Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, North Carolina November 16, 2015 Randall L. Foulke, PE, BCEE, LEED AP, AECOM Patrick C. Jennings, PE, Old North Utility Services, Inc. Adam J. Paukovich, PE, AECOM

Historical Background of Project Determination of Surge Forces Analysis and Recommendations for Controlling Surge Results of Recommendations Lessons Learned from Project Questions Presentation Outline

3 Lift Station 1 constructed in 1993 – Replaced an existing lift station Replacement consisted dry pit / wetwell type lift station – 40 foot deep dry pit /wetwell lift station Flowrate approximately 14 MGD Replacement included four shaft driven driven centrifugal pumps, hydraulic in-line grinders, check valves, and 14-inch suction and discharge headers. Two of the pumps were replaced in 2007 with Flygt dry pit submersibles UP privatization commenced in early 2008 – ONUS took over ownership and O&M responsibility for LS 1 Historical Background Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

4 Much of the equipment was not operational including the grinders, two of the shaft driven pumps, parts of the electrical system and controls, valves, sluice gates in wetwell. Water Leaking in the Lower Level through the walls. What ONUS found at LS 1when we took over Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

5 Ventilation system was failing Water Hammer – so bad that the entire lift station shook when the pumps shutoff & some of the support structure for the discharge lines had sheared. Cross line starters were causing harsh startup and shutdown, contributing to the water hammer issue. Due to force main size, pumping at a high TDH, which also contributed to the water hammer issue. What ONUS found at LS 1when we took over Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

6 Determination of Existing Surge Forces Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

7 V = Velocity Change, fps W = Weight of Water, 62.4 pcf g = acceleration due to gravity, 32.2fps 2 K = Bulk modulus of elasticity of water, 294,000 psi d = pipe diameter (nominal), inches t = thickness of pipe wall, inches E = Modulus of elasticity of pipe material, 24,000,000 psi of DI pipe L = Length of pipeline, ft T x = Closure or stop time, sec Formulas do not depend upon friction losses or c factors Formulas very dependent upon fluid velocity, pipe size & type, length of pipeline Formulas are exponential as a result of square root function in the denominator of the wave velocity Determination of Existing Surge Forces Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

8 Project Conditions –18 inch diameter DI pipe –Class 250 pipe with pipe thickness of 0.31 inches –Velocity of water at 3,300 gpm of 4.16 fps –Length of force main is 12,115 lf Calculations – a = 3,562 fps (note speed of sound = 1,126 fps) – T c = 6.8 sec (travel time in force main is 3.4 sec) – P s = 199 psi (actual pressure at lift station is ≈ 258 psi) Determination of Existing Surge Forces Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

9 Force on 14” blind flange at lift station = 20 tons Determination of Existing Surge Forces Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC Force of 20 tons travelling at 3 times the speed of sound – why it is called water hammer

10 Surge Control Approaches –Variable Frequency Drives (VFDs) Can control critical closure time & velocity in pipe, dependable and ease of maintenance, additional benefit of reduced electrical cost Adds complexity to electrical and controls, loss of power eliminates ability to control surge Analysis and Recommendations for Controlling Surge Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

11 Surge Control Approaches Continued –Specialty Check Valves Can control critical closure time & velocity in pipe Generally very reliable Adds complexity to mechanical system, adds to maintenance requirements Analysis and Recommendations for Controlling Surge Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

12 Surge Control Approaches Continued –Surge Relief Valve Mitigates pressure wave and provides last line of defense Has a finite life, may not necessarily completely eliminate water hammer, requires proper location for discharge from valve Analysis and Recommendations for Controlling Surge Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

13 Surge Control Approaches Continued –Vibration Isolation (Rubber Expansion Joints Protects equipment and piping from significant stresses Can be damaged easily & has a finite life –Adequate pipe supports and bracing Restrains forces Requires proper design and installation, & periodic inspection and maintenance –Force Main Improvements Analysis and Recommendations for Controlling Surge Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

14 Recommendations for ONUS Lift Station 1 –Install VFDs for each pump (3) Ramp down time of 1 minute Significantly reduces velocity change & increases time of closure resulting in the P s from 199 psi to 11 psi –Install slow closing, oil-cushioned check valve on each pump (3) Further reduces velocity change & increases time of closure, resulting in an additional reduction to ≈ 7 psi Analysis and Recommendations for Controlling Surge Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

15 Recommendations for ONUS Lift Station 1 Cont. –Install rubber expansion joints on suction and discharge of each pump (3) Eliminates or reduces stresses on pumps and piping, especially at joints –Maintain existing surge relief valve Provides last line of defense Provides surge protection during power outage –Loss of power results in a lag until generator is up to full power –Force main and piping improvements Working air/vacuum relief valves Thrust restraint (restrained joint pipe) Pipe supports and bracing Analysis and Recommendations for Controlling Surge Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

16 Phased construction with force main improvements completed ≈ 1 year prior to the pump station improvements –Surprising reduction in surge pressures and vibration at lift station –Working air/vacuum relief valves reduced the amplitude of the negative portion of the sine wave, dampening the amplitude of the positive portion of the sine wave Results of Recommendations Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

17 VFDs and oil-cushioned check valves provide for a smooth shutdown of pumping system Rubber expansion joints protect pumps and piping Surge relief valve set at 175 psi, protects pumps and piping, but does not relieve prematurely Pipe supports and bracing provide for restraint in case of excessive forces from surge pressure Results of Recommendations Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

18 Since lift station was being completely rehabilitated, implementation was easy Pumps, piping, and controls have been in operation for approximately 4 months System is experiencing little to no vibration and no water hammer noise during shutdown Pumps ramp down smoothly Check valves close smoothly and with no stress Results of Recommendations Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

19 Surge pressure and vibration significantly reduced with operable air/vacuum relief valves Pressure wave velocity at over 3X the speed of sound was eye-opening Ramping down of the pump at 1 minute duration reduced the surge pressure by 95% (T c < T x ) Surge relief valve required for loss of power and starting of backup generator Slow closing valves can be undependable Rubber expansion joints can be compromised easily Lessons Learned Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC

20 Surge (water hammer) results in significant forces with large diameter, long length force mains Surge forces are dependent upon velocity in pipeline, pipe size & type, and length of pipeline Conclusions Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, NC Multiple methods are necessary to control surge and protect the pumping system Use of VFDs to control pumps is an important tool Air/vacuum relief valves cannot be taken for granted and need to be maintained

Questions or Comments? November 16, Randy Foulke Pat Jennings