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How to Mitigate Risk in Your Sewer Bypass Design

A temporary sewer bypass is often needed when working to repair aging underground infrastructure. Aging sewer infrastructure poses a danger to public health, as well as the environment.

If cracked or corroded lines leak or break, municipalities may be subject to steep penalties for the illegal discharge of raw or partially treated sewage. Trenchless sewer line repair processes allow for faster, cheaper pipe rehabilitation or replacement with less damage to the landscape and less risk to workers.

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A temporary sewer bypass is typically necessary to divert the flow around the work area. But bypasses themselves can be subject to spills and leakages. The optimal sewer bypass design can minimize the risks and help contractors and owners complete sewer rehab projects without a hitch.

Following these 10 tips increases the chance of trouble-free sewer bypass operation.

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1. Create a Professional Drawing

A licensed professional engineer should create a professional drawing to set the project up for success from the get-go. The drawing should show what the system will look like, including the placement of all pumps and piping, and how it will address specific project needs as well as site-specific considerations, including space constraints.

A site visit is critical for assessing potential design challenges and reducing change orders. Where should access points for maintenance and fueling be placed? How will workers reach isolation gate valves? Is other construction or utility work going on in the area that should be accounted for in the design and location of the sewer bypass?

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2. Independently Confirm Project Specs

A site visit provides the opportunity to verify data provided by the system owner about the existing system’s average daily flow rate, pipe sizes, the distance from the pump site to the discharge point, the grade of the slope in the line and other measurements. Designing a system based on inaccurate data increases the risk of pump breakdown or sewage overflow.

3. Size the Sewer Bypass System for the Average and Peak Flow

If the real-world peak flow exceeds the specified peak flow, the system may fail, so getting the peak gallons per minute (GPM) right is essential. A flow study may be necessary. The use of undersized pumps or piping increases the risk of costly failures and spills. Similarly, oversizing the pumps for the average or minimum flow might lead to intermittent pumping and pump breakdown due to repeated starting and stopping. Based on the expected flow variance, a licensed professional engineer can design a pumping system with multiple pumps instead of one bigger pump.

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4. Determine the Suction Lift

One of the most important considerations in the design of a sewer bypass system is the suction lift, or the vertical distance from the water level to the eye of the pump impeller. If the sewer line runs through a manhole, vault or deep structure, the suction lift will be a major factor.

Generally speaking, the maximum total dynamic suction head (TDSH), which reflects the suction lift and friction losses during operation, should not be more than 28 ft at sea level. A TDSH outside those limits may lead to a loss of performance, pump cavitation and pump failure. An engineer should analyze the suction piping, site elevation and pump performance to determine the maximum feasible suction lift with the required Net Positive Suction Head margins to avoid damaging cavitation. Based on site conditions and an economic analysis, an engineer can recommend solutions such as benching or surcharging.

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Allow for the fact that water levels fluctuate in sewer systems. If the water level drops below the 28-ft point, a centrifugal pump won’t prime at startup. An electric or hydraulic submersible pump may be necessary. These pumps have no suction lift limitations.

5. Plan for Velocity Control

Flow velocity in a suction pipe should be limited to prevent cavitation risk and uneven loading of the impellers. To reduce the risk of damage to discharge pipes, the velocity of the carried fluid should not exceed certain limits, typically around 18 ft per second, that can be calculated accurately based on the pipe material, discharge pressure, fluid properties and pipe lengths. A minimum velocity of at least 2 or 2.5 ft per second should be maintained to prevent solids from settling.

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6. Build in the Right Amount of Ventilation

To reduce the risk of a discharge pipe flattening or rupturing, carefully calculate the amount of air that must be introduced and/or evacuated into the sewer bypass system. Air and vacuum release valves should be sized appropriately based on pipe size, elevation changes, flow rates and liquid properties.

7. Build In Redundancy

When designing the bypass system, specify the backup pumps and hoses, as well as spare parts such as valves and elbows, that must be kept on-site to reduce downtime in the event of a system failure. Backup pumps should be plumbed in and ready to go.

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Perform a cost-benefit analysis of building in redundancy by installing two pipes instead of one (for a two-pump system) so the second pipe can immediately accommodate the flow if the first becomes damaged during operations. The municipality may specify the degree of redundancy required.

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8. Add Spill Containment Measures

To avoid the risk of environmental damage from the spills of fuel, oil or wastewater, specify that the bypass system includes spill containment mats under relevant equipment.

9. Add Instrumentation and Telemetry for Automatic Pump Operation and Remote Monitoring

A pump system designed with instruments such as level floats, level transducers and flow meters allow for automatic pump control and operation including pump start, stop and speed control for optimal performance and seamless transition between primary, secondary and backup pumps based on fluctuations in system flow rates.

With remote monitoring via telemetry, there’s no need to maintain personnel on-site round-the-clock to ensure the bypass system works properly. Today’s telemetry systems offer a reliable means of monitoring pump fuel levels, oil pressure, coolant temperature, flow rate, suction and discharge pressure and more. Because these devices detect problems almost immediately — often sooner than a human would — and send alerts when measurements approach predetermined thresholds, they are a powerful tool for reducing the risks of a bypass operation.

10. Solicit Engineering Help

Designing a successful sewer bypass is a complex undertaking, one that requires myriad calculations and strategic problem-solving. For contractors who don’t have their own professional engineers with deep expertise in bypass system engineering, a temporary equipment vendor with in-house professional engineers can help. Based on years of experience and insights gained by working on countless other sewer bypass projects, they can provide guidance that enables contractors and owners to be more proactive in their approach to risk reduction.

Vignesh Santhanagopalan is a senior engineer-fluid solutions at United Rentals.

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