Introduction to Sewer Bypass Pumping Design
by Harrison Steed, P.E.
Pipeline rehabilitation via the cured-in-place pipe (CIPP) rehabilitation method is frequently used to extend the life of a deteriorating sewer. This method requires completely dewatering the pipeline to create a new pipe inside of an existing deteriorated pipe. To date, no commonly accepted standard for sewer bypass design criteria has been developed. The level of detail provided in the design documents can vary significantly from one engineer to another. It is not uncommon for the flow bypass design required by a sewer pipeline project to be considered incidental and left entirely up to the contractor. It is also not uncommon for the engineer to omit any information regarding flow values, primarily to avoid underestimating the flows and thus becoming vulnerable to errors and omissions. This is not a major consideration on small diameter projects, but it becomes significant for medium and large diameter projects. While limiting designer/owner liability is an important part of what an engineer does, there are proactive steps a designer can take on projects requiring bypass pumping to help save the owner money and make projects more successful. They include: generate good flow data, develop sound design criteria, and confirm adequacy of contractor’s bypass plan.
GENERATE GOOD FLOW DATA
Leaving flow data collection up to the contractor in the low-bid contracting environment may not provide a conservative bypass pumping system design. The contractor that estimates the least amount of sewage flows has an advantage and a high probability of being the winning bidder. In contrast, designer generated flow data creates a level playing field for all bidders and minimizes the advantage of an under-estimating contractor. Including the flow data in the design, the owner/engineer can confirm installed bypass systems are adequate for measured flows.
The key to generating good flow data is to minimize the difference between “measured” and “actual” flows during flow monitoring. The three most common methods of acquiring gravity sewer flow data are:
- Temporary flow monitoring by non-specialized subcontractor. Temporary flow monitoring completed by non-professionals without calibrating the results of the meter and/or use of marginally accurate equipment for the flow monitoring. This type of flow monitoring can be very inaccurate (± 50%). Beyond providing a reasonable gauge for peak and minimum flow times each day, it is not recommended for a major sewer rehabilitation project.
- Temporary flow monitoring by specialized subcontractor. This method commonly provides the most accurate flow data, in part due to a minimum of two to three meter calibrations in a short time period, typically one or two weeks. Unfortunately, temporary flow monitoring may provide very accurate data that is not reflective of the actual long term average peak flows, so this must be weighed against other seasonal considerations.
- Permanent flow monitoring maintained by a third-party flow monitoring company. The advantage of this method is that collecting data over an extended time period provides a good understanding of the true historical peak flows. The disadvantage is that calibration efforts, even when originally completed by qualified specialists, are typically too infrequent to provide confidence in the data. Submerged flow meters, which have the potential to be the most accurate, can be fouled by debris collecting on the meter.
There are many types of flow meters used to measure gravity sewer flow. It is important to understand the meter type and its limitations in order to adequately define the safety factors for the temporary bypass. The following is a short list of meter types:
- Ultrasonic Area/Velocity: This is the most common meter type and is installed in the pipe invert. It provides two points of measurement, velocity (ultrasonic) and level (pressure transducer).
- Ultrasonic Level: This flow meter measures the depth of flow only (ultrasonic) and is typically hung from a ladder rung or the underside of the manhole cover.
- Radar: This meter type is hung just above the flow and measures both level (ultrasonic) and velocity (radar).
- Pulse Acoustic Doppler: This is meter is installed in the pipe invert. It provides measurement of both velocity (ultrasonic) and level (pressure transducer). This meter has the potential to be the most accurate for the right application. Its accuracy comes from its ability to measure the velocity at multiple points in the cross section. All other meters measure one point velocity in the flow cross section.
Discuss your options with a trusted consultant or flow monitoring services company regarding the best meter type for your specific application.
DEVELOP SOUND DESIGN CRITERIA
Having obtained good flow data for the bypass design, the engineer can now move forward developing sound design criteria.
- Redundant Pumping Capacity. This relates to uncertainty about flows and thus is considered in tandem with the type, accuracy and duration of flow monitoring. Less accurate flow data requires greater pumping redundancies, another argument that good flow data results in true project savings. This safety factor typically ranges from 1.25 to 2.0.
- Redundant Piping Capacity. This is more relevant with long bypass lengths and larger flows. Designers should ask themselves if a leak can be repaired without compromising the ability to convey the peak flows. An example would be a recent project with a 10,000 foot long bypass project consisting of (4) 18-inch pipes: an additional 18-inch pipe for was required for redundancy.
- NPSH Calculations. Often overlooked or misleading is the rated capacity of the temporary pumps. Commonly these should be de-rated significantly due to large suction depths required for large diameter sewer interceptors and turbulence and vortexing which occur at the inlet. The manufacturer provided pump data and curves must be adjusted for field conditions. Commonly used centrifugal self-priming pumps do not perform well at suction lifts over twenty three feet. Inadequate pumping capacity leads to surcharging of the pipe which can result in flooding in basements and upstream connections. In addition, there are limitations to how much head an inflatable plug can withstand without being damaged.
- Pipe Material. High Density Polyethylene is most commonly used for bypass piping. It is durable and the joint fusion process provides leak free joints and a high level of flexibility during pipe layout including easy field cuts.
- Constrained Piping Corridor. Bypass pumping and piping systems are by definition temporary. Each contractor has his own concept of how he would design the same bypass due to differences in equipment, labor, project experience, and preferences. For these reasons a good designer will leave as much of the details of the actual bypass design to the contractor’s means and methods. Conversely, a pipeline owner wants to see the design in sufficient detail so that their internal departments, the roadway entity, and the local utilities can review the plan to be implemented during construction. Showing a dimensioned “constrained bypass corridor” on the plans is one way to do this. This defined corridor can be helpful in coordinating with public and private property owners during the design phase. We have found that projects that have large diameter (≥ 36”) pipes, tight schedule constraints, high profile locations, long bypass lengths, and deep depths often require a defined corridor. For smaller bypasses, a constrained corridor becomes more of a burden than a benefit to the project. The constrained corridor is an encroachment upon the means and methods of the contractor for a temporary system. The risks and benefits of such should be evaluated by the owner and designer.
CONFIRM ADEQUACY OF THE CONTRACTOR’S BYPASS PLAN
While the contractor assumes all liability for the effectiveness of his bypass plan, the owner and designer want to confirm that his plan is sound and in accordance with previously developed design criteria. Bypass pumping submittals often overestimate the pump discharge capacity. Pumps are tested in clear water under factory conditions, not duplicated in the field. To confirm that these calculations are correct requires the reviewer to have enough detail to understand the pump and piping configuration.
- How many discharge pipes are proposed in the bypass plan?
- Are the pipes manifolded for equal flow distribution?
- What is the velocity of the peak flow if the flow is evenly distributed?
Are the pipes manifolded and valved such that any one pipe may be isolated for repair? They should be.
- Is the flow capacity of the redundant pipe included in determining the adequacy of the pumping capacity? It should not be.
- What the elevations of suction and discharge?
Submittals provided with adequate information allow the engineer to review the proposed bypass details to confirm the soundness of the plan.
About the Author
Harrison Steed is a registered professional engineer in the states of Nevada and Arizona. He has a BS degree in civil and environmental engineering from Brigham Young University and an MS Degree in Civil and Environmental Engineering from the University of Las Vegas, Nevada. In the last 8 years Harrison has worked on the design of more than a dozen sewer rehabilitation and replacement projects with peak bypass design flows ranging from 15 mgd to 150 mgd. The majority of these projects were completed in the Las Vegas valley.