Nelson Irrigation Corp. 848 Airport Rd. Walla Walla, WA 99362-2271 USA Tel: 509.525.7660 Fax: 509.525.7907 E-mail: [email protected] Web site: www.nelsonirrigation.com
Control Valve
Application
Guide
Large pieces of trash and debris (1/2 inch or bigger) can get caught in the upstream side of the valve cage. The result is
partial blockage and extra pressure loss through the valve. If this condition happens then the material can be easily removed
by taking the valve out of the line and cleaning out the debris.
4.5 VALVES USED IN LOW TEMPERATURE
If the valve is to be used during low temperature (frost control for example) then the controls need to be covered. The
surrounding temperature must be high enough to keep ice from freezing in the control tube. Remember there is very little
water flow in the 800 Series controls. The result of ice forming in the control tube is that the valve can not operate correctly.
See Appendix B for information on draining external filters.
5.0 ATTENTION TO THE DANGER OF WATER HAMMER AND SURGE
5.1 WATER HAMMER REDUCED BY SMOOTH OPERATION OF THE 800 SERIES
Water hammer is the shock caused by suddenly arresting the flow of water in a pipe. This could happen by rapidly filling a
long pipe or in the case of pump shut down where water begins moving back toward a pump before a check valve can close.
The most likely time for water hammer to occur is the period of time during start-up and shut down of a system. The valve
ON/OFF speed has a great affect on water hammer. Entrapped air rapidly moving within a system can also be a cause of
water hammer. The rapid start up of a pump can cause water hammer in the case where no valve is used.
Severe water hammer can cause damage to pipes, pump, pressure
regulators and seals. The water hammer shock waves move at the
speed of sound in water (4660 ft./sec.). The time to control the wave
is during the design. General rules of thumb are: (1) control the
start-up so that in filling a long pipe system, no more than 1½ times
operating capacity (flow) is used; The Nelson
RATE
-
OF
-
FLOW
can be
used to automatically fill the pipe at a rate 10%-15% above
operating flow. (2) limit flow velocity in the pipe to 6 ft./sec. or lower.
(3) Select the Nelson 800 Series valve that has been designed for
smooth operation on opening and closing (See Figure 6 showing
closing speed of Nelson compared with other brands).
5.2 USE AIR VENTS TO PREVENT AIR ACCUMULATION
The sudden release of air in a system can cause pipe failure. Air control valves are necessary to allow air to escape and to
prevent a vacuum pressure which can collapse plastic pipe. The Nelson ACV200 air control valve should be at high points
in the piping, at extreme increases in pipe grade, downstream of valves which could possibly slam shut, and downstream of
pipe where the flow could trap air. The ACV200 is a combination air relief and air release valve design to solve both air
entrapment and vacuum problems.
5.3 SOME THINGS WHICH CAN HELP REDUCE THE DANGER OF WATER HAMMER
Select a slow opening/closing control on the valve or manually throttle the valve during opening.
Use a two stage opening electric timer to fill the system. See SPECIAL ON/OFF modes (section 2.4.2)
Use check valves to keep water in the bulk of the pipe system in order to minimize filling time.
Use combination air controls which both release air and act as a vacuum breaker. Place vents at all needed locations.
Figure 7
WATER HAMMER AND SURGE
Figure 6
12
