January 28, 1986 was the day when NASA shuttle challenger exploded shortly after its launch, leading to the deaths of seven astronauts. The catastrophic event was caused due to the failure of a pair of O-Rings in the space shuttle challenger.
This dark disaster took place when the shuttles right solid rocket booster O-Ring had failed at lift off, allowing pressurised hot gas to escape from the booster, causing the hardware to breakdown. ln view of the findings, the failure was due to a faulty design unacceptably sensitive to a number of factors. These factors were the effects of temperature, physical dimensions, and the character of materials, the effects of reusability, processing, and the reaction of the joint to dynamic loading.
This versatile sealing device offers many benefits to designers, engineers, maintenance staff, plant operators, but any compromise in the quality of O-Rings can lead to disastrous event like that of space shuttle challenger.Hence, it should be regularly assessed and quality appred otherwise can led to cataclysmic accidents.
So this brings us to an important question.
What causes the failure of O-Rings ?
For an O-Ring to function properly, it has to deform. Once an O-Ring is installed, it is then compressed resulting in zero clearance. It’s the zero clearance that seals the flow of fluids and gases. The actuating pressure increases so does the forces acting on surface resulting in tighter seal and deforming the O-Ring. Thus for sealing to work well, careful design, selection and installation process should be adopted.
The common causes for typical O-Ring failures are:
Improper installation - It is one of the major causes of seal failure. This failure can be noticed through short nicks and scratches, or peeling on the surface of O-Ring.
Abrasion - It is most common in dynamic sealing application. This failure produces a flattened surface on the sides of the O-Ring body subjected to movement.it occurs when either operating temperature are too high for the material, no lubrication in design or the system fluid is contaminated with abrasive particles.
Compression set - This failure occurs due to poor compression set resistance in the O-Ring material. It produces flat surfaces on both sides of O-Ring that is compressed.
Explosive decompression - Is sourced by gas permeating the O-Ring leading to rapid decompression which in turn causes micro explosion. Random ruptures small slits and crater like pores indicate this kind of failure.
Heat hardening and oxidation - Failure through this can occur due to temperature higher than recommended for material oxidation or elastomers becoming dry and evaporating. This can be identifies through cracked, hardened and pitted areas throughout the entire body of O-Rings.
Extrusion - The sources of extrusion includes excessive systematic pressures, material being too soft, improper machined grooves or due to much clearance between the mating parts. Ridges, nibbles and small missing pieces between inner and outer diameter can be noticed.
Chemical degradation - Is caused due to incompatibility with chemicals and thermal environment. The seal exhibits signs of cracks, discoloration or voids
Weathering and ozone cracking - Sources for this failure are attack of the polymer chains, destructing the material by cracking. This happens due to exposure of O-Rings to weather, pollution, and ultraviolet that have little resistance to ozone.
Spiral damage - Failure due to this damage can occur due to misfit components, uneven surfaces, unsuitable lubrication, side loads causing excessive clearance or material being too soft. The failure can be noticed when the surface of O-Rings appears to be twisted or rolled in its groove.
Hence it is essential that the quality of O-Ring is assessed and reassured with consistently safe and sterile production. If quality is compromised, it will lead to early contamination of the products requiring frequent changes and regular shut down. It can also result in the product losing its capabilities much faster causing leak in the system leading in undesired and unwanted breakdowns of machine it is used in.
This brings us to our next big question.
How to choose the right type of O-Ring ?
- First of all, the nature of your application needs to be recognised to select appropriate O-Ring.
- The next step is choosing the right material for your O-Ring. Thischoice is done on various aspect of the application of your O-Ring. The top three questions to ask are :
o Temperature – Whats going to be the minimum and maximum temperature acting on the O-Ring?
o Pressure – What pressure is the O-Ring going to be subjected to?
o Medium – what liquid or gas is the O-Ring used to seal?
The answers to these three questions helps you determine the material to be used for manufacturing the O-Ring.
- After determining the right material, O-Ring squeeze ratio has to be considered. This will tell you the force applied against the hardware that results from tightening the seal.
- Consider the designing details of your hardware so that the right O-Ring type is chosen according to tolerance of compression set and pressure.
- It is very important to consider the temperature and pressure level as the durability of O-Rings depends on its extreme operational conditions. Consideration of physical factors like these has the advantage of being operational for long time.
- For the selection of O-Rings, adaptation to storage conditions should be taken in account. For example, some O -Rings cannot stand hot air under pressure, some fail to perform well due to continuous exposure to wet atmosphere.
Choosing the right type of seal will get a fully functional seal for your application.
However, most vulcanised rubbers change their properties during storage and eventually become unusable due to excessive hardening or deformation. Hence the shelf life of an O-Ring constitutes an important factor. Storage should be done in dry container in a relaxed state with no overcrowding of container. Moreover, when in transit, seals should be packed in opaque bags.
The inadequate dual O-Ring design did not only cause this disastrous accident and changed the history of NASA , it also bought about transformation in rubber industry. O-Rings now come in individually packaged and labelled with material, cure date and batch information.
Moreover new advances will increase the importance of O-Rings (Nanotechnology is one such frontier).