Too much pressure!
Here’s what happens when you have more pressure than a valve can handle. Note the distorted threads where the two halves of the fitting said bye bye:
Not that these failures are particularly Plasson’s fault. We were advised by the guy that sold them to us that the fittings would work in our situation. Unfortunately the valves are rated to 800 kPa, while we have 1100 kPa on the lowest paddock.
When the pipes are still lying on the ground an they heat up to 45 degrees C or so, the rating goes down to 500 kPa or so.
Result? 24,000 litres of water sprayed on paddocks at random, as the valves fail.
Now we have pressure relief valves all over the place and are down to 500 kPa in the lowest spots. Problem solved