Pressure rising quickly

Great forum learned alot from reading but I am now stumped, Please HELP. I have a tagelus TA-50 sand filter with a 1.5 hp pump on a 20ft ag. I changed the sand in the filter before opening pool this year and yes used correct sand. When the system is on filter the pressure starts at 13psi with good flow back to pool. After about 30 mins. pressure rises to around 20psi and the flow almost stops. I have been running pool on recirculate but i need to filter so i can try and clear up this cloudy water. THANKS for any suggestions

First, Welcome to the forum.
Got a couple questions. To get the pressure back to the starting point of 13 psi do you just shut the pump down for a while or backwash, or something else? Is the pressure and flow steady when in recirculate mode? Before you changed the sand did it behave like it should? According to a filter data sheet I looked at it is rated at 50 GPM and I'm thinking your pump is capable of more than that. 1.5 HP is too big for your pool, but that doesn't answer your question. I think Ben mentioned this in another thread recently, not sure, but he suggested with an oversize pump you may be forcing sand into the lateral slits and effectively plugging them. Backwashing would most likely unplug them and the process starts over. To be honest I'm not sure of the various grades of filter sand but maybe you had a coarser grade before. Something else doesn't make much sense to me and that is with the flow almost stopped I would think the 1.5 HP pump would cause a reading up around 30 psi or greater in a blocked condition, not just 20 psi.
Al

To get the pressure back to 13psi I just unplug and leave off a bit and it will start over at 13 when I start back up. Pressure and flow are fine in recirculate mode. After backwashing same problem pressure rises from 13 to 20-23 in around 30 mins.

Thanks again

Right now I don;t have any ideas other than the sand type that I mentioned above. Seems obvious it is a filter problem and one that takes time to occur. About the only thing that I can think of that matches that criteria would be fine sand eventually plugging the lateral slits.
Anyone with comments about grades and types of filter sand and this problem?
Al