Water Exchange - CYA Reduction

[mld213]

Hello, all.

Short question up front: What percent reduction in CYA is reasonable for a given percent of pool water exchange if Delta T is very close to zero?

With all the rain that Northern CA has gotten I'm tackling a water exchange to reduce CYA (currently about 150, maybe higher).
My fill water (55 F) is only about 4 degrees cooler than the pool water (59 F) so I expect Delta T is not significant (800 salt, 440 CH so please correct me if I'm wrong). Still, I'm drawing from a shallow step while filling to the bottom opposite end 40 feet away. I have a good pump and strong supply and am able to exchange at 50 gpm.

If I target 100% exchange (15 hours) should I expect CYA to come down by 50% (since there will be some amount of blending as Delta T is low)? I really don't have any experience here and am appreciative of any that you may provide.

I'd prefer not to stop once I start, and I'd rather overshoot and hit a lower CYA, but at the same time don't want to needlessly spend $ on more new water than necessary.
Of course, if I don't get it low enough and have to exchange more, it's not the end of the word.

Thanks,
Matt

 

[mknauss]

Did you test CH and salt to see what the delta on those are?

How fast will you be able to do the exchange? I am sure the pool water is still somewhat more dense than the fill water, so be sure to pump from deep end, add to surface.

 

[mld213]

Yes, somehow I missed putting those in:
Fill water CH = 75 and salt <200 (so using 100 as an estimate)

So that makes Delta T approximately -3.36 (I know I likely didn't capture all the dissolved solids in the pool).

In the end I figure I'm likely still: | DeltaT | < 5.0 so there will be a lot of mixing.

I can do 100% exchange in about 15 hours. (45,000 gal at 50 gpm.)

I reconfigured to pump from the deep end (9.5 feet deep) and add to the surface of the shallow end 45 feet away. As the sun goes down, and the surface cools off, the Delta T will become a slightly larger negative number.

Thanks!

 

[mknauss]

I was asking about CH and Salt in the pool. Likely higher than you imagine. Especially salt if you have been using liquid chlorine or trichlor to chlorinate.

You have hoses that can do 50 gpm? That is really high. You will need a gas powered sump pump to achieve that rate of effluent.

 

[mld213]

Oh, yes, the pool CH and salt was in my original post: CH = 440 and salt = 800.

Yes, the benefit of living on a farm is that I have a bit of equipment to handle large(ish) jobs. [Although it seems that I frequently tackle jobs where I wish that I had just a slightly lager ______________________ (insert: tractor, excavator, dump-truck, you-name-it) I cannot complain at all.]

For removing water from the pool, my Honda gas-powered pump will discharge 75 gpm, but with a 1.5" discharge hose it slows to about 60 gpm.

I have a 2" feed of clean water at 110 psi so at the main I get about 70 gpm, but the run to the pool is 700 feet of 1.5" so it slows to about 50 gpm.

 

[woodyp]

Y'all made me Google Delta T.

 

[mld213]

I estimate that I'm about 75% exchanged, drawing from the deep end (9 feet) and adding to the surface of the shallow end.

I decided to measure CYA at two points:
a. Discharge from the deep end (9 feet down) the CYA is: ~55
b. At the surface of the deep end the CYA is ~50

So there has been blending, as expected, but I'm getting close, I think.

 

[mknauss]

If your CYA was in excess of 150 ppm, it stayed pretty stratified until just recently.

 

[mld213]

Yes! I was pretty pleased. I'm going to run another sample from deep end and surface and then likely turn the pump on.
I had let the FC get low (about 10.0) and a day with the cover open and the sun on it before starting the exchange resulted in a nice green algae bloom. It's a much milder green now with the fresh water--a SLAM should fix it (and that should be much easier to do with CYA lower).

 

[mld213]

Just to wrap up this thread, in case anyone needs a reference in the future...

In the end I estimate I did about 100% water exchange (45,000 gal) and was able to drop CYA from 150 to 35. (Much more than I would have predicted.)

A few minor hiccups along the way but nothing major: pump stalled at one point without my realizing it so I sent some fresh water into the fields; I decided to stop for the night so it didn't happen again while I was sleeping. The gas tank only holds about 2 hours of fuel anyway so that meant I didn't have to get up every 2 hours.

Most all levels are looking good, pH and TA were high but got them down, CH is a bit low but will correct that in the next day or two.

Overall I'm happy and looking forward to not having to maintain FC at 20+ ppm.

 

[mknauss]

If you shut down during the process, that hampers its efficiency.

Glad it worked for you.