Pump sizing help for new inground design

[Drewskii]

I need some help designing the most efficient pump configuration for the following new pool.

Inground gunite pool capacity 8,700 gallons
Main drain -32" Channel Drain in the floor: 236 GPM at 3.4 fps using a 2" Pvc dedicated pipe to the pump pad. (35')
2 Skimmers - Hayward SP1084 Auto Skim Series Skimmer 2-Inch Pvc dedicated pipe for each to the pump pad. (27' & 56')
Three 2" pvc returns with a dedicated pipe for each to the pump pad. (43', 35', & 60')
One dedicated vacuum line 2-Inch Pvc dedicated pipe for each to the pump pad. (45')
Three Pentair Colorcascade bubblers - 30 Gpm each - 1.5" Pvc to each bubbler. (55', 60', and 65')

Sand Filter
Heat Pump heater

The use of the bubblers is intermittent not constant.

Using a 7 hour turnover rate I estimated 21 GPM for the pool.

It would seem either a two speed pump or a single pump with a booster might be indicated but the truth is I don' really know how to do the calcs, particularly the TDH.

Should the bubblers be plumbed individually back to the pad, or is some form of branch piping the best way?

 

[mas985]

If you are truly looking for an efficient design, then you might want to consider a cartridge filter over a sand filter. Sand filters, as with DE filters, use backwash valves which have a lot of head loss. But I understand there are other issues to the choice.

The bubblers have a flow rate requirement and together they require 90 GPM which indicates a fairly large pump (2 FRHP/2.5 URHP). So if you are interested in efficiency, then you might consider lower flow rate bubblers. Or you can just plan on running on low speed most of the time. But most cleaners might require higher flow rates than low speed can provide so you could end up running on high speed more than you think. Because of the varying requirements of your setup, you might benefit from a VS pump which would take all the guesswork out of sizing a pump and allow you to optimize the flow rate for efficiency.

Also, be aware, the because you have the same number of return lines as suction lines and they are the same size, you run the risk of a loud pump and even cavitation. This has happened to a couple of forum members. If you go with that configuration, then I would up size the suction lines so the suction side has much less head loss than the return side. You can also go with smaller eyeballs to boost the return side head loss but then that kind of defeats the whole efficiency objective.

 

[Drewskii]

Thanks for the input Mark.

I can already tell the PB's have no clue as one said a 3/4 Hp would work just fine and the other suggested a 1 HP pump. No mention of the obvious dilemma posed by needing to supply such a huge difference in flow.

TBF is an amazing resource.

Sand filters, as with DE filters, use backwash valves which have a lot of head loss.

What would you say the head loss value for a backwash valve is?

So if you are interested in efficiency, then you might consider lower flow rate bubblers.

I have checked on the net and can't seem to find a low flow bubbler. The few I have found all seem to require 30 Gpm to produce a 21-24" plume in 6" of water. Would you know of any sources for bubblers?

But most cleaners might require higher flow rates than low speed can provide so you could end up running on high speed more than you think.

By most cleaners do you mean the equipment used to clean the pool surface such as a Polaris?
Could you define the RPM for low speed and high speed?

Because of the varying requirements of your setup, you might benefit from a VS pump which would take all the guesswork out of sizing a pump and allow you to optimize the flow rate for efficiency.

What about the possibility of either a booster pump or a separate pump for just the bubblers? It seems I could actually get away with a single speed 1/2 HP pump for the pool assuming a 21 GPM flow rate. Would the price difference between the required VS pump and a 1/2 HP pump be greater than the cost of a booster or separate pump?

Also, be aware, the because you have the same number of return lines as suction lines and they are the same size, you run the risk of a loud pump and even cavitation. This has happened to a couple of forum members. If you go with that configuration, then I would up size the suction lines so the suction side has much less head loss than the return side. You can also go with smaller eyeballs to boost the return side head loss but then that kind of defeats the whole efficiency objective.

I can modify the system. Would adding another return line solve the problem? I suppose deleting a skimmer would solve the problem but I would prefer not to do that.
Is there a value, or percentage, for the minimum difference in head loss to be maintained between the suction side and the return side?

