Low flow pumps - Is Air in Solar system ok?

[tomfrh]

Needsajet,

I agree with our comments re. filter head loss and Venturi effect.

in reality most loss occurs before the T.

the water level rises ~40mm with each 50rpm increment, and pressure rises approximately 3kPa

 

[mas985]

You are ignoring suction head loss which is .35m for the lift alone plus is usually at least 1/3rd of the total dynamic head loss in a typical pool system.

Again 2m @150 LPM is well beyond normal ranges for typical pool plumbing and on far right of the head curve which is beyond were most pools will operation. A more reasonable operating point would be 2.5m @ 120 lpm. This would be a normal total head loss.

So for the fittings and filter you listed above, at 120 lpm, those should have a head loss of .75m or close to 1/2 of what you are getting.

I'm not sure on this, Mark, but is the water traveling in the return line through the T exerting a small negative pressure on the static inlet of the T, by venturi effect?

No. A TEE is not the same as venturi. There is head loss but nothing else.

 

[tomfrh]

There is Venturi head loss at the T.

the water past the the T is faster than inside the filter. That increase in velocity head equates to pressure loss.

happy to assume 120lpm if preferred. 150lpm was a guesstimate simply to check that 1.4m is in the ballpark.

How did you calculate 0.75m total?

 

[mas985]

There is Venturi head loss at the T.

the water past the the T is faster than inside the filter. That increase in velocity head equates to pressure loss.

That is head loss and does not have anything to do with a venturi. A venturi is narrower in the middle which causes an increase in velocity and drop in pressure but only in the middle area. This is a venturi:



This is not the same a TEE.

 

[tomfrh]

V1,P1 at the filter. V2, P2 at the T

the Venturi effect

 

[mas985]

Yes, pressure and velocity changes with diameter of the pipe, but that still is not a venturi. A venturi is a very specific device used for injection of air or chemicals. A tee is not the same as venturi and there is no extra "Venturi" head loss. There is still head loss though and the head loss is dependent on both water velocity and pipe diameter. The head loss through a TEE has been well characterized and you can look at any hydraulic handbook which has coefficients for calculating the head loss.

http://www.waterlinefountains.com/wp-content/uploads/2017/09/Crane-410.pdf

 

[tomfrh]

there is Venturi loss.

the velocity in the filter is less than the velocity at the T. There is a corresponding reduction in pressure at the T, relative to in the filter, due to the difference in velocities, aka the Venturi effect.

 

[needsajet]

Needsajet, the water level rises ~40mm with each 50rpm increment, and pressure rises approximately 3kPa

Yeh I caught the incremental rise, but was wondering about the total height of the water column inside the solar pipes relative to top of the main return pipe (with solar off and static), and the new total kpa at the filter gauge for the new low speed.

 

[mas985]

the velocity in the filter is less than the velocity at the T. There is a corresponding reduction in pressure at the T, relative to in the filter, due to the difference in velocities, aka the Venturi effect.

I think you are confusing the Bernoulli effect with a venturi. The change in velocity and pressure with a change in diameter of the pipe is due to the Bernoulli effect.

Bernoulli's principle - Wikipedia

A venturi makes use of the Bernoulli effect for injection.

 

[tomfrh]

Just admit there’s a Bernoulli/Venturi pressure loss there and we can move on ok?

 

[mas985]

Just admit there’s a Bernoulli/Venturi pressure loss there and we can move on ok?

Yes but that is not the same as head loss. In a venturi, the pressure loss is due to the change in diameter. But a change in pressure is not always a permanent loss. As water travels up in elevation, it loses pressure but gains pressure on the way down. In a venturi, pressure drops with a decrease in pipe diameter, but it grows again with an increase in pipe diameter. That has nothing to do with head loss.

Head loss is due to the friction on the walls of pipes and fittings. When calculating head loss in pipes, the diameter of the pipe is only used to determine the velocity of the water in the fitting/pipe. The head loss is then determined from the friction coefficient and the water velocity. The change in pressure due to a change in pipe diameter is not a permanent loss. Only the friction loss is permanent.

But yes, please move on. The head loss I calculated above shows an abnormal amount of head loss from the filter to the tee.

 

[tomfrh]

There is a pressure loss at the T relative to the filter due to the venturi effect, because the pipe itself is the constriction. The water in the pipe is moving quickly, the water in the filter isn't, QED we should expect less pressure at the T (due to bernoulli/venturi effect). You can't just add up pipe and fittings.

How did you calculate 0.75m? What sizes and factors?

