CENTRIFUGAL PUMP MECHANICAL SEALS - Notes, Mehanical Engineering Notes | EduRev

: CENTRIFUGAL PUMP MECHANICAL SEALS - Notes, Mehanical Engineering Notes | EduRev

 Page 1


2.2.3
CENTRIFUGAL PUMP 
MECHANICAL SEALS
JAMES P . NETZEL
2.197
Mechanical seals have been used for many years to seal any number of liquids at various
speeds, pressures, and temperatures. Today plant operators are benefiting from improved
seal technologies driven by the U.S. Clean Air Act of 1990, and the American Petroleum
Institute (API) Standard 682. These new seal technologies are based on advanced com-
puter programs used to optimize seal designs, which are then verified through perfor-
mance testing at simulated refinery conditions required by the API. The results to date
indicate not only an improvement in emissions control, but also a major increase in equip-
ment reliability .
CLASSES OF SEAL TECHNOLOGY _____________________________________
Emerging seal technologies are providing clear choices for sealing. Various plant services
require the application of these new technologies for emissions control, safety, and relia-
bility . Sealing systems are now available that are based on the preferred method of lubri-
cation to be used. These classes of seals are as follows:
1. Contacting liquid lubricated seals:
• Normally, a single seal arrangement is cooled and lubricated by the liquid being
sealed. This is the most cost-effective seal installation available to the industry .
• Dual seals are arranged to contain a pressurized or non-pressurized barrier or
buffer liquid. Normally , this arrangement will be used on applications where the liq-
uid being sealed is not a good lubricating fluid for a seal and for emissions contain-
ment. These arrangements require a lubrication system for the circulation of
barrier or buffer liquids.
Page 2


2.2.3
CENTRIFUGAL PUMP 
MECHANICAL SEALS
JAMES P . NETZEL
2.197
Mechanical seals have been used for many years to seal any number of liquids at various
speeds, pressures, and temperatures. Today plant operators are benefiting from improved
seal technologies driven by the U.S. Clean Air Act of 1990, and the American Petroleum
Institute (API) Standard 682. These new seal technologies are based on advanced com-
puter programs used to optimize seal designs, which are then verified through perfor-
mance testing at simulated refinery conditions required by the API. The results to date
indicate not only an improvement in emissions control, but also a major increase in equip-
ment reliability .
CLASSES OF SEAL TECHNOLOGY _____________________________________
Emerging seal technologies are providing clear choices for sealing. Various plant services
require the application of these new technologies for emissions control, safety, and relia-
bility . Sealing systems are now available that are based on the preferred method of lubri-
cation to be used. These classes of seals are as follows:
1. Contacting liquid lubricated seals:
• Normally, a single seal arrangement is cooled and lubricated by the liquid being
sealed. This is the most cost-effective seal installation available to the industry .
• Dual seals are arranged to contain a pressurized or non-pressurized barrier or
buffer liquid. Normally , this arrangement will be used on applications where the liq-
uid being sealed is not a good lubricating fluid for a seal and for emissions contain-
ment. These arrangements require a lubrication system for the circulation of
barrier or buffer liquids.
2.198 CHAPTER TWO
FIGURE 1 C’Stedy
SM
fluid film model for a mechanical seal (John Crane Inc.)
1
Service Mark of John Crane Inc.
2. Non-contacting gas lubricated seals:
• Dual non-contacting, gas-lubricated seals are pressurized with an inert gas such as
nitrogen.
• Dual non-contacting, gas-lubricated seals are used in a tandem arrangement and
pressurized by the process liquid being sealed, which is allowed to flash to a gas at
the seal. A tandem seal arrangement is used on those liquids that represent a danger
to the plant environment. For non-hazardous liquids, a single seal can be used.
Each of these solutions has been used on difficult applications to increase the mean
time between maintenance (MTBM).
