Backflow Preventers

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Backflow Preventers

  1. 1. Backflow Preventers
  2. 2. What is a Backflow Preventer? <ul><li>Backflow is the unwanted reverse flow of water </li></ul><ul><li>The preventer physically blocks the potentially contaminated water in the irrigation system from going back into the domestic water </li></ul>
  3. 3. Air Gap <ul><li>Water can’t jump back up into the pipe </li></ul>
  4. 4. Selection <ul><li>Could be city mandated </li></ul><ul><li>Check with the Water Department to be sure your selection is allowed </li></ul><ul><li>Sizing based on flow demanded by your system </li></ul><ul><li>Should fall somewhere near the center of the pressure loss curve on the chart for the size you’ve selected </li></ul>
  5. 5. Example <ul><li>Using a flow of 40 gpm, look at the charts </li></ul><ul><li>Rainbird Design Book page 69 </li></ul><ul><li>5 different size Pressure Vacuum Breakers: ¾; 1”, 1 ¼”, 1 ½”, 2 </li></ul><ul><li>Look at the flow on the bottom and see where 40 gpm fits on the curve </li></ul><ul><li>Which has the 40 gpm in the middle of the curve? </li></ul>
  6. 6. Types of Backflow Preventers <ul><li>Atmospheric Vacuum breaker - AVB </li></ul><ul><li>Anti-Siphon Valve – ASV </li></ul><ul><li>Pressure Vacuum Breaker – PVB </li></ul><ul><li>Reduced Pressure Backflow Preventer –RP Unit </li></ul><ul><li>Double Check Backflow Preventers </li></ul>
  7. 7. Atmospheric Vacuum Breaker <ul><li>Least expensive </li></ul><ul><li>Must install one after every control valve </li></ul><ul><li>Must be installed at least 6” higher than the highest sprinkler head </li></ul><ul><li>Usually not economically practical if using 6 or so valves – then consider PVB </li></ul><ul><li>Some cities don’t allow AVB’s </li></ul>
  8. 8. Anti-Siphon Valve <ul><li>Manual or automatic control valve with a build-in atmospheric vacuum breaker </li></ul><ul><li>Install 6” higher than highest sprinkler head </li></ul><ul><li>Must be no other control valves between it and the sprinklers </li></ul><ul><li>Most common type on residential systems </li></ul>
  9. 9. Pressure Vacuum Breaker <ul><li>Similar to AVB but you only need 1 installed on the mainline leading to the control valves </li></ul><ul><li>6” higher than the highest sprinkler head </li></ul><ul><li>Some municipalities don’t allow it to be used with drip irrigation systems </li></ul>
  10. 10. Reduced Pressure Backflow Preventer (R.P. Unit) <ul><li>Expensive </li></ul><ul><li>King of backflow preventers </li></ul><ul><li>Made for high hazard uses </li></ul><ul><li>Install 12” above ground </li></ul><ul><li>Used often for commercial applications </li></ul>
  11. 11. Double Check Backflow Preventers <ul><li>May be legal or illegal to use </li></ul><ul><li>2 manual valves </li></ul><ul><li>Sealed </li></ul><ul><li>Installed below ground </li></ul><ul><li>Stryker says ‘ Don’t use them’ </li></ul>
  12. 12. Dual Check Device <ul><li>Not a backflow preventer </li></ul><ul><li>Stripped down version of the Double Check Backflow Preventer </li></ul><ul><li>Is a flow control device </li></ul>
  13. 13. Pressure Loss to Use Between 3-5 PSI, use 5 Check Valves and Dual Checks Between 3-5 PSI, use 5 Double Check Backflow Preventer Between 8-12 PSI, use 12 to be safe Reduced Pressure Backflow Preventer Between 2 – 5 PSI, use 5 to be safe Pressure vacuum Breaker 0 Anti-Siphon valve 2 PSI Atmospheric vacuum breaker
  14. 14. Mainline <ul><li>Stryker recommends SCH 40 PVC pipe </li></ul><ul><li>Take gpm needed for the system and look at the chart to see what size will let the gpm through without exceed 5 ft/second </li></ul><ul><li>Example: chart on page 95 of Rainbird guide </li></ul><ul><ul><li>40 gpm of schedule 40 PVC pipe </li></ul></ul><ul><ul><li>2” is the smallest size </li></ul></ul><ul><ul><li>Pressure loss is 1.15 per 100’ </li></ul></ul>
