Wastewater Conveyance - Wastewater Operators - page 210

4.13.3.1 Pneumatic and Hydraulic Valve Operators
Pneumatic and hydraulic valve operators are similar in appearance
and work in much the same way. Hydraulic cylinders using either plant
water pressure or hydraulic fluid frequently operate valves in treatment
plants and pumping stations (AWWA, 1996). In a typical pneumatic ball-
valve actuator, the cylinder assembly is attached to the ball-valve stem
close to the pipe. A piston inside the cylinder can move in either direc-
tion. The piston rod is linked to the valve stem, opening or closing the
valve, depending on the direction in which the piston is traveling. As
a fail-safe feature, some of these valves are spring loaded. In case of
hydraulic or air pressure failure, the valve operator returns the valve to
the safe position.
Note: Valve operators and positioners usually require more maintenance
than the valves themselves (Casada, 2000).
4.13.3.2 Magnetic Valve Operators
Magnetic valve operators use electric solenoids. A solenoid is a coil
of magnetic wire, roughly in the shape of a doughnut. When a bar of iron
is inserted as a plunger mechanism inside an energized coil, it moves
along the coil because of the magnetic field that is created. If the plunger
(the iron bar) is fitted with a spring, it returns to its starting point when
the electric current is turned off. Solenoids are used as operators for
many different types of valves used in water/wastewater operations; for
example, in a direct-operating valve, the solenoid plunger is used in place
of a valve stem and hand wheel. The plunger is connected directly to the
disk of a globe valve. As the solenoid coil is energized or de-energized,
the plunger rises or falls, operating or closing the valve.
4.13.4 valve Maintenance
As with any other mechanical device, effective valve maintenance
begins with its correct operation. As an example of incorrect operation,
consider the standard household water faucet. As the faucet washers age,
they harden and deteriorate. The valve becomes more difficult to operate
properly, and eventually the valve begins to leak. A common practice is
simply to apply as much force as possible to the faucet handle. Doing so,
however, damages the valve stem and the body of the valve body. Good
maintenance includes preventive maintenance, which, in turn, includes
inspection of valves, correct lubrication of all moving parts, and the
replacement of seals or stem packing.
4.14 PiPing sysTeM ProTeCTive deviCes
Piping systems must be protected from the harmful effects of unde-
sirable impurities (solid particles) entering the fluid stream. Because
of the considerable variety of materials carried by piping systems, an
equal range of choices in protective devices is available. Such protective