Gas pockets in a wastewater rising main: a case study

Area of Science:

• Civil Engineering
• Environmental Engineering
• Fluid Mechanics

Background:

• Wastewater rising mains (WWRM) are critical infrastructure for sewage transport.
• Pipe failures in WWRMs can lead to significant environmental and economic consequences.
• Prestressed concrete cylinder pipes (PCCP) are commonly used in WWRMs but can be susceptible to failure.

Purpose of the Study:

• To investigate the cause of a catastrophic failure in a 1.2 m (48 in) PCCP wastewater rising main.
• To identify the contributing factors to an extreme transient event resulting from a simultaneous pump power failure.
• To provide insights into the operational risks associated with gas accumulation in WWRMs.

Main Methods:

• Detailed in situ observation of the affected WWRM system.
• Experimental investigation using a dedicated setup.
• Comprehensive hydraulic analysis and fluid transient simulations.
• Assessment of the structural strength of the WWRM.

Main Results:

• An extreme transient event, triggered by simultaneous pump power failure, caused the rupture of the PCCP.
• Stationary small gas pockets accumulated at high points within the WWRM were identified as the principal contributory factor to the failure.
• Simulations confirmed the significant role of gas pockets in exacerbating transient pressures.

Conclusions:

• Operating a WWRM with gas pockets at its high points poses a significant risk of catastrophic failure.
• Understanding and mitigating gas accumulation is crucial for the reliable operation of WWRMs.
• This case study serves as a critical warning for WWRM management and design.