How long does the pump really need to run?

[Anonymous [GDPR] European in the UK]

Thanks Mark, What sort of pressure shows on the gauge at those flow rates? I usually fit sand filters but have a customer who specifically needs a cartridge filter and to get my system to run well the system pressure doesn't want to be too high.

Regards
John

[mas985]

High speed filter pressure is about 10 PSI without solar and 14 PSI with solar.

But I don't understand what you are trying to say here:

and to get my system to run well the system pressure doesn't want to be too high.

Cartridge and DE filters have about the same head loss independent of filter size. Only sand filters, because of the small area, have a head loss dependency on size.

Also when sizing a filter, you really should use the APSP-15 specs and don't worry too much about pressure. APSP-15 specifications use a rate of 0.375 GPM/sqft for cartridge filters.

[Anonymous [GDPR] European in the UK]

Happy new year Mark and thanks for the update. Yes with sand filters they only present a small surface area ( pi x r2 of the filter size) as the depth of the bed isn't considered for some reason so I would have expected the pressure to be lower with the large surface area of a pleated cartridge or DE grids. You say the headloss is about the same regardless of filter size, therefore they are very poor performers hydraulically speaking which doesn't surprise me as the pool industry designers are pretty lazy/useless/untrained?

Helpful to know though thanks, On your slow speed I would have expected the headloss to drop quite a lot especially as your filter is quite big.

[mas985]

You say the headloss is about the same regardless of filter size, therefore they are very poor performers hydraulically speaking which doesn't surprise me as the pool industry designers are pretty lazy/useless/untrained?.

Actually it means just the opposite. Cartridge and DE filters have the lowest head loss so are are very good performers hydraulically. The head loss doesn't change with size because even the smallest size is "large". What causes the head loss in these types of filters is just the internal fittings and for DE, the backwash valve adds a lot of head loss but the media does not add much to the head loss. I can take the cartridges out of my filter and the pressure does not change which means they don't add any head loss. Only the internal plumbing fittings add head loss which is why it is so low. However, for sand filters, the sand area is quite small and that does have an impact on head loss but only for filters smaller than about 5 sq-ft.

Here is a table with several different types of filters and filter sizes (sand only). For a given filter line with cartridge and DE, the head loss does not change with size but as you can see, the cartridge has the lowest head loss and by quite a bit of margin.



Also, I didn't post the low speed pressure because it doesn't actually read a pressure. But theoretically, it is about 2 PSI.

[Anonymous [GDPR] European in the UK]

Many thanks for that Mark, Chart saved for future reference.

What you have said about cartridge having the lowest head loss is what I was expecting. As you say the internal fittings are where the head loss occurs which is why I said the "designers" are lazy etc because it could easily be improved and needs to be given that we are going Eco. I have fitted a 15 PSI gauge to my filter as I needed to see the 2 PSI I run at also. I notice the almost exponential graph as the flow increases, which fits with my empirical data and is where I am investigating the head loss. Is it through the media or bad plumbing? Not an issue in the past as a powerful pump always makes sure the water gets where it's going but reducing the power brings on new challenges.

Thanks for you help on this, it's appreciated.

Regards
John

[Anonymous [GDPR] European in the UK]

Having had a chance to look over the data from your table Mark. If you look at the TR140 sand filter with 2" multiport and compare it with the TR50 or 60 with 2" multiport, the additional filter media in a TR140 which holds nearly three times what a TR60 and even more than that in a TR50 yet it has the lowest headloss figures same thing with the TR100 so it isn't the filter media causing the headloss it's poor hydraulic design with the laterals. This is the area I am concentrating on at the moment.

Thanks again for the data.

Regards
John

[mas985]

Head loss in a fixed plumbing system follows the pump affinity equations where head loss is proportional to the square of the flow rate. This is why any plumbing system can be fit to a plumbing curve that follows this equation:

Head Loss (ft) = Cp * GPM ^2

Where Cp is the plumbing curve constant. Typical 2" plumbing has a curve constant that is close to 0.0082 while 1.5" plumbing is closer to 0.0167.

This is useful when trying to determine the operating point of a new pump on an existing plumbing system. You just have to draw the plumbing curve over the pump's head curve and where they intersect is the new operating point.