issue with Pressure

[mbconner]

I have an above ground pool. We have a Pentair pool pump and a Pentair Sand Dollar Filter (SD 40). I replaced the sand this year. The pump and filter works fine but in past years, the pressure stays around 18. It is constantly around 13. When I first set it up this year, it works fine for about 30 minutes then the flow of water decreases. I turn it to backwash then start filtering again and it does fine for another 30 mins. I bought a new pump and a new valve assembly for the filter and now it runs great for about 8 hours, then the pressure of the flow decreases (but the pressure gauge stays around 13). Please help. We have tried everything. There are no leaks in the pipes or fittings.

[Teapot1]

It would suggest the new sand is the issue, maybe wrong grade with too much fine grains so causing the reduction in flow as the filter ripens. Overall the pressure is not the issue its the flow. Your previous years at 18psi is really too high, that forces dirt through the filter and back out again as it reduces the size of the dirt, literally sanding it down. The other point with high pressure is the strain on the pump and the extra electricity it uses for no gain whatsoever.
My system runs at 0.5psi and the water is filtered through DrDrydens Activate AFM, that is much much better than sand and never needs changing.