Pump inlet minimum straight length.

czb182

Well-known member
Oct 20, 2020
63
North Virgina
I know pool code and manufactures recommended minimum pipe length going into pump inlet , but not sure if that is undisturbed length or can include check valves like I have in picture below. 2” pipe, so should there be 8” of straight run after the check valve? They just installed pad today so want to know if I should bring this up.

1905A343-1DBF-4B82-9D1A-70239890650A.jpeg
 

JamesW

TFP Expert
Mar 2, 2011
25,052
In my opinion, it's fine the way it is.

What is the model number for the pump?

Why do you have the check valve?
 

czb182

Well-known member
Oct 20, 2020
63
North Virgina
It’s the Jandy flo pro 2.7hp dual voltage model, forgot exact model number. I think they did it for priming. The pump is about 4 or 5 feet above pool water level unfortunately.

edit

 

czb182

Well-known member
Oct 20, 2020
63
North Virgina
It’s weird when I think about it because I assume it’s for laminar flow to impeller, but what about the pool basket, doesn’t seem like it matters , but at the same time why is recommended, there must be a reason
 

JamesW

TFP Expert
Mar 2, 2011
25,052
I would put the check valve after the pump and use a Jandy 2 way valve before the pump.

You can remove the check valve part and install a Neverlube valve part in the check valve body so you don't have to replumb.

After the pump, install a Jandy check valve and a three-way valve so that you can go to waste if you want.

In my opinion the suction lines are probably too small depending on the maximum flow you need.
 

czb182

Well-known member
Oct 20, 2020
63
North Virgina
Ok thanks I will check. Not sure if there is room for check valve between pump and filter though. There is the check valve between heater and swcg, would that achieve same purpose?

why would the two way before pump be better? I would have to manually close to keep water in case i open basket?

Max is 80 gpm, needed by spa. The spa suction is 2.5” all the piping is 2” between equipment but that is restricted by size of unions on equipment.
 

JamesW

TFP Expert
Mar 2, 2011
25,052
The check valve has about 2 feet of head loss at 80 gpm.

100 feet of 2.5" pipe is about 4.2 feet of head loss at 80 gpm, which gives you 6.2 feet for the pipe and check valve.

Add 5 feet for the static head loss and your total is 11.2 feet.

2 feet from the check valve is not bad but it's better to have it on the pressure side rather than the suction side, especially since the pump is higher than the water, which creates static head loss in addition to the dynamic head loss of the pipe and the check valve.

Also, the check valve before the pump will be more likely to get debris inside compared to after the pump basket which will strain out the larger debris.

It's not critical that you change it to after the pump if you don't want to.



For suction, you want to keep the water velocity below 6 ft/sec. For returns, you want to keep the water velocity below 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
4”...........234.............313 gpm

2.5" pipe is ok up to 90 gpm.

Check valve .jpg
 
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czb182

Well-known member
Oct 20, 2020
63
North Virgina
That’s great info , thanks for that link, I was very curious about that, since they used a 1.5” to 2” 90 degree check valve at heater which I was worried about more head loss, but I guess it’s same as 2-2.5” one.

regarding static head of elevated pump, is this only added to Dynamic head on suction side? I know TDH should be lower on suction than return but don’t know how much lower is required to avoid issues.

I’m going to also ask what is the check valve on spa return for. Actually I bet I know, it’s elevated spa so water will be higher, so I bet to prevent water flowing backwards when pump off. I think anyway.
 

Darin

Well-known member
May 29, 2015
75
Muscatine/IA
Why do you need a check valve befor or after pump in a single pump scenario? What are you trying to hold back from getting to the pump?
 

czb182

Well-known member
Oct 20, 2020
63
North Virgina
I’m not 100% sure. Here is my guess of purpose.
On inlet - if opening pump basket open to clean, will hold water on suction for easy priming.
On outlet- not sure actually

I think even with elevated pump, with no check valves, water would not move in either direction once pump shut off , correct me if I’m wrong. I was think only needed when allowing air in system , like pump basket or cleaning filter. I’m guessing the one right before swcg is just precaution , if water flows just a bit back might have concentrated chlorine on heat exchanger
 

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JamesW

TFP Expert
Mar 2, 2011
25,052
The 90 degree check valve is 3 feet of head loss at 80 gpm, so not a big deal.

The suction should ideally not exceed 8 feet, but it can be more.

