September 2021

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Reliability: Calculating the value of upgrading: Shortcut estimations find savings and payback

Bearings are precision components; to survive, they require clean lubricants in adequate amounts.

Bloch, Heinz P., Hydrocarbon Processing Staff

Bearings are precision components; to survive, they require clean lubricants in adequate amounts. Even seemingly small amounts of contamination can greatly reduce equipment reliability and uptime. Using bearing housing protector seals as a typical example, this article shows how the business case can be made for improvements.1

By the late 19th century, lip seals of various configurations were developed as bearing protector components. A 1940 vintage lip seal activated by a garter spring is shown above the simulated centerline in FIG. 1. However, lip seals are prone to wear and, since about 1970, have been largely superseded by modern rotating labyrinth seals. Today’s advanced or “best available” models are often engineered with axially moving O-rings, and one such product is shown below the centerline in FIG. 1.

FIG. 1. Typical lip seal (top) and advanced rotating labyrinth seal (bottom). Source: AESSEAL Inc.

Many texts make a compelling case for preventing water intrusion into bearing housings. Once saturation is reached, the oil will absorb no additional water. Added water will exist as “free water,” and bearing life will be greatly reduced. Making the business case for seals to keep out water may become necessary. Proving the value of rotating labyrinth seals is made possible by examining local maintenance records or by using shortcut-style, generally applicable “rules of thumb.” These rules are readily demonstrated in a few examples.

Comparing cost of ownership

Contrasting the total cost of ownership of a set of lip seals with that of two labyrinth seals will prove revealing. Our comparison assumes a cost of $20 for two lip seals. However, lip seals wear relatively rapidly and typically require replacement after only 2,000 operating hours. Accordingly, and in this example case, they will need to be replaced four times per year.

Comparison with a pair of rotating labyrinth seals at $600 and, conservatively, surviving 4 yr is rather telling. In either case, the cost of maintenance labor would be $1,000 per event. Lip seal replacements would cost [$20 + $(1,000 × 4)] = $4,080 per year, or $16,320 over 4 yr. Rotating labyrinth seals would cost ($600 + $1,000) = $1,600 over a 4-yr period, or $400/yr. Here, the true cost of lip seals would be 10 times that of modern rotating labyrinth seals!

Calculating upgrade justification

Upgrades justified by calculations based on rules of thumb use a different approach. While perhaps not as precise as the one just submitted in the lip seal vs. rotating labyrinth comparison, three rule-of-thumb calculations have proved useful in making the business case for upgrades in the past.1

An empirical assessment, our first rule of thumb assumes implementation of a single upgrade measure, perhaps selecting advanced bearing protector seals. These could include the hybrid product shown in FIG. 2, or the considerably simpler product shown in FIG. 3. Both can easily extend safe operating life by factors ranging from 1.1 to 1.4. Implementing two different upgrade measures would extend safe operating life by factors from perhaps 1.5 to 2.5; with three low-cost improvement measures tending to achieve from 2.6-fold to roughly 3.3-fold operating life.

FIG. 2. Hybrid lip seal mated with advanced rotating labyrinth protector seal designed for special applications. Source: ASSEAL Inc.
FIG. 3. A narrow-fit advanced rotating labyrinth seal. Source: AESSEAL Inc.

In this example case, an average pump repair costs $12,000 and occurs every 4 yr. We take $3,000 as the per-year repair outlay and purchase two rotating labyrinth seals for $600, thereby avoiding one such repair. The likely payback would be $3,000 ÷ $600 = 5 times/yr, or approximately every 10 wk. We also could have said that repair frequencies go from one every 4 yr to one every 5.5 yr. Instead of an imputed $3,000/yr, we now only spend $3,000 ÷ 1.4 = $2,150/yr. We can be certain that a $600 set of two advanced bearing protector seals will last 6 yr = $100/yr. The payback ratio is $2,150 ÷ 100, or about 20:1. One-twentieth of 1 yr equals about 3 wk.

A second rule of thumb uses an exponential approach. It assumes that if a fully upgraded machine has a reliability of 1.0, then a missed upgrade will lower the machine’s reliability (or life) to 90% of 1.0 = 0.9; two missed upgrades lower reliability to 90% of 0.9 = 0.81; three missed upgrades to 90% of 0.81 = 0.73; four missed upgrades to 90% of 0.73, equaling only 0.66, and so forth. We consider this elementary rule of thumb rather optimistic; reliability or operating life expectancy with four deficiencies is probably less than 50% of what would be achieved with better bearings, better mechanical seals, better couplings or whatever other upgrades are available today.

