Gaskets,
Packings and Seals
A gasket
is a material that is used to seal two faces of a machine.
Gaskets can be made of a soft materials, such as asbestos or
elastomers, or they can be made of harder materials, such as metal
ring gaskets made of iron, steel and other materials. Combinations of
materials such as spiral-wound metal/asbestos-filled gaskets
are also common.
Gaskets
are designed for compressibility and sealability. Compressibility is
a measure of the gasket's ability to deflect and conform to
the faces being sealed. Gaskets compressibility compensates
for surface irregularities, such as minor nicks, non-parallelism,
corrosion and variations in groove depth. Sealability is the measure
of fluid leakage through and across both faces of a gasket.
Most of the leakage for a properly installed face to face connections
will occur through the gasket.
Types
of Gaskets
Gaskets
are classified usually either by the material from which they are
made and the type of construction or design or in some cases both.
Flat
Gasket
Flat
gaskets are cut from flat stock of gasket material. The
material can be elastomer, either natural ( rubber ) or synthetic (
hypalon, viton, SBR ). Fiber material is often formed with a binder
into flat stock gasket material. Flat stock comes in standard
thickness ranging from 1/64” to 1/4”. Flat stock gaskets
are often cut into full face gaskets, in which the gasket
design incorporates the appropriate bolt-hold pattern and the flange
bolts are used to center the gasket in place. The full face
design is most often used with the flat face flange design. Ring face
gaskets are cut so that the outer diameter of the gasket
rest inside the bolt pattern and the gasket is centered by
resting it on the flange bolts.
Envelope
Gasket
Envelope
gaskets consist of an elastomeric material protected by
another material as a gasket. TFE is common envelope material
because of its resistance to many chemicals. Envelope gaskets
are usually ring face sized so that the gasket is centered by
resting on the flange bolts. Different metals can be used as the
envelope material.
Spiral-Wound
Metal-Filled Gaskets
Spiral-wound
metal-filled gaskets are the most common and popular. They
consist of a thin metal spiral separated by different filler
materials depending on service conditions. They are available in full
face design. The spiral-wound section of the gasket exists
only where the flange faces meet. The remainder of the gasket
consists of a backing ring. Special spiral-wound gaskets with no
backing rings are used in some tongue-and-groove and male-and-female
joint designs.
Grooved
Metal and Solid Flat Metal Gaskets
Grooved
metal and solid flat metal gaskets are most often employed in
tongue-and-groove and male-and-female joint design.
Metal
Ring Joint Gaskets
Metal
ring joint gaskets are either oval or octagonal in cross
section. They are used exclusively in ring joint flange
configurations.
Choosing
a Gasket
When
choosing a gasket, remember that it must perform under the
system temperature and pressure conditions. Gasket
manufacturers commonly list the maximum temperature and maximum
pressure ratings of their gasket materials. They also give the
maximum pressure times temperature ( P x T ) rating.
Gasket
Materials' Physical Properties
Material
|
Max.
Temp. ( °F )
|
Max.
Pressure ( psi )
|
P x
T, Max.
|
Natural
Rubber
|
200
|
100
|
15000
|
SBR
|
200
|
100
|
15000
|
Neoprene
|
250
|
150
|
20000
|
Nitrile
|
250
|
150
|
20000
|
EPDM
|
300
|
150
|
20000
|
Asbestos/rubber
binder
|
900
|
3000
|
350000
|
Asbestos/SBR
binder
|
750
|
1800
|
350000
|
Asbestos/neoprene
binder
|
750
|
1500
|
350000
|
Asbestos/nitrile
binder
|
750
|
1500
|
350000
|
A
seal may be described as a device for controlling the movement
of fluids across a joint or opening in a vessel or assembly. Packings
are a form of seal commonly associated with condition where
relative motion occurs for example the sealing of moving parts either
rotating or reciprocating at the point of entry to a pressure
chamber. Various types of packing material are also used to
seal stationary parts, such as cylinder heads and flanges.
Packing
Material and Construction
Compression
packing is manufactured from various forms of fibers, such as
vegetable, animal, mineral, or synthetic, combined with binders and
lubricants. The four principal types of construction are twisted,
square-braided, braid-over-braid and interlocked-braided.
Twisted
Construction
The
twisted type is the simplest and a widely used of packing, is
usually made from twisted strands of asbestos or cotton lubricated
with mineral oil and graphite. Twisted packing is not as
strong as the braided type but it is quite easy to install. One size
of packing may be adapted to many stuffing box sizes.
Square-braided
Construction
Square-braided
construction is made of asbestos, cotton, plastic or leather and
sometimes includes metal wires such as lead or copper. It is usually
grease or oil impregnated. It is made by gathering a number of yarns
into a strand. The strands usually eight in number, are plaited into
a square shape or cross section. The loose structure permits
flexibility and easy adjustment. They are generally graphited
throughout.
Interlocked-braided
Construction
Interlocked-braided
packing has some of the characteristics of both braided and
braid-over-braid. All yarns in this type of construction are
interlocked to form a solid square braid that is extremely resistant
to unraveling. It is most frequently supplied with oil lubrication
and is available in asbestos, cotton, plastic and other fibers.
Seals
Seals
are used to prevent passage of oil along a rotating shaft. It is
necessary when a shaft extends from a housing containing oil, such as
pumps and gearboxes.
Mechanical
Seal
The
sealing of rotating shafts at the point of entrance to a pressure
chamber has for years been accomplished with a stuffing box seal.
A serious deficiency of the stuffing box type of seal is its
leakage requirement for satisfactory operation. Still widely used
because of its simplicity and ability to operate under extremely
adverse conditions, the mechanical seal has replaced it in
many applications. The mechanical seal is an end face seal
designed to provide rotary seal faces that can operate with
practically no visible leakage. The principle of mechanical seal
is to use two replaceable anti-friction mating rings – rotating and
stationary.
These
mating rings are statically sealed, the rotating ring to the shaft
and the stationary ring to the housing.
O-Ring
The
o-ring is a squeeze type packing made from synthetic rubber. It is
manufactured in several shapes, the most common being the circular
cross section from which it derives its name. It is very simple
device and probably was not developed earlier because suitable
materials and precision molding were not available. The principle of
operation of the o-ring can be described as controlled deformation. A
slight deformation of the cross section, called mechanical squeeze
deforms the ring and places the material in compression.
V-ring
Seal
V-ring
seal have two different sections of rubber, divided into thick
base and thinner flap that extends out forming a right angle space
between the two pieces. When the seal is placed on a pipe, the
outer flap bends in, contracting forming a v-shape with the base
rubber section. It is easily installed even without a perfect fit.
This seal withstand high pressure applications before
breaking.
Labyrinth
Seal
Labyrinth
seal are made of parallel grooves carved into the rubber
making it a series of seals in the shaft. This creates
obstruction of oil to leak since it will tightly pass multistage
sealing of grooves. This is very common in rotating high speed
members.
Shaft
Seal
Shaft
seal is applicable for extremely high temperature and high
pressure conditions. It is made of a synthetic rubber and spring
loaded to ensure tightness and prevent oil passage on rotating shafts
in pumps and gearboxes.