Below shows the boiling point of water vs. pressure.

At 80 degrees, the boiling point is 29” Hg. of vacuum

29” Hg. = -14.2435 psi.

29 Inches of mercury = 32.8556 Feet of water (Lift).

If a pump could produce a perfect vacuum, the maximum height to which it could lift water at sea level would be 33.9 feet.

However, the water would begin boiling (cavitating) at 32.9 feet of suction head loss at the impeller eye.

The eye of the impeller has a lower pressure than the suction head loss, so you want to avoid creating suction head loss as much as possible to stay well away from the cavitation point.

In my opinion, I would avoid going over 12 feet of head loss on the suction side.

I would put the check valve after the pump and change the valve in front of the pump to a two way valve by removing the check valve part and installing a NeverLube valve part.

If you come out of the pump and go into a 2” x 2.5” 3-way valve, you can use the 3-way valve as the 90 going to the filter.

To the left, you will add a 1.5” x 2” check valve into the 3-way valve and, to the right, you will use that as waste.Vacuum vs pressure.jpgJandy check valve head loss.png
 
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JamesW

TFP Expert
Mar 2, 2011
25,052
I think even with elevated pump, with no check valves, water would not move in either direction once pump shut off , correct me if I’m wrong. I was think only needed when allowing air in system , like pump basket or cleaning filter.
Theoretically, if the system is perfectly airtight, the water will stay in the system without draining out.

However, I would use a check valve to avoid problems, especially if the filter is a DE filter.

What is the filter model number?
 

czb182

Well-known member
Oct 20, 2020
63
North Virgina
Any idea equivalent foot of this type of reducer in picture. This is spa suction , 3” reduced to 2.5”
 

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JamesW

TFP Expert
Mar 2, 2011
25,052
regarding static head of elevated pump, is this only added to Dynamic head on suction side? I know TDH should be lower on suction than return but don’t know how much lower is required to avoid issues.
Static head is added to the suction and subtracted from the return for a net of zero.

The key for suction is to stay well away from the cavitation point.

I would use 12 feet as the maximum acceptable suction head loss.

What you lose "pulling" the water up is gained as the water "falls" back to the pool.

This also creates a problem for the heater pressure switch, which requires about 3 psi to close the switch.

Since the head loss is subtracted from the returns, you need at least 5 psi to close the pressure switch instead of the normal 3 psi.

If we use 6 psi as a target head loss for the return after the filter to reliably close the heater pressure switch, that’s about 14 feet of head loss from the exit of the heater to the pool.

If you have 100 feet 2” PVC from the heater to the pool, you don’t hit 14 feet of head loss until the flow is about 100 gpm.

Even a 400,000 btu/hr heater only requires 40 gpm.

So, unless you install a 40 gpm flow switch instead of the pressure switch, you will have to run the pump much faster than necessary just to make the heater work.

You can install eyeballs on the returns to create extra back pressure, but I would go with a flow switch.

Once the heater goes more than 4 feet above or below the water level, I would recommend adding a flow switch set to the correct required gpm instead of the pressure switch.

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2 inch PVC pipe head loss.png
A 400,000 btu per hour heater requires a minimum of 40 gpm.

Below is the temperature rise for a 400,000 btu per hour heater vs. flow in gpm.
400,000 btu per hour heater flow vs. temp rise.png
 

JamesW

TFP Expert
Mar 2, 2011
25,052
Any idea equivalent foot of this type of reducer in picture. This is spa suction , 3” reduced to 2.5”

I would estimate it at about 5 feet of pipe.

PVC fitting equivalent length.png
 
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czb182

Well-known member
Oct 20, 2020
63
North Virgina
Ok using jxi400, says operates from 30 to 100 gpm / 2 to 50 psi . Says factory pressure switch set at 7 psi is good for up to 2 feet static head, adjustable up to 6 feet of static head, so seems like it should work for my case.
 

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JamesW

TFP Expert
Mar 2, 2011
25,052
Note that pipe length is not the same thing as head loss.

For example, if you have 100 feet of 2" PVC pipe, the head loss depends on the flow as shown in an earlier chart.
 

Darin

Well-known member
May 29, 2015
75
Muscatine/IA
If you really feel tht your system will not self prime, then put a simple ball valve in ahead of the pump. Then you could prime your pump by adding your water hose directly into your strainer drain for priming.
 

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