Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, North...

Solving Significant Surge and Water Hammer in a Large

Wastewater Lift Station at Fort Bragg, North Carolina

November 16, 2015

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Randall L. Foulke, PE, BCEE, LEED AP, AECOMPatrick 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 Outline01

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• 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

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• 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


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• 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


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• Surge pressure (also known as water hammer)• Formulas

= Surge pressure above initial pressure, psi (when Tc ≥ Tx)

= Wave velocity, fps

= Wave velocity of fluid = 4,660 fps for water

= Critical time in sec for pressure wave to travel 2L

= Surge pressure above initial pressure, psi (when Tc < Tx)

Determination of Existing Surge Forces


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• V = Velocity Change, fps• W = Weight of Water, 62.4 pcf• g = acceleration due to gravity, 32.2fps2

• 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

• Tx = 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



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• 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)

– Tc = 6.8 sec (travel time in force main is 3.4 sec)

– Ps = 199 psi (actual pressure at lift station is ≈ 258 psi)



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Force on 14” blind flange at lift station = 20 tons



Force of 20 tons travelling at 3 times the speed of sound – why it is called water hammer

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• 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



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• 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



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• 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



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• 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



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• 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 Ps 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



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• 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



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• 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



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• 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



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• 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



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• 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% (Tc < Tx)

• 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


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• 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


• 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 orComments?

November 16, 2015

919-637-3344 │[email protected] Foulke

Pat Jennings910-495-1311 │[email protected]

Solving Significant Surge and Water Hammer in a Large Wastewater Lift Station at Fort Bragg, North...

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