Seals and gaskets must be correctly selected, as the failure of seals and gaskets can endanger human life, as well as potentially causing significant financial losses, particularly in critical applications, says mechanical contractor Brimis Engineering executive director Moeketsi Mpotu.
Brimis Engineering specialises in repair and refurbishment of critical valves and pumps in various heavy duty industries, and also provides engineered solutions. The company also offers a range of mobile testing facilities on critical valves.
The company uses its specialised experience in the valves and pumps industry to provide its customers with in-depth guidance to ensure that they select the appropriate seals and gaskets to suit their unique requirements.
Failures in Seals
Mpotu explains that, owing to the critical function that seals perform in pumps and valves, it is important for customers to be aware of the key failure modes and prevention when selecting seals.
One such key failure is a chemical swell, which occurs when the wrong seal material is chosen and is, therefore, incompatible with the environment or media.
Another key failure is when the applied pressure is more than the allowable pressure for a specific seal which impacts the compression setting. All seals have recommended pressure to be applied. If not, the seal will be permanently deformed and not go back to its original shape or form.
Mpotu adds that trained artisans are critical when performing such installations, along with engineers overseeing the selection of the correctly rated seal.
Thermal degradation sets in when the selected seal is below the operating temperature. Seals have recommended and design temperatures for best results. Therefore, if a seal that is selected is not in the right temperature range, it will fail prematurely. As a result, some seals will have radial cracks, while others may harden and lose their characteristics.
Therefore, the technical team has to allow for occasional changes in operating temperature and should consider an operating temperature margin so that thermal degradation does not occur.
Mpotu says installation-related failures – when a seal is installed incorrectly or prepared incorrectly – can also occur. As a means of prevention, he recommends that, typically, a seal that is stretched more than 50% should never be used.
Further, surface preparation should also be considered, as uneven surfaces can lead to premature failures.
To reduce or avoid this type of failure, the training of the technical team is very important; the team should ensure that there are no sharp edges, seals are not overstretched and that all seals are cut at the correct angle.
Customers also need to be aware of extrusion-related issues, which typically occur when the seal is overtightened, then starts to extrude outside the sealing surface. This is typical of overfilled glands, which the technical team should always avoid when gland packing. Gland packing refers to a braided rope-like material that is packed around the shaft of the pump to fill any space and prevents fluid loss from around the shaft.
Gland packing is often used instead of a mechanical seal, as it is cost effective, easy to install and can be used in various products.
A key benefit of gland packing, compared with a mechanical seal, is having an engineered solution with specific operating conditions taken into consideration.
“Gland packing can [also] reduce plant downtime and increase availability – this is a huge efficiency benefit when one considers maintenance planning,” Mpotu adds.
Failures in Gaskets
There are two broad failure modes for gaskets – catastrophic or instantaneous blowouts, and slow or chronic leaks.
“Engineers are tasked with eliminating both of these modes at all material times,” adds Mpotu.
Brimis Engineering is involved in critical applications – with pressures higher than 300 bar and temperatures above 500 ºC – and catastrophic or instantaneous blowouts is, therefore, of critical concern to the business.
Some commonly missed, root causes of gasket failures are chemical attacks, temperature- related and vibration-induced issues, as well as movement of pump component such as shafts.
He notes that such failures can result in loss of life and disabling injuries, as well as major financial losses.
Therefore, engineers have to consider key parameters, such as seal size, material selection, surface type and the amount of compression required, when selecting gaskets.
Brimis Engineering engineers have vast experience in the selection and optimisation of gaskets for critical applications, says Mpotu.
The company continues to partner with original-equipment manufacturers in any maintenance-related work it undertakes, as the importance of using original parts can never be overstated, he concludes.