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Sizing Pool Pumps


Sizing Your Swimming Pool Pump
The pump consists of two principal parts: A motor and the wet end. The motor provides the power to move the components located in the wet end. The wet end consists of a trap, and impeller, which is responsible for moving the water.

The pump is probably the most important single piece of equipment in the circulation system. Selecting a pool pump is determined by a long list of factors. It is best to review this list even if you are replacing an existent pump with the same size. If you are tempted to increase the HP of the pump, you must read and understand this.

Total dynamic head (TDH) is the sum of the resistance experienced by water flowing through the pipes and filter system. Each elbow, foot of pipe, fittings and anything else that is in the circulation system increases the Total Dynamic Head. The TDH will affect the flow rate. For a new pool this is somewhat more easy to calculate, but for an existing pool, with piping below the ground it is much more difficult.

Typical Calculation:

1. Friction Loss (vacuum Side)

Common Head loss factors
       
Vacuum (at pump) -
       Assumes 2" suction pipe not to exceed 40 feet.
       Minimal fittings - One 2" valve and full rated pump.

Total Dynamic Head

Horsepower of Pump
Feet of water
3/4    
4.5 - 5.5 ft of water
1  
7.0 - 9.0 ft of water
1-1/2      
10.0 - 12.5 ft of water
2      
13.5 - 16 ft of water

       Let's use a 1 HP pump to perform a calculation, Reading the table above we have 9 feet of head.

2. Friction loss (pressure side): _____(clean filter pressure) psig. x 2.31 ft of head / psig = ______ft of head (pressure side). Typically filter pressure should be 10-15 lbs.

Using 10 feet of pressure we will have 23 ft of head on the pressure side.

3. The Total Dynamic Head: __9_(Vacuum)__ + _23(Pressure)_  =     ____32__

Practically speaking 30-40 feet of head is a good estimate to size a pump. which can be adjusted upward or downward depending on the circumstances.

Pipe characteristic (maximum Flow)

Pipe Size (PVC)
Pump Output (GPM) versus Total Resistance to Flow (feet of Head)
 
Suction Line (vacuum)
Return Line (pressure)
1-1/2 " 50 GPM 65 GPM
2 85 GPM 105 GPM

A typical performance chart

Pump Size
Pump Output (GPM) versus Total Resistance to Flow (feet of Head)
  20 ft 30 ft 40 ft 50 ft 60 ft 70 ft
1/2 55 45 29     -
3/4 67 58 47 31    
1 85 76 65 50 27  
1-1/2 97 90 80 67 50 10
2 116 111 99 85 70 51

NOTES:
   
Numbers in table represent typical information.
   Consult specific information for the pump you intend to use.
   Move up to next level of resistance 32 -> 40 feet and specify the pump that exceeds the calculated flow rate.       Select a filter based on the flow required (always going to larger filter size).

Filter Capacity and Flow Rates

DE Filter
sq-feet
Cart Filter
sq-feet
Sand Filter
sq-feet
Design 
Flow Rate (gpm)
Turnover in Gallons
8 hours 10 hours
  25    25  12,000 15,000
    1.75  35  16,800 21,000
    2.20  44 21,200 26,400
24      48 23,040 28,800
  50    50 24,000 30,000
    2.64  52 24,960 31,200
    3.13  62 29,760 37,200
36      72 34,560 43,200
  75    75 36,000 45,000
48      96 46,080 57,600
    4.91 98 47,040 58,800
  100    100 48,000 60,000
60      120  57,600 72,000
72     120 57,600 72,000
  150    150 72,000  90,000


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