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Selecting steam turbines in a ‘lean’ environment

02.01.2012  |  Bloch, H. P.,  Hydrocarbon Processing Staff, 

Keywords: [steam turbine] [double-shell machine] [cross-compound machine] [direct-drive turbine] [maintenance]

We received a nice compliment recently from a reader in South America. He wrote: “I am a mechanical engineer working on power plant designs at 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 added, “I am writing you because I could not find all the answers in your steam turbine text.1 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 110 bar and 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, 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. Accordingly, I would like to know your opinion about this machine. Is this solution technically feasible? Are there many operating and maintenance (O&M) problems?”

I drafted an answer agreeing that the recent Bloch-Singh steam-turbine book gives little guidance on the matter.1 It does, of course, describe similar machines. However, the book may have added to the reader’s confusion by mentioning not only cross-compound double-casing machines, but also double-shell steam turbines.

More information needed.

The only way one could make a definitive judgment is to:
a) Look at the guaranteed efficiencies of the two different offers and keep in mind the overall steam balance of the facility
b) Make a decision as to how well trained the operators will be
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) requires considerable diligence, time and effort. The reviewer should add to this a thorough check of the gearbox design and should accept 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. It is always prudent to solicit bids from manufacturers that have ample experience with both direct-drive generator turbines and the more complex compound/reheat multi-casing machines.

With time permitting, consider including a few bidders who can 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. These machines require less attention from the operator. However, during the maintenance cycle, this steam turbine does need very competent maintenance skills.

“Cross-compound” machines are probably found on shipboard, but predominantly at inlet pressures slightly lower than 110 bar. Again, substantial inquiring should be done before a decision can be made. As regards items to be reviewed, one might investigate 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. Investigate who makes the gearbox and how the gears are lubricated.

Total cost issues. Initial cost, operating cost (efficiency) and long-term reliability expenses are of interest, and the total must be considered as part of the life-cycle cost. 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 their 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 are reviewed.

Don’t get caught in the ‘lean and mean’ craze.

A perceptive reader may have seen how our answer alludes 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 steam turbine drivers, and multiple catastrophic blade failures have occurred since then.

It seems that the equipment owner was unaware of the need to look at the vibration modes of the blades for these steam turbines. A Campbell diagram, or interference diagram (Fig. 1) is used to indicate what speeds to avoid and 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. A Campbell diagram is a graph with turbine speed (r/min) plotted on the horizontal axis and the frequency, in cycles/sec, 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. Stress magnitudes are greatly amplified at resonance.

 
  Fig. 1. Campbell or interference diagram
  for a partial steam turbine stage.  

Over the past few years, the mindless interpretations given to “lean and mean” thinking have often led to costly oversights. No time or budget is allocated to understanding what happens when steam turbine speeds are re-set for operations 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 or green, or whatever. Evaluating interference diagrams and steam turbine blade stresses is a mandatory task that can never be overlooked in a modern plant

Likewise, let your specifications reflect attention to seemingly small issues; include such items as keeping lube oil from exiting the bearing housing, or steam leakage from entering into a bearing housing. Review how best-of-class companies have systematically solved these problems by using advanced bearing protector seals (see HPIn Reliability, August 2010) or by scrupulously avoiding outdated or risk-prone old-style components (see HPIn Reliability, October 2007 and HPIn Reliability, May 2009). Include details on field erection requirements in your specification; HPIn Reliability, February 2008 commented on these. Avoid carbon seal rings in steam turbines (HPIn Reliability, April 2008) and use only the most advantageous seal configurations in turbine-support pumps (HPIn Reliability, January 2009). These are just some of the items that can allow you to achieve lowest possible cost of ownership. HP

LITERATURE CITED

1 Bloch, H. P. and M. P. Singh, Steam Turbines: Design, Applications and Re-Rating, 2nd Ed., McGraw-Hill, New York, New York, 2009.

The author 

Heinz P. Bloch is Hydrocarbon Processing’s Reliability/Equipment Editor. 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. 

 



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ZAHEER A MIRZA
02.16.2012

This is a great article for the engineers working on power plants selection and sourcing of machinery.

Regards.

Zaheer

Luiz Diogo de Vasconcellos
02.15.2012

I would like to contact the engineer who presented the question, if possible, in order to help him. I'm a rotating equipment specialist which years of experience in selection and application of turbines for chemical, petrochemical, refinery and power plants in Brazil. I would be glad to exchange ideas and help him. I dont mean selling him anything, let alone engineering manhours. I would be glad to help just by the interchanging of ideas and experiences.

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