Page 165 - Steam Turbines Design, Applications, and Rerating

Governors and Control Systems 145

ing the nozzle area and, consequently, the steam flow. The resulting
relationship was shown earlier in Fig. 2.8.

8.4.2 Multivalve turbines
Multivalve turbines usually use a type of valve entirely different from
single-valve machines. Such a valve is commonly called a venturi valve.
It is used for higher pressures and flows because of its superior flow lift
characteristics. A series of such valves are then used (that is, 4, 5, 6, or
7) for finer control and better part load economy. Valve sizes vary up to
a nominal 5-in diameter.
Grid type valves are also used, usually for extraction service. This
type of valve offers very high flow areas required for extraction pres-
sures of less than 50 psig. Occasionally, grid valves have also been used
for inlet service. Venturi valves are used for extraction pressures
greater than 50 psig (3.5 bar).
As was discussed earlier, on multivalve machines venturi valves are
located in the top half of the steam chest casing and are lifted by a bar
or cam mechanism. In bar mechanisms the valves and stems are set in
the bar with different stem heights to allow them to be lifted from
their seats in the proper sequence and move positively in the opening
direction. Motive force to move the valves in the closing direction
comes from the weight of the valve itself and the unbalanced steam
pressures across it. This is again illustrated in the bar lift arrange-
ment of Fig. 8.5. In cam mechanisms, Figs. 2.4 and 2.6, each valve is
individually opened or closed through the direct contact action of a
contoured cam.
In a multivalve turbine, the steam flow is divided and directed to
several nozzle groups. Each nozzle group is controlled by an individual
valve. An increase in steam flow through the turbine is obtained by
opening the valve on successive nozzle groups, thereby increasing the
nozzle flow area. The lifting beam has the valve stem length so gradu-
ated that the minimum number of nozzle groups are passing steam to
satisfy the load requirements. When these individual valves are open,
there is no throttling.
In essence, the multivalve turbine changes steam flow by cutting in
or cutting out small increments of nozzle flow area. Since little, if any,
throttling is done, the multivalve turbine is more efficient, particularly
at part loads, or under varying load conditions.

8.5 PG Governors
Automation and process requirements are expanding and demanding
more and finer control. Thus, users have more need for a governor sys-