chill said:
1) Nothing - Keep the system in the same configuration. Replace pumps as necessary.
2) Keep the four pump, four filter system, but re-evaluate the plumbing to make it more efficient.
3) Switch to a one pump system (i assume keeping the four filters).
4) Switch to a salt-generator system and replace the necessary plumbing.
1) It's a mess, and impossible to service. If it were mine I would redo it.
2) Every state, county, city, etc has its own health codes. Most States I have dealt with require a 6 hour turnover in a pool of this type. So for the sake of having a place to start I will assume that this is how your .gov sees it
120,000/360 (minutes in a 6 hour turnover) = 333.33~ GPM
Judging by those photos the filters are pentair/pacfab TR140 filter.
They are 7.06 square feet. Most states have a maximum of 20gpm per sqft 7.06*20= 141.2 gpm per filter or 564.8GPM total would be the maximum usually I like 15gpm per SQFT so 423 GPM
So you definitely have enough filter for your 333 gpm flow rate.
Judging by the spacing on the flange bolts I am guessing its 6" on the suction and 4" on the return.
6" pipe @333 GPM = ~3.72fps which is a great number.
4" pipe @333 GPM = ~8.17fps which is above most health department standards but still will work (just inefficiently)
3) A 10hp pump is going to be WAY TO BIG! A 7.5 would fit the bill nicely but do cost significantly more than smaller pumps.
2 of the Pentair whisperflow XF should be able to do the job, but it would be hard to calculate this exactly without going over the entire system.
4) Salt works great. It isn't for every pool, but it works very well on most pools. The #1 key factor to a salt system operating properly is the attentiveness of the operator. If they are inspecting and cleaning the cells at regular intervals all should for the most part be good.
You can plumb cells in series. I have done this with many different products, but especially the Poolpilot systems that Poolsean deals with (my brand of choice).