Properties
One of the more desirable properties of an effective gasket
in industrial applications for compressed fiber gasket material is the ability to
withstand high compressive loads. Most industrial gasket applications involve bolts
exerting compression well into the 14 MPa (2000 psi) range or higher. Generally
speaking, there are several truisms that allow for best gasket performance. One
of the more tried and tested is: "The more compressive load exerted on the gasket,
the longer it will last".
There are several ways to measure a gasket material's ability to withstand compressive
loading. The "hot compression test" is probably the most accepted of these tests.
Most manufacturers of gasket materials will provide or publish the results of these
tests.
Gasket design
Gaskets come in many different designs based on industrial
usage, budget, chemical contact and physical parameters:
Sheet gaskets
The premise is simple in that a sheet of material has the
gasket shape "punched out" of it. This leads to a very crude, fast and cheap gasket.
In previous times the material was compressed asbestos, but in modern times a fibrous
material such as graphite is used. These gaskets can fill many chemical requirements
based on the inertness of the material used and fit many budgetary restraints. Common
practice prevents these gaskets from being used in many industrial processes based
on temperature and pressure concerns.
Solid material gaskets
The idea behind solid material is to use metals which cannot
be punched out of sheets but are still cheap to produce. These gaskets generally
have a much higher level of quality control than sheet gaskets and generally can
withstand much higher temperatures and pressures. The key downside is that a solid
metal must be greatly compressed in order to become flush with the flange head and
prevent leakage. The material choice is more difficult; because metals are primarily
used, process contamination and oxidation are risks. An additional downside is that
the metal used must be softer than the flange — in order to ensure that the flange
does not warp and thereby prevent sealing with future gaskets. Even so, these gaskets
have found a niche in industry.
Spiral-wound gaskets
Spiral-wound gaskets comprise a mix of metallic and filler
material. Generally, the gasket has a metal (normally carbon rich or stainless steel)
wound outwards in a circular spiral (other shapes are possible) with the filler
material (generally a flexible graphite) wound in the same manner but starting from
the opposing side. This results in alternating layers of filler and metal. The filler
material in these gaskets acts as the sealing element, with the metal providing
structural support.
These gaskets have proven to be reliable in most applications, and allow lower clamping
forces than solid gaskets, albeit with a higher cost.
Constant seating stress gaskets
The constant seating stress gasket consists of two components;
a solid carrier ring of a suitable material, such as stainless steel, and two sealing
elements of some compressible material installed within two opposing channels, one
channel on either side of the carrier ring. The sealing elements are typically made
from a material (expanded graphite, expanded polytetraflouroethylene (PTFE), vermiculite,
etc.) suitable to the process fluid and application. Constant seating stress gaskets
derive their name from the fact that the carrier ring profile takes flange rotation
(deflection under bolt preload) into consideration. With all other conventional
gaskets, as the flange fasteners are tightened, the flange deflects radially under
load, resulting in the greatest gasket compression, and highest gasket stress, at
the outer gasket edge.
Since the carrier ring used in constant seating stress gaskets take this deflection
into account when creating the carrier ring for a given flange size, pressure class,
and material, the carrier ring profile can be adjusted to enable the gasket seating
stress to be radially uniform across the entire sealing area. Further, because the
sealing elements are fully confined by the flange faces in opposing channels on
the carrier ring, any in-service compressive forces acting on the gasket are transmitted
through the carrier ring and avoid any further compression of the sealing elements,
thus maintaining a 'constant' gasket seating stress while in-service. Thus, the
gasket is immune to common gasket failure modes that include creep relaxation, high
system vibration, or system thermal cycles. The fundamental concept underlying the
improved sealability for constant seating stress gaskets are that (i) if the flange
sealing surfaces are capable of attaining a seal, (ii) the sealing elements are
compatible with the process fluid and application, and (iii) the sufficient gasket
seating stress is achieved on installation necessary to affect a seal, then the
possibility of the gasket leaking in-service is greatly reduced or eliminated altogether.
The Patent for the constant seating stress gasket is held by JJENCO, Inc., Charlotte,
North Carolina, USA. Various constant seating stress gasket designs for different
applications are sold by that company under the tradename 'PerfectSeal'.
Double-jacketed gaskets
Double-jacketed gaskets are another combination of filler
material and metallic materials. In this application, a tube with ends that resemble
a "C" is made of the metal with an additional piece made to fit inside of the "C"
making the tube thickest at the meeting points. The filler is pumped between the
shell and piece. When in use the compressed gasket has a larger amount of metal
at the two tips where contact is made (due to the shell/piece interaction) and these
two places bear the burden of sealing the process. Since all that is needed is a
shell and piece, these gaskets can be made from almost any material that can be
made into a sheet and a filler can then be inserted. This is an effective option
for most applications.
Kammprofile gaskets
Kammprofile gaskets are used in many older seals since they
have both a flexible nature and reliable performance. Kammprofiles work by having
a solid corrugated core with a flexible covering layer. This arrangement allows
for very high compression and an extremely tight seal along the ridges of the gasket.
Since generally the graphite will fail instead of the metal core, Kammprofile can
be repaired during later inactivity. Kammprofile has a high capital cost for most
applications but this is countered by long life and increased reliability.
Flange gasket
A flange gasket is a type of gasket made to fit between
two sections of pipe that are flared to provide higher surface area.
Flange gaskets come in a variety of sizes and are categorized by their inside diameter
and their outside diameter.
There are many standards in gasket for flanges of pipes. The gaskets for flanges
can be divided in major 4 different categories:
- Sheet gaskets
- Corrugated metal gaskets
- Ring gaskets
- spiral wound gaskets
Sheet gaskets are simple, they are cut to size either with
bolt holes or without holes for standard sizes with various thickness and material
suitable to media and temperature pressure of pipeline.
Ring gaskets also known as RTJ. They are mostly used in offshore oil- and gas pipelines
and are designed to work under extremely high pressure. They are solid rings of
metal in different cross sections like oval, round, octagonal etc. Sometimes they
come with hole in center for pressure equalization.
Spiral wound gaskets are also used in high pressure pipelines and are made with
stainless steel outer and inner rings and a center filled with spirally wound stainless
steel tape wound together with graphite and PTFE, formed in V shape. Internal pressure
acts upon the faces of the V, forcing the gasket to seal against the flange faces.
|
Polytetrafluoroethylene (PTFE) gasket
|
|
|
|