Suppose, for example, that we had requested the purchase of a $600 set of advanced bearing protector seals after hearing that a neighboring “Refinery X” is routinely doing this for a critical pump. We know that our critical pump has a mean time between repairs (MTBR) of 3 yr, and assume that Refinery X’s MTBR reaches 3 ÷ 0.9 = 3.3 yr. They, too, spend money on pump repairs. We spend $12,000 ÷ 3 = $4,000/yr, and “Refinery X” spends $12,000 ÷ 3.3 = $3,600/yr. Over a 6-yr period, we spend $24,000; they spend $21,600. Their $600 upgrade returns $2,400. Their mindset puts “Refinery X” on the track to routinely do these and similar upgrades. They either are, or likely will soon become, a best-in-class performer. Their market valuation probably tells the story of doing things smart and with forethought.

A third rule of thumb is also worth sharing. Again, reasonable assumptions are made; a probable 20% improvement in failure avoidance, repair cost reduction or life extension is thought to result from each upgrade. In such upgrade examinations, the first such initiative will move equipment reliability from 1.0 to 1.2, a second (different) upgrade would capture 1.2^2=1.44; further upgrades yield 1.2^3 = 1.73, and 1.2^4 = 2.07. The implementation of four proven upgrade measures would cause the MTBR to be extended two-fold. Yearly repair expenditures would be 50% of what they were before; employees previously involved in repairs would spend time on repair avoidance tasks. Safety would go up and community goodwill would be given a boost, as would worker morale.

Using pressure-balanced constant-level lubricators

In this example, we opt for routine upgrades by (a) using sets of advanced bearing protector seals ($600), (b) switching to extended-life synthetic lubricants (incremental cost of $200 per charge), (c) installing pressure-balanced constant level lubricators (FIG. 4) (incremental cost of $100) and (d) purpose-designed stress-relieved (annealed before final machining) brass or bronze oil rings ($200). It would be reasonable to assume that the four upgrades totaling $1,000 will shift the asset’s operating life from 3 yr to 3 × 2.07 = 6.2 yr.

FIG. 4. Pressure-balanced constant-level lubricator. Source: TRICO Manufacturing Co.

Suppose that our records showed it costs $18,000 to repair the process pump in this example, and repairs would be made every 3 yr. Distributing $18,000 over 3 yr equals $18,000 ÷ 3 = $ 6,000/yr. A one-time expenditure of $1,100 results in spending only $18,000 ÷ 6.2 = 2,900/yr. Prorated savings are about $3,100 during each of the next 6 yr. Near-term payback time will be $1,100 ÷ $2,900, which is less than 5 mos.

Where contaminants originate

Let us return to our bearing protector seals and an examination of where contaminants originate. Moisture and dust often enter bearing housings through old-style, ineffective labyrinth seals or worn lip seals. Moisture is airborne water vapor; it could also be a stream of water from hose-down operations. Contaminants frequently enter through a breather vent, or from the widely used non-pressure-balanced constant-level lubricators depicted in just about every pump text in print today.1,2 These references describe how abraded oil ring material becomes an often-overlooked source of oil contamination.

How to stop the contamination. There should be no communication or connection between the housing interior and the surrounding ambient air. Breather vents normally supplied at the top of bearing housings should be discarded and the threaded port fitted with the connector shown in FIG. 4. Note that the traditional open-to-surrounding-air constant-level lubricator has been upgraded to the balanced lubricator shown in this image.

Surprisingly, even the pressure-balanced lubricator in FIG. 4 suffers from a seldom-recognized water intrusion path: the caulking that bonds the oil-filled glass bulb to its support casting. Sooner or later, caulking will develop tiny fissures or cracks, allowing water to enter through capillary action. This makes a compelling case in favor of oil mist.

The use of a face seal, along with recommendations such as deleting the breather vent and using balanced constant-level lubricators or upgrading to oil mist2 will prevent the entry of virtually all external contamination into the housing. However, none of these measures will avoid contamination from inadequate oil rings and potential defects introduced by an old-style bearing housing protector seal. HP

LITERATURE CITED

  1. Bloch, H. P, Fluid Machinery: Life Extension of Pumps, Gas Compressors and Drivers, DeGruyter, Berlin, Germany, 2020.
  2. Bloch, H. P., Optimized Equipment Lubrication: Conventional Lube, Oil Mist Technology and Full Standstill Protection, DeGruyter, Berlin, Germany, To be published December 2021.

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