Wastewater Conveyance - Wastewater Operators - page 204

Each of these valves is designed to control the flow, pressure, and direc-
tion of fluid flow or for some other special application. With a few excep-
tions, these valves take their names from the type of internal element
that controls the passageway. The exceptions are the check valve, quick-
opening valve, regulating valve, relief valve, and reducing valve.

4.13.2.1 Ball Valves

Ball valves, as the name implies, are stop valves that use a ball to
stop or start fluid flow. The ball performs the same function as the disk
in other valves. As the valve handle is turned to open the valve, the ball
rotates to a point where part or all of the hole through the ball is in
line with the valve body inlet and outlet, allowing fluid to flow through
the valve. When the ball is rotated so the hole is perpendicular to the
flow openings of the valve body, the flow of fluid stops. Most ball valves
are the quick-acting type and require only a 90° turn to either com-
pletely open or close the valve; however, many are operated by planetary
gears. This type of gearing allows the use of a relatively small hand
wheel and operating force to operate a large valve; however, it increases
the operating time for the valve. Some ball valves also contain a swing
check located within the ball to give the valve a check valve feature. The
two main advantages of using ball valves are that: (1) the fluid can flow
through it in either direction, as desired; and (2) when closed, pressure
in the line helps to keep it closed.

4.13.2.2 Gate Valves

Gate valves are used when a straight-line flow of fluid and minimum
flow restriction are necessary; they are the most common type of valve
found in a water distribution system. Gate valves are so named because
the part that either stops or allows flow through the valve acts somewhat
like a gate. The gate is usually wedge shaped. When the valve is wide
open, the gate is fully drawn up into the valve bonnet. This leaves an
opening for flow through the valve the same size as the pipe in which the
valve is installed. For these reasons, the pressure loss (pressure drop)
through these types of valves is about equal to the loss in a piece of pipe
of the same length. Gate valves are not suitable for throttling purposes
(i.e., controlling the flow by means of
intermediate steps between fully open
and fully closed). The control of flow is
difficult because of the design of the
valve, and the flow of fluid slapping
against a partially open gate can cause
extensive damage to the valve.

Key Point: Gate valves are well suited to service
on equipment in distant locations, where they
may remain in the open or closed position for a
long time. Generally, gate valves are not installed
where they will have to be operated frequently
because they require too much time to operate
from fully open to closed (AWWA, 1996).

4.13.2.3 Globe Valves

Probably the most common valve type in existence, the globe valve
is commonly used for water faucets and other household plumbing. As
illustrated in Figure 4.33, the valves have a circular disk (the globe) that