1 Scope ………………………………………………………………………………………………………….
2 References …………………………………………………………………………………………………..
3 Definitions…………………………………………………………………………………………………..
4 General ……………………………………………………………………………………………………….
5 Process Considerations …………………………………………………………………………………
6 Compressor Casing Design Options ……………………………………………………………….
7 Shaft Seals …………………………………………………………………………………………………..
8 Comparison of Shaft Seals …………………………………………………………………………….
8.1 Labyrinth Seals ……………………………………………………………………………………
8.2 Carbon Ring Seals……………………………………………………………………………………
8.3 Liquid-Film / Bushing Seals ………………………………………………………………………
8.4 Circumferential Seals ………………………………………………………………………………..
8.5 Mechanical (Contact) Face Seals ………………………………………………………………..
8.6 Noncontacting Gas Face Seals ……………………………………………………………………
8.7 Summary ……………………………………………………………………………………………….
9 Maintenance Recommendations …………………………………………………………………..
FIGURE 1 – Bolting of Casing Split Surfaces ……………………………………………………….
FIGURE 2 – Typical Compressor Sealant Supply System ………………………………………
FIGURE 3 – Typical Compressor Seal Leak System………………………………………………
FIGURE 4 – Buffered Labyrinth Seal and Plain Labyrinth Seal …………………………..
FIGURE 5 – Carbon Ring Seal ……………………………………………………………………………
FIGURE 6 – Liquid-Film Seal with Cylindrical Bushing ………………………………………..
FIGURE 7 – Liquid-Film Seal with Pumping Bushing …………………………………………..
FIGURE 8 – Circumferential Seal with Segmented Carbon Rings…………………………..
FIGURE 9 – Single Mechanical (Contact) Seal ……………………………………………………..
FIGURE 10 – Double Mechanical (Contact) Seal ………………………………………………….
FIGURE 11 – Double-Face Mechanical (Contact) Seal ………………………………………….
FIGURE 12 – Noncontacting Gas Seal (Tandem Arrangement) …………………………..
TABLE I – Comparison of Compressor Seals and Support Systems for Controlling
Emissions …………………………………………………………………………………………………………
10 Revision History ………………………………………………………………………………………..
1 Scope
1.1 This standard provides recommendations for reducing fugitive emissions from rotary
motion compressors. It applies to centrifugal axial, and rotary screw designs.
1.2 The primary focus is on shaft seal selection and the expected environmental
performance of each seal option.
1.3 This standard discusses several machine design options, shaft-end seal designs, and
the characteristics of each.
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, and Control-oil Systems and Auxiliaries
G04-S01 Centrifugal Compressors for Process Services
G08-S01 Rotary Type Positive Displacement Compressors
Society of Tribologists and Lubrication Engineers (STLE)
Special Publication SP-32 – Guidelines for Meeting Emission Regulations for Compressors
with Advanced Sealing Systems
3 Definitions
For the purpose of understanding this standard, the following definitions apply.
Fugitive Emissions. All releases of gases, solids, or liquids not confined to a stack. Major
categories include equipment leaks, secondary emissions, windblown dust, and other
miscellaneous emissions from process sources.
Settling-out Pressure. Equilibrium pressure in the compressor system when the compressor
is shut down.
4 General
4.1 The potential sources of fugitive emissions are the shaft/casing interface and the
casing split surfaces of the compressor.
4.2 To achieve compressor installations with low emissions, the process, compressor
casing design options, and shaft-end seal designs shall be considered.
4.3 See SES G04-S01, G08-S01, and G03-S01 for details of compressor casing design
shaft end seal selection and auxiliary support system.
5 Process Considerations
To help determine the safety, health, and environmental impact of a process gas, information
about the gas and the compressor application shall be compiled. The factors relating to gas
properties and process conditions shall be considered. See 5.1 to 5.3.
5.1 Process Gas. The following information is required for a process gas:
a. Whether it is flammable, toxic, or carcinogenic.
b. Chemical composition.
c. Physical properties.
d. Effect on construction materials.
e. Required reliability – for critical or intermittent operation?
f. Requires nominal or absolute containment?
g. Tolerance for foreign fluids, for example buffer gas, oil vapors, water, water
vapor (steam), and condensate.
5.2 Pressure Conditions. Pressure shall be determined for the following cases:
a. Start-up.
b. Design operation.
c. Normal operation.
d. Shut down.
e. Settling-out.
f. Upset.
g. Maximum design – process relief valve setting plus the accumulation on the
compressor inlet side within the first block valve.
