Hydraulics 101 - Have you lost your head?

SPP

LifeTime Supporter
Apr 6, 2008
306
Indonesia
Now the 2 HP is 12 hours per turnover, too slow to maintain best water clarity when divers use the pool.
Hence I have back up 1.5 HP pump on trolley and use it with the 2 HP to get near 4 hrs turnover after each pool used by swimmers or divers.

Sanitation distribution concern and being addicted to very clear water, no choice, 2HP pump will never stop. :mrgreen:
 

mas985

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SPP said:
Now the 2 HP is 12 hours per turnover, too slow to maintain best water clarity when divers use the pool.
Are the divers complaining about clarity or does the pool cloud up after use? The particle size that an average filter removes from the water is too large to remain suspended in the water (will normally sink) so if your water is cloudy, then it is probably something growing in the water rather than simple dirt so it could be a chemistry issue rather than filtering.

Also, if you want to simulate a 50 HZ pump with the spreadsheets, just use 2875 RPM instead of 3450.
 

SPP

LifeTime Supporter
Apr 6, 2008
306
Indonesia
Hi Mark,

If 7 divers hang out for 4 hours, clarity drop a bit, usual.
No one is complaining, its just me being so used to see maximum achieveable clarity and I want to keep it that way always, and fast.

That is why the trolley pump ( 2nd pump ) with 20 micron cartridge and 1 micron slime bag downstream is for, faster recovery. I am stuck with a poor plumbing design on the main 2HP pump.. :rant:

Its not chemistry thingy, all can be filtered to achieve clarity approx 1.5 NTU.

My pool is 13.2 feet deep on one end, white tiles, so any minor clarity drop is easy to see.
One of the common reference I like to use is strand thin female hair ( black color ) at 13.2 feet deep is highly visible from 1.5 meters above the pool.

Ok me check the pump spreadsheet, many many thanks.

Later,
SP
 

robinasu

Well-known member
Jun 23, 2014
45
CA
Great thread! Thanks to all for putting this information out there. Motor ratings are such a disaster!
 

JamesW

TFP Expert
Mar 2, 2011
23,224
"High water velocity in suction lines and main drains can increase the risk of entrapment."

How does water velocity affect risk of entrapment?

The momentum of the water is the same for the same gpm through equal lengths of different size pipe. Wouldn't that determine the force exerted on someone blocking a suction outlet?

The kinetic energy is higher for equal gpm through equal lengths of different size pipe. Would that be the reason?

For example, 80 gpm through 100 feet of 2" pvc pipe is about 569 lbs of water at 7.8 feet per second. 80 gpm through 100 feet of 1.5" pvc is 343 lbs of water moving at 12.9 feet per second. The momentum is the same, but the kinetic energy of the water in the 1.5" pipe is 1.66 times higher.

Also, this is only relevant to the initial blockage. The suction would reach equalibrium based on the pump curve and the new total dynamic head. The equilibrium suction should be roughly equal regardless of the pipe size.

If we look at the suction in the main drain pot, the suction would be the same for equal water flow through the grate regardless of the pipe size. Only once we get further into the pipe towards the impeller does the pressure diverge for different pipe sizes.

If we assume that the initial blockage of the grate is sudden and complete, then the force would come from decelerating the moving water. Force = mass x acceleration. The water in the 2" pipe has more mass but lower velocity. The mass is about 1.65 times more but the acceleration should be 1.65 x less.
 

mas985

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There are several major sources of entrapment that the VGBA calls out:

Hair entanglement: hair can get caught in a faulty or broken drain cover
Limbs: arms, legs and fingers can become lodged in a suction opening
Body: any body part that can cover a drain can be held down by suction
Evisceration: sitting on a broken or uncovered drain can cause injuries or disembowelment
Mechanical: jewelry or bathing suits can become entangled in a drain cover

That comment was directed to the first and last in that list and I believe the reason why the regulations recommend a larger pipe for the MD split (on new builds) than the rest of the suction pipe. With higher water velocity in the pipe, there is more turbulence near the entry point of the pipe and more chance of hair or other loose things getting entangled in the cover.

But regarding body entrapment and suction (2-4), once entrapped, the pipe size would no longer matter. At that point, only the maximum pump suction matters.
 

mas985

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Yes which is why listed as one of the concerns for high water velocity.



Water Velocity

There are three reasons to be concerned about water velocity in plumbing:

  • High water velocity can result in high head loss.

    High water velocity in suction lines and main drains can increase the risk of entrapment.

    High water velocity can also increase the risk of hydraulic shock (water hammer) which can cause damage to plumbing weld joints.
 

JamesW

TFP Expert
Mar 2, 2011
23,224
I was thinking about the effect of water hammer on a person. Perhaps the initial shock wave would cause extra bodily harm. I would also suspect that it could cause extra mechanical entrapment by pulling a limb in deeper.
 

