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Water hammer in sewer pipelines – engineering causes, analysis and protection methods

  • 2 days ago
  • 3 min read

Hydraulic transients, commonly referred to as water hammer, are among the most complex and frequently underestimated phenomena affecting pressurized sewer systems. Although water hammer is typically associated with potable water distribution networks, transient hydraulic events can be even more critical in wastewater pipelines due to the presence of entrapped air, dissolved gases, suspended solids, and highly variable operating conditions.

In practice, sudden power failures, emergency pump shutdowns, and improperly designed check valves are among the leading causes of pipeline failures, equipment damage, and costly service interruptions in wastewater infrastructure.


transient profile
transient profile

Water hammer is an unsteady hydraulic phenomenon that occurs when a rapid change in fluid velocity generates a pressure wave propagating throughout the pipeline system.

When the velocity change is sufficiently large, pressure can increase or decrease by tens of meters of water column within fractions of a second. Under certain conditions, negative pressures may develop, resulting in column separation followed by water column rejoining. The subsequent collapse of vapor cavities can generate pressure peaks significantly exceeding the system's design pressure.

These transient events often remain invisible to operators while causing severe structural damage over time.


Why Are Sewer Systems More Vulnerable?

Unlike potable water transmission systems, pressurized sewer pipelines operate under considerably more complex hydraulic conditions.

Key factors include:

  • Entrapped air and sewer gases;

  • Two-phase flow conditions;

  • Suspended solids and sediment transport;

  • Highly variable flow rates;

  • Frequent pump start-stop cycles;

  • Significant elevation differences along the pipeline;

  • High points where air pockets accumulate.

Even relatively small volumes of entrapped air can substantially alter wave propagation velocity and significantly increase the complexity of transient hydraulic behavior.


Common Causes of Hydraulic Transients

Experience from wastewater utilities shows that most failures result from a combination of several adverse conditions, including:

  • Sudden power outages;

  • Improper pump selection;

  • Rapid closure of check valves;

  • Incorrect variable frequency drive (VFD) control strategies;

  • Insufficient air valve installation;

  • Inadequately sized pipelines;

  • Failure to perform transient hydraulic analysis during the design stage.


Potential Consequences

Water hammer does not always result in immediate pipeline failure. More commonly, repeated transient pressure cycles gradually weaken system components until structural failure occurs.

Typical consequences include:

  • Failure of PVC, HDPE, ductile iron, or steel pipelines;

  • Damage to mechanical and electrofusion joints;

  • Failure of manholes and ancillary structures;

  • Check valve malfunction;

  • Damage to pump impellers, bearings, and shafts;

  • Cavitation;

  • Groundwater infiltration through compromised joints;

  • Increased energy consumption;

  • Reduced service life of mechanical and hydraulic equipment.


Hydraulic Transient Analysis

Modern engineering practice strongly recommends performing transient hydraulic analysis during the design phase of every pressurized wastewater system.

Specialized software commonly used includes:

  • Bentley HAMMER

  • InfoWorks ICM;

  • WANDA;

  • AFT Impulse.

These applications enable engineers to simulate numerous operating and emergency scenarios, including:

  • Complete power failure;

  • Emergency shutdown of one or multiple pumps;

  • Check valve malfunction;

  • Rapid or controlled valve closure;

  • Vacuum formation;

  • Air admission and release throughout the pipeline.

Simulation results provide maximum and minimum pressure profiles along the entire system, allowing engineers to identify critical locations and select appropriate surge protection measures.


Scientific literature

  1. Wylie, EB, & Streeter, VL Fluid Transients in Systems .

  2. Chaudhry, MH Applied Hydraulic Transients .

  3. Thorley, ARD Fluid Transients in Pipeline Systems .

  4. Mays, LW Water Distribution Systems Handbook .

  5. AWWA Manual M11 – Steel Pipe: A Guide for Design and Installation .

  6. AWWA Manual M51 – Air-Release, Air/Vacuum and Combination Air Valves .

  7. IEC 61362 – Guide to specification of hydraulic transient studies.

 
 
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