Page 160 - Steam Turbines Design, Applications, and Rerating

140 Chapter Eight

unbalanced forces over the normal lift range. A properly designed
double-seated valve will exhibit the same magnitude of unbalance at
any valve lift point. For high flows and pressures a venturi type valve is
used. This valve has superior flow lift characteristics over the double-
seated type valve.
If controls are combinations of simple elements, why is there trouble
with them and about them? Like any other mechanical or hydraulic
device, governors are prone to certain deterrents. Principally, these are:
■ Friction
■ Lost motion
■ Misunderstanding

Friction cannot be eliminated entirely, as we well realize, but it can be
reduced to a low acceptable value (i.e., proper alignment, lubrication,
proper valve stem design and adjustment). Most steam chest covers are
designed with a valve steam leakoff arrangement and connection to
reduce valve steam leakage without excessively increasing the pressure
of packing on the valve stem. Frictional binding sensed by the packing
can upset an otherwise reliable control system.
Similarly, lost motion can be controlled. For example, excessive toler-
ance in connector pins will cause lost motion, which is nothing more
than the governor or sensing element responding to a small change in
speed although not great enough to effect a change in the steam admis-
sion valve due to the linkage pin or connector tolerance. This problem
can usually be traced and corrected.
Principally, an understanding of a governor and/or system is most
necessary for its proper application and installation. Even an excel-
lently designed control system with proper linkage and adjustment can
offer poor or unacceptable performance if the selection of the system
was not properly made. An attempt will be made to detail various types
of governors and controls along with a classification, specification, vari-
ation, performance, and recommended application of each type.

8.2 Governor System Terminology

As a means of rating the performance of a governor or control system, the
requirements are usually specified in terms of percent speed regulation.

8.2.1 Speed regulation
Speed regulation, expressed as a percentage of rated speed, is defined
as the change in speed when the power output of the turbine is grad-
ually changed from rated output to zero output, under the following
conditions: