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Improving Service to Customers by Decommissioning an Elevated Water Storage Tank

Improving Service to Customers by Decommissioning an Elevated Water Storage Tank. Jon C. Ford P.E. – Seattle Public Utilities AWWA-PNWS Conference April 30, 2008. Overview. Seattle Water System Description Maple Leaf 530 Pressure Zone Improvements Analysis Recommended Improvement.

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Improving Service to Customers by Decommissioning an Elevated Water Storage Tank

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  1. Improving Service to Customers by Decommissioning an Elevated Water Storage Tank Jon C. Ford P.E. – Seattle Public Utilities AWWA-PNWS Conference April 30, 2008

  2. Overview • Seattle Water System Description • Maple Leaf 530 Pressure Zone • Improvements Analysis • Recommended Improvement

  3. Seattle Regional Water System

  4. Seattle Public Utilities Water System • 1.3 million direct service and wholesale customers in King and Snohomish County • 188,000 retail/100 wholesale service connections • Two surface water sources, one (emergency) groundwater source • 1,600 mi. distribution/159 mi. transmission mains • 15 reservoirs, 7 elevated tanks, 11 standpipes

  5. Low Pressures in Seattle’s Water System • Approximately 45 services have Peak Hourly Demand (PHD) pressure less than 20 psi (<0.1%) • Approximately 1,500 services have PHD pressure between 20 and 30 psi (0.8%)

  6. Locations of Low Pressures

  7. Maple Leaf 530 PZ PHD Pressure

  8. Base Case PHD Pressure – Low Pressure Area

  9. Low Pressures Due to Maple Leaf Tank Elevation

  10. Maple Leaf 530 PZ MDD Fire Flow

  11. Base Case MDD Fire Flow – Low Pressure Area

  12. Project Objectives • Increase low pressures to at least 20 psi • Determine most cost-effective option of raising pressures above 20 psi • Determine future of Maple Leaf Tank as water storage facility

  13. Improvement Alternatives • Alternative 1 – Upgrade existing tank • Alternative 2 – New higher tank • Alternative 3 – Decommission tank • Alternative 4 – Decommission tank plus booster pump station

  14. Comparison of Alternatives

  15. Comparison of Alternatives

  16. Alternatives – PHD Pressure Results

  17. Alternatives – MDD Fire Flow Results

  18. Alternatives Comparison • Net Present Value • Benefits • Increased Pressure • Increased Fire Flow • Decreased Risk • Costs • Capital Costs • O&M Costs

  19. Storage Analysis

  20. Benefits Components

  21. Supply Sources to Maple Leaf 530 Pressure Zone

  22. Benefits Comparison(5% Discount Rate, 100-yr return)

  23. Benefits of Increased Pressure Value ($/ERU/Yr) is area under curve $151/ERU/Yr

  24. Benefits of Increased Pressure (5% Discount Rate, 100-yr return) Alt 3 Alt 2 Alt 4 Alt 1

  25. Planning-Level Fire Flow Requirements(Available Flow at 20 psi Residual Pressure) • Single Family: 1,000 gpm • Multi-Family: 2,000 gpm • Schools: 2,250 gpm • Commercial: 4,000 gpm • Downtown/Industrial/Major Institutions: 8,000 gpm

  26. Fire Flow Benefits Calculation • Reduced risk of deaths and injuries and property damage

  27. Benefits of Increased Fire Flow (5% Discount Rate, 100-yr return) Alt 2 Alt 3 Alt 3 Alt 2 Alt 4 Alt 4 Alt 1 Alt 1

  28. Storage Analysis

  29. Maple Leaf 530 Pressure Zone Storage Analysis • Operational & Equalizing Storage – not needed • Fire Suppression Storage – provided in Lake Forest Park Reservoir • Standby Storage – need reduced by source availability: • Roosevelt Way RCBV – 6,000 gpm • Roosevelt Way PS – 3,000 gpm • View Ridge High Service Pump – 2,000 gpm • Roosevelt Turbine – 800 gpm

