Filters in Tandem/Parallel - Commercial Pool

[Bogo]

Hi, long time member/lurker with first post here. I’m looking for opinions/advice on running 2 identical filters (primarily sand) in tandem(parallel).

Common arrangement I’ve seen are with valve provisions for backwashing only. I don’t know if additional plumbing and valves are used elsewhere to provide functions normally found on multi-port valves i.e., rinse, drain to waste and recirculate. I've searched other threads here and I could not find this being discussed.

Question:
Is it typical to use multi-ports on each filter if used in tandem or more advisable to plumb their functions once on the main input and output side of the 2 filters? Or simply forego them?

Thanks for any feedback and great forum!

 

[Jimrahbe]

B,

Why? What is the point?

Thanks,

Jim R.

 

[proavia]

Welcome to (finally) posting TFP!

What is your reason for wanting to use two filters on the same pool?
If in parallel, the water will take the path of least resistance (well, water will always take the path of least resistance). So if one filter is dirtier than the other, more water will flow thru the cleaner filter.

 

[JoyfulNoise]

The only reason to have 2 filters is if your pool volume is huge, like 100k gallons. In that case you’d pick the filtration system that makes most sense and then plumb them in parallel to handle the high volumes. You would only put filters in series if you are trying to achieve a certain solids reduction target - you want the coarsest filtration first so that the fine filter doesn’t load up quickly and cause serious head loss. In a series filtration system you’d be backwashing/cleaning your coarsest filter more frequently.

In potable water systems they spend a lot of time and energy upfront in the process reducing the solids content as much as possible through sedimentation processes and coarse filtration before the water ever gets chemically sanitized and/or fine polishing.

 

[Bogo]

Thanks for the replies. I was not sure as to how much detail I should go into this because it involves commercial pool codes (NY) and the forum is geared more towards residential users. I thought my inquiry could be replied to without getting over complicated. I did read the forum rules and it seemed an OK topic but if outside the bounds of the forum my apologies.

This involves an older HOA pool (1974) where I live that needs to be brought into compliance. It’s a long story!

Basically, the system flow rate needs to be increased such that it will require either (2) 34” or (1) 46” filter. It’s a 42,000G 4/skimmer pool that needs 172GPM flow rate. The present system uses (2) 30” filters each with a multi-valve in a non-tandem configuration. Hence my question regarding how the valves are typically handled for tandem pump setups.

 

[Jimrahbe]

B,

We assumed that your request had something to do with trying to get rid of algae, which is not a filter issue.

Now that we know it is a code issue, then your question makes a lot more sense.

I know nothing about commercial pools, but it seems to me that you'd want a multiport on each filter. with the two filters installed in parallel.

That said, let's see what @mas985 has to say about it...

Thanks,

Jim R.

 

[proavia]

What size suction and return piping does this pool have?
What pump(s) are being used?

I doubt there are any residential type pool filters that could handle that flow rate - especially two filters in series. Even in parallel, that would be 86 gpm per filter to attain the 172 gpm flow rate you state is required.

 

[cowboycasey]

The THS series filters from Pentair can handle that with no problems at all..

The THS Filter is an NSF - approved 50 psi rated tank with flow rates up to 535 gallons per minute. They are a BIT more expensive though...

THS Series® Horizontal Sand Filter

Pentair offers a complete line of commercial pool filters that keep your pool or spa water sparkling clean. View all products here.

www.pentair.com

 

[mas985]

The present system uses (2) 30” filters each with a multi-valve in a non-tandem configuration.

Does this mean the filters are in series plumbed one right after the other?

For high flow rate applications, the two filter/pump combos should be in parallel with completely separate plumbing loops (i.e. completely independent systems). So each pump/filter combination would have it's own suction lines and return lines which given the setup seems possible as long as there are homeruns to each skimmer and at least two return lines.

Also, are you sure about the 4 hour turnover rate being a mandatory requirement?

This says 6 hours for newer pools and 8 hours for older pools (pre-1973):

Section 6-1.11 - Treatment | New York Codes, Rules and Regulations

regs.health.ny.gov

 

[Bogo]

Thanks everyone for your input.

@proavia
Main drain line is 4”
Skimmers 3” branching down to 2”, 1.5” if old drawings are believable
Returns 3” branching down to 2.5”, 2”, 1.5” “
Pumps = 3HP Pentair Intelliflo SVRS

@mas985
The present configuration has one 30” filter w/own pump on skimmers and one 30” filter w/own pump on main drain. The 2 filter outputs are joined at the return line. So 50% of flow is via skimmers and 50% via main.
Hopefully this formats OK. (guess not, middle line spaces get dropped - sorry)

Skimmers >-----------P1---------F1-------\
|-----> Return
Main >-----------------P2---------F2------/

There is another side issue because the 4 skimmers must carry 4 x 30 GPM = 120 GPM. The system was configured many years ago with the skimmers and main drain junction feeding a parallel connection of a pump with filter.

