Calculated Turnovers - Old Pump, Clean to Dirty Filter

[ride525]

I now have a VF pump and was curious if some of the pump experts here might have some ideas of flow and turnover changes on my old 1 hp single speed pump. I have single runs of 1.5" piping, and no solar, heater, or other features. Based on flow and pressure on my VF, I'm guessing that the old pump probably pumped about 50 GPM, showing about 15 psi on my CLEAN 48 sq ft DE filter.

When dirty, the filter could get as high as about 30 psi, with the old single speed pump. The flow was really low then. Since the flow was really low with the old pump with a really dirty filter, I'm guessing the turnovers decreased greatly over time from clean to dirty filter.

1. Any guesses from the experts, how low the flow may have been with the dirty filter at 30 psi? (The clean filter at 15 psi was about 50 GPM I think, comparing to new VF.) My guess is 20 or less GPM, but would like to hear anyone else's thoughts.

2. Would the GPM decrease on a linear relationship from the clean filter at 50 GPM, and 15 PSI? That is, say the clean filter (50 GPM, and 15 psi) went to a dirty filter at 30 psi, (say a drop of 30 GPM to 20 GPM, for the 15 psi increase). Would this be a linear change, so every psi rise of 1 psi, would mean a 2 GPM flow decrease?

This means any turnover estimates I had done using only clean filter would be too high (on average), as the GPM would steadily be decreasing until the dirty filter.

Thanks for your thoughts,

Jeff

 

[ride525]

So, what I'm suggesting....is a calculated turnover really a turnover? It is on a clean filter.

Let's say you have one speed pump that pumps 50 GPM with a clean filter, and the clean filter pressure is 15 psi. So that's 50 x 60 or 3,000 gallons per hour. And if you pump six hours that's 18,000 gallons a day.

But say the filter is dirty, with the psi at about 25 psi. The flow might be much less, say 25 GPM. So, in this example, with a dirty filter, you would be turning over half, or 9,000 gallons a day.

So, the calculated turnover, would be accurate the first day, but steadily decrease until the dirty filter was cleaned again.

But, I think I read on Pool School, that one good way to check your needed flow, it just to adjust the number of hours, until you find a value that gives you clear, sparkling water.

Jeff

 

[mas985]

Every 1 PSI increase in filter pressure is adding about 2.3' of head. So if the clean pressure is 15 PSI and you don't clean it until 30 PSI, you have nearly doubled your head loss. So if you had 40 feet of head and 5 was the suction, it would go to about 70' of head at 30 PSI. That can pretty much kill the flow rate on any pump.

This is why I would suggest never allowing the filter PSI to rise more than 25% from the clean value. Otherwise, you are really sacrificing a lot of flow rate to delay cleaning a filter. The rule of thumb of 8-10 PSI rise is really way too much and can cost a lot in lost efficiency. However, you kind of have to balance that with the frequency of cleaning since you probably don't want to be doing that more than once a week or two either.

I have a cartridge filter and I clean it before it gets even a 1 PSI rise. Of course, that is only twice a year so not too bad.

oops forgot your original question.

#1 Very low flow rate but to get exact numbers, I would need a lot more details on the pump and plumbing.

#2 The increase in head loss due to pressure rise would follow the pumps head curve to the left. So as head increases, flow rate decreases. There is more or less a quadratic relationship between the two.

 

[ride525]

I have a cartridge filter and I clean it before it gets even a 1 PSI rise. Of course, that is only twice a year so not too bad.

Darn, maybe I should have bought a big cartridge filter, instead of recently replacing my DE grids!

Clean you filter only twice a year! And only less than 1 PSI rise !

Amazing!

I want one too!