fig 1

Figure 1: Assets and human lives are at stake when there are pump fires in refineries or petrochemical plants.

It was known (in 1974) that for every 1,000 refinery pump repairs, there was a pump-related fire incident. More recently, and in an oil refinery with approximately 2,000 installed pumps, the acknowledged mean time between repairs (MTBR) was six years. This would allow us to calculate that approximately 333 pumps underwent repair each year. Since that particular facility experienced five pump-related near-disasters in the span of 14 years, doing the simple math tells us that its rate of major pump issues tracked the 1,000-per-1 rule for presumably API-compliant pumps within 6% accuracy.

Failure statistics tell the story
An airplane has about 4,000,000 parts, an automobile approximately 10,000 parts and a centrifugal process pump only about 200 parts. It's fair to say that if a machine is made up of a large number of parts, more parts could malfunction. However, this does not mean that more parts will, in fact, malfunction during an operational cycle. So, what's the point of this reminder?

As we think about the reasons why the average process pump requires a repair after approximately six years, we realize that not all of its components are designed, fabricated, assembled, maintained, operated, or perhaps installed with the same diligence as aircraft components.

It doesn't have to be that way. Alloys can be upgraded and better components are sold to owner-purchasers that insist on such upgrades. Advanced computer-based and reasonably priced design tools are available for the pump hydraulic assembly. It has been shown that computational fluid dynamics (CFD) can be used to define the improvement potential of impellers and stationary passages within pumps; Figure 2 certainly attests to that.

fig 2

Figure 2: Relative velocity plot of an optimized vertical pump stage (Source: Pump Design, Development & Diagnostics; gregcase@pdcubed.net)

But the mechanical assembly (drive end) of some pumps also deserves attention, especially since this portion of the pump has been neglected in some brands or models. Fortunately, expert advice is available for the specification and selection of better drive end geometries for process pumps (see author's note below regarding availability of full-length text).

Thoughtful specification and selection used to be par for the course at best-of-class companies and there is really no reason why this thinking should have undergone change. What we see lacking today is an awareness of the precise steps that are needed for such specifying and selecting. Management has fallen prey to consultant-conceived generalities, including "lean and mean" and similar catchy utterances.

Opportunities in sealing and environmentally-friendly technology
Superior dual seals and environmentally-friendly sealing systems (Figures 3 and 4) are now available and routinely selected by reliability-focused owner-purchasers. The primary use of cost-effective sealing is likely on ANSI and ISO-style pumps. Many slurry services in the mining, pulp and paper, and power generation industries also have benefited from adopting dual seals (Figure 3) with closed, water-containing seal support systems (Figure 4). At the pump, the sealing water is totally confined in the space between the bore diameter of the two pairs of seals and the outside diameter of the shaft sleeve. The amount of makeup water required can be as little as 40 liters per year and many systems pay for themselves in less than one month.

fig 3

Figure 3: A dual mechanical seal. The space between the sleeve and the inside diameter of the two sets of seal faces is filled with a pressurized barrier fluid, usually clean water. (Source: AESSEAL Inc., Rockford, TN and Rotherham, UK)

fig 4

Figure 4: Pre-engineered self-contained water management system found highly effective for dual mechanical seals in slurry services (Source: AESSEAL Inc., Rockford, TN and Rotherham, UK)

Taking advantage
We obviously believe that good managers must lead and ought to reinforce a failure avoidance culture. That said, the user industry should get away from the obviously flawed approach whereby people are instructed to run equipment to failure and to then try to resurrect it to a better life than ever before. It also makes no sense to wait for things to go wrong and then heap praise on super-human efforts to rebuild. The old adage of an ounce of prevention being worth a pound of cure is as valid as ever. So, consider this a call to do something about your repeat failures and start asking questions. Better yet, offer some of the solutions that have consistently kept best-of-class performers in the top rankings for safety and profitability. They have stayed at the top because they take advantage of best available technology.

Author's note: This material was excerpted and condensed from Bloch, Heinz P., Pump Wisdom-Problem Solving for Operators and Specialists, New Jersey: John Wiley & Sons, 2011 (ISBN 978-1-118-04123-9). The hard-bound text is available from Amazon.com (List price: $49.95). It highlights highly effective techniques to prevent repeat failures of process pumps. Among other issues, it divulges and discusses superior bearing protection and sealing issues that not all pump manufactures share with their clients.

authorHeinz P. Bloch is a practicing consulting engineer with 50 years of applicable experience. He advises process plants worldwide on failure analysis, reliability improvement and maintenance cost avoidance topics. A frequent contributor to Uptime magazine, he has authored or co-authored 18 textbooks on machinery reliability improvement and over 500 papers or articles dealing with reliability-related subjects.

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