Remember when your dad used to tell you “Don’t slam the door closed. It could break something.”? That same logic applies to your valves in your plumbing. Whether is the valves in your main lines or the valves on your controller, you should always be opening and closing them slowly.
We just recently saw an instance where a constant slamming of the valves repeatedly actually caused one the electrodes on a CLO (free chlorine) probe to break off after only about a month of usage. That probe should have lasted over 3 years. Now an expensive piece of equipment has to be replaced.
We have also seen in the past where a customer slammed open a valve causing over 200 gallons per minute worth of pressure shoot into a UV reactor which shattered all the quartz sleeves and lamps instantly. They had to immediately shut down the pool and drain it to make sure there was no glass in the pool. Then it must be refilled, which is a spendy process, re-adding salt and chemicals to rebalance your water and heat it, which takes time. If you are running a commercial pool, that time isn’t free either.
These are just two examples of the time and money that you could be flushing down the drain if you don’t just take a few extra moments and open your valves slowly to allow your equipment adequate time to refill with water properly after a shut down or isolation event. Please, stay safe and take your time when you are dealing with any of these aspects of the pool.