  15. 15. Calculate Pressure loss <ul><li>Example: </li></ul><ul><ul><li>schedule 40 PVC pipe </li></ul></ul><ul><ul><li>20 gpm </li></ul></ul><ul><ul><li>Run 30 feet </li></ul></ul><ul><ul><li>Chart – 1 ¼ inch pipe </li></ul></ul><ul><li>PSI loss per 100 feet (psi/100 ft) is 2.28 </li></ul><ul><ul><li>2.28 x 30/100 =68.4/100 =.684 pressure loss </li></ul></ul>
  16. 16. Valves <ul><li>Emergency shut-off valve </li></ul><ul><li>Control valves - Turn on and off the sprinklers </li></ul><ul><ul><li>Globe or angle valve </li></ul></ul><ul><ul><li>Anti-Siphon valve </li></ul></ul>
  17. 17. Globe or Angle Valve <ul><li>Globe - Most commonly used valve </li></ul><ul><li>Has no backflow preventer </li></ul><ul><li>Many sizes </li></ul><ul><li>Installed underground in a box or vault </li></ul>
  18. 18. Anti-Siphon Valve <ul><li>Only in ¾” and 1” size </li></ul><ul><li>Jess Stryker recommends for homeow&quot;ners </li></ul><ul><li>Has a backflow preventer in it </li></ul><ul><li>Must be installed above ground and must be at least 6” higher than the highest sprinkler </li></ul>
  19. 19. Mainline depth <ul><li>Should be 18” below ground </li></ul>
  20. 20. Valve Operation <ul><li>Manually </li></ul><ul><li>Electric *** what everyone uses </li></ul><ul><li>Hydraulic </li></ul><ul><li>Good to have automatic valves with a manual flow control on it. </li></ul><ul><ul><li>Just in case you need it </li></ul></ul>
  21. 21. Valve Material <ul><li>Come in brass and plastic </li></ul><ul><li>Most used are plastic </li></ul><ul><ul><li>Glass reinforced nylon is best, tougher </li></ul></ul><ul><ul><li>PVC – costs less </li></ul></ul>
  22. 22. Sizing Valves <ul><li>The pressure loss is the energy used by the valve to open and close </li></ul><ul><li>If there is not enough pressure, it won’t work properly </li></ul><ul><li>Most valves won’t work with less than 5 gpm </li></ul>
  23. 23. Rules <ul><li>#1 Don’t want pressure loss to be more than 10% of the static pressure available in the main line </li></ul><ul><li>#2 valve should be either the same size as the largest pipe in the lateral it serves or no more than one nominal size smaller </li></ul><ul><li>#3 The valve should not be larger than the pipes in the lateral, unless a high flow results from a split lateral </li></ul>
  24. 24. Pressure Loss in valves <ul><li>Example PEB series valve pg 68 </li></ul><ul><li>Flow 40 gpm </li></ul><ul><li>Pressure in main line of 75 psi </li></ul><ul><li>Rule # 1 10% of 75 psi = 7.5psi </li></ul><ul><li>Look at chart 40 gpm </li></ul><ul><ul><li>1 in pipe loss is 9.3 </li></ul></ul><ul><ul><li>1 ½ in pipe loss is 1.9 </li></ul></ul><ul><ul><ul><li>Use this one </li></ul></ul></ul>
  25. 25. Rule #2 <ul><li>Sample Circuit #1 calls for valve to feed a 2” pipe </li></ul><ul><li>The 1 ½” pipe in the first example is only one size smaller and fits the rule </li></ul>
  26. 26. Rule #3 <ul><li>Look at Circuit 2 </li></ul><ul><li>The 1 ¼” pipe flows to a 2” main </li></ul><ul><li>Because it is a split lateral and there is more flow, this is okay </li></ul>
  27. 27. Lateral Pressure Loss <ul><li>Stryker recommends CL 200 PVC pipe </li></ul><ul><li>Use the chart to find out the pressure loss for the laterals </li></ul><ul><li>Friction Loss in PSI is PSI per 100 feet of pipe </li></ul>
  28. 28. Sprinkler Pressure Loss <ul><li>Use the operating pressure (30 psi) of one sprinkler </li></ul><ul><li>The loss is the water coming out of the sprinkler </li></ul>
  29. 29. Table <ul><li>See Jess Stryker Sprinkler 14.htm </li></ul><ul><li>http://www.irrigationtutorials.com/sprinkler14.htm </li></ul>

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