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By Heinz P. Bloch, P.E., Process Machinery Consulting
If you’re looking for reliable process plant machinery, please realize that almost any machine can be made more reliable by expending additional design, engineering, fabrication, installation, and maintenance effort. Unfortunately, with equipment offers generally being evaluated on as-proposed initial costs alone, manufacturers are often under pressure to leave off desirable reliability enhancements. A sensible approach to machinery procurement would, therefore, invoke compliance with well-thought-out design, manufacturing, and installation specifications.
Many purchasers and users of machinery are, of course, opting for this specification approach. However, even such well-known specifications as API 610 (Centrifugal Pumps for General Refinery Services) are not always sufficient. First, the user of this specification should realize that, like other industry specifications, API 610 contains a large number of clauses which require that the purchaser exercise an option. The pertinent paragraphs typically start with: “When specified …” or “With the purchaser’s approval ….” Obviously, these caveats should compel the specifying entity to take a very close look at this or, for that matter, any other industry specification for process machinery.
API or other industry specifications are not used as stand-alone specifications by the world’s best petrochemical and oil refining plants. The leading users of, say, centrifugal pumps, recognize that specification supplements or amendments may be in order. These users are cognizant of the various qualifying clauses given in the foreword to one of the typical editions of API: “The purchaser may desire to modify, delete, or amplify sections of this standard” or “Standards are not intended to inhibit purchasers or producers from purchasing or producing products made to specifications other than those of API.” Appropriately, the American Petroleum Institute (API) expressly disclaims any liability or responsibility for loss or damage resulting from the use of its standards, or for the violation of any regulation with which the standards publication may conflict.
The API’s stance is entirely reasonable, but even a well-written industry specification will often merit amendments and additions if extended life of machinery, reduced maintenance frequency, and lower equipment repair costs are desired.
Figure 1: Many oil rings tend to misalign and then touch portions of the housing interior. As abrasion occurs, the lubricant will be contaminated.
A case in point would be API Ninth Edition (January 2003) Paragraph 3.33, which doesn’t list oil rings (Figure 1) among the wear parts to be replaced during pump overhauls. Of course, the API standard also doesn’t mention when, where, or why oil rings are not the preferred lube application method for reliability-focused users. The standard thus leaves the choice to the pump manufacturer, and low manufacturing cost becomes the driver.
Experience shows, however, that unless a shaft system is installed perfectly horizontal, oil rings run downhill and often make intermittent contact with a stationary interior part of the bearing housing. They then slow down and become unstable. Worse yet, they now tend to abrade, and you can see loss of chamfer after a few months of operation. The wear product contaminates the lube oil, which causes bearings to fail very rapidly. The lube oil often turns gray in color, which is a sign that oil ring debris is present.
While aligning shafts, we usually put shims under part of the pump or its driver. Rarely do we ensure shaft system horizontality. Also, oil rings become unstable at certain shaft surface velocities. And this causes reliability-focused users to question the applicability of oil rings in modern pumps. Oil rings are immersion-sensitive — that is, more oil, more drag. They are viscosity-sensitive, meaning thicker oil results in more drag. They are also extremely sensitive to both out-of-roundness and rough RMS surface finish. They will become out-of-round unless they are first rough-machined, then stress-relief-annealed and, finally, finish-machined.
Combine all of the typical field deviations and you'll probably end up with a “safe” DN-limit (inches of shaft diameter times rpm) of somewhere around 6,000. That's an experience-based limit; it was also used in a multinational oil company’s lube marketing-internal training courses. Invoking this safe limit will perhaps allow a certain amount of deviation from perfect shaft horizontality, ideal ring depth of immersion, oil viscosity, ring concentricity, and ring I.D. surface finish (recommended to be 32 RMS). Unfortunately, nobody knows the extent to which every possible deviation from ideal values occurs in operating equipment or which combination of deviations will bring on oil ring instability.
This is just one of hundreds of examples of why it’s often to the purchaser’s advantage to amend or add specification paragraphs. In such areas as cooling of bearings and pedestals, an up-to-date user would improve equipment life and even reduce the risk of pump fires by disallowing cooling water in rolling element bearing housings and in pump support pedestals.
|Heinz P. Bloch, P.E., is owner of Process Machinery Consulting (www.heinzbloch.com) in Westminster, Colorado, and the author of numerous articles and books, including “Improving Machinery Reliability” and “Pump Wisdom.”|
Pedestal cooling is not required for any centrifugal pump in petrochemical plants. Pumping services with fluid temperatures as high as 740 °F (393 °C) require only hot alignment verification between driver and pump. Also, even in very high temperature pumping services, the rolling element bearings operate at sufficiently low temperatures without cooling. It presupposes that the pump owner uses a properly formulated synthetic lubricant and the lubricant is properly applied.
Note again that these two reliability improvement measures — no cooling of pedestals and rolling element bearings — were known 30 years ago. Books and articles explained the issue as early as 1977.
So, to stay on top of cost savings, preservation of resources and reliability improvement opportunities, look to the industry leaders and supplement your equipment specifications by making use of their experiences. Look at the right books for this knowledge and steer clear of anecdotal stories that are passed on by people for whom repeat failures have become the accepted norm. Best-of-class companies view every maintenance event as an opportunity to determine if upgrading — that is, future failure avoidance — is possible. If the answer is affirmative, best-in-class companies will always take tangible steps to upgrade the equipment.