Flow between pool and surge tank

[Abdulellah]

Greetings of respect to every member and moderator in this wonderful forum .

I hope to find what I need through your advice

I'm having a problem with the flow between the pool and the overflow tank, when the pump is shout-off mode

In the simplified hydraulic system attached to the pictures, what is the method through which the flow of pool water can be prevented from causing an overflow of the balance tank water?

Note that
-valves cannot control the value of hydrostatic pressure
-When a natural non-return valve similar to a Hartford loop is implemented that rises above the pool water level, the problem of the appearance of a back siphone remains.
-Sometimes we need a full flow from the pool for the purpose of water circulation and for another purpose mode related to a locally invented heating system, and this is done by closing the water flow valves of the balance tank.
-There is a problem with the type of spring non-return valve. It is not possible to achieve full crack pressure that allows the valve to be opened completely, which necessitates opening the pool floor water drainage valves by more than 60%.
-When the pool water flow valves are not opened more than 60%, there is a significant decrease in the available NPSH value.
-I hope I have conveyed the idea to you completely. What I want is a simple, traditional method that allows you to prevent the pool water from causing an overflow of the balance tank water without manual intervention or operation.

Also how can I use and understand the included spring check valve spreadsheet

I live in a country where there are not many types of valves with advanced technology, and I want a simple solution that meets the desired purpose.

Thank you. I have always learned a lot from this forum and I hope this problem will be solved

 

[YippeeSkippy]

Hello! I am going to page one of our experts in this topic- @mas985

 

[mas985]

I'm having a problem with the flow between the pool and the overflow tank, when the pump is shout-off mode

I am not clear, does the check valve shown in the first picture not work or is there currently no check valve installed? A check valve is really the only practical solution. Do you have access to Jandy check valves? They do not have as much head loss as the spring loaded check valves you are showing.

https://www.jandy.com/en/products/pool-valves/check-valves/check0

As you pointed out, a hartford loop doesn't work when full of water as there will be a siphon in place. They are only useful when the siphon is broken.

 

[Abdulellah]

Hello! I am going to page one of our experts in this topic- @mas985

Thank you

 

[Abdulellah]

I am not clear, does the check valve shown in the first picture not work or is there currently no check valve installed? A check valve is really the only practical solution. Do you have access to Jandy check valves? They do not have as much head loss as the spring loaded check valves you are showing.

https://www.jandy.com/en/products/pool-valves/check-valves/check0

As you pointed out, a hartford loop doesn't work when full of water as there will be a siphon in place. They are only useful when the siphon is broken.

Thank you for your response. Let us consider it a general case in which the balance tank and the pump may be located below the pool’s water surface level. I hope that a valuable discussion will continue between us regarding this topic. The main idea is that the difference in hydrostatic water pressure between the two sides of the valve will consume a lot of the TDH value. For the pump, and you can imagine having more than one valve. Any solution or trick from the seventies is acceptable to me. We do not have advanced equipment.

 

[JamesW]

A ball check valve or a swing check valve will probably be a better choice.

What is the pipe diameter, pipe length and flow rate?

What pump do you have?

https://www.cepex.com/wp-content/uploads/2017/07/6-cepexcheckvalves56.pdf

https://www.cepex.com/wp-content/uploads/2017/07/3-Cepex-check-valves-Ball.pdf

 

[JamesW]

When the pool water flow valves are not opened more than 60%, there is a significant decrease in the available NPSH value.

Are you measuring the suction pressure?

If yes, what are the readings?

What is the NPSH required by the pump?

What are your calculations for the NPSH Available?

For the D-90 (3") ball valve, the head loss is not too bad.

The ball check valve requires some back pressure if horizontal.