 

[mas985]

There is a pressure loss at the T relative to the filter due to the venturi effect, because the pipe itself is the constriction. The water in the pipe is moving quickly, the water in the filter isn't, QED we should expect less pressure at the T (due to bernoulli/venturi effect). You can't just add up pipe and fittings.

Yes, you can. The filter adds head loss but so does every component in the plumbing. Every fitting, valve, pipe and piece of equipment has head loss that is dependent on the flow rate through them. This is how you calculate head loss in plumbing. Again, just pick up any hydraulics textbook or handbook and they outline the methodology for you. I have been doing this for over a decade on this forum and have accurately predicted head loss for many many pools.


How did you calculate 0.75m? What sizes and factors?

I assumed 2" plumbing and the loss through a typical large sand filter. But I can use any values you provide. I just need to have the pipe diameter and the sand filter size.

 

[tomfrh]

40mm pipe.

Typical large sand filter.

i know how to add up fittings, what I meant was you can’t ignore the Venturi effect if you want an accurate ansnwer.

there *IS* Venturi head loss at the T (relative to filter gauge). This is part of the 1.4m difference.

If you’re going to nitpick and calculate head to within 10mm then include the venturi effect between the filter and the T.


after the T there is:

-SWG
-Branch into three 40 pipes, each with valve, each travelling about 3m to a return, each with about 4 elbows before the return.

 

[mas985]

First, I did not mean to imply that a change in pipe diameter does not cause head loss. It definitely does. The link I posted above has the coefficients for head loss in a step transition and for all fittings. Don't get me wrong, the head loss of a step transition, TEE, 90, 45, valve, etc. and the pressure drop in a venturi are all related to each other by the Bernoulli equation but they all differ by the coefficients used to determine the pressure drop. It is just that I have never seen "Venturi Effect" used to describe the head loss in fittings. That is generally used exclusively used for venturis. So the best I can do is to say they are related.

However, the head loss of a filter (published by the manufacture) is referenced at the pipe connections of the multi-port valve. There are no other diameter transitions to take into account because it is already included in the published numbers and the rest of the plumbing is 40 mm.

But because of the 40 mm plumbing, it is likely the operating point is closer to 100 LPM @ 3 m of head than the numbers I used earlier which were for 50 mm plumbing. Using this flow rate instead and 40 mm pipe/fitting/valve diameter, the plumbing between the filter gauge and TEE would now be closer to 1.4 m of head loss. My mistake for assuming 50 mm.

But if you ever have the desire, you can confirm the flow rate by measuring the head loss at the pump's drain plugs with suction and pressure gauges to measure total head.

 

[tomfrh]

Ok, so close to 1.4m at the T.

where does the other 1.6m come from?

Id have thought the triple 40s wouldn’t offer much restriction compared to filter and a single pipe.

I haven’t actually calculated anything...



lm tempted now to measure flow rate properly!

 

[mas985]

where does the other 1.6m come from?

Suction side head loss, head loss between pump and filter and the rest of the return head loss post TEE which may be small if it is split into three pipes close to that point. But if you measure head loss at the pump drain plugs, it includes everything and usually pretty accurate although at low speeds, the pressures are close to atmospheric where the gauge error is the highest. The best thing to do is to measure head loss at full speed and then scale by RPM using the affinity equations.

New Head = Old Head * (New RPM/Old RPM) ^2

Or if you have a full schematic with all pipes and fittings and dimensions, I can estimate it.

 

[needsajet]

Main back pressure after that point is the return eyeballs, and the pipe distance would be next I believe.

But the reason I'm asking my other question is that I believe the height of the water in the solar pipes (with solar off) is the pressure in the main return line at that point. It might not be because of the VRV, but it would have to be very close if you unscrewed the cap off the VRV.

 

[mas985]

But the reason I'm asking my other question is that I believe the height of the water in the solar pipes (with solar off) is the pressure in the main return line at that point. It might not be because of the VRV, but it would have to be very close if you unscrewed the cap off the VRV.

Yes, that is true. But there is also an elevation drop before entering the pool where the pressure would rise again. So that pressure is not all dynamic head loss but both dynamic & static loss. Static loss nets out in a pool system since the suction and return go back to the same body of water.


EDIT

I forgot to include the height difference between the filter and the solar pipe. I have to re-estimate the operating point.

 

[tomfrh]

Needsajet,

i removed the VRV as a test to remove pressure from that pipe. It made neglogible difference to water level in the solar return. (The VRV opens very easily). So yes, the solar return tube is a simple pressure stand pipe, in this case slight negative pressure

Howwver i I tried a different VRV which has a stiffer spring, and that makes a big difference. The water stays much higher in the solar return pipe, due to vacuum in the pipe.