Engineering for Legionella Control at Cooling Towers. Clive Broadbent AM, FIE Aust.

Transcription

1 Engineering for Legionella Control at Cooling Towers Clive Broadbent AM, FIE Aust.

2 Legionella in the environment

3 Chain of Causation

4 Total Microbial Control Temperature Biofilm Protozoa, bacteria Nutrients

5 Heat Rejection by Evaporation

6 Towers on Roof

7 Towers Indoors

8 Industrial Applications - small

9 Larger

10 Very Large

11 Inside

12 Warning

13 At Mines

14 Refining metals

15 Co-located

16 Single Flow for cleaning

17 Single Flow

18 Power Stations

19

20

21 Variation of Legionella Counts with Basin Water Temperature

22 kw v. Cooling Tower Water Temperature

23 Total Microbial Control Temperature Biofilm Protozoa, bacteria Nutrients

24 Poor Practices

25 Stagnant water - deadleg

26 Duty/Standby changeovers

27 Nutrients - algae on the top deck

28 Overflow chemical loss

29 Uncontrolled Water Losses

30 Windage

31 Air intakes

32 Location Location Location

33 Outbreak Lessons

34 Outbreak - chillers

35 Pumps

36 Tower 1

37 Tower 2

38 Biocide dosing

39 Two Independent Systems CT1 CT2 P1 P2 C1 C2

40 Two Interconnected Systems Scheme 1 CT1 Auto Valves CT2 Auto valves interlocked with pumps P1 Auto Valves P2 Allows P1 to serve C2 or P2 to serve C1 in event of failure C1 C2 Design water flow maintained through each cooling tower

41 Two Interconnected Systems Scheme 2 CT1 P1 C1 Manual Valves Auto Valves CT2 P2 C2 Auto valves at towers replaced with manual isolating valves On auto operation, uncontrolled water losses occur when one pump/chiller/ tower is sufficient for load

42 Piping (no balance line)

43 Chemical dosing point (return lines)

44 Details Site dust load high Piping/pumping irregularities No balance line between towers Return piping at high level No anti-syphon trap Overflow on pump shutdown Plant, incl. chillers, on manual control Contd

45 Contd Details Chemical injected at return line Componentry (dosing pump) unreliable Only one biocide in use non-oxidising low concentration Contractual difficulties Major L.D. outbreak

46 Examples of Developments

47 Safety

48 Example

49 Example

50 Snow making

51 Air Compressors

52 Towers in the Snow

53 Dry Basin

54 Water Saving

55 Strategies Understand and improve plant operation (best practice) Uncontrolled water losses (identify, remove) Fix basin leaks Increase cycles of concentration if relevant Clean mud out of fill

56 Splashguards

57 Eliminators

58 Sheets rolled back

59 Cooling Tower Makeup Water with 5.5 C Range Water Savings For a cooling tower rejecting 2,000 kw, return water 35 O C, supply water 29.5 O C, recirculation is L / s 5.5 x Evaporation is 2000 x 996 2,430 x L / s (where 996 is water density, 2430 is enthalpy of vaporisation). Drift is 0.002% of flow = L/s Take TDS as 30 ppm (Melbourne) 0.82 x 30 For 2 cycles, bleed required = (60 30) used is = 1.64 L/s 0.82 L / s x L / s x L / sand water For 6 cycles, bleed = 0.82 x 30 (180 30) 0.16 L / s and water used is = 0.98 L/s

60 Cooling Tower Makeup Water with 5.5 C Range Cycles Water Use L/s

61 Cooling Tower Makeup Water with 5.5 C Range Makeup L/s Cycles of Concentration

62 Reclaim System at Office Cost - $140,000 Payback 2 years Water saving 1,135,520 litres reclaimed in 2 months Reuses tower blowdown Reclaim tank/flusher tank

63 Reclaim System at Office Cooling water system primary disinfection by bromine generated at site. Reclaim system secondary disinfection by electrochlorination converts high chlorides in blowdown water into chlorine

64 Riverside Centre

65 Reclaim Tank

66 Standards

67 AS/NZS Microbial Control Part 1 - Design, installation, commissioning Part 2 - Operation, maintenance Part 3 - Performance-based maintenance of cooling water systems

68 Design features for towers (prescriptive Part 1) Convenient accessible openings Components that can be removed Sumps that can be readily drained Materials compatible with use of disinfectant and hosing with water jets Use of components that minimise corrosion Efficient drift eliminators (0.002% loss) Minimal internal components such as structural brackets which can collect sediment Surfaces which can be readily cleaned Protection of wetted surfaces from direct sunlight

69 Operation & maintenance features (prescriptive Part 2) Layout of system Correct and safe operating procedures Maintenance, cleaning and disinfection procedures and their frequency Regular water treatment regimes Bleed rate Testing requirements ph total dissolved solids or conductivity bacterial counts Disinfectant levels Safety precautions Person or contracting agency responsible for: overseeing and recording the work ensuring that the plant operates normally

70 Performance-based maintenance (Part 3) Assessment the problem Preventive measures known controls Operational procedures continuing controls Verification monitoring the controls Incident reporting evidence

71 Companion Standard AS Power station cooling tower water systems management of Legionnaires disease health risk

72 Other Hazards

73 Artesian Bores

74

75 Mine

76 Are we there yet? While there is an environmental pathogen and there are people susceptible to infection, there may always be cases of disease. Legionella control is based on a partnership of disciplines including engineering. Scientific understanding and wisdom have already come a long way. But we are still on a journey of discovery. May all travel well.

77 Thank you for your attention