To recap, the author’s previous article for The RAM Review (Feb. 6, 2022, see link below) discussed a letter from mechanical engineer in South America who was seeking guidance on steam turbines for a combined-cycle-power-plant. The suitability of one type of steam turbine over another was an important issue. In his response to, Heinz listed three “must dos” in the making of a definitive judgment. Among other things, he also pointed out that making the right decision would require “considerable diligence, time, and effort.” This week, he’s expanding on that advice. Read on.
Time permitting, the engineer in South America should seriously consider adding a few bidders willing to comment on the very advisability of double-shell machines. A double-shell construction machine prevents inlet steam coming into direct contact with the outer casing joint. Double-shell machines are (probably) less in need of careful operator attention but, during the maintenance cycle, might require rather competent maintenance skills.
“Cross-Compound” machines are probably found on shipboard, but predominantly at inlet pressures slightly lower than 110 bar. Again, there is much inquiring to be done before a decision can be made. As regards items to be reviewed, one might investigate, among other things, the lubrication system. In a cross-compound machine the input and output shafts are at different levels and the lubrication system serves not only the turbine and generator bearings, but also the gearbox. Who makes the gearbox and how are the gears lubricated?
Initial cost, operating cost (efficiency), and long-term reliability are of interest and must be considered as life-cycle cost factors. All are of equal concern and, without making a final judgment one way or the other, many different options should be explored before reaching a conclusion.
Although one should make good use of vendor input and defer to the vendor’s demonstrated experience, expect double-shell machines to cost more money and cross-compound machines to require more than the average maintenance commitment. And the “simple” machine would also stay in the running until all the data have been put on paper.
DON’T GET CAUGHT IN THE ‘LEAN & MEAN’ CRAZE
A perceptive reader may have noticed that our answer the our correspondent in South America alluded to the subject of suitability analyses or pre-purchase selection work that needs to be done. We were reminded of the pitfalls of “lean and mean” when another facility experienced several extreme failures on smaller two-stage back-pressure mechanical drive steam turbines.
For several years, these turbines had been driving refrigeration compressors without incidents. Then, about two years ago, the refrigeration gas composition was changed to accommodate new (and well-justified) environmental concerns. The new gas conditions mandated a speed change for the turbine drivers and multiple catastrophic blade failures have occurred since then. Apparently, the equipment owner was unaware of the need to look at the vibration modes of the blades in these steam turbines.
A Campbell Diagram, or interference diagram, is used to indicate what speeds to avoid so as to safeguard blade life in a particular stage. Because almost all blade failures are caused by vibratory stresses, many reliability-conscious purchasers are requesting Campbell Diagrams with turbine quotes or orders.
Note: A Campbell Diagram is a graph with turbine speed (r/min) plotted on the horizontal axis, and frequency (in cycles per second) plotted on the vertical axis. Also drawn in are the blade frequencies and the stage-exciting frequencies. When a blade frequency and an exciting frequency coincide or intersect, it is called resonance. “Schaufelsalat” (mixed salad made with turbine blades) is a common result because stress magnitudes are greatly amplified at resonance.
WORDS TO THE WISE
Over the past few decades, the often mindless interpretations given to “lean and mean” thinking have led to some costly oversights. Little time or budget has bee allocated to understand what happens when steam turbine speeds are re-set for operation away from the original governor adjustment range. The result has been a much higher probability of steam-turbine blade failures. Consider this comment a plea to know if and when it is proper to be lean, green, or whatever.
Evaluating interference diagrams and steam-turbine-blade stresses is a mandatory task. It can never be overlooked in today’s plants.TRR
Editor’s Note: Click Here To Download A Full List Of Heinz Bloch’s 24 Books
ABOUT THE AUTHOR
Heinz Bloch’s long professional career included assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. A recognized subject-matter-expert on plant equipment and failure avoidance, he is the author of numerous books and articles, and continues to present at technical conferences around the world. Bloch holds B.S. and M.S. degrees in Mechanical Engineering and is an ASME Life Fellow. These days, he’s based near Houston, TX.
Tags: reliability, availability, maintenance, RAM, professional development, steam turbines, Campbell Diagrams
