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)
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 readers
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
a) Look at the guaranteed efficiencies of the two
different offers and keep in mind the overall steam balance of
b) Make a decision as to how well trained the operators
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
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.
Dont get caught in the lean and mean
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
1. Campbell or interference diagram
for a partial steam turbine
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
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
1 Bloch, H. P. and M. P. Singh, Steam
Turbines: Design, Applications and Re-Rating, 2nd Ed.,
McGraw-Hill, New York, New York, 2009.
Heinz P. Bloch is Hydrocarbon
Processings 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