role in the selection, design, installation and reliable operation of compression systems however, Figure 12
summarizes the important sub-elements of these items.
10.3 See SES G06-S01, G07-S02, and G19-S01 for details on ancillary/support equipments.
Ancillary and Supporting Systems for Compressor Applications
FIGURE 12
Figure 3 – Typical Single-Stage Compressor
Figure 4 – Typical Multi-Stage Compressor
Figure 5 – Typical Axial Compressor
Figure 6
Typical Piston-Type Reciprocating Positive Displacement Compressor
Figure 7
Typical Diaphragm-Type Reciprocating Positive Displacement Compressor
Figure 8
Typical Flooded Screw-Type Rotary Compressor