5.3 Controls, Alarms, and Interlocks. It shall be determined whether the controls, alarms,
and interlocks will be direct or closed loop.
6 Compressor Casing Design Options
6.1 The casing split surface interface is a potential fugitive emissions source from axial
split compressors. To ensure that axially split casings with a metal-to-metal joint,
using a suitable joint compound, will not leak, the surfaces shall be tightly clamped
with bolting to achieve a
1/3
overlap of the contact stress zone radius produced on the
casing split surface by the bolts (see Figure 1).
6.2 When the nature of the gas requires the highest degree of confidence in sealing, for
example flammable or toxic gases or high inlet pressures, radially split casings using
confined, controlled-compression gaskets or O-rings shall be selected.
6.3 Examples of other low emission designs include centrifugal machines with double
inlets, where the maximum gland pressure is also the inlet pressure; internal
discharge-to-suction pressure balances, which reduce gland pressure to the inlet
pressure; and gland scavenging eductors, which further reduce gland pressure on
the seal.
7 Shaft Seals
7.1 The most likely source of fugitive emissions is the point where the shaft penetrates
the casing. Shaft seals are necessary to restrict or prevent process gas leakage to
the atmosphere, or seal fluid leakage into the process gas stream, throughout the
entire range of operating conditions determined in 5.2. Seal operation shall be
suitable for variations in the compressor inlet conditions that may occur during startup,
shut
down,
or
settling-out.
7.2 When consideration of the process and casing design is complete, the shaft seal
system shall be selected. The selection process depends on the operating conditions
determined in 5.2, the economics, and the various seal types and arrangements that
the compressor design can incorporate. Where buffer gas or liquid sealants are
involved, the seals can perform no better than the associated sealant system (see
Figures 2 and 3).
7.3 A seal selection shall not be made until all aspects of the process conditions, sealant
system, and compressor design are examined. The following factors shall be
considered during the seal selection process:
a. Operating conditions:
(i) Gas properties
(ii) Pressure
(iii) Temperature
(iv) Compressor rotational speed
b. Initial, operating, and maintenance costs of seal options.
c. Sealing redundancy.
d. Reliability.
e. Failure mode.
f. Shut down sealing requirements.
g. Support system requirements:
(i) Buffer fluid, see Figure 2 for sealant supply system:
Type – gas, liquid
Compatibility with process gas
Cost
Availability (alternate power supply, system monitoring to warn of
impending failure)
Backup (auxiliaries, spares, sealant rundown tank)
Quality
Pressure and temperature
Dew point
Cleanliness
Alternative/emergency supply
(ii) Disposal of liquid sealants, see Figure 3 for seal leak system:
Into process
Reduced to atmospheric pressure, and degassed and recirculated or
reclaimed Treatment or disposal requirements for contaminated fluid
(iii) Disposal of gas sealants:
Inner leakage – into process or vent collection system
Outer leakage – to atmosphere, low pressure point in process, or
flare
Treatment or disposal requirements for contaminated fluid
(iv) Utility reliability:
Steam
Water
Electricity
Plant air
(v) Allocation for shaft seals:
Axial length
Diameter
Maximum seal rotating weight
7.4 Shaft Seal Types. The seal types for a rotating shaft are the following:
a. Labyrinth.
b. Carbon ring.
c. Liquid-film (bushing).
d. Pumping bushing.
e. Circumferential.
f. Mechanical (contact) face.
g. Noncontacting gas seal
7.4.1 None of these is a perfect seal by itself, they are all ‘controlled leakage
devices’. This means the seals allow a certain amount of process gas or an
inert buffer gas or liquid to leak past the sealing area to avoid damaging face
contact or permit lubrication and cooling. Sometimes a combination of two or
more seal types is used to achieve secondary sealing or to deal with
incompatibility between a process gas and a seal oil. Any of these seal types
may be equipped with a manual shut down seal. This seal acts as a backup,
after the compressor has stopped, by compressing a stationary elastomeric
seal, that is an O-ring or packing, against a shoulder on the shaft sleeve. To
help in the selection process, Table I shall be used, which is a summary of
seal types and their characteristics.
7.4.2 To further aid in seal selection, a cost factor is given for each type of shaft
seal described in 8.1 to 8.6. The reference or base case is the buffered
labyrinth seal (cost factor equals 1). The cost factors represent an initial
purchase price (including the auxiliary sealant system) and do not reflect
operating costs.
