Yeah I'm sorry, I had a feeling I was going to freak everyone out with that above 20 psi warning. No worries though...It won't cause any significant immediate damage. I only heed the warning because I can hear my motor and impeller straining to circulate at a faster rate, but can't because of the back pressure. I have absolutely 0 experience with sand and de filtration systems. As the pool I inherited when I bought our home 6 years ago made me a first time pool owner. I have no idea how long the previous owner had this pump for or how long it was service for, but she's been going strong every year with no issues. I understand all the filtration methods and how and what equipment uses them, but for my pool its exclusively cartridge filter. The only reason why I heeded that warning is because I've heard here at TFP that you shouldn't let cartridge filters get up to 25-30 psi because it may damage the pleats. Myself being involved in the electrical and solar trades, when I listen to the strain that's being placed on the impeller on the motor, although it hasn't been proven as of yet, I just suspect and assume that eventually with prolonged use like several years of continually operating at 30 psi that it will overheat the brushes and coils and wear down the motor with time and repeated prolonged use operating it like that.
For all I know tolerances and back pressure could be different with a Sand Filtration or DE filtration system, so just as a rule of thumb I recommend to keep the back pressure below 20 psi. Not trying to scare anyone here into not utilizing a good proven method in filtration. You should be fine using the bag the and the skimmer sock. Just do your diligence with it that's all. If your pool is cloudy blue and you don't have as much Gunk to filter out, you should be running around 12 to 14 PSI with the duda bag and the skimmer sock. If your pool is green and you have more algae to filter out then it will hit 20 psi which is still fairly safe. Again because I only have a one speed pump, what I do to alleviate back pressure is I throttle back to return ball check valve to 1/4 turn off, and I do the same for the inlet ball check valve to even out the pressure. The pump and filter filters slower like this, but that's okay too because when you're trapping really fine sediment you want the filtrate a little bit slower anyway, so it catches a little more gunk each time.
Again just a rule of thumb and guideline if your filtering out a lot of green Gunk just check it every hour and check the PSI every hour and keep cleaning and rinsing the sock and the bag. Once your pool clears it should maintenance itself at 12-14 psi with the bag and skimmer sock. When I just use my skimmer sock without the bag I usually run about 9 to 10 PSI clean pressure.
The theory behind filtration, any filtration whether it be Sand, DE, or Cartridge is that you want it to load up a little bit and run it a little dirty so it can filter out the finer particles. The side effects is as the secondary filters such as your skimmer sock or duda bag loads up and filters out finer particles is that more back pressure is created. I guess the best way to explain this is. If I was to just run the skimmer sock eventually it will get clogged with debris if the debris is floating around in the pool. So what happens is it creates a negative pressure, vaccum, or a higher amount of suction on the skimmer basket itself which is plastic. Over the time and course of a season I have cracked the ring on my skimmer baskets if I let my skimmer sock load up too much. No big deal I keep a couple of extra handy. They are only 8 bucks. Because my Duda bag is post pump, and not pre pump like the skimmer basket. The duda bag is on the return jet post pump and cartridge filter. When it loads up it creates positive pressure, so you have to continuously clean the secondary filters meaning the skimmer sock and the bag. Once your pool is as clear as glass, and you no longer see any residue on your skimmer sock or your duda bag, you are crystal sparkling clear. From there on out you only have to run the skimmer sock to prohibit frequent primary filter cleanings such as you're sand, DE, or cartridge. I only clean my cartridge twice a year or season. Once after I run the pool Crystal Clear with the bag and the skimmer sock, and then I clean it at the end of the season before I put it away for the winter.
Realistically, if I wanted to I could just eliminate the cartridge filter all together and just run the pool on The Duda bag. I use the Duda bag for polishing the water. Its like the skimmer sock filters at 40 micron, or whatever it is, the cartridge filter filters out of 10 micron, then The Duda bag filters out at 5 micron. So it really is over redundant filtration, but it gets me that clear as glass sparkling water. Once I am crystal clear I really no longer need the Duda bag. It's just handy for polish in the water when it looks dull and filtering out really fine algae, when and if I have it, that I would otherwise flocculant to the bottom of the pool, if I didn't have the bag. I hope this explains the relationship of things and a better understanding of how things work.
Here are some pictures of my setup for those who are visual learners.
