A Contrarian View: Rediscovering And Forgetting Lessons Learned

By Heinz P. Bloch, P.E.

The term “integrally geared compressor” describes equipment consisting of a large-diameter, direct-driven input gear (the “bull gear”) and one or more (to a maximum of six) driven, smaller-diameter pinions. Because each pinion end can be fitted with a bladed impeller, 12 stages of compression can be obtained. When a reader asked about the feasibility of using variable-speed drives for these integrally geared machines, we advised against it because an infinite number of resonant vibration frequencies could harm one or more of the slender, three-dimensional, contour-machined impeller blades.

The answer generated feedback. One reader recalled an interesting installation he encountered while working for a major manufacturer in the 1990s. He was visiting a customer in South America with a steam turbine-driven Plant Air Package (PAP) compressor. This workhorse was a fine, integrally geared unit with a successful 1-, 2- or 3-stage design dating from the 1960s.

The reader’s company had quoted a digital control system for the PAP, but the customer chose to purchase a more expensive one from an independent third-party. Asked why, the customer explained that the owners were adjusting the turbine speed to move the operating curve and meet plant demand at a more optimal efficiency. There were other advantages, to be sure, but when he was warned that the equipment could be operating near resonance on occasion, the customer didn’t seem concerned. The reader added that he wondered how the customer’s machine had operated in the years since.

This discussion, in turn, reminded me of a legal deposition I attended that involved testimony by an OEM vice president. The gentleman explained calmly—and truthfully—that his company routinely rediscovered and forgot “lessons learned” in 30-year cycles. Some end-user organizations exhibit similar tendencies: They, too, have short memories and work in cycles.

From personal experience, I am familiar with PAPs owned by the affiliates of a major multi-national petrochemical company. We believed in simple controls for them. The experienced manufacturer of these compressors gave the correct advice when cautioning against running PAPs at variable speed—no criticism there. Even the most impressive electronic controls might not be able to track changes in blade resonance due to a mighty combination of compressor fouling, blade erosion and the failing memory of a night shift after celebrating whatever it is that night shifts have occasionally celebrated.

I am also somewhat familiar, from personal experience, with wisdom, and the lack thereof. Wisdom is the application of knowledge. In short, all people with wisdom have knowledge, but not all people with knowledge have wisdom. And so it is that we get ourselves in trouble when we encounter a lack of either knowledge or wisdom. Wisdom must be acquired via the laborious “gaining-knowledge-through-learning” route. Alas, learning takes effort, and not everyone is willing to expend such effort.

In summary, much of whatever knowledge or wisdom we might acquire through decades of work, including in the reliability arena, may: a) not be passed on to others; b) fall prey to operations that are being encouraged to be “lean and mean”—often mistakenly viewed as an invitation to use shortcuts; c) not be acknowledged by those who are embarrassed by their own oversights and mistakes; and d) in the case of success, may not be cleared for publication for fear of giving away competitive advantage. This gets us back to our original point: We, too, wonder how that South American PAP-user fared in the long run. MT

heinzbloch@gmail.com