SEAL DESIGN _______________________________________________________
Advancing the state-of-the-art sealing systems are new suites of computer programs such
as C’Stedy
SM(1)
used to analyze the performance of both contacting and non-contacting seal
designs during steady-state and transient conditions. This type of finite analysis consid-
ers all of the operating conditions, the fluid sealed, the materials of construction, and seal
geometry . The outputs from the program are seal distortion, temperature distribution, fric-
tion power, actual PV (pressure  velocity), leakage, the percentage of face in liquid or
vapor, and fluid film stability (see Figure 1). This type of analysis requires accurate fluid
and material properties. The results from the program can predict the success or failure
of a given installation.
For example, a mixture of liquid hydrocarbon made up of ethane, propane, butane, and
hexane has to be sealed. This is a new application. The operating condition is 1,300 psig
at 70°F. The shaft speed is 3,600 rpm. To determine the performance of this seal prior to
installation, an analysis must be made. The results of this study indicate stable operations
for a contacting seal, as shown in Figure 2. This study was used to predict seal perfor-
mance. Actual field results from this difficult service were excellent at startup and during
equipment operation.
Page 3


2.2.3
CENTRIFUGAL PUMP 
MECHANICAL SEALS
JAMES P . NETZEL
2.197
Mechanical seals have been used for many years to seal any number of liquids at various
speeds, pressures, and temperatures. Today plant operators are benefiting from improved
seal technologies driven by the U.S. Clean Air Act of 1990, and the American Petroleum
Institute (API) Standard 682. These new seal technologies are based on advanced com-
puter programs used to optimize seal designs, which are then verified through perfor-
mance testing at simulated refinery conditions required by the API. The results to date
indicate not only an improvement in emissions control, but also a major increase in equip-
ment reliability .
CLASSES OF SEAL TECHNOLOGY _____________________________________
Emerging seal technologies are providing clear choices for sealing. Various plant services
require the application of these new technologies for emissions control, safety, and relia-
bility . Sealing systems are now available that are based on the preferred method of lubri-
cation to be used. These classes of seals are as follows:
1. Contacting liquid lubricated seals:
• Normally, a single seal arrangement is cooled and lubricated by the liquid being
sealed. This is the most cost-effective seal installation available to the industry .
• Dual seals are arranged to contain a pressurized or non-pressurized barrier or
buffer liquid. Normally , this arrangement will be used on applications where the liq-
uid being sealed is not a good lubricating fluid for a seal and for emissions contain-
ment. These arrangements require a lubrication system for the circulation of
barrier or buffer liquids.
2.198 CHAPTER TWO
FIGURE 1 C’Stedy
SM
fluid film model for a mechanical seal (John Crane Inc.)
1
Service Mark of John Crane Inc.
2. Non-contacting gas lubricated seals:
• Dual non-contacting, gas-lubricated seals are pressurized with an inert gas such as
nitrogen.
• Dual non-contacting, gas-lubricated seals are used in a tandem arrangement and
pressurized by the process liquid being sealed, which is allowed to flash to a gas at
the seal. A tandem seal arrangement is used on those liquids that represent a danger
to the plant environment. For non-hazardous liquids, a single seal can be used.
Each of these solutions has been used on difficult applications to increase the mean
time between maintenance (MTBM).
SEAL DESIGN _______________________________________________________
Advancing the state-of-the-art sealing systems are new suites of computer programs such
as C’Stedy
SM(1)
used to analyze the performance of both contacting and non-contacting seal
designs during steady-state and transient conditions. This type of finite analysis consid-
ers all of the operating conditions, the fluid sealed, the materials of construction, and seal
geometry . The outputs from the program are seal distortion, temperature distribution, fric-
tion power, actual PV (pressure  velocity), leakage, the percentage of face in liquid or
vapor, and fluid film stability (see Figure 1). This type of analysis requires accurate fluid
and material properties. The results from the program can predict the success or failure
of a given installation.
For example, a mixture of liquid hydrocarbon made up of ethane, propane, butane, and
hexane has to be sealed. This is a new application. The operating condition is 1,300 psig
at 70°F. The shaft speed is 3,600 rpm. To determine the performance of this seal prior to
installation, an analysis must be made. The results of this study indicate stable operations
for a contacting seal, as shown in Figure 2. This study was used to predict seal perfor-
mance. Actual field results from this difficult service were excellent at startup and during
equipment operation.
