Hydraulics THE BASICS OF PUMP/FILTER SIZING Hydraulics is the study and understanding of the behavior of liquids at rest and in motion. We are concerned with water, and the following characteristics of our application: 1. How much water do we have (Pool Capacity)? 2. How fast can we safely move the water (Turnover Rate and Water Velocity)? 3. How much resistance will this water meet while moving through the system (Friction Loss)? 4. How will we overcome this resistance (Pump/Filter Sizing)? Following are step-by-step instructions to answer these four questions and ultimately determine the proper size pump or filter for virtually any installation. Below each step is a calculation based on the following example: 16 ft by 32 ft rectangular pool, 3 ft to 8 ft deep, 2" suction side and return side plumbing. Existing 1 H.P. pump; filter gauge reads 10 PSI (clean). 1. Pool Capacity To determine total gallons, we must first calculate the surface area of the pool in square feet: 1A. Surface Area ____________ ft 2 (surface area) Surface Area: 16 ft x 32 ft = 512 ft 2 Next, multiply the surface area by the average depth to determine the appropriate volume of the pool. 1B. Average Depth (____________ ft + ____________ ft ) ÷ 2 = ____________ ft (depth, shallow end) (depth, deep end) (average depth) Average Depth = (3 ft + 8 ft) ÷ 2 = 5.5 ft 1C. Volume ____________ ft 2 x ____________ ft = ____________ ft 3 (surface area) (average depth) (volume) Volume = 512 ft 2 x 5.5 ft = 2,816 ft 3 Next, multiply the pool’s volume in ft3 by 7.5 to get the pool capacity in gallons: 1D. Pool Capacity ____________ ft 3 x 7.5 gallons/ft 3 = ____________ gallons (volume) (pool capacity) Pool Capacity = 2,816 ft 3. x 7.5 gallons/ft 3 = 21,120 gallons Some of the more common pool sizes are: Above-Ground Size Gallons* 15 ft Round 5,293 18 ft Round 7,622 21 ft Round 10,374 24 ft Round 13,550 12 ft x 24 ft Rectangle 8,626 27 ft Round 17,149 *Average Depth: 4 ft In-Ground Size Gallons* 12 ft x 24 ft Rectangle 11,861 16 ft x 32 ft Rectangle 21,086 18 ft x 36 ft Rectangle 26,687 20 ft x 40 ft Rectangle 32,947 *Average Depth: 5.5 ft 2. Flow Rate While the actual flow rate of a pump is based on the total resistance of the system as described below, the desired flow rate must be calculated to verify it will satisfy Turnover Rate and Water Velocity requirements. 2A. Turnover Rate The turnover rate for a swimming pool is the amount of time required to circulate the entire volume of water through the system once to meet reasonably clean, safe water standards. Check with local regulations for the minimum required turnover rate. Based on the pool’s capacity and the desired turnover rate, the minimum rate at which the water must be circulated in Gallons Per Minute (GPM) is calculated as follows: A. Minimum Flow in Gallons per Hour (GPH) ______________ gallons ÷ ______________ hours = ______________ GPH (pool capacity) (desired turnover rate) (minimum flow, gallons per hour) Minimum Flow: 21,120 gallons ÷ 10 hours (assumed) = 2,112 gallons per hour B. Minimum Flow in Gallons per Minute (GPM) ___________ gallons per hour ÷ 60 minutes per hour = ___________ GPM (minimum flow, (minimum flow, gallons per hour) gallons per minute) Minimum Flow: 2,112 gallons per hour ÷ 60 minutes per hour = 35 gallons per minute 21 P u m p s
