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Flow Sensors and Pressure Switches to Shut Off the Pump

 

Q. I am currently in the process of converting my entire lawn irrigation system into an electronically controlled system using a control and relay setup for the pumps. I currently have two centrifugal pumps that pump water from a pond about 150 yards away. The system has seventeen zones and I have already ordered all the valves needed as well as the controller and pump relay and am in the process of installing it all. I am concerned with the fact that if one of the valves fail to open then I may have a problem with too much pressure and would like to know what kind of setup you suggest in order to overcome this. I researched pressure relieve valves and such but I feel that a flow sensor combined with a high pressure sensor to turn the pumps off would be the safest route in order to minimize damage to the pumps. How could this be done to cut pumps off if there is too much pressure or no flow at all?

A. They make flow sensors that use paddle wheels, they can actually measure the flow rate in the pipes in GPM or cu ft/min. They are a great way to go for this. They require that you have a fancy irrigation controller that can work with them, so you may need to return your controller and upgrade it. The irrigation controller measures the flow and compares it to the pre-programmed flow that should be present in the system for the valve that is currently open. The controller then makes a decision based on that flow. If the flow is too low or too high it can shut down the pumps or close a master valve that shuts off the water to the entire system.

The sensor needs to be installed in a tee on a straight length of pipe. The length of the straight pipe should be 5x the pipe diameter before the sensor and 5x after it. This is to reduce water turbulence in the pipe caused by turns, the turbulence can cause inaccurate pressure readings.

You can also use a pressure sensor and pump logic controller to turn off the pumps at high pressure or very low pressure. You should be able to get what you need at a specialty pump supplier. The sensor is a bit different from the typical pressure switch. A standard switch turns the pump on at low pressure and off at high pressure. The logic controller is basically used as a detector and timer. The timer would only turn off the pump if the high pressure was present for maybe 4 minutes or so. It is normal to have a pressure spike as the system changes from one valve to another, you don’t want the pump to shut off during the switch of valve zones. You also need a delay to allow the pump to start up, since there will be no pressure until it gets going (so the switch would never allow the pump to start!) The pressure sensor also needs to be on a straight pipe section like the flow sensor.

If you wnat to use a pressure sensor you should also do a quick test to make sure your pumps are capable of producing a high enough pressure to detect. Some pumps don’t produce very much increased pressure, even at no flow. So you need to make sure your pump will, if it doesn’t you need to use a different method of detecting no flow, like a flow sensor. Run the pump as normal with the smallest valve circuit open and check the pressure. Now shut off the valve and watch the pressure (don’t let it run for more than 3-4 minutes without flow! Don’t want to overheat the pump.) Ideally you want to see a pressure increase of 5 or more psi. The more pressure increase you have the less likely you are to get a false alarm caused by a small pressure spike.

If the pump doesn’t produce enough pressure to measure the increase at no flow you will need to use a flow switch to detect flow. A flow switch is nothing more than a paddle that sticks down into the pipe. When the water is flowing it presses against the paddle and the switch opens/closes (depending on how you have it set.) It’s very simple. Unfortunately flow switches also break pretty easy, so they have to be frequently replaced. That’s why I don’t use them as my first choice.



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