Improve plant equipment reliability by choosing right bearing guard

February 17, 2014 6:33 am

 Proper bearing seal is a surest way to maximise rotating equipment effectiveness and reducing plant failures
 In today’s highly competitive environment, pressure to keep the equipment performing increases year on year. Equipment inefficiencies cannot be passed onto the consumer through elevated selling prices; therefore, these inefficiencies eat into the plants profit margins, endangering jobs, communities and livelihoods.
Plant downtime, lost production, increased plant maintenance and increased refurbishment costs are real concerns for reliability engineers around the world.
Research into bearing failuresReliability by numbers: 52 per cent of bearing failures. A 2007 study into equipment reliability conducted at a major US oil refinery has published statistics on causes of equipment failure. They concluded that 40 per cent of overall rotating equipment failures (pumps, mixers etc.) were due to bearing failure.
They further estimated that 48 per cent of all bearing failures were due to particle contamination and a further 4 per cent were due to corrosion (caused by liquid in the oil).
Therefore, 52 per cent (total contamination cause) of 40 per cent (bearing failures) = 20.8 per cent of all rotating equipment failures are caused by contamination of the bearing oil.
Cause categories equipment failureIt is, therefore, important that fitting proper bearing seals is a fast and sure way to maximise rotating equipment effectiveness and reduce up to 20 per cent of the total plant failures.
What are “proper” bearing seals?The first place to look is the premier pump specification, API610 Ed10, section 5.10.2.7, which states “replaceable Labyrinth-type” or “magnetic-type end seals” states that “lip-type seals shall not be used”.
The word “replaceable” is an important word to note as it generally provides the first distinction between a bearing seal specifically designed for the reliability-focused individual and one which is an afterthought, which has typically been machined integrally into the bearing plate by the OEM (30 years ago), to save cost.
Section 5.1.25, of the specification goes on to state that said devices “….shall be designed to minimise contamination by moisture, dust, and other foreign matter during periods of operational and idleness.”
The design differences of Labyrinth sealsThere are many types and configurations of non-contacting Labyrinth seals.
Labyrinths specifically work when the rotating equipment is dynamic as they rely on a close fitting, non-contacting interface between the rotor and stator.
The first important aspect to observe is whether a sealing member is present between the Labyrinth rotor and stator, and what exactly the manufacturers claims are, relating to this. If a sealing member is not present, the design is probably about as much use in blocking vapour as using a fishing net to stop mosquito bites.
Certain manufacturers add a static fibre sealing member or lip seal between the rotor and stator that supposedly blocks the rotor to stator gap. A plant engineer will quickly understand that such a static, wearing element will leak over time, add frictional heat to the equipment and pass worn contaminates into the sealed lubrication fluid. Other manufacturers incorporate an O-ring that moves radial in and out of a groove, supposedly overcoming frictionally resistance between the two parts. Again, a plant engineer will understand that in order for something to radial move, it needs to be compliant. Yet if, at the same time, it is expected to perform a sealing function between two counter rotational parts, it needs to be rigid and wear resistant. This results in a conflict of functions and a compromise is made to the seal performance.
On top of this, if the same O-ring holds the rotor and stator axially together as well as sits against the edges of an abrupt groove, its life expectancy must be questionable, given that the O-ring is being axially sliced during installation.
One manufacturer that has gained favour with European engineers has a Labyrinth design that incorporates a dynamic O-ring which makes sealing contact, during equipment idle conditions, against something that is similar in shape to a valve seat.
The sealing O-ring is offered in a wear resistant, low co-efficient of friction material and is energised into sealing engagement by a radial moving O-ring made of a compliant material.
LabTecta 66 bearing protectorsThis may help in explaining why plant engineers favour this design, given that the sealing elastomer theoretically should exhibit near zero wear, thus increasing its life exponentially. This device may also explain why the global manufacturers of rotating Labyrinth seal with integral, self-adjusting axially energized shut-off O-ring have independent IP55 product certification. (IP55 is the premier water and dust protection standard as specified by the IEEE 841-2001 electric motor standard.)
It is also evident that some rotating Labyrinth designs and configurations are decidedly not field-repairable whereas other manufacturers have made it their business to design this “in-place repairability” into rotating Labyrinth seals. This is an important issue for both the maintenance engineer and the buyer as the buyer is typically responsible for managing the spare-parts costs also.
In essence, these technical developments show that technological advances exist in bearing seals, and it is no longer acceptable to be using lip seals in rotating equipment.
Initial cost barriers to change are no longer an economical restraint, given the fact that high-performance, replaceable bearing protectors are manufactured and readily available worldwide.
That said, reliability-focused users, both OEM and end user alike, need to embrace low-cost, maximum-reliability-gain products such as non-contacting, replaceable Labyrinth bearing seals.