 

[mas985]

Sand filters, as with DE filters, use backwash valves which have a lot of head loss.

What would you say the head loss value for a backwash valve is?

You can see the effect in the following chart that is also part of the Hydraulics 101 sticky:

So if you are interested in efficiency, then you might consider lower flow rate bubblers.

I have checked on the net and can't seem to find a low flow bubbler. The few I have found all seem to require 30 Gpm to produce a 21-24" plume in 6" of water. Would you know of any sources for bubblers?

I can't help you there. Maybe somebody else has an idea.

But most cleaners might require higher flow rates than low speed can provide so you could end up running on high speed more than you think.

By most cleaners do you mean the equipment used to clean the pool surface such as a Polaris?
Could you define the RPM for low speed and high speed?

For a two speed pump/motor, high speed is 3450 RPM and low speed is 1725 RPM. So low speed flow rate is about 1/2 of high speed.

Because of the varying requirements of your setup, you might benefit from a VS pump which would take all the guesswork out of sizing a pump and allow you to optimize the flow rate for efficiency.

What about the possibility of either a booster pump or a separate pump for just the bubblers? It seems I could actually get away with a single speed 1/2 HP pump for the pool assuming a 21 GPM flow rate. Would the price difference between the required VS pump and a 1/2 HP pump be greater than the cost of a booster or separate pump?

A VS is probably going to be much less than two pumps. An Intelliflo VS is about $900 while a two speed is going to cost you close to $500-$600 depending on model#. A large single speed is about the same. So you really are much better off with a single VS in this case.

Also, be aware, the because you have the same number of return lines as suction lines and they are the same size, you run the risk of a loud pump and even cavitation. This has happened to a couple of forum members. If you go with that configuration, then I would up size the suction lines so the suction side has much less head loss than the return side. You can also go with smaller eyeballs to boost the return side head loss but then that kind of defeats the whole efficiency objective.

I can modify the system. Would adding another return line solve the problem? I suppose deleting a skimmer would solve the problem but I would prefer not to do that.
Is there a value, or percentage, for the minimum difference in head loss to be maintained between the suction side and the return side?

Actually, that makes the problem worse. You want LESS head loss on the suction side than the return side. Pumps have problems with high suction head combined with low return head. So the easiest solution is to simply use larger pipe on the suction side plumbing. That lowers the head loss on the suction side.

 

[Drewskii]

Thanks Mark,

Is there a value, or percentage, for the minimum difference in head loss to be maintained between the suction side and the return side?

 

[mas985]

It is hard to say for sure, I just know that the closer they are the worse the problem. However, I also know that having a return to suction head loss of at least 2:1 should eliminate any issue. But predicting head loss is not easy to do because the majority of head loss is in the equipment and the fittings so it depends heavily on how the plumbing will be laid out.

 

[Drewskii]

Regarding the cartridge filter, would it have to be sized based on the higher flow rate of 90 Gpm or somewhere between the pool flow rate, 21 Gpm and the bubbler rate, 90 Gpm?

 

[jblizzle]

Filters are generally sized based on the pool size ... and if properly sized, generally they are rated at a higher enough flow rate to not be a problem. The issue is that is seems
that pool builders often install pumps that are too big and filters that are too small.

That said, you have a small pool, so the flow rate requirement could certainly be the driving factor.
For a 8700 gallon pool, these are the minimum filter sizes:
130 sqft cartridge ; 1.6 sqft (17.3" diameter) sand ; 24 sqft DE

So look for a cartridge filter around 130 sqft and see what the flow rate limits are ... realize that for cartridge filters, there is no way to go too big.

 

[mas985]

For 90 GPM, you probably want 240 sq-ft or larger.

 

[Drewskii]

Let's see if I understand this correctly.

Based on the information below, using a variable speed pump, when the bubblers are on:
The pool will turn over in 1.6 hours and the bubblers will reach a height of 23"?
The water velocity will be approx.. 9ft/sec?
The required filter area (@20/sq/ft/gallon filtered) is 4.5 sq/ft?


