Troubleshooting Flow Switch Cooling Tower Issues

Lakewood Instruments Flow Switch

Have you ever arrived at a customer site and noticed something wasn’t quite right? Maybe the controller says the system conductivity is well above your set point but the blowdown valve is closed. That doesn’t make sense. Or maybe the cooling tower system is shut down but the blowdown valve is open. There’s definitely something very wrong. Did someone change the controller set point? Did the valve fail? Is the controller malfunctioning? Should I call Lakewood? You can certainly call us, we’re happy to help. The first thing we’re going to ask is “Do you have flow”?

If you’re in the mechanical room you might see/hear the condenser pump and know it’s on. Or you’re on the roof and you can see/hear water spraying down in the tower.  Those are good indications that there is actual flow in the cooling tower system. On the contrary, it’s easy to tell if the system is not operating and there is no flow. The next question we’ll ask is, “Does the controller think there’s flow”? 

Below is a schematic of a simple cooling tower system. The larger piping around the outside is the main cooling tower or condenser “loop.” For the cooling tower to work at all, the condenser pump must be running. The smaller piping and devices on the inside comprise the cooling tower water treatment system. We’ll be focusing on the sample line.  

cooling tower system

The main component of the sample line is the plumbing assembly, which performs two functions. The first is housing the system conductivity sensor. The second is sensing flow.  The controller will not operate if it does not sense flow.

Flow Switch Operation

The image below shows the parts that make up the flow switch. The float sits in the upper half of the plumbing assembly. The float sight is secured by the top lock-ring.  The molded end of the reed switch fits into a recess in the back of the plumbing assembly. The float contains a magnet that forces the reed switch to close when raised to the upper position. The top of the float is also visible in the sight when raised.     

flow switch parts

What Causes a False Flow Status?

Under normal operation, flow through the plumbing assembly lifts the float to the top of the plumbing assembly, resulting in a reed switch closure that tells the controller there is flow. The specified flow rate range for the float to operate is 1 to 5 GPM. We recommend at least 2 GPM for reliable float movement. If the flow rate is too low, the float will not raise enough to engage the reed switch. More than 5 GPM can damage the float, causing it to deform and possibly get stuck in the float sight. Such situations result in incorrect flow indications to the controller.

The most common cause of a false flow reading is a buildup of scale in the plumbing assembly. Such buildup can prevent the float from rising enough, resulting in a false “NO FLOW” indication. We recommend using a small brush to clean the interior of the plumbing on a monthly basis.

Another cause is a chemical reaction with the backflow prevention o-ring, pictured below. This is located inside the plumbing where the flow float is seated. If chemicals have interacted with the o-ring, over time it may swell, preventing the float from dropping all the way down. This could result in a false “FLOW” indication. The short term fix for this issue is to remove the o-ring, but it should be replaced to maintain backflow prevention.  

backflow prevention o-ring

As mentioned above, if the flow float becomes deformed it will no longer drop down into the plumbing, resulting in a false “FLOW” indication. Replacing the flow float will solve this problem.

If none of these steps fix the issue, it may be time to replace the magnetic reed switch in your plumbing. Remove the reed switch from the back of the plumbing. Hold the flow float directly against the reed switch. If the flow light does not come on (Model 140) or the “NO FLOW” alarm doesn’t clear (Model 1575/3175), the reed switch has failed.  

How Lakewood Can Help

Regular cleaning of the flow switch portion of the plumbing assembly should result in many years of reliable operation.  However, if you ever notice that the controller flow status does not match actual conditions, you now know a few things to check.  All of the components that make up the flows witch are readily available and easy to replace.  

Call us at (800) 228-0839.