CEMP-E
EI 11C201
1 March 1997
installed on the pump discharge line, and did not close properly. Reverse flow fully accelerated
through the pump could cause transient flows and pressures well above maximum design
conditions. A swing check valve which stuck open temporarily, and then slammed shut under
these conditions, would result in a large pressure surge as given by paragraph c above. In order
to control and limit these surge phenomena, the following practices will be followed.
(1) Gravity check valves. For simple cases involving small to medium sized pump stations
with gradually rising force mains (no intermediate high points) of less than 300 meters (1000 feet)
in length, and with static discharge heads of less than 15 meters (50 feet), a gravity operated
check valve will usually be sufficient. Gravity type check valves may be either swing checks
utilizing outside lever and weight (or spring) set to assist closure, or then may be ball checks.
Swing check valves are usually installed horizontally, while ball check valves may be either
vertical or horizontal. For additional protection, a pressure relief valve may be installed in
conjunction with check valves to allow reversing flow to reenter the wet wall. Pressure relief
valves must be specially designed for sewage applications. As an alternative to relief valves, a
hydro pneumatic tank may be utilized.
(2) Automatic control valves. In situations where long force mains are required, pipe
profiles must conform to existing ground elevations for economic reasons. This normally will
result in high points in the force main, with the possibility of water column separation at the high
points during pump shutdown or power failure. The pressures generated when these separated
columns come to rest against closed valves or against stagnant columns may be large, and are
again determined by paragraph c above. In general, where force mains are greater than 300
meters (1000 feet) in length or contain intermediate high points, and where pumping stations are
large in capacity, or static discharge heads are greater than 15 meters (50 feet), control valves
will be automatically operated (1) cone, (2) plug, (3) ball, or (4) butterfly valves. Normal operation
of these valves upon pump shutdown, is to slowly close the valve while the pump continues to
run. When the valve is closed, a limit switch then stops the pump motor. On power failure, an
emergency hydraulic or other type operator closes the valve slowly. The time of valve closure is
of utmost importance. Valves should be half closed when the velocity in the force main has
dropped to zero. The time required to reach zero velocity can be calculated with the following
formula:
t
= LV/gHav
where:
t
=
time in seconds
L
=
length of force main in meters (feet)
V
=
velocity of fluid in pipeline in meters (feet) per second
g
=
Hav
=
average decelerating head including pipe friction in meters (feet).
The types of valve operators most often utilized are hydraulic, electric and pneumatic. Valves
and operators specified for use will be fully adjustable for closure times ranging from t to 4t
minimum. In some large pumping stations, the use of automatically controlled valves alone will
not be sufficient. Extremely long force mains (over 1.6 kilometers (1 mile)) may require very long
valve closing times, and thus result in excessive backflow to the wet well and reverse rotation of
the pump and motor. To solve these problems, a pump bypass with surge relief valve will
generally be required. Valves used for surge relief will be automatically controlled cone or
butterfly valves, similar to the pump discharge valves. Normal operation upon pump shutdown
7-11
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