GASKET FAILURES AND MATERIALS

GASKET FAILURES AND MATERIALS

Introduction
Baskets are used in electrical and mechanical equipment for sealing removable covers, to prevent leakage of compressed air or any other fluids and entry of dust, etc. They are also used in boilers, reactors and other types of pressure vessels. Some examples are gaskets between:
- transformer tank and tank cover
- pipe flange and compressor output opening
- traction motor carcass and inspection cover
- air-flow relay flange and air duct
- cylinder head and cylinder block of an automobile engine
- inspection covers, pipe connection flanges etc. and pressure vessels.

Gaskets are generally treated as trivial components. They are inexpensive, replaceable and to be used and discarded. However, failures of gaskets can cause severe and expensive damages.
There is not much technology involved in their installation but it is very important to ensure that certain simple precautions are taken.

Failure Modes for Gaskets
There are two main failure modes. These are:
- Leakage of fluid through the joint over or under the gasket.
- Fracture and blowing of gasket and leakage of fluid.
It is necessary to assess both the type of damage to the gasket and also when it occurs. Early failures that exhibit localized damage to the gasket are very likely to be due to inadequate or uneven bolting, flange damage or misalignment. Damage or disintegration around the entire circumference are more indicative of the wrong type of gasket, such as excessively thick or inadequate material, or
potentially overloaded and crushed gasket.

Leakage when initially pressurized
● Bolting incorrect, insufficient compression of gasket.
● Flanges misaligned giving uneven compression of gasket.
● Damaged flange providing leak path.
● Wrong type of gasket, insufficient pressure capability, review thickness and material.

Leakage during first temperature cycle
● Bolting incorrect, insufficient compression of gasket.
● Wrong type of gasket, insufficient pressure/temperature capability, review thickness and material
These problems may be compounded by:
● Flanges misaligned giving uneven compression of gasket.
● Damaged flange providing leak path.

Premature failure after few hundred hours or limited number of thermal/pressure cycles
● Bolting incorrect, insufficient compression of gasket.
● Creep or ageing of gasket, review load compression and seal ability data for gasket material.
● Creep of bolts, check material specification for creep at elevated temperature.
● Buckling of spiral wound gasket, 
These problems may be compounded by:
● Flanges misaligned giving uneven compression of gasket.
● Damaged flange providing leak path.

Leakage after an extended period, but inadequate service life
Failure after a long period of service is potentially due to either one or a combination
of factors. Potential contributory factors include:
● Degradation of gasket material with time
● Long-term relaxation of gasket material, load compression data available from premium suppliers should assist with selection of an alternative with improved characteristics.
● Long-term creep of bolt material.
● Change of system conditions, fluid content, thermal/pressure cycles, vibration, external environment (e.g. removal of insulation, other causes of external heating/cooling).
● Individual event: Pressure surge, temperature cycle, external event causing temperature shock or vibration.
● Incorrect initial bolting may still be a factor due to less than ideal compression of the gasket.
● Misalignment can accelerate the effect of all the above factors and may be a contributory cause

Indicators to type of gasket degradation
● Potential creep, gasket material has flowed outside original inside and/ outside dimensions. Plastic flow, particularly with PTFE-based materials. Localized creep indicates uneven bolt loading. The pressure, temperature and flange loading need to be considered with a review of the gasket material.
● Entire gasket has hardened/disintegrated on removal. Degradation of binder and/or fibre has left aged material unable to provide an effective seal. Review both fibre type and binder material and their proportions. A high proportion of binder phase is unsuitable for all but low temperature
applications
● Localized damage such as blow out in one area. This is probably caused by local flange damage or uneven bolt loading.
● Degradation appears to progress from inside diameter of gasket. This is an indication of chemical attack of material by the sealed fluid.
● Degradation is most pronounced at the outside diameter of the gasket. This indicates that the external environment is the primary factor causing degradation. A prime example is the oxidation of graphite gaskets at high temperature, Figure 6.10. Similar but potentially less obvious problems
could occur by for instance oxidation of binder phase if a fibrous gasket is used at elevated temperature.
● Localized degradation or corrosion on the atmospheric side of a gasket. A section of the exterior of the flange may be exposed to some environmental condition that creates a corrosive environment, such as salt spray or rain. Depending on operating conditions, gasket and flange materials this
may cause additional degradation or potentially crevice corrosion around the gasket. Various protective shield devices are available to clad flange joints for protection from the environment.

