Irrigation control valves should always be placed up stream of the pressure regulator and filters. There are times, however, when this is not possible. If such is the case, the pressure loss through the valve must be calculated and the “real pressure” to the first emitter determined.
In addition, each irrigation valve has a certain flow rate operating range. Care should be taken to ensure that the valve selected is compatible with the system flow rate with the emitters in both the “flush” state and the “regulated” state.
Flow controls on valves can often time's lead to problems. Inexperienced maintenance personnel may turn the flow control valve down to reduce the flow not realizing that they are increasing the pressure loss of the system. With pressure compensated emitters, the emitters themselves are the "flow control”. For the same reason, preset pressure regulators are often preferred over adjustable types. Be sure to match the preset pressure regulator to the pressure and flow needed for the drip system to operate properly; this information should be clearly marked on the regulator itself.
Experience has shown that, generally speaking, it is preferred to use an irrigation valve, regulator and filter in combination, rather than a pressure regulating control valve. These components together are called a “Control Zone”; they are available pre-assembled, which really increases the efficiency of the installation process. Depending on your design, they are available in anti-siphon and high volume commercial configurations. As the filter collects contaminates the pressure will decrease downstream so be sure that the regulator is in the down stream position from the filter.
Keep in mind that it is important to choose the individual components or the “Control Zone” package on quality. In most situations the prices of competing manufacturers Control Zones are about the same, however, the quality of the components will determine their true value.