Gary Taylor, GT Irrigation Services
The time of year is upon us when we need to winterize our irrigation systems. In the Spring issue, I discussed starting up your sports field irrigation system. I stated, “How well you winterized the system in the fall will translate in large part to how well the startup goes. If you blew out the piping network thoroughly with compressed air then pipe breaks should be non-existent or minimal unless we have a severe Winter with deep frost. If you blew out the system excessively then there may be sprinkler and valve issues to deal with.”
In this issue, I will describe the proper way to winterize a sports field irrigation system to ensure that the Spring startup is pain free subject to Mother Nature. Winterizing an irrigation system involves blowing out the system (piping network, zone valves and sprinklers) with compressed air. Unfortunately there are no courses or certifications required for blowing out a system with compressed air which is a shame because compressed air can cause major damage to the integrity of all components of an irrigation system. Typically we just do it the way we were instructed by our predecessors. The same can be said of the golf course industry where the potential for damage is exponentially greater.
So, why do we need to blow out an irrigation system anyways? Why can’t we just turn it off and relieve the pressure in the mainline? The reason is that in our part of the world the ground freezes. If you can remember back to science class, when water freezes it expands. In fact, it can expand up to 10% when frozen. If we leave pipe full of water it will break. PVC pipe shatters in a herringbone pattern, PE pipe splits, ABS pipe shatters in a spiral pattern. Sprinkler head bodies will crack, internals will break apart, electric valve bonnets and bodies will crack and diaphragms will rip and tear.
Since our pipe is installed (or should be installed) a minimum of 12” (30.5 cm) below grade, it will freeze as the frost penetrates the ground. While the depth of frost can vary by soil type, region and year depending on temperatures, snow cover and other factors, we proactively winterize our systems to prevent costly and time consuming damage. Some people believe pipe should be installed below the frost line which is typically at or near where we install large diameter
(6” (15.2 cm) and larger) pipe. Even if we did install pipe this deep, the sprinklers, swing joints and electric valves would be still be affected.
When we winterize our irrigation systems, do we actually blow every last drop of water out of the piping infrastructure? In reality we do not. We blow out the majority of the water so that any water that is left in the pipe can dissipate over distance and expand with freezing and not cause any problems. This is why once we have finished blowing the system out, we open any and all section (isolation) valves on the mainline to ensure there are no blockages where any of the remaining water
The key to water removal is volume not pressure. Volume is measured in CFM (cubic feet per minute). In metric terms it is measured in cubic meters per minute although for the most part the industry still uses the CFM measurement. Since compressors are designed for the construction trades, they produce pressure to increase the efficiency of the tools and implements that are attached to them. Typically they are set to operate at 100 PSI or higher. We require minimal pressure to blow water out of pipes, electric valves and sprinklers. In fact, only 35 PSI or less is required to pop up a sprinkler. Therefore, it is imperative to reduce the pressure at the outlet of the compressor to prevent damaging the irrigation system components.
While compressors come with a regulating valve, it generally will not regulate the pressure low enough for our requirements. Adding an air pressure regulator downstream from the compressor discharge will accomplish this feat. These specialized pressure regulators can regulate air pressure from 5 to 125 PSI.
You want to have a maximum of 50 PSI of air in the pipe. If sprinklers won’t pop up at that pressure due to elevation or other factors, then you may need to increase it slightly but be very cautious. Pressure regulators on electric valves are designed to regulate water pressure not air pressure so they cannot be relied upon to regulate the pressure to the sprinklers.
So how big of a compressor do you need? This is calculated by the size and layout of your system(s). In the Spring issue
I recommended the installation of a quick coupling valve (QCV) at the end of each mainline. A 1” (25 mm) quick coupling valve with a key installed can pass up to 150 CFM of air. A sprinkler with a 12 GPM nozzle will typically pass +/- 30 CFM. Therefore, five sprinklers operating on a zone passes approximately the same amount of air as the quick coupling valve. A 185 CFM compressor will allow you to blow the water out of the mainline through a quick coupling valve and then progress through the individual stations with the QCV key removed. A 260 CFM compressor will allow you to blow out one station at a time with the QCV installed.
