For the first time since 03 when the pool was put in, I noticed that the top swelled when I turned on the pump. it swelled up about to about 1/4 " taller. I had backwashed the pool and started it back up. the De was going back in to the pool also . So I backwashed the filter again and it did the swelling a little less. Also the DE did not go into the pool the 2nd time. I think maybe the DE was not mixed enough and it clogged things up momentarily . I have not had any problems with high pressure. The pressure stays in the low 20's all week .Is this swelling normal or is it just time to replace the case after 13 years ? sure don't want it to blow up and explode.