Experimenting with pipe sizing/length and flows - considerations for new construction

[mvoltin]

I had posted earlier about chasm between the theory and practice in pool plumbing (especially when it comes to finding 2.5" compatible equipment). So, I conducted little non-scientific science experiment with different pipe sizes/length and their impact on flows. Hopefully, this will help others as they design their own pools.

Some general takeaways:

1. If you are going to need more than 30GPM, go with 2.5" piping even if you think you don't need per formal GPM recommendation - it will pay big dividends in the long run. (especially if you need more than 30GPM)
2. Even if you cannot sustain 2.5" piping throughout (small SWCG and tiny inlet/outlet on pumps, etc.) keep the rest of the piping 2.5". As others have stated more eloquently on this forum: it's not just about a restriction but its about restriction over distance.

So, about the experiment:

The diagram below shows the pool plumbing... (I know, I easily win the worst diagram in the history... ). Blue are the 2" pipes and orange are the 2.5" pipes. I ran the pump at three different speeds (1000, 1600, 2200) and measured GPM in three different scenarios:

A - pump to filter and back (3-way heading towards the SWCG and heater are closed - they are bypassed).
B - pump to filter and then to SWCG (heater is bypassed). Section heading to SWCG (about 3' section each way) is 2" piping but the rest is 2.5".
C - pump to filter to heater to SWCG (heater is only about 1.5' further away from SWCG).

SuperFlow speed	Using pool drain and returns only (2" system around pool)	Using both pool and spa drains and returns (2.5" system throughout - except around SWCG and heater). Overall pipe length is the same but the drain/return sections are "wider" when comparing to the previous column.
1000RPM	A- 28GPM B - 26GPM C - less than 20GPM - unable to measure	A- 29GPM B - 26GPM C - less than 20GPM - unable to measure
1600RPM	A- 46GPM B - 43GPM C - 38GPM	A-52GPM B-49GPM C-40GPM
2200RPM	A-61GPM B-58GPM C-51GPM	A-71GPM B-66GPM C-61GPM

I had flow meters in three different locations (I marked them in red dots):
FlowVis at the first check valve before the pump.
FlowVis at the check valve between heater and SWCG
Blue-white flow meter at just before 3-way diverter for the heater (the one that would send flow to the SWCG only or to heater (and then it would go to SWCG).

Couple more observations:
1) Blue-white and FlowVis matched very well around 45GPM and below. Around 60GPM Blue-White consistently showed 4-5GPM lower value. Not 100% sure which one is more accurate but, I would argue, for home-owner, both are good enough.
2) I had regular Jandy CV outside the heater and bought Flow-Vis retrofit. The retrofit seems to be exactly the same as the full flow-vis kit (tested by exchanging them). Thus, getting the actual flowvis (whole thing rather than just retrofit for Jandy) seems much better deal since you are getting an external body for only $10 more. You can just take out the flap and use it and keep the body if you need later on.
3) FlowVis flap has noticeably higher resistance than standard Jandy CV but when tested both in the same location, FlowVis only reduced flow by around 1-2GPM. Not sure how much Blue-white reduces the flow...

P.S. Pool and spa are combined (they share the same space/water). On rare occasions when I need to use the spa, I put divider between the spa and the rest of the pool (2mm plastic sheet), switch circulation to spa only and heat it up quickly. Otherwise, it's one combined space with two sets of drains and two sets of returns. yes, its idiosyncratic but serves our specific needs.

 

[mas985]

Definitely, more paths to/from the pool reduce head loss but I am not exactly clear on what you are comparing. Isn't the 2.5" pad pipe the same in both setups? It looks like the pad equipment is 2.5" and the rest underground to the pool is 2". So to me, you are not really comparing 2" to 2.5" but more like comparing dual 2" lines to a single 2" line although that is not exactly true either because you are also adding entry/exit points which affects head loss too.

Return eyeballs and jet orifice size have a big impact on head loss so when you put the returns in parallel, some of the head loss difference may be due to extra return orifice area and not the pipe size itself. MD/Skimmer losses behave in a similar fashion. The more entry/exit points to/from a pool reduces head loss independent of the pipe size or the number of pump to pool lines. Having the extra pipes reduces head loss as well but not as much as you might think. All the components that touch water add head loss so this why sometimes changing just the pipe size alone may not result in much benefit. It depends on how much head loss (relative to total head loss) is in those pipes to begin with.

Also, did you record the pump power draw for those measurements? The higher flow rates will result in higher energy use so the benefit is somewhat muted when you compare the economics of the two situations.

Note too that check valves plumbing curves change with flow rate (i.e. higher plumbing curve at lower flow rates). So this may be why you saw less of a difference at 1000 RPM as the head loss of the check valves became a larger portion of the total head loss.

 

[mvoltin]

Definitely, more paths to/from the pool reduce head loss but I am not exactly clear on what you are comparing. Isn't the 2.5" pad pipe the same in both setups? It looks like the pad equipment is 2.5" and the rest underground to the pool is 2". So to me, you are not really comparing 2" to 2.5" but more like comparing dual 2" lines to a single 2" line although that is not exactly true either because you are also adding entry/exit points which affects head loss too.