8 Comparison of Shaft Seals
8.1 Labyrinth Seals
Cost factor is 1.
8.1.1 Labyrinth seals, which consist of several ‘knife-edges’ or fins, are the
simplest seal design. The knife-edges and the mating surfaces together form
throttling orifices that reduce the escape of process gas. The seal can be of
various lengths to achieve effective sealing at moderate pressures. The
leakage is relatively high (1 to 2 percent of compressor flow). Labyrinth seals
are normally used without buffer gas injection when a certain amount of the
process gas is allowed to leak.
8.1.2 Variations have successfully reduced or even eliminated process leakage.
Using knife-edges of hard material rotating against a stationary sleeve made
from softer material permits a design with near zero clearance. If possible, a
nongalling material combination should be selected. The best materials for
corrosion resistance for a specific process application may not be suitable for
the best design if the knife-edges and sleeve are in contact during operation.
Figure 4 shows two labyrinth seals; the upper half is a buffered labyrinth seal,
and the lower half is a plain labyrinth seal.
8.1.3 When process gas leaks have to be prevented, a buffer gas shall be injected
between two labyrinths. See upper half of Figure 4. A vent connection (not
illustrated) shall be used between the process gas and the buffer gas to
further reduce the risk of process gas leaks to the atmosphere. For high inlet
temperature compressors using buffered labyrinth seals, the buffer gas
temperature and thermal distortion of compressor parts shall be considered
in the seal design. It may be necessary to preheat the buffer gas to avoid the
thermal distortion that could occur if ambient temperature buffer gas is used.
The pressure differential shall be controlled between the buffer and process
gases as much as possible, to prevent process gas leaks and reduce the
flow of buffer gas. Process dynamics can make accurate control difficult. This
type of seal can also be furnished with eductor systems on the vent
connection – this improves seal reliability and reduces buffer gas
consumption.
8.2 Carbon Ring Seals
Cost factor is 2. See Figure 5. Carbon ring seals consist of several rings of carbon or
other suitable material, mounted in retainers. Like labyrinth seals, they are a closeclearance
design, but leakage is less. The seal may be operated without buffer gas
injection, as in the labyrinth type. When process gas leaks have to be prevented, a
buffer gas shall be injected between the ring sets. This seal can also be furnished
with vent or eductor systems.
8.3 Liquid-Film / Bushing Seals
Cost factor is 20. See Figure 6.
8.3.1 High-pressure compressors have successfully used liquid-film (or bushing)
seals backed up with a high-pressure seal oil. The two bushings float and
follow the shaft movement, and seal oil is injected between the two bushings
to maintain pressure at 34 to 102 kPa (5 to 15 psi) greater than that of the
process gas pressure. Typically, an elevated tank is used to maintain the
head differential. High pressure bushing seals, and mechanical contact
seals, may require an analysis of the cross-coupled forces and the effect on
rotor stability, particularly if there has been no successful demonstration of
the seal design.
8.3.2 The seal oil flows in two directions: through an inner bushing to internal
process gas pressure and a high-pressure drain, and an outer bushing to an
atmospheric drain. Oil leaking to the atmospheric side of the seal is returned
to the reservoir. The flow of seal oil through the inner bushing toward the
process gas prevents any outward gas flow. If this oil absorbs some process
gas, it shall be recovered and handled appropriately. In some cases, it may
be degassified and returned to the reservoir. Using this seal oil system with
its controls and safety provisions is complicated and costly, but usually
results in a very reliable seal.
8.3.3 The liquid-film seal may also be designed with a pumping bushing, see
Figure 7. This seal type uses a cone-shaped surface on the rotating shaft
sleeve to reduce oil leaks into the compressor. Thus, the amount of oil
leaking to the process side is smaller than with the cylindrical bushing seal.
8.4 Circumferential Seals
Cost factor is 5. See Figure 8. Circumferential seals are segmented carbon rings held
together with a spring and enclosed in a metal housing. Circumferential seals are
shaft rubbing devices that are highly effective as gas seals. Their design permits use
as a single- or multiple-ring installation with a buffer gas supply.
8.5 Mechanical (Contact) Face Seals
Cost factor is 28.
8.5.1 The mechanical (contact) face seal is an axial sealing device that requires
pressurized seal oil as a lubricant and buffer fluid since the life of the seal is
not predictable when run dry. The fluid is supplied in sufficient quantity and at
high enough differential pressure above the gas to effect positive sealing and
lubrication of the rubbing faces. This seal may be a single seal with an
outboard or inboard bushing, a double seal, or a double-face seal design with
an outboard and inboard bushing.
