We recently received a compliment from someone in South America. In his introduction, he wrote, “I am a mechanical engineer working on power-plant designs for a major corporation and admire your work as a writer of turbomachinery books. Your texts are much respected, and I usually refer to them to find answers to my equipment questions.” He then proceeded as follows:
“I am writing to you because I could not find all the answers in your (2009) steam-turbine text. My aim is to clear up some doubts related to steam turbine technical specifications. More specifically, the corporation is developing a combined-cycle power-plant project that includes an 86 MW condensing-type steam turbine with one reheat entry. The HP inlet steam is at 110bar/540 C and the reheat is being designed for 24 bar.
We are communicating with several respected steam turbine manufacturers and some of them are proposing a ‘Standard Type’ machine; in other words, they offer a turbine with a single casing and a single rotor direct-coupled to the generator. But there are also some manufacturers that propose a ‘Cross-Compound Type’ machine, in other words, a turbine with two casings and two rotors. In one offer, the HP rotor is coupled to the generator by gearbox and the IP/LP casing is direct-coupled to the generator.
Personally, I am not comfortable with the ‘Cross-Compound’ machine, as described above. Accordingly, I would like to know your opinion about this machine. Is this solution technically feasible? Are there many O&M (operating and maintenance) problems?”
I sent an answer agreeing that the Bloch-Singh Steam Turbine book gives relatively little guidance on the matter. Although it does, of course, describe similar machines, the book may have added to the reader’s confusion by not only mentioning cross-compound, double-casing machines, but double-shell steam turbines as well. So, in essence, the only way one could make a definitive judgment is to:
(a) look at the guaranteed efficiencies of the two different offers keep the
overall steam balance of the facility in mind
(b) decide as to how well trained the operators will be; and
(c) closely examine the respective field and service experience histories of
the two different turbine offers.
Complying with the basic requirements of (a), (b), and (c) above requires considerable diligence, time, and effort. To this, the reviewer should add a thorough check of gearbox design and accept the fact that time is needed to draw up a comprehensive comparison between the two offers. It would even be appropriate to ask if the original inquiry went to the right bidders. Also keep in mind that when it comes to picking steam turbines, it is always prudent to solicit bids from manufacturers that have ample experience with both direct-drive generator turbines and more complex compound/reheat multi-casing machines.
My next article for The RAM Review will focus on “Selecting The Right Steam Turbine When Given Enough Time.”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