Gasket Materials

Natural Rubber		110 0C Max Temp.	-	-
Good Mechanical Properties. Resistant to Water.  Fair to good resistance to acids and alkalis.  Poor resistance to oils petrol.  Poor Resistance to Weathering.
Butyl		150 0C Max Temp.	-
Very good resistance to water, alkalis, many acids. Poor resistance to oils, petrol, and most solvents.
Nitrile		150 0C Max Temp.	-
Very good resistance to water.  Fair resistance to alkalis, acids. Excellent resistance to oils and petrol.
Polysulphide		660C Max Temp.	-
Very good resistance to water.  Good resistance to alkalis, fair resistance to acids. Excellent resistance to oils, petrol, aliphatic and aromatic hydrocarbon solvents .  Poor mechanical Properties
Neoprene		1200C Max Temp.	-
Good resistance to water and alkalis, fair resistance to acids. good resistance to non-aromatic pertroleum, fatty oils, solvents (except aromatic, chlorinated or ketone types).  Excellent mechanical Properties
Acrylic		2300C Max Temp.	-
Good heat resistance.  Poor low temperature properties. Poor resistance to water, alkalis and some acids.  Poor resistance to steam steam at high temperatures.  Fair to good resistance to alkalis and acids. Good resistance to oils, petrol, aliphatic and aromatic hydrocarbon solvents
Hypalon-Chlorosulphonated Polyethylene		2300C Max Temp.	-
Good Mechanical properties  Excellent resistance to oxidising chemicals, ozone weathering. .Relatively good resistance to oils and greas.  Poor resistance to oils, petrol, aliphatic and aromatic hydrocarbon solvents
Viton/Kel-F/Fluoroelastomer		2300C Max Temp.	-
Good Mechanical properties  Excellent resistance to ozone weathering. .Can be used at high temperatures with with many fuels, lubricants, hydraulic fluids, solvents.
Asbestos Derivatives		6000C Max Temp.	-
Large number of composites and combinations- Asbestos is not really acceptable for use in UK for health reasons
Cork Composites		1200C Max Temp.	-
Low Cost.  Compressible allowing substantial deflection with negligible side flow. Will conform to irregular surfaces.   High resistance to oils.  Good resistance to water and many chemicals.  Should not be used with inorganic acids and alkalis, oxidising solutions, live steam
Cork Rubber		1500C Max Temp.	-
Low Cost.  Defined compressiblity. good fatigue resistance. Chemical resistance dependent on rubber used
PTFE/Teflon/ PolyTetrfluoroethylene		2500C Max Temp.	-
Excellent resistance to allmost all chemicals and solvents.   Good heat resistance. Exceptional low temperature properties.  Low compressibility. Low resilience tends to creep under stress.
Filled PTFE/Teflon		up to 2500C Max Temp.	-
Improved mechanical properties - however filling material can impair chemical properties
PTFE/Teflon composites(lined)		up to 2500C Max Temp.	-
Chemical properties comparable with virgin PTFE. Inner gasket material providing improved resiliance and deformability
Polythene		700C Max Temp.	-
Resists most solvents- Poor heat properties
Neoprene Impregenated wood fibre		800C Max Temp.	-
No porous - good for glycol, oil, and petrol
SBR bonded Cotton		1050C Max Temp.	-
Good Water resistance
Nitrile Rubber - Cellulose fibre		1050C Max Temp.	-
Oil Resistant up to reasonably high temperatures
Inorganic Fibre		10000C Max Temp.	-
Excellent heat properties - poor mechanical properties
Graphite /Carbon Fibre		10000C Max Temp.	-
Excellent heat properties - Excellent chemical and mechanical properties based on product selected.
Felt-Pure		-	-
Resilient, compressible, and strong but not impermeable.  Resists medium strength mineral acids and dilute mineral solutions if not intermitently dried.  Resists oils greases, waxes, most solvents. Damaged by alkalis.
Felt-PTFE Impregated		1300C Max Temp.	-
Good chemical and heat resistance
Lead		2600C Max Temp.	-
Good general chemical resistance. Best conformity of all metals
Tin		2600C Max Temp.	-
Good resistance to neutral chemicals . Attacked by acids and alkalis
Aluminium		4300C Max Temp.	-
High corrosion resistance. Slightly attacked by strong acids and alkalis
Copper & Brass		4300C Max Temp.	-
Good corrosion resistance at moderate temperatures
Nickel		7600C Max Temp.	-
High corrosion resistance
Monel		6500C Max Temp.	-
High corrosion resistance.   Good for use with wide range of acids and alkalis.  Attacked by strong hydrochloric and oxidising acids.
Inconel		10000C Max Temp.	-
Excellent heat and oxidising resistance.
Stainless Steel		6000C Max Temp.	-
High corrosion resistance depending on grade used.
Leather		1000C Max Temp.	-
Low cost.  Limited chemical and heat resistance.   Not recommended against pressurised steam, acid or alkli solutions.