Page 101 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

PIPING 2.51

FIGURE 2.22 Force required to open a gate valve against unbalanced pressure. (Data compiled by The
Chapman Valve Manufacturing Company.)

required at breakaway, reduces to about 30 percent of breakaway in the half-open position
and increases to about 90 percent of breakaway at closure.
Actuators are classified by their source of power: electric, pneumatic, or hydraulic.
Electric Actuators. These are the most commonly used power source. They can be sole-
noid or motor operated. They usually have the least total installed cost because the source of
electricity is generally available throughout a facility and the wiring and control instrumenta-
tion is relatively simple. Solenoid operation is generally limited to smaller lines, ON/OFF
service, and systems in which the water hammer produced by quick closing can be easily
attenuated because of the systems’ low pressure and velocity requirements. Motor-operated
actuators tend to be bulky and slow, particularly when large gear reduction is used to increase
torque. Their advantage is that the torque output is constant throughout their stroke, and their
response is linear. For critical systems, there must be an emergency power supply.
The speed at which a valve closure member is operated is important. The figure gener-
ally used for a gate valve closure member is 12 in (300 mm) per min and for a globe valve
closure member, 4 in (100 mm) per min. Higher rates are available, but exceeding the
maximum specified speed will damage the seat and disk. Gate and globe valves are torque-
seating valves when closed. In the open direction, a limit switch is often provided to protect
the seat against backseat overtightening. Quarter-turn valves are position-limited open and
closed because seating is based on position, not force.
Electric motors do not stop instantaneously, but coast to a stop. The use of a solenoid
brake to prevent the motor from overtightening the closure member should be ascertained
from the manufacturer of the valve on which the actuator is installed.

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