on valve, so that no steam can reach
the actuating piston of the secondary valve until it has passed through
the primary valve. When the pilot valve is closed, the pressures
equalize above and below the piston N and the valve remains upon its
seat. When the load upon the turbine exceeds its rated capacity, the
pilot valve moves upward so as to connect the space above the piston
with the exhaust L, relieving the pressure upon the upper side and
allowing the greater pressure below to force the valve open, which
admits steam to the secondary stage of the turbine.

It would do no good to admit more steam to the first stage, for at the
rated capacity that stage is taking all the steam for which the blade
area will afford a passage. The port connecting the upper side of the
piston N with the exhaust may be permanently closed by means of the hand
valve Q, to be found on the side of the secondary pilot valve chest,
thus cutting the secondary valve entirely out of action. No dashpot is
necessary on this valve, the compression of the steam in the chamber W
by the fall of the piston being sufficient to avoid shock.

The timing of the secondary valve is adjusted by raising or lowering the
pilot valve by means of the adjustment provided. It should open soon
enough so that there will not be an appreciable drop in speed before the
valve comes into play. The economy of the machine will be impaired if
the valve is allowed to open too soon.

Safety Stop Governor

This device is mounted on the governor end of the turbine shaft, as
shown in Figs. 52 and 53. When the speed reaches a predetermined limit,
the plunger A, having its center of gravity slightly displaced from the
center of rotation of the shaft, is thrown radially outward and strikes
the lever B. It will easily be understood that when the plunger starts
outward, the resistance of spring C is rapidly overcome, since the
centrifugal force increases as the square of the radius, or in this case
the eccentricity of the center of gravity relative to the center of
rotation. Hence, the lever is struck a sharp blow. This releases the
trip E on the outside of the governor casing, and so opens the steam
valve F, which releases steam from beneath the actuating piston of a
quick-closing throttle valve, located in the steam line. Thus, within a
period of usually less than one second, the steam is entirely shut off
from the turbine when the speed has exceeded 7 or 8 per cent of the
normal