Control Valve
Application
Guide
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
3.2 SIZING PRESSURE CONTROL VALVES
To size pressure control valves, both the pressure loss and the operating conditions must be considered. Generally the
pressure control valve is only partially open. The sleeve reacts to the pressure regulator which responds to the changes in
the irrigation system pressure. To correctly size the 800 Series valve size, three things must be considered: 1) the pressure
differential, 2) the flow through the valve and 3) the expected downstream pressure.
When doing a design, determine the worst case pressure differential operation point. A large pressure differential and a
low flow condition will position the sleeve very near the center seat. This condition should be avoided. It is generally best
to select the smallest valve which will work.
If cavitation occurs it could cause wear and deterioration of the valve parts. Nelson Irrigation can make no guarantee that
the valve will work under all conditions because of the many variables which can be encountered. Ongoing tests are being
conducted at this time to exactly identify the areas of operation that are safe. Each valve style has unique performance.
Contact the Nelson Irrigation factory for the latest test results and information on the valves.
A NOTE OF CAUTION:
Significant fluctuation of pressure can be a problem in some hydraulic systems if a cyclic pressure
wave develops. Abrupt pressure change or flow change is a potential cause which could upset the valve. It is also possible
that extra long pipe, either upstream or downstream, or valves in series can create a condition where the regulator "hunts"
and is not stable. This condition is difficult to predict. Many times the problem can be avoided by using a
RATE
-
OF
-
FLOW
control slowly
filling the line or slowly changing flow rate. If the unstable situation is due to pressure control responsiveness, then see Section
4.3 or contact the Nelson factory for assistance regarding the pressure control sensitivity options available which have been
designed to solve such problems. Impeller type flowmeters cause turbulence that can affect the valve sleeve. Do not locate
flowmeter immediately upstream of the valve. If a flowmeter is to be used then it is best to locate it downstream of the valve.
4.0 UNDERSTANDING HOW THE PRESSURE CONTROL REGULATOR WORKS
The regulator used on the NELSON 800 Series Control Valve is a three-way (3-way) regulator. The regulator adjustment screw
is used to establish the pressure set point. If the pressure is higher than the internal spring then water is directed through the
regulator to change the sleeve position.
Other two-way (2-way) regulators function differently from the Nelson 3-way regulator. See the discussion on differences in
section 4.1. The following page (Section 4.2) shows in Figure 4 how the Nelson 3-way control regulator functions and the
resulting affect on the 800 Series valve under different situations.
4.1 DIFFERENCES BETWEEN 3-WAY AND 2-WAY REGULATORS
The 800 Series pressure control is a 3-way, spool-type pilot designed to serve pressure regulating purposes. The pressure
control is a self contained regulator with no external accessories needed. It works very well for both pressure reducing and
pressure sustaining or relief. While many other manufacturers use 2-way pressure control pilots that depend upon continuous
water flow through them, the 3-way pilot on the NELSON 800 Series Control Valve operates by directing very small amounts
of water in and out of the sleeve chamber.
The 3-way pilot on the NELSON 800 Series Control Valve has the following benefits:
LOWER PRESSURE LOSS: The 3-way pilot reduces pressure loss across the valve because when turned to open; the valve
completely opens.
LESS FILTRATION: Water moves through the controls only when there is a change in line pressure. At steady pressure
conditions, there is no water flow through the control pilot.
SIMPLE ADJUSTMENT: Setting of the desired pressure is simple with the Nelson 3-way regulator. Only a single adjusting
screw is used and no flow control auxiliary devices are required. See Appendix A for pressure adjustment control instructions.
Due to the features of the 3-way pilot, maintenance of the NELSON 800 series pressure control pilot is minimum and many
times troubleshooting of the controls is simple and can be done with little training.
MORE ACCURATE: The 3-way pilot on the NELSON 800 Series Control Valve has a pressure control regulator which is
extremely reliable and sensitive to slight pressure changes.
LESS COMPLICATED: The Nelson 3-way pilot eliminates the need for flow regulating needle valves reducing maintenance
requirements. The 3-way regulator filter may never need cleaning because of the low quantity of water flow required for
the control pilot. This helps in the reliability of the entire control package!
HOW PRESSURE CONTROL WORKS
9