2.2.3 CENTRIFUGAL PUMP MECHANICAL SEALS 2.199
FIGURE 2 C’Stedy output for a successful seal on high pressure light hydrocarbon service (John Crane Inc.)
FIGURE 3 The basic components of a mechanical seal
These state-of-the-art computer tools not only predict performance, but they also can
be used to determine any short seal life. By using a series of calculations per second, this
type of analysis can be used to create an animation that will visibly show changes to a seal
at startup and during fluctuations in operating conditions. A seal can be examined for sta-
ble and unstable operations. This is a useful analysis tool for critical applications.
DESIGN FUNDAMENTALS _____________________________________________
Contacting Liquid Lubricated Seals The basic components of a mechanical seal are
the primary and mating rings. Together they form the dynamic sealing surfaces, which
are perpendicular to the shaft. The primary ring is part of the seal head assembly, while
the mating ring and static seal form a second assembly, making a complete installation
for a pump. These basic seal parts are shown in Figure 3. For slower and normal shaft
speeds, the seal head assembly will rotate with the shaft, while on high shaft speeds, the
seal head assembly will be held stationary to the equipment.
The only difference between contacting and non-contacting seal technologies is found
in the design of the seal faces. Each system has the same type and number of parts. Each
has its own area of application for maximum sealing efficiency. Non-contacting seal tech-
nology will be discussed later.
Page 4


2.2.3
CENTRIFUGAL PUMP 
MECHANICAL SEALS
JAMES P . NETZEL
2.197
Mechanical seals have been used for many years to seal any number of liquids at various
speeds, pressures, and temperatures. Today plant operators are benefiting from improved
seal technologies driven by the U.S. Clean Air Act of 1990, and the American Petroleum
Institute (API) Standard 682. These new seal technologies are based on advanced com-
puter programs used to optimize seal designs, which are then verified through perfor-
mance testing at simulated refinery conditions required by the API. The results to date
indicate not only an improvement in emissions control, but also a major increase in equip-
ment reliability .
CLASSES OF SEAL TECHNOLOGY _____________________________________
Emerging seal technologies are providing clear choices for sealing. Various plant services
require the application of these new technologies for emissions control, safety, and relia-
bility . Sealing systems are now available that are based on the preferred method of lubri-
cation to be used. These classes of seals are as follows:
1. Contacting liquid lubricated seals:
• Normally, a single seal arrangement is cooled and lubricated by the liquid being
sealed. This is the most cost-effective seal installation available to the industry .
• Dual seals are arranged to contain a pressurized or non-pressurized barrier or
buffer liquid. Normally , this arrangement will be used on applications where the liq-
uid being sealed is not a good lubricating fluid for a seal and for emissions contain-
ment. These arrangements require a lubrication system for the circulation of
barrier or buffer liquids.
2.198 CHAPTER TWO
FIGURE 1 C’Stedy
SM
fluid film model for a mechanical seal (John Crane Inc.)
1
Service Mark of John Crane Inc.
2. Non-contacting gas lubricated seals:
• Dual non-contacting, gas-lubricated seals are pressurized with an inert gas such as
nitrogen.
• Dual non-contacting, gas-lubricated seals are used in a tandem arrangement and
pressurized by the process liquid being sealed, which is allowed to flash to a gas at
the seal. A tandem seal arrangement is used on those liquids that represent a danger
to the plant environment. For non-hazardous liquids, a single seal can be used.
Each of these solutions has been used on difficult applications to increase the mean
time between maintenance (MTBM).
