Sprinklers

1. General Considerations
2. Control Clocks and Valves
3. Types of Sprinkler Heads
4. Drip Irrigation
5. Backflow
6. Pressure vs Flow

I. General Considerations

You may think that it rains a lot in Seattle. Well, we have a lot of hours of gray sky and moisture in the air, but many parts of the country have more inches per year of rainfall than we do. Especially in the "dry" parts of our summer. Many plants benefit for supplemental watering in the Seattle area during the summer. Many lawns will go dormant and turn brown in the Seattle summer without added water (but they usually come back with the fall rains).

We also often have concerns about adequate water supplies, and occasionally have water rationing. A sprinkler system can be a useful tool to help manage these concerns. It can be set to apply a minimum amount of water at set intervals to keep plants alive then shut itself off, or used to pamper your gardens into beauty. There are many brands and models of clocks, valves, and heads. Most all operate on the same low voltage and are interchangeable (just as a GE timer could operate a Westinghouse light bulb in an Italian lamp).

II. Control Clocks and Valves

Occasionally we install an underground sprinkler system with manual valves - no control clock; but usually we put a control clock in the garage connected to electric valves in the ground that turn on one sprinkler area at a time and then shut off. The clock can be set to operate the valves on specified days, starting at a specified time, and each valve stays on for the set number of minutes, then turns off and the next valve comes on.

III. Types of Sprinkler Heads

There are many to choose from, even within one brand. There are large area heads such as stream rotors and Rainbird Impact heads with up to 35 foot radius or more; smaller spray heads for 6 foot to 15 foot radius; and several other options to choose from. Most heads can be either popup for lawn areas or non-popup that fit on the top of a pipe for planting beds. Usually we don't use popup heads in beds, as they have more moving parts that require more service over the years.

IV. Drip Irrigation

Again many types to choose from. Great for pots. Usually needs a pressure reducer to operate at lower pressure, and a filter to clean debris from the water that could plug the fine drip fittings.

Drip can be used for watering a yard, but it has two drawbacks. One is that it can plug up or get damaged more easily and has many more small parts, so requires more routine maintenance. The other is that you can't always see when and where it is operating, so an emitter might be plugged or a valve might be stuck on or off for weeks or months and you might not notice until plants are dying from drought or flooding. The advantage is in lower water use.

V. Backflow

Backflow devices are required by code to prevent water from flowing backwards from the sprinkler system into the household water or even back into the city water line. The concern is that in unusual situations (city water pressure is temporarily zero or negative) ground water, that could be contaminated with lawn weed killer or whatever, could flow back into the system, so a backflow device only allows the water to flow forward.

There are generally two types of devices in use. Atmospheric Vacuum Breakers must be placed on each valve above the level of the heads, so there are usually several of this type above ground. The other common type is a Double Check Valve. It is usually placed underground in a box, and only one is needed for the entire system.

VI. Pressure vs Flow (boring stuff)

Residential static water pressure (no water flowing) is usually between 40 pounds per square inch and 100 psi. Somtimes the pressure in the city main is too high and each house will have a pressure reducer to bring the pressure to under 100 psi. Usually static pressure is not our problem when we design a sprinkler system. The problem is more often a water flow rate of under 10 gallons per minute.

Imagine that you have a water pipe to your house that is only 1/4" diameter. You would still have full pressure when no water was being used, so your static water pressure might be 60 psi. But when you tried to use the water, the flow would be restricted and the pressure would rapidly drop to near zero. If we tried to measure water flow, we would find a low flow problem.

The same thing happens as old galvanized pipes corrode and the internal pipe diameter shrinks due to rust building up. Often after 40 to 50 years, galvanized pipes buried in the ground have lost much of their capacity for water flow and may need to be replaced. Copper and plastic water lines do not have this problem.

Before we design a sprinkler system, we measure the static pressure and the water flow. This often gives us information on how many gallons per minute are available. By knowing the flow needed for various heads, we can design a system that will work with the available water flow. Then we calculate flow through various parts of the system and design the pipe and valve sizes to provide the right flow. I warned you that this was boring.

Back to Top