SURGE CONTROL STRATEGIES FOR WASTEWATER CONVEYANCE SYSTEMS. water resource specialists

Transcription

1 SURGE CONTROL STRATEGIES FOR WASTEWATER CONVEYANCE SYSTEMS water resource specialists

2 Overview Consequences of surge events Events that create pressure surges Surge control strategies Surge analysis objectives and criteria Surge analysis projects Concluding remarks / Q&A

3 Consequences of pressure surges High positive pressure can rupture pipe Increased thrust force can damage joints Negative pressure can collapse pipe Can cause flow reversal

4 Events that create pressure surges in wastewater systems Pump power failure (creates the worstcase surge events) Pump shutdown/startup Pipeline rupture

5 Pump operating at 1420 gpm (v = 4 ft/s) Time = 1 sec Air valve Air valve Tank Pump Station 6800 ft long, 12-inch diameter PVC pipeline

6 Drop in pressure proportional to reduction in flow velocity Pressure drop wave Joukowsky Equation: Drop in pressure head (Δh) = a/g Δv Time = 2.3 sec Δh a = acoustic wavespeed for PVC pipe (1300 to 1500 ft/sec) g = gravitational acceleration Δv = change in velocity (v) of flow in pipeline

7 Pressure drops low enough to open air valve Time = 3.8 sec 1 st air valve opens

8 HGL drops sufficiently below pipeline to create vapor pressure Time = 4.6 sec 2 nd air valve opens Vapor cavity formation and water column separation

9 Water hammer wave reflection repressurizes pipeline between air valves Time = 5.6 sec Pipeline re-pressurized by water hammer wave reflection Water columns rejoin and Vapor cavity collapse

10 Water column rejoinder causes vapor cavities to collapse Positive pressure spike createdimpossible to accurately predict Time = 6.4 sec Vapor pressure should be avoided

11 Controlled venting sewage vacuum relief valves Regulates air outflow to ensure gentle closure of air valves (Courtesy of Vent-O-Mat) (Courtesy of ARI) (Courtesy of APCO)

12 Flywheel Motor Flywheel Vertical drive shaft to pump on lower level

13 Pressurized Surge Tank

14 Pressure / Surge Relief Valve Opens quickly when pressure exceeds set point pressure and closes slowly when it subsides

15 Variable Frequency Drive

16 Backup Generator (Courtesy of MTU Onsite Energy)

17 Surge Tower/Standpipe

18 Objectives of Surge Analysis Identify potential adverse surge pressures If needed, recommend additional surge control measures Air entrainment in wastewater Recommend safe startup and shutdown procedures for pumps

19 Surge Control Design Criteria Limit positive surge pressures to ~30 percent over the larger of static, working, or rated pressure Eliminate large negative pressures and vapor pressure

20 Kihei 10 Wastewater PS Renovation (County of Maui, HI) Parallel 12 diameter, 2,700 long, proposed PVC and backup DIP force mains 3 X submersible pumps ( ) Swing check valves VFDs Sewage combination air/vacuum valve on discharge header

21 Movie - Power failure without additional surge protection (PVC)

22 Power failure without additional surge protection (PVC)

23 Surge control alternatives Provide satisfactory protection for both PVC and DIP force mains scenarios Device involved Strategy 1 Strategy 2 (preferred) 7 Controlled venting VVs 100 ft 3 surge tank with a controlled venting VV Negative pressure heads Pros/cons High level of maintenance force mains subjected to less stress; prolongs effective life of infrastructure; less maintenance

24 Movie - Power failure with surge tank (PVC), negative pressure heads controlled to -10 ft

25 Power failure with surge tank (PVC)

26 Movie - Power failure with controlled venting vacuum relief valves (PVC), negative pressure heads controlled to -16 ft

27 Power failure with vacuum valves(pvc)

28 Main Wastewater PS (Lexington, MA) Noise from force main Possible causes of noise - transient events created by Pump shutdown (no VFDs) Accumulated air (manual air release valves) 24 dia., 5,850 long, DIP force main 3 X vertical centrifugal pumps ( )

29 Movie - Pump shutdown (power failure) without surge protection

30 Pump shut down (power failure) without surge protection

31 Surge control alternatives Device involved Negative pressure heads Effectiveness at controlling pressure/noise Pros/cons Strategy 1 Strategy 2 Strategy 3 (preferred) 8 Controlled venting VVs and a surge relief valve 2.5 dia. X13 thick flywheels Least effective Requires the highest level of maintenance Additional recommendation: automatic air release valves at high points Moderately effective Introduces harmonic issues when installed with VFDs 396 ft 3 surge tank with 2 controlled venting VVs Most effective force main subjected to less stress; prolongs effective life of infrastructure; less maintenance

32 Movie Pump shutdown (power failure) with surge tank, negative pressure heads controlled to -13 ft

33 Pump shutdown (power failure) with surge tank, negative pressure heads controlled to -13 ft

34 Comparisons of pressure heads at Pump Station Loss of power to pumps 250 Pressure Head (ft) ft No surge protection Surge Tank Flywheel Sewage vacuum valve with controlled venting feature Time (sec)

35 Large diameter, multiple pump stations system near San Francisco, CA Four wastewater pumps stations (Q total = 89 MGD) Force main diameters ranging between 30 and 60 HDPE and DIP force main alternatives

36 Movie - Power failure without surge protection (HDPE)

37 Power failure without surge protection (HDPE)

38 Surge control recommendations Force main Material HDPE DIP Device involved 1500 ft ft 3 surge tanks at 3 out of 4 pump stations 3 controlled venting VVs 1500 ft ft 3 surge tanks at all pump stations 2 controlled venting VVs Negative pressure heads -11-8

39 Movie - Power failure with surge protection (HDPE)

40 Power failure with surge protection (HDPE)

41 Concluding Remarks Many surge control options available Proper selection and installation are important Surge control strategy could involve multiple devices Surge control device can maximize effective life of infrastructure

42 Northwest Hydraulic Consultants 80 South Lake Avenue, Suite 800 Pasadena, California Tel: (626) Contact: Nami Tanaka, P.E. water resource specialists

43 Valve closure creates upsurge and downsurge Tank Tank Flow Control Valve

44 Valve opening creates downsurge and upsurge Tank Tank Flow Control Valve

45 Dampened Swing Check Valve Oil controlled bottom buffer recommended when surge tank installed

46 Pressure Surge Analysis Computer Model Method of Characteristics (MOC) Highest level of accuracy Extensively verified in lab and field Thousands of real-world systems Well suited to multi-core processors

47 Data Requirements 1. Pump performance curves 2. Pump and motor polar moment of inertia 3. Pipe length, diameter, friction factor, wavespeed 4. Pipeline alignment and elevation drawings 5. High and low tank/reservoir/clearwell levels 6. Pump station plan and elevation drawings 7. Diameter, manufacturer, and model number for existing valves (e.g., check, air, relief, etc.) 8. Whether or not pumps have VFD s, soft start/stops or control valves 9. Junction demands

48 Pump Startup Static HGL