SEAL DESIGN _______________________________________________________
Advancing the state-of-the-art sealing systems are new suites of computer programs such
as C’Stedy
SM(1)
used to analyze the performance of both contacting and non-contacting seal
designs during steady-state and transient conditions. This type of finite analysis consid-
ers all of the operating conditions, the fluid sealed, the materials of construction, and seal
geometry . The outputs from the program are seal distortion, temperature distribution, fric-
tion power, actual PV (pressure  velocity), leakage, the percentage of face in liquid or
vapor, and fluid film stability (see Figure 1). This type of analysis requires accurate fluid
and material properties. The results from the program can predict the success or failure
of a given installation.
For example, a mixture of liquid hydrocarbon made up of ethane, propane, butane, and
hexane has to be sealed. This is a new application. The operating condition is 1,300 psig
at 70°F. The shaft speed is 3,600 rpm. To determine the performance of this seal prior to
installation, an analysis must be made. The results of this study indicate stable operations
for a contacting seal, as shown in Figure 2. This study was used to predict seal perfor-
mance. Actual field results from this difficult service were excellent at startup and during
equipment operation.
2.2.3 CENTRIFUGAL PUMP MECHANICAL SEALS 2.199
FIGURE 2 C’Stedy output for a successful seal on high pressure light hydrocarbon service (John Crane Inc.)
FIGURE 3 The basic components of a mechanical seal
These state-of-the-art computer tools not only predict performance, but they also can
be used to determine any short seal life. By using a series of calculations per second, this
type of analysis can be used to create an animation that will visibly show changes to a seal
at startup and during fluctuations in operating conditions. A seal can be examined for sta-
ble and unstable operations. This is a useful analysis tool for critical applications.
DESIGN FUNDAMENTALS _____________________________________________
Contacting Liquid Lubricated Seals The basic components of a mechanical seal are
the primary and mating rings. Together they form the dynamic sealing surfaces, which
are perpendicular to the shaft. The primary ring is part of the seal head assembly, while
the mating ring and static seal form a second assembly, making a complete installation
for a pump. These basic seal parts are shown in Figure 3. For slower and normal shaft
speeds, the seal head assembly will rotate with the shaft, while on high shaft speeds, the
seal head assembly will be held stationary to the equipment.
The only difference between contacting and non-contacting seal technologies is found
in the design of the seal faces. Each system has the same type and number of parts. Each
has its own area of application for maximum sealing efficiency. Non-contacting seal tech-
nology will be discussed later.
2.200 CHAPTER TWO
FIGURE 4 Processes involved at contacting seal faces
In a contacting seal, as the shaft begins to rotate, a small fluid film develops, along
with frictional heat from the surfaces in sliding contact. These processes occurring at
the seal faces are shown in Figure 4. The amount of heat developed at the seal faces
must be removed to prevent the liquid being sealed from flashing or beginning to car-
bonize. Seal heat can be removed with a seal flush located at the seal faces. To analyze
the performance of a seal and determine amount of cooling, the following calculations
can be made.
Seal Balance The greatest concern to the seal user is the dynamic contact between the
mating seal surfaces. The performance of this contact determines the effectiveness of the
seal. If the load at the seal faces is too high, the liquid at the seal faces will vaporize or
carbonize and the seal faces can wear out. Damage to the seal faces can occur due to unsta-
ble conditions. A high wear rate from solid contact and leakage can occur if the bearing
limits of the materials are exceeded. Seal balancing is a feature that is used to avoid these
conditions and provide for a more efficient installation.
The pressure in any seal chamber acts equally in all directions and forces the primary
ring against the mating ring. Pressure acts only on the annular area a
c
(see Figure 5a), so
that the force in pounds (Newtons) on the seal face is as follows:
where p  seal chamber pressure, lb/in
2
(N/m
2
) and
a
c
 hydraulic closing area, in
2
(m
2
)
The pressure in lb/in
2
(N/m
2
) between the primary ring and mating ring is
F
c
 pa
c
Page 5


2.2.3
CENTRIFUGAL PUMP 
MECHANICAL SEALS
JAMES P . NETZEL
2.197
Mechanical seals have been used for many years to seal any number of liquids at various
speeds, pressures, and temperatures. Today plant operators are benefiting from improved
seal technologies driven by the U.S. Clean Air Act of 1990, and the American Petroleum
Institute (API) Standard 682. These new seal technologies are based on advanced com-
puter programs used to optimize seal designs, which are then verified through perfor-
mance testing at simulated refinery conditions required by the API. The results to date
indicate not only an improvement in emissions control, but also a major increase in equip-
ment reliability .