8.5.2 Single mechanical seals, see Figure 9, consist of a stationary seal face in
sliding contact with a rotating ring of compatible material, a spring to hold one
member against the other, and a secondary axial seal between the seal
member and the casing or shaft.
8.5.3 The pressure of the buffer oil is higher than that of the process gas, so the
carbon ring is always kept in contact with the rotating ring. A very low buffer
oil flow between the seal faces to the process side prevents process gas
leaks. This flow does not enter the main gas stream and is recoverable. In
the outboard direction, the bushing controls the flow of the buffer oil that
cools the seal. The process pressure keeps the seal closed, so this seal is
also effective when the oil system and the compressor are shut down. When
isolation between the process gas and the buffer oil is necessary or desired,
an optional buffer gas can be injected between the process gas and the seal.
Low differential pressures are involved, so the consumption of buffer gas is
small.
8.5.4 An alternative to the single seal is a double mechanical (contact) seal, see
Figure 10. It provides the added safety of the secondary seal. A very small
buffer oil flow between the seal faces to the process side prevents process
gas leakage. No process gas leaks to the atmosphere.
8.5.5 In the double-face arrangement, see Figure 11, the seal oil area and the
internal process gas are separated by a metal-reinforced carbon ring. This
ring is sandwiched between a stationary metal sleeve and a rotating seal
ring. A small amount of seal oil flows through the seal faces towards the
internal process gas to prevent outward gas flow. This flow does not enter
the main gas stream and is recoverable.
8.6 Noncontacting Gas Face Seals
Cost factor is 20. The noncontacting gas face seal (dry-running gas seal or gaslubricated
seal) requires no liquid for lubrication or cooling. The seal will leak a very
small amount of process gas; therefore, when necessary, for example when a toxic
process gas is being sealed, a double seal arrangement shall be applied. This
enables an inert gas buffer to provide a barrier to process gas leaks. The buffer gas
shall be maintained at a higher pressure than the process gas to ensure that the
buffer gas leaks into the process gas. Alternatively, these seals shall be applied in
tandem and intermediate leaks shall be diverted to a recovery or abatement system,
see Figure 12.
8.7 Summary
These seal types and their respective support systems all have advantages and
disadvantages (see Table I). Selection will be influenced by the process gas
conditions (pressure and temperature), availability of construction materials
compatible with the process gas and seal design, and economics. The selection
process should include consultation with the compressor manufacturer.
9 Maintenance Recommendations
9.1 Where buffered seals are used, the seals can perform no better than the auxiliary
sealant supply system. The purpose of this system is to continuously supply clean,
cool sealing fluid of buffer gas or seal oil to the seal interfaces at the correct
differential pressure. The auxiliary system for seal oil fluid will have numerous
components, for example reservoir, pumps, heat exchangers, filters, control valves,
and traps. An auxiliary system for a buffer gas fluid may be as simple as a differential
pressure controller on the source gas.
9.2 Periodic maintenance of these components is imperative to ensure a quality sealant
supply system. For critical applications, installed spares or means of bypassing
components may be necessary to allow maintenance or replacement without
interrupting the sealant flow. System monitoring shall be used to warn of impending
failures, for example higher sealant flow or higher pressure drop across the filter. An
instrumentation calibration program shall be maintained to ensure proper instrument
indication. It may be necessary to carry out a periodic functional check of the
instruments.
FIGURE 1 – Bolting of Casing Split Surfaces
FIGURE 2 – Typical Compressor Sealant Supply System
Note: For alternate arrangements of typical compressor sealant supply system, see
SES G03-S01.
FIGURE 3 – Typical Compressor Seal Leak System
FIGURE 4 – Buffered Labyrinth Seal and Plain Labyrinth Seal
FIGURE 5 – Carbon Ring Seal
FIGURE 6 – Liquid-Film Seal with Cylindrical Bushing
FIGURE 7 – Liquid-Film Seal with Pumping Bushing
FIGURE 8 – Circumferential Seal with Segmented Carbon Rings
FIGURE 9 – Single Mechanical (Contact) Seal
FIGURE 10 – Double Mechanical (Contact) Seal
FIGURE 11 – Double-Face Mechanical (Contact) Seal
FIGURE 12 – Noncontacting Gas Seal (Tandem Arrangement)
TABLE I – Comparison of Compressor Seals and Support Systems for Controlling Emissions
