Can someone help estimate my Total Resistance to Flow (Feet of Head)

[1PoolNoob]

I can help but I have to ask what is your objective and why do you need to know the head loss?


As for additional information:

Pump make/model

Filter make/model

Backwash valve size

Current filter pressure

Picture of the pad might be helpful too

Awesome thanks a lot. My current pump is all but dead (leaking etc.) and I am looking at replacing it with a Hayward Super Pump 3/4 hp (Model: SP2605X7A).

The issue is that the old pump is about 30 years old and pumps these days put out a lot more GPM for the same HP than they used to and I am concerned that the new pump may be too powerful for my setup. I was unable to find any specs for the current pump due to it's age for comparison purposes (I called Pentair, which purchased Pac-Fab, to see if they had any data on it, but their records did no go back that far).

I figure if I can get a reasonable estimation of what the Total Feet of Head is, I can look at the performance curve and see what the pump's resulting flow rate will be and whether that will be too much, or acceptable for my system. The last thing I want is to install a new pump, only to have something else break because it's too powerful.

I remeasured/calculated the pool size and it is closer to 8400 Gallons (not a whole lot different than before)

Setup:


Current Pump model:


Specs on motor of pump:


Sandfilter Specs (it is a 17-inch I believe, and old as well): (note I am in Canada If things are slightly different)


Current Filter pressure is around 13 psi after a backwash and 15 psi when its getting full.

Performance Curve for the new Hayward Super Pump:

[mas985]

Best I can tell, the current pump is producing around 52 GPM @ 54' of head and a new 3/4 HP Superpump would produce around 32 GPM @ 21' of head (note head loss is not a constant and depends upon flow rate). The lower flow rate is because the current pump is a full rated pump and the Superpump is an up rated pump.

Also, sand filter are much more robust than most filters so although your current pump is exceeding the recommendations, it is probably not enough to really damage the filter. However, the filter is very small and you might want to up size that at some point.

[mas985]

Sorry correction, the new Superpump would have an operating point around 44 GPM @ 39' of head. I used the wrong pump above.

[1PoolNoob]

Best I can tell, the current pump is producing around 52 GPM @ 54' of head and a new 3/4 HP Superpump would produce around 32 GPM @ 21' of head (note head loss is not a constant and depends upon flow rate). The lower flow rate is because the current pump is a full rated pump and the Superpump is an up rated pump.

Also, sand filter are much more robust than most filters so although your current pump is exceeding the recommendations, it is probably not enough to really damage the filter. However, the filter is very small and you might want to up size that at some point.

Sorry correction, the new Superpump would have an operating point around 44 GPM @ 39' of head. I used the wrong pump above.

Would you mind explaining a bit how you came up with those numbers, more specifically the GPM for the current pump and the feet of head for both...

As far as I can tell, the sand filter has a max flow rate of 20 GPM, if it was currently at 52 GPM wouldn't that be kind of through the roof? And also can't 1.5" piping only safely handle around 47 GPM?

Thanks again for your help.

[mas985]

The filter has a recommended flow rate of 20 GPM. There is no true maximum but there can be a point where it damages the filter. However, sand filters are fairly robust and it is difficult to damage them but it does happen once in a while. Also, with pipe, there is no maximum either. There is only increasing head loss with flow rate. It doesn't really make it unsafe but it does make in inefficient. The limits that you read about are actually recommendations to minimize head loss and water hammer but there are no fundamental limits.

As for the methods that I use to determine head loss, it is a little complicated but it is a two step process. The filter pressure gives you the head loss from that point out of the returns:

Return dynamic head loss = 13 PSI * 2.31 + 9.5 = 39.53' of head

9.5' is the height of the filter gauge above the water level.

The suction side is a little more difficult because you don't have a suction measurement so for that, I calculate the equivalent plumbing curve based upon the description of the plumbing you gave me. I then use the pump's head curve to find the operating point.

I have a spreadsheet that does the calculations in my signature under "Energy Efficient Swimming Pools".

[1PoolNoob]

Great, thanks.

So in essence, installing the Hayward 3/4hp SuperPump shouldn't pose a risk of damaging the system because it will actually output less GPM than the current pump that's in there now?

Also, would you recommend putting in a ball valve between the pump output and filter to manually reduce (throttle) the flow to the desired level? Is this safe for the pump's longevity? Does it cause a reduction in electricity usage?

[mas985]

The new pump should should pose LESS of a risk but if your current pump did not damage the filter, then I doubt this one will.

As for a throttling valve, I don't think you need one. If the old pump did not do any damage in 30 years, what makes you think the new one will? However, restricting water flow will reduce energy use but not by much.