CIRCULATION

This section is going to give you the overview of how a modern above ground pool should be plumbed and why:

1. We start with the pool gallons. 
2. The skimmer selection comes next. Skimmers need a minimum of 15 gallons of flow in order to operate. While many are rated up to 75 gpm, The wier is pulled down into the basket at this flow. 
3. For best energy efficiency, we want the water speed inside the suction pipe to be less than 6 ft. per sec. For 1 -1/2" pvc pipe, this is 32 gpm. Remember the skimmer, the bottom water fitting is only 1-1/2" pipe. Therefore, the skimmer rating is 15 gpm to 32 gpm.  
4. Pool cleaner: I recommend a suction side pool water vacuum that uses the pump suction to operate. Using this type still provides filtration while also cleaning the pool. There also isn't a bag on the unit that I have to empty since the debris will be collected in the pump strainer basket that is much easier to clean. Install a separate wall fitting for the pool cleaner. Plumb the cleaner and skimmer together using a 3-way diverter valve. You can also install a motor actuator on this 3-way valve so that you can control it remotely. 
5. Wall fittings or pool returns are determined: For a big pool, you will want one directional fitting to move the water across the pool and into the skimmer. Sometimes angling it slightly off center will cause a circular movement too. Install a 2nd wall fitting to move water around on the floor.  Install a 3rd wall fitting if needed for returning the filtered water. 
6. Directional eyeballs have a center hole that measures 3/8" to 1". The smaller the size, the higher the backpressure on the pump and thus the apparent illusion that it is more forceful. I have found that using larger eyeballs achieves better results without adding back pressure to the pump has there is more volume of water trying to move the pool water (higher mass). See pump section regarding reasons for keeping the filter pressure low. 
7. Two 1" eyeballs will be sufficient for returning the 32 gpm from the skimmer while still directing the water where you want it to go. 
8. Filter sizing: There are a couple of rules for filtration flow. 
9. Water flow thru a sand filter is about 15 gpm/sq. ft. of media. For a 10,000 gallon turning over the water volume once per day (6 hours) requires  2 sq. ft. of media. Thus the filter size is a 19" sand filter. For a 15,000 gallon pool and keeping the same size filter, the pump run time would be 9 hours per day at 30 gpm. Conversely, the filter size could be reduced if the flow decreased and the pump duration increased. Cleaning a sand filter takes about 3 minutes with no tear down of equipment. 
10. Cartridge filter flow should be around .375 gallons per sq. ft of media. In my opinion, the whole idea of upgraded equipment is less time caring for the pool. While it might seem that there is less maintenance using cartridge filters, cleaning the filters takes a long time and effort. This type of element is like your shirt material, it collects body oils and debris. They must be soaked in a cleaning solution and then hosed down. It isn't such a job in the hot summer but isn't my favorite thing to do in the off-season. I usually have a spare ready. 
11. Finally, the pump. You have determined the flow and run time. Trying to find a good in-expensive pump is the next task. Remember the filter, it takes pressure to force the water thru the media so we add some PSI (we call this head pressure required from the pump). The eyeball returns take some pressure to operate and piping/fittings slightly restrict the water movement. So we add all these flow restrictions up and call it required THD. We know the desired flow. You now go the pump curve supplied by your potential pump manufacturer. The THD is usually the left vertical part of the graph. Flow is the bottom horizontal part of the graph. Find where the THD and Flow intersect. You will now be looking for a pump curve that lies above your intersection point. This is your pump. I should warn you however, the majority of the pumps being offered for sale have pressures and flows far greater than what you will be needing. This is one of the reasons for this website, with the exception of variable speed pumps, the above ground pool industry doesn't offer pumps properly sized for the pools that are being sold. The consumer is lost in the "20 years of doing nothing". 
12. See the pumping section for additional information and details. 

The purpose of the above chart is to show that an 18,000 gallon pool (27 ft. Round) can be filtered with a 19" sand filter. Longer pump operating times are often preferred to collect leaves and floating debris in the skimmer before it has a chance to submerge to the pool floor.