If vertical, the back pressure required is lower.

y = 0.00005517X^2, y from 0 to 5 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.

www.wolframalpha.com

The minimum opening pressure for the spring check valve is only 0.025 bar or 0.836 ft WC (feet of water column) and the head loss is also not bad, so I do not see why you would have a problem with the spring check valve.

https://www.cepex.com/wp-content/uploads/2017/07/1-Cepex-check-valves-Spring.pdf

Check Valves | Products | Cepex a brand from Fluidra Group

Cepex designs and manufactures an extensive valve retention family useful for a wide range of applications that provide fluid control.

www.cepex.com

The swing check valve does not create a lot of head loss and it does not require much to open.

If vertical, it does not require much back pressure to keep it closed.

https://www.cepex.com/de/wp-content/uploads/2017/07/5-Cepex-Swing-check-valves.pdf

Overall, I do not think that you would have a problem with the Spring, Ball or Swing check valves.

The opening pressure is low and the head loss is also low for all three types.

If the pump is below the pool and the pipe is sized correctly, I can't see a problem with excessive head loss reducing the NPSH Available to a level low enough to cause any issues.

 

[JamesW]

The main idea is that the difference in hydrostatic water pressure between the two sides of the valve will consume a lot of the TDH value.

How much flow do you want from the pool main drain and how much flow do you want from the balance tank?

Is the pump single speed or variable speed?

Can you post the pump performance curve?

What is the pipe size and length from the pool to the T and what is the pipe size and length from the balance tank to the T?

 

[JamesW]

S1 = 100 feet of 3” PVC pipe.

S2 = 100 feet of 3” PVC pipe.

H1s = 8 feet

H1d = 0.0001904F1^2

H1T = 0.0001904F1^2 - 8 feet.

H3s = 3 feet

H3d = 0.0001904F2^2 + 0.00005517F2^2 = 0.00024557F^2

H3T = 0.00024557F^2 - 3 feet

F1 + F2 = FT

H1T = 0.0001904F1^2 - 8 feet.

H2T = 0.00024557F2^2 - 3 feet

H1t = H2t

If the flow through the pool’s main drain suction pipe is 250 GPM (F1), then the dynamic head loss is 11.9 feet and the total head loss is 11.9 - 8 feet = 3.9 feet.

3.9 + 3 = 0.00024557F2^2

F2 = sqrt(6.9/0.00024557)

F2 = 167.62 GPM.

FT = 250 + 167.62 = 417.62 GPM.

F1 = 60%.

F2 = 40%

Based on the difference in height and the extra resistance of the check valve, the flow will be lower for the surge tank suction line than the pool main drain suction line but you can add resistance to the main drain line by closing the valve on the line some.

 

[JamesW]

If the water was the same height, then the difference between F1 and F2 would be.

0.0001904F1^2 = 0.00024557F2^2

F1 = sqrt(0.00024557/0.0001904) F2

F1 = 1.135675F2

F2 = F1/1.135675

If F1 = 250 GPM, then F2 = 250/1.135675 = 220 GPM.

However, the water in the pool is higher, so there is more pressure pushing the water to the pump for the pool suction line vs. the Surge Tank suction line.

 

[JamesW]

The surge tank water surface is 1.4 meters below the pool water surface, which is 4.59318 feet (2 psi).

The water level differential creates 2 psi of closing force on the check valve when the pump is off.

The equipment room is 3.4 meters (11.1549 feet) below the pool water surface.

The equipment room is 2 meters (6.56168 feet) below the surge tank water surface.

If we assume that both lines are 100 feet of 3” PVC pipe, the flow just by gravity will be:

Pressure differential............Flow in GPM

Pool 11.15 feet...........................223

Surge 6.56 feet............................168 (55 GPM less)

Total = 391 GPM.

Pool = 57%

Surge = 43%

So, before you even get to any negative pressure, you can get 391 GPM.

If the pump begins to run and produces 5 feet of suction:

Pressure differential............Flow in GPM

Pool 16.15 feet...........................276

Surge 11.56 feet.........................228 (48 GPM less than the pool).

Total = 504 GPM.

