Environmental Engineering Reference
In-Depth Information
in a system with a fast flow reversal and a slow moving check valve (i.e.,
swing-check valves). From a limiting transient pressure standpoint, a fast acting,
spring-actuated check valve that closes at about the same time as the flow comes
to rest (just before reversing direction) would be ideal. Examples of fast-closing,
spring-actuated check valves included nozzle, silent, duo, double-door, and lift
check valves.
5.3 Air Valves
Air valves release unwanted air volumes that collect in a pipe system (typi-
cally at high points) and admit air into the pipe when the pressure becomes
sub-atmospheric. Negative pressures can occur during normal operation if the
hydraulic grade line or piezometric head (elevation plus pressure head) falls
below the pipeline profile, such as at a high point. Intentional or unintentional
(pipe rupture) draining of the pipeline can also result in negative local pressures.
Large, moving air pockets generated through the pipe-filling process, unsub-
merged pipe inlets, or air admitted by air valves can result in the generation
of dangerous transient pressures when passing through flow restrictions such as
control valves.
5.4 Air/Vacuum Valves
There are three types of automatic air valves: (1) air/vacuum valves, (2) air
release valves, and (3) combination valves. The air/vacuum valve is designed for
releasing large quantities of air while the pipe is being filled and admitting air
when the pipe is being drained. Air/vacuum valves typically contain a float, which
rises and closes a discharge port as the valve body fills with water. Once the
line is pressurized, the float cannot reopen to remove air that may subsequently
accumulate. If the pressure becomes negative during a transient or while draining,
the float drops and admits air into the line. At least one of the air/vacuum valves
in the system should be sized for the maximum discharge associated with a full
pipe break. Air/vacuum valves, when sized based on vacuum service, should be
sized to ensure protection against negative pressures caused by line breaks. Note
that the pipe fill rate should be controlled by the water inflow rate rather than
the discharge capacity of the air/vacuum valve(s) to avoid transients when the
air/vacuum valve closes.
5.5 Air Release Valves
Air release valves contain a small orifice and are designed to release small quan-
tities of pressurized air that are not released by the air/vacuum valves during
the pipe filling or normal operation. The small orifice is controlled by a plunger
activated by a float at the end of a lever arm. As air accumulates in the valve
Search WWH ::
Custom Search