Many water systems maintain a very high pressure in their main lines which can be up to 200 psi. This pressure is necessary for water towers, fire fighting, elevation increases in the topographical layout of their service area, or even for elevation increases in tall buildings. Once that water enters a home's plumbing or irrigation system, it needs to be reduced to a manageable level. Many plumbing codes specify use of pressure reducing valves for pressures over 80 psi, and that's a great guideline for irrigation as well. Even though the pipe and fittings are generally rated for 150 psi, this high pressure can wear out components such as valve diaphragms and o-rings, cause pinhole leaks in improperly cured solvent welds, and cause
water hammer. It also uses more electricity to open the solenoids, although that's not a huge consideration. More importantly for the financial and ecological bottom line, high pressures cause loss and inefficiency through misting at the sprinkler heads.
It should be noted that pressure reduction is not the same as flow reduction. Partially closing a ball or gate valve reduces the flow, not the pressure. A spring loaded diaphragm mechanism is needed to control the pressure and attempt to mitigate any pressure fluctuations from the source.
Adjusting a pressure reducing valve can be a little counter-intuitive. A downstream pressure gauge is needed to monitor your adjustments. First, loosen the locknut and rotate it to give the main pressure adjusting screw room to move. One would think that the principle of “righty – tighty” would apply here, but actually a clockwise rotation raises the downstream pressure. Rotating the adjusting screw counter-clockwise results in a reduction of the downstream pressure. Once you achieve the desired pressure, re-tighten the locknut.