Page 37 - Steam Turbines Design, Applications, and Rerating

18 Chapter One

valves with a bar lift mechanism. The extraction valves illustrated for
the first extraction are internal spool valves. With spool valves, a hori-
zontal, external cam shaft lifts four vertical stems. Each stem positions
two internal spool valves, one each in the upper and lower halves of the
turbine.The valves are designed to open sequentially, providing the effi-
ciency advantage of multiple partial arc admissions to the immediate
downstream stage group.An axial-flow extraction valve is shown for the
second extraction. Axial-flow or grid valves are appropriate for low-
pressure applications and high-volume flows. The additional benefit is
the relatively short span that is required. Shown on the low-pressure
end, which is the drive end, is a turning (barring) gear used to rotate the
shaft when the turbine is cooling down. These turning devices are often
utilized on high inlet temperature turbines over 850°F (455°C) and
when the bearing span is over 150 inches (3810 mm).
The double automatic extraction turbine shown in Fig. 14d has the
same inlet valve gear as the previous machine but has a cam-lifted
valve gear with a cast nozzle box for the first extraction. This is the
design typically used for moderate extraction pressures ranging from
650 to 250 psig (45 to 17 bar). The second extraction is a bar lift, spool
valve design.
Steam volume increases rapidly as the steam expands to condenser
pressure. Thus the length of the buckets (blades that make up the tur-
bine rotor) increases rapidly between the inlet of the LP section and
the last-stage buckets.
Today there is a wide range of bucket designs applied to steam tur-
bines. Traditional tangential dovetails may not provide adequate cen-
trifugal capability on high-speed machines such as those applied on
syn-gas turbines. Here, axial entry dovetails and features such as inte-
gral and double-covered designs provide the long-term reliability and
wide speed range necessary for mechanical drive applications. Tall con-
densing section buckets have seen the evolution from aircraft engine
and gas turbine technology. Features such as axial entry dovetails and
Z lock covers are used to dampen vibration and lower response factors
to allow greater bucket loads with higher reliability. Examples of
impulse buckets are shown later in this text.
Operation over a wide speed range adds a significant degree of com-
plexity for the turbine designer. Accurate prediction of blade frequency
and stress is necessary to ensure the level of reliability expected today.

1.3.4 Basic steam control considerations
The selection of a particular turbine type is influenced by the nature of
the driven load as well as the need for power and process heat. One set
of valves can control only one parameter at a time: speed/load, inlet
pressure, extraction pressure, or exhaust pressure. The control of a sec-