CLASSES OF SEAL TECHNOLOGY _____________________________________
Emerging seal technologies are providing clear choices for sealing. Various plant services
require the application of these new technologies for emissions control, safety, and relia-
bility . Sealing systems are now available that are based on the preferred method of lubri-
cation to be used. These classes of seals are as follows:
1. Contacting liquid lubricated seals:
• Normally, a single seal arrangement is cooled and lubricated by the liquid being
sealed. This is the most cost-effective seal installation available to the industry .
• Dual seals are arranged to contain a pressurized or non-pressurized barrier or
buffer liquid. Normally , this arrangement will be used on applications where the liq-
uid being sealed is not a good lubricating fluid for a seal and for emissions contain-
ment. These arrangements require a lubrication system for the circulation of
barrier or buffer liquids.
2.198 CHAPTER TWO
FIGURE 1 C’Stedy
SM
fluid film model for a mechanical seal (John Crane Inc.)
1
Service Mark of John Crane Inc.
2. Non-contacting gas lubricated seals:
• Dual non-contacting, gas-lubricated seals are pressurized with an inert gas such as
nitrogen.
• Dual non-contacting, gas-lubricated seals are used in a tandem arrangement and
pressurized by the process liquid being sealed, which is allowed to flash to a gas at
the seal. A tandem seal arrangement is used on those liquids that represent a danger
to the plant environment. For non-hazardous liquids, a single seal can be used.
Each of these solutions has been used on difficult applications to increase the mean
time between maintenance (MTBM).
SEAL DESIGN _______________________________________________________
Advancing the state-of-the-art sealing systems are new suites of computer programs such
as C’Stedy
SM(1)
used to analyze the performance of both contacting and non-contacting seal
designs during steady-state and transient conditions. This type of finite analysis consid-
ers all of the operating conditions, the fluid sealed, the materials of construction, and seal
geometry . The outputs from the program are seal distortion, temperature distribution, fric-
tion power, actual PV (pressure  velocity), leakage, the percentage of face in liquid or
vapor, and fluid film stability (see Figure 1). This type of analysis requires accurate fluid
and material properties. The results from the program can predict the success or failure
of a given installation.
For example, a mixture of liquid hydrocarbon made up of ethane, propane, butane, and
hexane has to be sealed. This is a new application. The operating condition is 1,300 psig
at 70°F. The shaft speed is 3,600 rpm. To determine the performance of this seal prior to
installation, an analysis must be made. The results of this study indicate stable operations
for a contacting seal, as shown in Figure 2. This study was used to predict seal perfor-
mance. Actual field results from this difficult service were excellent at startup and during
equipment operation.
2.2.3 CENTRIFUGAL PUMP MECHANICAL SEALS 2.199
FIGURE 2 C’Stedy output for a successful seal on high pressure light hydrocarbon service (John Crane Inc.)
FIGURE 3 The basic components of a mechanical seal
These state-of-the-art computer tools not only predict performance, but they also can
be used to determine any short seal life. By using a series of calculations per second, this
type of analysis can be used to create an animation that will visibly show changes to a seal
at startup and during fluctuations in operating conditions. A seal can be examined for sta-
ble and unstable operations. This is a useful analysis tool for critical applications.
DESIGN FUNDAMENTALS _____________________________________________
Contacting Liquid Lubricated Seals The basic components of a mechanical seal are
the primary and mating rings. Together they form the dynamic sealing surfaces, which
are perpendicular to the shaft. The primary ring is part of the seal head assembly, while
the mating ring and static seal form a second assembly, making a complete installation
for a pump. These basic seal parts are shown in Figure 3. For slower and normal shaft
speeds, the seal head assembly will rotate with the shaft, while on high shaft speeds, the
seal head assembly will be held stationary to the equipment.
