Page 24 - Steam Turbines Design, Applications, and Rerating

Introduction 5

Since there is theoretically no pressure drop across the moving
blades (and thus no reaction), internal clearances are large, and no bal-
ance piston is needed. These features make the impulse turbine a
rugged and durable machine that can withstand the heavy-duty ser-
vice of today’s mechanical drive applications.

1.2.1 Steam turbine staging can vary
First, let’s consider velocity-compounded (Curtis) staging. A Curtis
stage consists of two rows of moving blades. Stationary nozzles direct
the steam against the first row; reversing blades (not nozzles) then
redirect it to the second row.
The large pressure drop through the nozzle produces a high-speed
steam jet. This high velocity is absorbed in a series of constant pressure
steps (see Fig. 1.4). The two rotating rows of blades make effective use
of the high-speed jet, resulting in small wheel diameters and tip speeds,
fewer stages, and a shorter, more rugged turbine for a given rating.
In pressure-compounded (Rateau) staging, the heat energy of the
steam is converted into work by stationary nozzles (diaphragms) direct-
ing the steam against a single row of moving blades.As in a Curtis stage,
pressure drops occur almost entirely across the stationary nozzles.

1.2.2 Modern impulse design
The importance of steam turbine efficiency has continued to increase
over the last decade. Today, there is no pure impulse turbine. Manufac-
turers are using a combination of reaction and impulse design features
to further improve turbine efficiency. The traditional impulse turbine
manufacturers, who utilize the basic wheel-and-diaphragm construc-
tion, have been able to meet, and many times exceed, the performance
of a pure reaction turbine. This is done on high-pressure stages by
adding a small amount of reaction to improve the performance, without
the need for tight leakage controls or increasing thrust forces. Tall, low-
pressure buckets are designed with more reaction than ever before
using advanced aerodynamic codes for these complex blade forms.
The generous clearances of the wheel-and-diaphragm construction
decrease the dependence on tight leakage control. Field data have
shown that these modern impulse turbines will sustain their high level
of performance over time and are much more tolerant to fouling, which
can have a significant impact on thrust loads.

1.2.3 Single-valve vs. multivalve
construction
Single-valve units (Fig. 1.5) are available when justified by plant eco-
nomics. When used, individual nozzle ring segments are controlled by