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Unidirectional Flushing

Unidirectional Flushing. Thomas F. Clark Monroe County Water Authority. Introduction. Why bother to flush ? What is unidirectional flushing? What are the benefits of unidirectional flushing?. Flushing….why bother?. Customer complaints on “stale” water. Low chlorine residuals. Dead ends.

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Unidirectional Flushing

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  1. Unidirectional Flushing Thomas F. Clark Monroe County Water Authority AWWA Seminar

  2. Introduction • Why bother to flush ? • What is unidirectional flushing? • What are the benefits of unidirectional flushing? AWWA Seminar

  3. Flushing….why bother? • Customer complaints on “stale” water. • Low chlorine residuals. • Dead ends. • Distribution system chemistry changes. • Loss of “C” factor. • Sediments/corrosion products. AWWA Seminar

  4. Where do sediments come from? • Everything from microscopic debris from the finished water leaving the plant, to material brought into the system from main repair and tank outflows to the corrosion of cast iron pipe. AWWA Seminar

  5. What are some of the other factors which cause water to degrade? • Age • Poorly managed storage facilities • Biofilm/microbial growth • Nitrification • Cross-connections/backflow • Permeation/leaching AWWA Seminar

  6. Flushing….why bother? • High flow events or main breaks within the system will eventually cause problems with turbidity. • Low flow mains will eventually lead to taste and odor problems, iron and sediment accumulation, etc. AWWA Seminar

  7. Flushing….why bother? • Periodically remove biofilm. • Maintain/increase disinfectant residual. • A chance to exercise valves and hydrants. • Respond to consumer complaints. AWWA Seminar

  8. Flushing…… • There are a number of reasons NOT to flush….. • Water availability • Time and people • Cost of water • Property damage • Complaints from consumers CAUSED by flushing. • Other job tasks that are waiting to be done. AWWA Seminar

  9. Spot Flushing • Historically, spot flushing has been done. Generally, flushing velocities of < 5 fps are used. • Often this is done in conjunction with other maintenance tasks associated with hydrants and valves. • Random opening of hydrants – often not in an organized manner. • Does not have a lasting beneficial effect. • BUT it is quick and easy !!!! AWWA Seminar

  10. Procedures….. • Normally, one opens a hydrant and hopes for a sufficient pipeline velocity. • Or, a sufficient number of hydrants are opened to raise pipeline flow. • Discharge water until it runs clear. • Inspect the hydrant. • Maybe conduct water quality checks such as turbidity, chlorine residual, clarity. AWWA Seminar

  11. Unidirectional Flushing • Water flows in one direction only. • Method is systematic and pre-planned. • Produces more effective scouring. • Use less water in general. • Improved quality is longer lasting. AWWA Seminar

  12. Principles of Unidirectional Flushing • Work outwards from a clean source of water. • Use a prearranged pattern. • Work from a large diameter main to a small diameter main. • Close valves where necessary to ensure that flow is from a clean source on a one way path and to isolate pipe sections. • Flush until water reaches your target water quality. AWWA Seminar

  13. Principles of Unidirectional Flushing • Maintain 20 psi system pressure everywhere. • Ensure that a sufficient flow rate is achieved to produce a pipeline flow velocity of 4- 5 ft/second. AWWA Seminar

  14. Pipeline Velocities at Various Flushing rates AWWA Seminar

  15. Conventional Flushing AWWA Seminar

  16. Unidirectional Flushing AWWA Seminar

  17. Field Parameters for Flushing Disinfectant Residual Depends on level entering the system Free chlorine of 0.5 mg/l – 1.0 mg/l Turbidity less than 2 ntu AWWA Seminar

  18. Field Parameters for Flushing Volume turnover Try to turn over 3x the pipe diameter Evaluate each flushing effort as conditions dictate. AWWA Seminar

  19. Pipeline Volume Info • Pipe Size Volume per 1000 ft InchesGallons 4 653 6 1,470 8 2,610 10 4,080 12 5,875 16 10,444 18 13,218 24 23,500 AWWA Seminar

  20. Calculate Pipeline Residence Time • Calculate residence time (flow through) of 1,000 LF of12 inch pipe when the main is delivering 820 gpm. • Since we have 1000 ft of 12 inch pipe, we’re talking about 5,875 gallons of water that needs to be displaced. • 5,875 gallons/flow rate = residence time • 5,875 gallons/820 gallons/min = 7.2 minutes AWWA Seminar

  21. Impact of unidirectional flushing AWWA Seminar

  22. Impacts on the Environment From Flushing • Watch for free/total chlorine levels • Neutralize chlorine by use of sodium sulfite, sodium bisulfite, sodium thiosulfate, etc. • Don’t use excessive sulfite chemical as it will create an oxygen demand in receiving waters. • Watch for TSS levels • Impound sediment laden water to allow for settling prior to discharge. Catch basin sumps, lagoons, sand bagging on a road, etc are options. AWWA Seminar

  23. General Considerations in Flushing/Flowing Water • Ten States Standards • 8.1.1 -Pressure - The system shall be designed to maintain a minimum pressure of20 psi at ground level at all points in the distribution system under all conditions of flow. • Hydrants should be spaced from 350 ft – 600 ft depending on the nature of the area. AWWA Seminar

  24. Needed Fire Flow for a System • For one and 2 family dwellings not exceeding two stories, we use the following flows: • Distance between BuildingsNeeded Fire Flow • Over 100 ft 500 gpm 31 ft – 100 ft 750 gpm 11 ft – 30 ft 1,000 gpm less than 10 ft 1,500 gpm Ref: AWWA Manual M-31 AWWA Seminar

  25. A Cautionary Note….. • Flushing will result in increased flow velocities….. • Increased flow velocities in the water mains result in reduced system pressures….that means • You need to be aware of the concept of the hydraulic grade line. AWWA Seminar

  26. Hydraulic Grade Line • What is it? • It’s the level that water would rise in a thin glass column were you to tap such a glass tube into your main. AWWA Seminar

  27. Hydraulic Grade Line Friction loss Pump head added 50 psi PRV 85 psi 60 psi AWWA Seminar

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  30. 55 psi 7 psi 35 psi (20 psi in main)

  31. Elevation + (Pressure x 2.31) = HGL AWWA Seminar

  32. Elevation + (Pressure x 2.31) = HGL AWWA Seminar

  33. Fire Flow Test Residual Hydrant Flow Hydrant 70 psi Static 50 psi Residual 19 psi Pitot AWWA Seminar

  34. 69.3 feet = V2 / 64.4 V = 66.8 fps 30 psi = 69.3 feet Pitot Pressure P = 30 psi V = 0 (- 10 %) P = 0 psi V = 60 fps Q = V x A Q = 920 gpm AWWA Seminar

  35. Fire Flow Calculation • Conduct Flow Test • Extrapolate flow at desired residual pressure AWWA Seminar

  36. 0 gpm 750 gpm 1230 gpm 70 psi 50 psi Example Resid. Press. 20 psi • Static pressure = 70 psi • Residual Pressure = 50 psi • Pitot Pressure = 19 psi • Actual Flow Rate • Fire Flow at 20 psi Hydrant Flow

  37. Where to Get More Information • Other training sessions • List books, articles, electronic sources • Consulting services, other sources AWWA Seminar

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