One of the major components of any automatic irrigation system is the remote control valve. When it receives a signal from the irrigation controller, the valve opens, allowing water to move down the lateral piping system to the sprinklers or drip tubing. When it fails, usually in the open position, the contractor gets a frantic call from the owner of the system.

Therefore, it is very important that irrigation contractors use the proper procedure when selecting which valve type and size to use.

When choosing a valve the designer or contractor must know the following:

1. How much water (GPM) will flow through the valve

2. What the static and working pressures (PSI) will be at the valve being selected

3. The condition of the water passing through the valve

Flow

The flow through the valve is probably the single most important condition in properly sizing the valve. If you look in the valve section of any irrigation manufacturer's catalog, you will see written specifications for each type and size of valve. There you will see a maximum flow as well as a minimum flow listed for each valve.

What most contractors don’t realize is it's just as important for a valve to pass enough water for proper operation, as it is not to exceed the maximum safe flow. Insufficient flow through a valve leads to a condition where the diaphragm movement is so minimal that the valve has trouble functioning. The major symptom of insufficient water flow is that the valve will have trouble shutting down and remain in the open position. Even worse, this problem usually occurs intermittently. The contractor will get a call from the owner about a sticking valve. When he gets there the problem has either stopped or he cleans the valve out and the system works fine only to get stuck a week or month later with the same problem.

Many contractors size a valve by making it the same size as the pipe it is being cut into. This is called "line sized." This practice leads to oversizing valves, which costs the customer more for the larger valves as well as additional service calls.

Two-inch SDR21 PVC Pipe can handle a flow of approximately 55 GPM without exceeding the industry standard maximum velocity of 5 feet per second. If we put a 2-inch valve (line sized) with a minimum flow of 80 GPM on this line, the valve will not operate properly. The correct valve size should therefore be 1.5 inch.

If 20 GPM was going to be the flow through the valve, and using the example chart above, the correct valve size to select would be 1 inch. If the flow was to be 30 GPM then either a 1 inch or 1.5 inch valve could be selected. Note that 30 GPM is the minimum flow for the 1 .5 inch valve. However, because it is right on the line, the 1.5 inch size should not be selected. The center of each range is the optimum flow for that valve (the valve will operate best in that range). For the 1.5 inch valve in the chart above the best range would be from 60 to 80 GPM. A good rule of thumb is to add 25 percent to the minimum operating flow and subtract 25 percent from the maximum. Using this adjustment the operating range for the 1.5 inch valve would now be 37.5 to 82.5 GPM.

One thing that also affects our selection is that the higher the flow through a valve the higher the pressure loss. If you have to keep your pressure losses to a minimum, stay in the low range of the valve without falling below the minimum flow required.

Pressure

The static and working pressures help to determine which type of valve is selected. Today, most of the valves used in irrigation are plastic. They have a proven track record and will survive in all but the worst conditions. The important thing we are looking at is the pressure rating of the valve, again located in the manufacturer's catalog. Most residential and light commercial valves have an operating pressure range of up to 150 PSI. Some commercial plastic and brass valves are up to 220 PSI. For higher pressure, brass, bronze and cast iron valves should be used.

Select a valve with a rating higher than your anticipated pressure. In order to reduce damage from water hammer, a rule of thumb is to pick a valve rated at a pressure twice the operating pressure. If the operating pressure were 60 PSI a valve with a pressure rating of at least 120 PSI should be selected.

If the pressure entering a valve is higher than the design pressure of the zone, a pressure-regulating valve should be used. These valves monitor the incoming pressure and reduce to a dialed-in pressure. If the pressure goes up, the valve still reduces to the pressure set. Note that there is usually a pressure differential of at least 10 PSI required between the inlet pressure and the set outlet pressure. Today, most manufacturers allow this water-conserving feature to be added on before or even after the valve is installed.

Condition

The type of valve selected is also affected by the condition of water flowing through it. This is usually determined by its source. Does that water come from a pond, river, well, or is it supplied through the city or other provider? If it is from a pond, river or well, is it filtered? Is it effluent water?

Most valves manufactured today have "self-cleaning tendencies" and if the water is supplied from a pond or river through a filter, well, or from a domestic supplier, the valves will perform well. If particulates and/or organic material are present in higher concentrations, a contamination resistant or "dirty water" valve should be selected. These valves usually have screened intake ports and are identified in the manufacturer's catalogs.

Other sizes and types

Most manufacturers of irrigation products provide lines of remote control valves from .75 to 3 inches. They cover approximately 99 percent of the average contractor and designers needs. For the larger remote control valves, traditional valve manufacturers are used. In addition, specialty valves may be required. Specialty valves include pressure relief, pressure sustaining, air release and flow control. These are usually associated with pump stations and large (3-inch and larger) main lines. The same procedure applies when sizing these valves. With the cost of these valves much higher in some cases, don’t be afraid to ask for the assistance of the manufacturer's representative when sizing and selecting.

When valves are properly selected and sized, an efficient, well-designed, headache-free irrigation system will be the result.