  30. Standby Storage Benefit Calculation • Value = Risk Cost = Probability of Service Outage x Consequences of Service Outage • Probability: 0.02/yr (outage event exceeding MDD) • Consequence: $280,000 per episode • Value = 0.02/yr x $280,000 = $5600/yr • PV = $100,000

  31. Storage and Supply Availability to Maple Leaf 530 Pressure Zone • Lake Forest Park Reservoir provides storage • Gravity feed through Roosevelt Way RCBV/PRV • Capacity > PHD • Storage volume (60 MG) • Backup sources available • Roosevelt Way Pump Station • View Ridge High Service Pump • Roosevelt Turbine

  32. Costs Comparison(5% Discount Rate, 100-yr return)

  33. Alternative 1 NPV = -$2,582,900 Alternative 2 NPV = -$1,774,400 Alternative 3 NPV = $4,359,200 Alternative 4 NPV = $2,936,500 B/C ratio = 0.46 B/C ratio = 0.75 B/C ratio = 42.5 B/C ratio = 2.21 NPV Comparison

  34. NPV Comparison

  35. Tank Decommission vs. Demolition • No longer needed as water system facility • Value as antenna tower • Costs of demolition and soil remediation • Possible landmarks issue • Communication to customers

  36. Recommendation • Recommend Alternative 3: Decommission Maple Leaf Tank and serve the Maple Leaf 530 Pressure Zone directly from the 550 Pipeline

  37. Conclusions • Decommissioning the Maple Leaf Tank increases pressures and fire flow • Supply redundancy provides for minimal outage risk • Decommissioning still preferred alternative if demolition costs are included

  38. Alt 3 PHD Pressure – Low Pressure Area

  39. Alt 3 MDD Fire Flow – Low Pressure Area

  40. Questions?

  41. Decommissioning - How Do We Make It Happen? Cheryl Capron, Senior Water System Operator Seattle Public Utilities PNWS-AWWA Conference April 30, 2008

  42. Overview • Operations input to asset management process • Compensating improvements • New procedures • Operational testing • Retirement

  43. Operations Input to Asset Management Process • How the tank and zone are actually operated • Reliability and condition of existing components • Identify all functions of tank • Venting & surge protection • Holds positive pressure in zone if pumps fail • Smooths diurnal demand on source • Provides opportunity to boost Cl2 residual

  44. Compensating Improvements • Pressure relief? • Booster chlorine capability replaced? • More instrumentation? • Upgrade, add, replace pumps? • Variable output to match demand • Hot standby power • Upgrade, add, replace flow control valves? • Adjust valve speed

  45. New Procedures • Revise minimum and maximum zone operating pressures • Hydraulic modeling • Response to high and low pressure alarms • When do we shut down gravity supply and switch to pumps? • Revise SCADA graphics & programming • Train the operators

  46. 3-Part Operational Testing • PRV Loop • Close pump discharge line and start pump • Pressure Zone Test Scenarios (Tank off-line) • Most likely to go wrong? • SCADA glitch - valve opens too far • Power fail – pump stops • Peak Season Test • Are we absolutely sure we won’t need this tank anymore?

  47. Operational Test Objectives • Verify new 6” PRV loop will function as needed • Verify an existing off-site 4” PRV functions as needed • Document location & effect of discharge flow to environment. • Revise settings so that new 6” PRV opens first. • Verify high & low psi alarm settings are ok

  48. Test Plan • Objectives • Prior to Test • Customers ready for pressure change • Customer service reps aware of pressure changes & dirty water from testing • Fire department aware pressure surges may trigger false alarms • SCADA & new operating procedure ready • All compensating improvements completed

  49. Test Plan • Preset the system • Reservoir levels • Valves positioned • Field staffing • At PRVs to observe & document • At tank inlet to put back on line quickly

  50. Test Plan • Step by step procedure • Establish supply to zone • Tank inlet closed • Slowly boost supply to force PRVs open • Verify they’re working • Slowly back off supply to close PRVs • Adjust pressure settings and alarm settings

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