The only difference between contacting and non-contacting seal technologies is found
in the design of the seal faces. Each system has the same type and number of parts. Each
has its own area of application for maximum sealing efficiency. Non-contacting seal tech-
nology will be discussed later.
2.200 CHAPTER TWO
FIGURE 4 Processes involved at contacting seal faces
In a contacting seal, as the shaft begins to rotate, a small fluid film develops, along
with frictional heat from the surfaces in sliding contact. These processes occurring at
the seal faces are shown in Figure 4. The amount of heat developed at the seal faces
must be removed to prevent the liquid being sealed from flashing or beginning to car-
bonize. Seal heat can be removed with a seal flush located at the seal faces. To analyze
the performance of a seal and determine amount of cooling, the following calculations
can be made.
Seal Balance The greatest concern to the seal user is the dynamic contact between the
mating seal surfaces. The performance of this contact determines the effectiveness of the
seal. If the load at the seal faces is too high, the liquid at the seal faces will vaporize or
carbonize and the seal faces can wear out. Damage to the seal faces can occur due to unsta-
ble conditions. A high wear rate from solid contact and leakage can occur if the bearing
limits of the materials are exceeded. Seal balancing is a feature that is used to avoid these
conditions and provide for a more efficient installation.
The pressure in any seal chamber acts equally in all directions and forces the primary
ring against the mating ring. Pressure acts only on the annular area a
c
(see Figure 5a), so
that the force in pounds (Newtons) on the seal face is as follows:
where p  seal chamber pressure, lb/in
2
(N/m
2
) and
a
c
 hydraulic closing area, in
2
(m
2
)
The pressure in lb/in
2
(N/m
2
) between the primary ring and mating ring is
F
c
 pa
c
where a
o
 hydraulic opening area (seal face area), in
2
(m
2
).
To relieve the pressure at the seal faces, the relationship between the opening and clos-
ing forces can be controlled. If a
o
is held constant and a
c
is decreased by a shoulder on a
sleeve or seal hardware, the seal face pressure can be lowered (see Figure 5b). This is
called seal balancing. A seal without a shoulder in the design is referred to as an unbal-
anced seal. A balanced seal is designed to operate with a shoulder.
The ratio of the hydraulic closing area to the face area is defined as seal balance b:
Seals can be balanced for pressure at the outside diameter of the seal faces, as shown
in Figure 5b. This is typical for a seal mounted inside the seal chamber. Seals installed
outside the seal chamber can be balanced for pressure at the inside diameter of the seal
faces. In special cases, seals can be double-balanced for pressure at both the outside and
inside diameters of the seal. Seal balances can range from 0.65 to 1.35, depending on oper-
ating conditions.
Face Pressure As relative motion takes place between the seal planes, a liquid film
develops. The generation of this film is believed to be the result of surface waviness in the
individual sealing planes. Pressure and thermal distortion, as well as anti-rotation devices
such as drive pins, keys, or dents used in the seal design, have an influence on surface
waviness and on how the film develops between the sliding surfaces. Hydraulic pressure
develops in the seal face, which tends to separate the sealing planes. The pressure distri-
bution, referred to as a pressure wedge, shown in Figure 6, can be considered as linear,
concave, or convex. The actual face pressure p
f
in lb/in
2
(N/m
2
) is the sum of the hydraulic
pressure p
h
and the spring pressure P
sp
designed into the mechanical seal. The face pres-
sure P
f
is a further refinement of , which does not take into account the liquid film pres-
sure or the mechanical load of the seal:
P
œ
f
b 
a
c
a
o
P
œ
f

F
c
a
o

pa
c
a
o
2.2.3 CENTRIFUGAL PUMP MECHANICAL SEALS 2.201
FIGURE 6 The pressure distribution can be considered linear, concave, or convex.
FIGURE 5 Hydraulic pressure acting on the primary ring: a) unbalanced, b) balanced
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