A prior configuration was:

Skimmers >---------\ /-----------P1--------F1--------\
| |-------> Return
Main >--------------/ \-----------P2--------F2--------/

The SVRS pumps in this arrangement would not provide the secondary protection (VGB) should the main drain be blocked. The pump(s) would still draw from skimmers and not shut off. A dedicated main pump/filter line shown in first figure ensured the pump shuts off releasing the suction on a blocked main drain.

Flow requirement is based on skimmers & main drain and must equal or exceed a 6 hour turn over. The requirements of the former easily exceed the turn over.

@cowboycasey
I’m aware of the THS series and other single large alternatives but cost and space become a significant factor.

I’ve received some feedback elsewhere that with these types of parallel configurations the rinse and recirculate functions are usually omitted and drain to waste only is provided by adding a valved line at input/output of tandem filters to bypass them. Conclusion seems to be that multi-ports in such configurations are general not used and benefits of a rinse cycle considered not worth it.

So maybe the subject should be closed because it can easily veer off into other matters? Lots of interesting subtleties in this and I could post a system diagram if thought helpful but it’ll only raise questions perhaps more appropriate in another thread.

 

[mas985]

@proavia
The present configuration has one 30” filter w/own pump on skimmers and one 30” filter w/own pump on main drain. The 2 filter outputs are joined at the return line. So 50% of flow is via skimmers and 50% via main.
Hopefully this formats OK. (guess not, middle line spaces get dropped - sorry)

Skimmers >-----------P1---------F1-------\
|-----> Return
Main >-----------------P2---------F2------/

While not ideal, that is a satisfactory setup. How far away are the individual lines and how are the splits done? Ideally, you could use a 3" header with individual tees feeding each return line. The two pumps could then feed each side of the header.

Flow requirement is based on skimmers & main drain and must equal or exceed a 6 hour turn over. The requirements of the former easily exceed the turn over.

Then technically, you only need 117 GPM total and not 172 GPM which is very reasonable for dual pumps.

 

[Bogo]

Then technically, you only need 117 GPM total and not 172 GPM which is very reasonable for dual pumps.

No, the required recirculation rate is determined by number of skimmers (30GPM per) and flow through the main drain (30% min. of the total flow)
Recirculation Rate = RR = #skimmers x 30 + .3 x RR = 120/.7 = 172 GPM
and RR must also supply the minimum turn over rate of 6 hours.

Our issue may have started from possibly day one - well before my time. The original pool design application/engineer report was for 3 skimmers. Health department wanted 4 and plans changed to 4. But looks like the recirculation/filter system was not recalculated, sized for 4 and apparently never caught.

 

[mas985]

I can't seem to find the 30 GPM/skimmer specifications in the regulations? Where is it listed? I can't see why they would regulate that.

 

[Bogo]

Here's code: Title: Section 6-1.29 - Swimming pool design standards | New York Codes, Rules and Regulations

9.5.2.3 Flow rate. Skimmers shall be designed for a flow-through rate of at least 30 gallons per minute or 3.75 gallons per minute per lineal inch of weir. The swimming pool recirculation rate is determined by the total of: design flow rate required by the number of skimmers, and the flow through the main drain system, as specified in Item 9.6.4, and must supply the minimum turnover rate required by Item 9.0.

I think it's pretty much standard for skimmers to be spec'd. Perhaps related to ensuring there's a 70/30 spilt between surface/main and that each skimmer is drawing the flow for the area (400 sq.ft) they are required to handle.
In our case the pool surface is 1,000 sq.ft. which technically should only require 3 skimmers but there is a caveat that more may be required to achieve effective skimming. With the pool being 20x50 I can somewhat understand and accept why 4 was required. However, the reason given in 1974 was that skimmers can be no further than 40' apart. I can not find that requirement anywhere.

 

[mas985]

To me, that means the "design flow rate" must be less than the skimmer maximum rating and that maximum rating must be at least 30 GPM/skimmer. But it does not mean that the flow rate per skimmer must be at least 30 GPM/skimmer. Just the rating needs to be at least that. So the "design flow rate" must be greater than the turnover rate but less than the rating of the skimmers combined. They are giving a range of design flow rates.

 

[Bogo]

I understand what you’re saying but not sure if correct. I read code as if the skimmer needs to handle 400 sq.ft. it’s based on its operating at 30GPM (or 3.75 per weir inch). This does raise the question if a skimmer is handling less than 400 sq.ft. then operating flow can be lowered proportionally. I sort of doubt that.

The recirculation rate is calculated from the number of skimmers and flow through the main (which is based on recirculation rate) with no mention of turn over except as setting the minimum recirculation rate. I view that as meaning that at a minimum the design (equipment sizing, piping, filter area, etc.) has to use 30GPM/skimmer.

I’ll be discussing this further with the health department engineer because we’re trying to get an exemption for operation this year as is. Unfortunately, there are some other issues which could be another thread. We are also in the initial planning for an entire facility replacement – pool, deck, equipment room, etc. – so I’ll have opportunity to learn more during that process.