Pool = 55%

Surge = 45%

Hazen-Williams Water Flow Formula: Head Loss, Data, Charts & Calculator

Friction head loss (<i>ft_H2O per 100 ft pipe</i>) in water pipes can be estimated with the empirical Hazen-Williams equation.

www.engineeringtoolbox.com

So, the pressure differential creates more flow in the pool main drain suction line than in the surge tank suction line.

The check valve adds to the head loss on the surge tank suction, but not that much.

If you need to balance the flow, then you need to restrict the flow on the pool main drain suction line.

I don’t see where you would have a problem with the spring check valve since it requires only 0.85 psi or 2 feet of head loss to fully open.

To get the spring check valve fully open, you need 2.85 psi differential or 6.58 feet of head loss.

I would probably put a swing check valve and a ball check valve on the surge tank suction line to have a redundant safety check valve and then a spring check valve after the pump as an additional safety.

What are the line sizes and lengths?

How much flow do you need from the pool and from the surge tank?

Here are some check valves from Praher.

https://www.praherplastics.ca/wp-content/uploads/2022/04/Praher-2022-Pool-Catalogue-April-2022.pdf

The Jandy Check Valve is a good choice for 3" PVC.

Pool Check Valve: Jandy Check Valve | Jandy

Looking for a Pool Check Valve? Find the Jandy Check Valve for the Perfect Pool Experience. Buy Now w/ a Pool Pro Near You!

www.jandy.com

https://www.poolsupplyunlimited.com/Products/Manuals/53125_2013811086.pdf

Pentair makes a 3” check valve. 263060 Check Valve CPVC 2.5 in. (3 in. slip outside)

https://www.pentair.com/content/dam/extranet/nam/product-related/product-manuals/pool-and-spa-equipment/valves/FullFloXF%202Way%20and%203Way%20Diverter%20Valve%20and%20Check%20Valve%20Installation%20and%20Operation%20Instructions%20English.pdf

FullFloXF® Diverter & Check Valves – 2-1/2" & 3"

Our larger FullFloXF 2-way, 3-way and check valves provide higher flow rates and produce much lower water pressure drop. These valves are ideal for diverting, shutoff, or mixing applications. The polytetraflouroethylene (PTFE) composite diverter seal requires no lubrication. Field-adjustable...

www.pentair.com

 

[Abdulellah]

A ball check valve or a swing check valve will probably be a better choice.

What is the pipe diameter, pipe length and flow rate?

What pump do you have?


View attachment 531892

https://www.cepex.com/wp-content/uploads/2017/07/6-cepexcheckvalves56.pdf

https://www.cepex.com/wp-content/uploads/2017/07/3-Cepex-check-valves-Ball.pdf

What is the pipe diameter, pipe length and flow rate?

What pump do you have?


View attachment 531892

https://www.cepex.com/wp-content/uploads/2017/07/6-cepexcheckvalves56.pdf

https://www.cepex.com/wp-content/uploads/2017/07/3-Cepex-check-valves-Ball.pdf

Assume two pipes 4" from balance tank , two self prim pumps ( astralpool maxim ; 66 cube meter per hour @ 10mH2o TDH ) 4.5 Hp

 

[Abdulellah]

How much flow do you want from the pool main drain and how much flow do you want from the balance tank?

Is the pump single speed or variable speed?

Can you post the pump performance curve?

What is the pipe size and length from the pool to the T and what is the pipe size and length from the balance tank to the T?

 

[Abdulellah]

Filteration mode Flow rate ( balance tank 70% main drain 30% )

 

[Abdulellah]

If the water was the same height, then the difference between F1 and F2 would be.

0.0001904F1^2 = 0.00024557F2^2

F1 = sqrt(0.00024557/0.0001904) F2

F1 = 1.135675F2

F2 = F1/1.135675

If F1 = 250 GPM, then F2 = 250/1.135675 = 220 GPM.

However, the water in the pool is higher, so there is more pressure pushing the water to the pump for the pool suction line vs. the Surge Tank suction line.