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mas985

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I suppose that it is possible if the conditions are right. Hydraulic shock is generally caused by quickly closing valves like a check valve slamming shut but it must be a very fast (<< 1 second) closure to create a powerful enough wave to cause any plumbing damage. The wave pressure intensity is inversely proportional to the closing time as well as water velocity. I think there would be a tendency to pull one's arm back as it is getting sucked in so I suspect it would probably be a slower process than needed to create a substantial pressure wave. Also, hydraulic shock tends to launch a pressure wave away from the closure point in both directions. So I am not sure that it would cause any more damage than just getting your arm sucked into a pipe which is bad enough. You could experiment with a rubber ball (that sinks) in the skimmer port and see if that is enough to cause hammer and if the ball is pushed backwards any due to a reverse wave.
 

windowman

Silver Supporter
Mar 23, 2016
7
West Chester, PA
Had an interesting one last year in my 25 year old pool. Air was being introduced into filter basket continuously at an intermediate level but pump could still pressurize filter. Dug up pipe below suction line at pump as digging at skimmer would require removing concrete. Noticed after turning off pump that hole would partially fill with water after a few hours then empty of water when pump was turned on. Could find no cracks in piping or gaps at fittings. Kept Digging so entire pipe was exposed at elbow and found problem. Over the years some small debris had been circulating at bottom of elbow at curve and had finally worn away the plastic of the pipe on the outer radius till water could escape or air could enter under suction.
 

possas

New member
Jul 27, 2017
1
Portugal
Hi.
First let me say that this post is amazing and i learn a lot. Thanks mas985 ;) (btw im portuguese , sorry for my bad english)

In next month i will build a natural swimming pool. Me and the pool guy (wich is a great guy with open mind ) are trying to get the most eficient design of pipes.
We will use flexible pipe that have 63mm outside and inside 55mm. That is a bit more than a 2" pipe, to make it more easy just let say its a 2" pipe. I would want bigger but here dont exist bigger than 63mm for flexible pool pipe and they are already expensive.

On suction side, the pool will have 2 skimmers + 1 drain , return side will have 4-6 returns. After i read your post the best option to have the lowest head loss possible would be 1 independent pipe for each skimmer and 1 for drain, total 3 pipes on suction side of 2" (wich according your table is equal to a single 3.47" pipe).

For return side, and considering this "On the return side of the pump, multiple lines can help reduce head loss as well but the equivalent size of all the return lines should never exceed the equivalent size of all the suction lines or pump cavitation could become an issue." its best to have 2 or 3 pipes of 2" ?

On pump house the pipe will be rigid PVC and im think using 3" pipe instead 2" pvc pipe to reduce head loss, that make any sense or is a waste of money/work?

Something like this :)
In reality the return side is a bit more complex since have one line for the return eyeballs and other for the biofilter but this way is more simple to drawn.
pool.jpg

 

mas985

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Any idea about what these numbers translate to, outside the United States where pipe is typically measured mm, energy used in Kilowatts and pressure in Kilopascals?
Which numbers? The units conversion is fairly straight forward.
 

mas985

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Pool Pump Tools now available macro free in both Excel and Google Sheets version! Also, includes a complete head loss model.

Hydraulics 101 - Pump and Pool Spreadsheet Tools

Disclaimer


These spreadsheets are provided as is without any guarantees or warranty.

License Agreement

To get access to these spreadsheets, you must agree to the following license agreement:

  • These spreadsheets are for personal use only and shall not be used for any commercial business without the author's permission.
  • These spreadsheets shall not be copied in whole or part into another spreadsheet or application without the author's permission.
  • These spreadsheets shall not be sold nor distributed in anyway other than through this website.

The following spreadsheets are available:

  • NEW Pool Pump Tools - Estimates pump operating points, pump energy costs and waterfall flow rates. Now Macro Free and Google Sheet Version! Excel Download, Google Download
  • Pool Heat Transfer Tools* - Estimates heat loss and gain from a full heat transfer model that includes the effects of the sun, solar panels, NG heater, heat pump, and environmental conditions. It will also estimate the amount of water lost per day/week due to evaporation. Included are tabs to estimate heat loss from pipes and how long it can take for a pipe to freeze. New cross-platform version: Excel Download
  • Chemistry Calculator* - Calculate chemicals required to balance pool chemistry and logs test results. Excel Download, Google Download

* Macros must be enabled to activate the workbook.

For questions regarding these spreadsheets, please PM me.




Reference

Motor Mastery University
Motor Doctor
A.O.Smith Troubleshooting Guide
Baldor Cowern Papers
PumpEd 101
 
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YonnyPiscinas

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Jan 10, 2020
132
Uruguay - SA
"The static head loss on the way up is directly offset by the static head gain on the way down so there is no net static head change due to the fact that the solar is installed on the roof -Suction head = 75% of return head for 1.5" suction and 1.5" return pipe "

Great info thanks.
Does having a vacuum release valve on the solar collectors affect this offset? I'm planning to move my collectors to a high roof and was wondering whether to install a VRV.
 

mas985

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Static head is related to the height of the water only (i.e. weight of the water). It doesn't require a sealed system (i.e. closed VRV). However, if the VRV does open, the pressure at the VRV becomes 0 PSI and the return side column of water will drain out at a faster rate than the supplied water replaces it.

But I would not be concerned with the above. The primary reason for a VRV is so the panels become self draining to prevent freeze damage. It is also useful if solar is turned off during the heat of the day. I would definitely plan on having a VRV. However, the placement of the VRV can be critical depending on the total plumbing design.
 
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