Sewer Bypass FAQ’s

MIDAS Flow Control has assembled a list of key questions and answers to help you better understand our capabilities.

What is the Sewer Line Size I am Bypassing?

This information helps determine the line’s maximum sewer flow. Other factors include: the scope of the sewer line, the type of pipe material (i.e. PVC or concrete), since pipe materials have a different friction coefficient.

Once the sewer line size is known then the appropriate mechanical plug can be selected to stop the sewer’s flow. Understanding the line size is a priority as it indicates the flow that the bypass must cover.

How Do I Determine the Flow?

Normal peak flows are between 6-9 am and 6-9 pm. If the sewer is in an industrial or commercial area, different peak times may apply due to production processes. Observing the flow during these times is crucial. The system must have the capacity to cover these peaks. Sanitary sewers often experience extremely high flows during storm events due to infiltration.

What is Considered Low Flow

Low flow times may be the longest periods of the day so the system should be designed to carry this flow in an efficient manner. Oversizing pumps resulting in a failure to consider these low flow periods can cause very inefficient pumping systems and can lead to problems.
It’s important to avoid pumps repeatedly cycling on for short bursts to drain the suction chamber and then restart a short time later and continue this cycle. The Best Management Practice is to combine pumps that cover these peaks and low flow periods.

What is the Average Daily Flow?

The Public Works department should have this flow information.The average daily flow combined with observations of peaks and low flow times will help determine primary pumping system parameters.

How Deep is the Suction Manhole?

Lift is a critical element in the assessment of a pumping system. This LIFT from the top of the liquid in the manhole to the eye of the impeller must be measured as it will help determine the type of pumps the system requires. Dri-Prime (self-priming) pumps can employ a suction LIFT of 28’ (at sea level). If the sewer is deeper than the suction pumps can operate then a different methodology must be used in determining pump type, such as a hydraulic submersible or electric submersible.

How Far From the Manhole Can You Place Your Pumps?

We like to determine the horizontal distance from the suction point to allow for additional suction hoses. This horizontal distance is significant in determining friction loss as it will add distance to the fluid traveling through the system. The additional suction distance will increase the time that the pump needs to self-prime, since there is more air to evacuate with the additional line added.

How Far is the Discharge Manhole?

Again, distance is a key design component, and is critical in determining friction loss while the fluid is pumped through the bypass to the receiving manhole. If the line is more than 1000 LF, it may be necessary to up-size the discharge line to reduce friction loss.

Will the System Run Constantly?

Is the system going to run overnight or 24/7? If so, MIDAS will continue supervision and monitoring. A fueling program may also need to be incorporated, as well.

Is Noise an Issue?

If noise is an issue then “silent” pumps can be specified to minimize the noise. Silent units are designed for 69 dBA at 30 feet. This level is similar to the sound level of a normal conversation.

At What Velocity is the Flow in the Sewer? Feet Per Second? Meters Per Second?

Velocity is an important factor in determining FLOW. If a sewer flowing at 50% full is also flowing at two (2) feet per second it will be half the flow of the same sewer flowing 50% full at four (4) feet per second. Velocity is a fundamental element in determining flow.

What Slope is the Existing Sewer?

Slope will affect velocity, as the steeper the sewer the faster the flow. The faster the flow in the sewer the more capacity that sewer has. Slope will correlate directly with velocity. Most sanitary sewers are designed to not run less than two (2) feet per second. This is the flow that keeps solids in suspension which inhibits the lines from clogging under normal conditions.

What Percentage Flow is the Sewer? Quarter Full, Half Full? What Time of Day is it?

Noting this and knowing the sewer size will help determine FLOW and peaks. This information must be recorded and used in the system design.

What Pressure is Desired at Discharge Point?

A bypass system will have to reach a certain TDH (Total Dynamic Head) to pump to a physical location. Once there, the fluid may only have to exit the end of the pipe and be influenced by gravity (down into a receiving manhole) or discharge area. However, if pumping into a pressurized system, it is important to also add additional pressure to the system TDH. This is done by multiplying the pressure requirement in psi by 2.31.

Is Traffic Control Required to Protect the Pumping System and Work Area?

A pumping system must always be protected from traffic. Special road ramps might have to be specified to allow traffic to pass over the system. A barrier system may be required to protect the work area.

What is the Planned Redundancy or Pump Back-Up Required for the System? Is the Back-Up Built Into the System?

Pumps are intricate machines and although they are built with reliability in mind sometimes components expire or bad fuel is used. A variety of issues can lead to a pump clogging or a line getting damaged. Redundant procedures should if the pump operation is continuous. For example, setting up three pumps in a system that only requires two. The third would operate automatically by float switch if it sensed the level was breached.

Is the Back-Up Only Mechanical Stand By (and Not Intrinsically Integrated)?

The third pump in the above example would not be connected to the system but would be on-site and ready to be hooked up in case one of the primary pumps failed.

Designing and operating bypass pumping systems that range from the most simple operation to the intricate and extremely complicated.