Return eyeballs and jet orifice size have a big impact on head loss so when you put the returns in parallel, some of the head loss difference may be due to extra return orifice area and not the pipe size itself. MD/Skimmer losses behave in a similar fashion. The more entry/exit points to/from a pool reduces head loss independent of the pipe size or the number of pump to pool lines. Having the extra pipes reduces head loss as well but not as much as you might think. All the components that touch water add head loss so this why sometimes changing just the pipe size alone may not result in much benefit. It depends on how much head loss (relative to total head loss) is in those pipes to begin with.

Also, did you record the pump power draw for those measurements? The higher flow rates will result in higher energy use so the benefit is somewhat muted when you compare the economics of the two situations.

Note too that check valves plumbing curves change with flow rate (i.e. higher plumbing curve at lower flow rates). So this may be why you saw less of a difference at 1000 RPM as the head loss of the check valves became a larger portion of the total head loss.

1. I was just trying to share some of the observations that may be helpful for someone building a new pool: even though 40-60GPM flow requirements would be satisfied by 2" piping, going 2.5" would be beneficial. I think that is the main point with all other things being equal.
2. second point was that even small diversion (towards SWCG and heater) imposes big penalties on flow rates. Yes, this is known from formulas but putting actual numbers from physical pool could be an useful illustration?

P.S. You are correct, there are lots of variables that need to be considered and this is not a scientific experiment but one can ascertain some info from imperfect scenario with all other things being equal. And agree, none of this is different from what is known but just putting practical examples...

Regarding the piping after the pad, pool has 2" piping (with 6 3/4" eye-lets) vs. spa has 2.5" that splits into two 2" piping (its always more than just a single 2") Same for the drains. So, the argument was simply more vs. less rather than exact comparison of 2" vs. 2.5": just because 2" is more than enough for 40GPM per standards, bigger pipes are still better.

 

[mas985]

So, the argument was simply more vs. less rather than exact comparison of 2" vs. 2.5": just because 2" is more than enough for 40GPM per standards, bigger pipes are still better.

However, bigger pipes are not necessarily "always" better. My point was sometimes, it doesn't make that much of a difference and sometimes, you can be worse off. I just want others to know that there are other factors at play.

-Pump's use more energy with less head loss (unless RPM can be reduced to compensate for the higher flow rates)
-Pipes larger than 2.5" increase material and labor costs.
-Too little head loss and the pump can enter run out were cavitation can occur.

There is a sweet spot in the pump's head curve where efficiency is maximum (BEP) but it is not at the highest flow rates and lowest head loss.

In your specific case, you showed a flow rate improvement but is there that much of a benefit? You may be able to save on energy IF you reduce the RPM of the pump enough to be below the energy use with the smaller pipe. Skimming may occur faster which results in shorter run times. But the net benefits may be very small.

Don't get me wrong, I will usually encourage pool owners to go with larger pipe when possible and where appropriate. But I often discourage pool owners replacing pad pipe with larger plumbing as it is rarely worth the cost and effort.

 

[JamesW]

If you are going to need more than 30GPM, go with 2.5" piping even if you think you don't need per formal GPM recommendation - it will pay big dividends in the long run. (especially if you need more than 30GPM)

The general rule is that suction should not exceed 6 ft/sec and return should not exceed 8 ft/sec.

Size.......6 ft/sec......8 ft/sec.

1.5"...........38...............51 gpm

2"..............63...............84 gpm

2.5............90.............119 gpm

3.0".........138.............184 gpm

Here is the head loss in feet for 100 feet of PVC pipe at different flow rates for 2” and 2.5” pipe.

GPM.........2”..............2.5”

30............1.6 ft..........0.7 ft

40............2.8 ft..........1.2 ft

50............4.2 ft..........1.8 ft

60............6.0 ft..........2.5 ft

70............7.9 ft..........3.3 ft

80..........10.2 ft..........4.3 ft

90..........12.6 ft..........5.3 ft.

As you can see, the head loss difference at 30 gpm is only about 0.9 feet, which is not very significant.

Even at 60 gpm, the head loss difference is only 3.5 feet per 100 feet of pipe.

I think that as long as you follow the 6 feet per second rule for suction and 8 feet per second for returns, the advantage of upsizing the pipe is negligible for pipe runs less than 100 feet in length.

For pipe runs that are over 100 feet, you would want to calculate the head loss and size the pipe accordingly.

If you want to be conservative, you can use the 6 feet per second rule for the return side as well.

PVC Pipes Schedule 40 - Friction Loss vs. Water Flow

Water flow in thermoplastic PVC and CPVC pipes Schedule 40 - friction loss <i>(ft/100 ft, psi/100 ft)</i> and flow velocities at dimensions ranging 1/2 to 16 inches.

www.engineeringtoolbox.com