Hopefully you have a point of connection close to your water source in your mainline to connect the compressor to ensure that water is blown out from the source out to the sprinklers. If the compressor needs to be connected at another place other than the water source, a drain valve will need to be installed at the water source to allow the water to be blown back and out. If you have a flow sensor, remove the internal paddle wheel assembly and install a “dummy plug” which can be obtained from an irrigation distributor. Blowing compressed air past the paddle wheel can cause the bearings to be damaged and result in product failure that will likely not be identified until the following Spring.
The phrase “air compresses, water doesn’t” that I used in the Spring issue applies even more to winterization. It is imperative to always have air venting from the piping network. Even if your pressure regulator at the compressor is set at 40 PSI, it is possible for the pressure at the end of the mainline and in the lateral lines to be substantially higher because air will compress in the pipe without any form of venting. It is also possible for the air and any water in the pipe to mix and create a dangerous situation. Remember, we never want more than 50 PSI in the piping network.
Always blow out the mainline(s) first. Using the quick coupling valve to blow out the mainline pipe prior to operating any zones will save pushing excessive air across the diaphragm on the electric valve. Pushing compressed air through the sprinklers requires that air to pass by the gear drive mechanism in the sprinkler which can spin much faster than designed and will heat up once the water has been evacuated. Imagine driving your car at 10,000 RPM – it can be done but likely not
How do you know when a zone is blown out? Is it required to have just air coming out of the sprinkler? The answer is no. When a sprinkler is misting it means that air is being pushed through the sprinkler. If a zone with 4 – 5 sprinklers has water still coming out of one head while the others are blowing air or misting, it is important to achieve a mist from the one sprinkler because it may be indicative of a low spot in the pipe.
Is it a good idea to go back and operate zones a second and third time to ensure that all of the water has been blown out? Again the answer is no. Operating sprinklers with an air/water mixture can cause dangerous product failure. Namely, blowing sprinklers out of the ground.
The actual process of blowing the system out is quite straight forward. Once irrigation has been completed for the season and prior to blowing out the system, shut off the main irrigation valve and release the pressure in the mainline by installing quick coupler keys and operating the stations multiple times. When you connect your compressor to the systems it is a good idea to include a short length of galvanized pipe between the pressure regulator and the access point to the pipe to help dissipate the heat created by the air compressor.
With a quick coupler key(s) installed in the mainline or a farthest zone open (if you don’t have quick coupling valves) start up the compressor and ensure the pressure regulating valve is set to 40 PSI. Open the ball valve to the system slowly and then go out into the field to monitor the water leaving the mainline. Once all water has been removed from the mainline or farthest zone, then you are ready to progress through your stations. Always ensure that something is open in the system to prevent the buildup of compressed air. Once you have completed progressing through the stations once then close the ball valve at the compressor and turn the compressor off.
Large systems with multiple fields and mainline looping generally utilize a larger compressor. It is recommended to install pressure gauge(s) on quick coupler keys in areas where zones will be activated to maintain the proper number of zones operating and ensure the desired pressure is not exceeded.
It is not a bad idea to operate the zones a few times after the system has been blown out to dissipate any water that may remain in the orifices of the actuating portion of the electric zone valve (including the solenoid and plunger). Just avoid operating the electric zone valve in freezing temperatures or the plunger seal may become damaged if it is actuated while it is frozen in place.
The electrical winterization is a quick and easy process. Disconnect the station common wire(s) in the controller. This is done to minimize any potential damage caused by lightning over the Winter. While station wires connect to individual valves, the common wire connects a series of valves. When the ground freezes, conductivity in the soil is nullified and grounding is virtually non-existent.
It is necessary to complete winterization at the water source as well. On potable water systems, the backflow preventer and water meter may need to be drained and/or removed especially if their location is not heated. Normally this is completed by another department or the utility company so it is a separate matter from the irrigation system. If a pump(s) is used at the source then this must be winterized separately as well, especially if it is a centrifugal pump which must be drained to prevent cracking of the volute in freezing temperatures.
Winterizing a sports field irrigation system can be a worry free process with the proper compressor and completed systematically. The phrase, “An ounce of prevention is worth a pound of cure”, definitely applies to the importance of winterizing an irrigation system for startup the next Spring.
Gary Taylor is owner of GT Irrigation Services, an independent irrigation consulting and water management firm specializing in golf and sports field irrigation design as well as central irrigation control for municipalities. •