Based on the information below, using a variable speed pump, when the bubblers are off;
The pool will turn over in 7 hours?
The water velocity will be 6ft/sec
The required filter area is not known because the flow rate is too low?

 

[jblizzle]

You are over thinking things. Estimating the flow rate is not as easy as you are trying to make it and turn over time is not important and the water velocity is really not that important either.

Your filter needs to be sizes based on your max desired flow rate of 90 GPM ... so that would put you in the 4.5 sqft range for a sand filter (which is much larger than the size that would be required strictly based on volume = 1.6 sqft)

I wonder if you could valve the bubblers such that they would bypass the filter and thus you would not need such a huge filter. Although when the bubblers were running, most/all of the water would not be filtered. I also do not know if bubblers would have a problem with unfiltered water.

 

[mas985]

In those charts, Hayward is using the old recommendation of 20 GPM/sq-ft. Current recommendations from APSP-15 are 15 GPM/sq-ft so in reality, you should be at 6 sq-ft.

The water velocity will be approx.. 9ft/sec?

It depends on how you setup the plumbing. Since you have three suction lines, the flow on the suction side would be more or less evenly split up. So 30 GPM on 2" plumbing is less than 3 ft/sec which is fine. However, for the bubbler return, you are correct. If you up size to 2 1/2" lines then the velocity would drop to less than 6 ft/sec.

New regulations in Florida limit flow rates on the return side to below 8 ft/sec and 6 ft/sec for the suction so you need to make sure you are compliant or it may not pass inspection.

 

[Drewskii]

water velocity is really not that important either.

The issue is meeting all the filter criteria for both operating modes, bubblers on and bubblers off. If the minimum flow rate of the cartridge filter far exceeds the flow rate when the bubblers are turned off, that's not a problem?

However, for the bubbler return, you are correct. If you up size to 2 1/2" lines then the velocity would drop to less than 6 ft/sec.

In that case, the bubbler return would be 6Ft/Sec, the other three returns would stay at 9Ft/Sec, and the suction lines would be at 3Ft/Sec?

Or did you mean to upsize all of the return lines?

Maybe I'm looking in the wrong places but I can't find a low flow bubbler anywhere.

 

[jblizzle]

There is no minimum flow rate required for any filter. The filter just has to be big enough to handle the highest flow rate condition. All filters work better at lower flow rates.

 

[mas985]

In that case, the bubbler return would be 6Ft/Sec, the other three returns would stay at 9Ft/Sec, and the suction lines would be at 3Ft/Sec?

Or did you mean to upsize all of the return lines?

The other three returns divide the flow rate so each is only 3 ft/sec +-. The only pipes that carry the full flow rate are the bubblers (single pipe) and the pad plumbing (single pipe). So those are the only ones that are required to be up sized. I would use 2 1/2" for the suction lines, bubbler line and the pad plumbing. Leave 2" on the return side.

As for bubblers, these can operate down to 15 GPM: http://www.pentairpool.com/pdfs/colorcascadeBR.pdf

 

[Drewskii]

There is no minimum flow rate required for any filter. The filter just has to be big enough to handle the highest flow rate condition. All filters work better at lower flow rates.

Thanks jb

I thought the "design flow" shown in the chart above was a minimum spec.

Wouldn't a lot of the issues I have be solved by your prior idea, by-passing the filter for the bubbler return/supply?

 

[mas985]

I would just opt for the lower flow bubblers I posted above.

 

[Drewskii]

The other three returns divide the flow rate so each is only 3 ft/sec +-. The only pipes that carry the full flow rate are the bubblers (single pipe) and the pad plumbing (single pipe). So those are the only ones that are required to be up sized. I would use 2 1/2" for the suction lines, bubbler line and the pad plumbing. Leave 2" on the return side.

Is this what you mean?

 

[mas985]

Yes, although I would use a manifold configuration made from TEES rather than a 4-way divide. Something like this:

 

[Drewskii]

I see...Is the order of the outgoing pipes based on the distance to the termination point...the most distant point closest to the pump?