Civil Engineering Reference
In-Depth Information
Pipes of composite materials, such as concrete-lined steel cylinder pipe, will have wave
velocities more nearly those of the inner material.
From the previous description of wave propagation for instantaneous valve closure,
it is apparent that if the valve were not completely closed by the end of the cycle
2
L
/
a
, the intensity of the pressure wave reflected from the valve would be suppressed.
The maximum intensity of the pressure wave for a valve closure time of longer than
2
L
/
a
may be obtained by nomograph charts.
19
Typical solutions to reducing pressure surge magnitude include:
Surge tank.
The top of the surge tank must be above the hydraulic grade line.
The surge tank accumulates water in response to the pressure surge and supplies
water to moderate the negative pressure.
•
Air chamber.
The lower portion of the air chamber contains water, and the upper
portion contains compressed air. Like the surge tank, the air chamber provides
water and a place to accumulate water during a surge.
•
Slow-closing check valve.
By a slow closing of the pump check valve over a
period of more than one cycle (i.e., more than 2
L
/
a
), the water hammer pressure
is moderated.
•
Surge suppressor
. A surge suppressor consists of a pilot-operated valve that opens
quickly upon pump outage through the use of a solenoid valve. The valve releases
water from the pipeline and is closed later at a slower rate by using the pipeline
pressure acting on a diaphragm valve.
•
Pump control valve
. Typically a ball valve with an air / oil or all-oil hydraulic
operator is used for the pump check valve. The opening and closing speeds of
the valve are adjustable, and limit switches on the valve are normally wired into
the control circuit of the pump. The valve acts as a slow-closing check valve in
power failure modes, and in normal operation the valve is 90-95 percent closed
when the pump is stopped.
•
TRACER STUDIES
As part of the Safe Drinking Water Act, there is a requirement to determine the CT
times for disinfection purposes. In order to determine this value accurately, a tracer
study is performed to determine the contact time through the basins of interest. The
contact time used, defined as
T
90
, represents the time in which 90 percent of the water
passing through the system being tested is retained.
A single flow rate may not characterize the flow through the entire system. There-
fore, tracer studies should be conducted at different flow rates. Ideally, tracer tests
should be performed for at least four flow rates that span the entire range of flows.
The flow rates should be separated by approximately equal intervals to span the range
of operation—with one near average flow, two greater than average, and one less than
average flow. The number of studies can be reduced based on site-specific restrictions
and resources available. Systems with limited resources can conduct a minimum of
one tracer test at the peak flow rate.
The water level in the basin being tested should be at or slightly below, but not
above, the normal minimum operating level. For ozone contactors, tracer studies should