Thank you for your valuable and worthly efforts. It seems that you made a great effort to analyze the current situation. Thank you again

 

[Abdulellah]

My only concern is to prevent the compensation tank from overflowing when the pumps are off, and also to ensure that the pump does not exhaust its capacity in suction side , since the non-return valve is exposed to high hydrostatic pressure due to the pool.

 

[mas985]

Thank you for your response. Let us consider it a general case in which the balance tank and the pump may be located below the pool’s water surface level. I hope that a valuable discussion will continue between us regarding this topic. The main idea is that the difference in hydrostatic water pressure between the two sides of the valve will consume a lot of the TDH value. For the pump, and you can imagine having more than one valve. Any solution or trick from the seventies is acceptable to me. We do not have advanced equipment.

The surge tank is below the pool water level so if you go with a combining TEE between the pool and surge tank drains, you will need a check valve on the outlet of the surge tank. Otherwise the surge tank will overflow when the pump is off.

Another option would be to convert the floor drain into a return since it really isn't needed. That way you can put the check valve on the return side of the pump where the head loss of the check valve does not matter as much and you don't have to be concerned with the imbalance between the two suction lines. Plus the overflow will be stronger.

The float valve idea might work but failure of the float valve is concerning as it would create an overflow condition.

If it were me, I would convert the pool floor drain into a return.

 

[JamesW]

Assume two pipes 4" from balance tank , two self prim pumps ( astralpool maxim ; 66 cube meter per hour @ 10mH2o TDH ) 4.5 Hp

66 cubic meters per hour is 290.6 gallons per minute.

For a 4” pipe, that is 7.43 feet per second

The suction should be kept below 6 feet per second

For this design, I would put 2 check valves on the suction from the surge tank at the red dots and a check valve after the pump .

In my opinion, the head loss from the check valves is negligible.

You can control the flow ratio from both lines with valves.

I think that you can use the spring, ball or swing check valves in 4".






In this design, I would put 3 check valves, 2 on the suction and one on the pump pressure side.

You can't use a check valve on the main drain line to the surge tank since the water has to go that direction anyway.






You want to keep the water velocity below 6 ft/sec on the suction and below 8 ft/sec on the pressure side of the pump.

For a single suction, I would do a 5" PVC Pipe.

Pipe Size	6 ft/sec	8 ft/sec
1.5"	38 GPM	51 GPM
2"	63 GPM	84 GPM
2.5	90 GPM	119 GPM
3.0"	138 GPM	184 GPM
4.0"	235 GPM	312 GPM

Hazen-Williams Water Flow Formula: Head Loss, Data, Charts & Calculator

Friction head loss (<i>ft_H2O per 100 ft pipe</i>) in water pipes can be estimated with the empirical Hazen-Williams equation.

www.engineeringtoolbox.com

 

[JamesW]

The equipment room is 2 meters (6.56168 feet) below the surge tank water surface.

This creates a flooded suction and the NPSH available before the pump turns on is about 40 feet.

The head loss at 300 GPM through a 4" pipe is less than 5 feet.

So, the NPSHa is going to be about 35 feet at 300 GPM through 4" PVC.

The common NPSHr is about 17 feet, so you should not be in any danger of dropping below the NPSHr.

 

[JamesW]

I would go with this design because it will allow you to control the flow to/from the surge tank.

I would make both lines 5".

That way, you can put the full flow through either line.

75 Cubic Meters per hour is 330.2 gal/min (gallons per minute).

You might need to shut down the surge tank for service and you can still circulate using the main drains.

Make sure that you use at least 2 main drains Teed together for safety and make sure that they are rated for the full flow plus a margin for safety.

I would want a minimum flow capability of 100 cubic feet per hour (440.3 gallons per minute) for the main drain pair (About 220 GPM (50 cubic meters per hour) each .

Or, you can close the main drain line and put the full flow on the surge tank if you want.