Page 66 - Fluid mechanics, heat transfer, and mass transfer

GASKETS AND MECHANICAL SEALS 43

& Others include grooved joints, expanded joints, sion, provides protection against blowout when used
V-clamp joints, seal ring joints, pressure seal joints, with raised facing.
soldered, brazed, cemented joints, and so on. . “Life of a gasket depends on the operating temperature
& Welding is another way to join piping, wherever the and halves for every 10 C rise in temperature.” True/

joints need not be disassembled for general mainte- False?
nance purposes. & True.
. What is a gasket? . What is an O-Ring? For what purpose it is used?
& Gaskets (made out of comparatively softer materials & O-ring seal joints are used for applications requiring
than the material of the pipe or section) are used to heavy wall tubing. The outside of the tubing must be
secure leakproof conditions in flanged joints in pip- clean and smooth. The joint may be assembled
ing or between two surfaces of different sections of repeatedly and as long as the tubing is not damaged,
an equipment that must be assembled together. Tight- leaks can usually be corrected by replacing the
ening the bolts causes the gasket material to flow into O-ring and the anti-extrusion washer. This joint is
the minor machining imperfections, resulting in a used extensively in oil-filled hydraulic systems.
fluid-tight seal. & Viton, fluorinated ethylene–propylene polymers
. What are the important factors that go into the selection (FEP), vulcanized rubber, perfluoro-elastomers, and
of gaskets? so on go into the manufacture of O-rings.
& Compatibility with process fluid so that the gasket . Name some materials that go into the manufacture of
will not swell or dissolve (physical effects). gaskets, O-rings, and other seal materials for valves and
& Should withstand thermal cycling, vibration, aging, other components used in piping systems.
and should not harden. & CAS (compressed asbestos sheet).
& Should not be corrosive to flange surfaces and/or & Rubber-based compounds (usually compounded
bolting. with a binder).
& Should be uniformly covering the flange surfaces. & SBR (styrene-butadiene rubbers) or NBR (nitrile),
& Must have high coefficient of friction to be held FPM/Viton (fluorinated rubber), and other synthetic
properly between the flanges. rubbers.
. What are spiral wound gaskets? What are their char- & EPDM (ethylene–propylene).
acteristics and applications? & TEFLON/PTFE (poly-tetra-fluoro-ethylene).
& Combination of metal and filler like asbestos or other & Viton.
material, spirally wound under high pressures. . “Rubber gaskets are used to prevent oil leakages from a
& Metals used include stainless steel, nickel, Monel,
pipe joint.” True/False?
Inconel, titanium, Hastelloy, and so on. & False. Rubber is not compatible with oil.
& Fillers used for spiral wound gaskets are asbestos,
. Name some of the most commonly used gasket materi-
PTFE, ceramic, graphite, and so on.
als used for mechanical seals for pumps in modern
& In view of the cancer risks involved in the use of
chemical plants and give their characteristics.
asbestos, other metal–nonmetal combinations, such & EPDM (ethylene–propylene copolymer): Good re-
as metal–TFE and metal–graphite spiral wound gas-
sistance for some ketones, alcohols, food products,
kets are marketed as substitutes to asbestos and may
ozone, hot water, and radiation. Not compatible
seal better than metal–asbestos gaskets.
with hydrocarbons, organic and nonorganic oils, and

& Used for higher temperature (up to 600 C) and
fats.
pressure services, including cyclic or difficultly con- & Nitrile/NBR (acrylonitrile-butadiene copolymer):
tained fluids.
Good general capabilities and excellent low temper-
& Density of the gasket is determined by the compact-
ature properties.
ness of windings and fillers. & PTFE: Near universal chemical compatibility but not
& Spiral wound gaskets are also used widely in high- elastic.
pressure steam services.
& Fluorinated rubber (FPM)/Viton): Resistance to most
& Spiral wound gaskets should preferably be used with chemicals and ozone. Not suitable for fluids such as
a smooth flange finish. hot water, alcohol, steam, lye, and acid.
& The spiral wound gaskets are furnished with a solid & Fluoro-elastomers (FEP, fluorinated ethylene–
metallic ring on the outside, to limit gasket compres- propylene): Best standard duty elastomer with