How do you
Design a Bypass Pumping System ?
Bypass pumping systems run the gamut from the simple to the intricate and extremely complicated. The following is intended to address several basic factors required to build a successful bypass pumping system.
Many bypasses are similar in appearance but it’s their subtle differences that require close attention to understand component performance parameters. Below are just some of the general questions users must ask to minimize risk and ensure a successful bypass, as each bypass is unique and requires close assessment and planning before implementation.
Understanding System Components
The repair area must be isolated from the suction area and the discharge location.
If sewer plugs are used, for example, they cannot be placed in a sewer line that has to be repaired. The plug must be installed one manhole upstream.
Sizing the Pumping System
What is the sewer line size that you must bypass?
The answer will help address the maximum flow that this specific sewer line can carry. Other factors contribute to the flow, including the scope of the sewer line, the type of pipeline (e.g. PVC or concrete), as different materials have a different co-efficient of friction. Once the size of the existing sewer line is known then the appropriate mechanical plug can be selected to plug the sewer. Understanding the line size is a priority as it indicates the flow the that the bypass must cover.
How much is the FLOW?
Normal peak flows are between 6-9am and 6-9pm. These times are the highest flow times for domestic sanitary sewer lines: the morning as everyone is starting their day and the evening when people arrive home and during the dinner hour. 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 as the system must have the capacity to cover these peaks. Sanitary sewers often respond with exponential flows during storm events due to infiltration.
What is the 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. Over-sizing 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?
Government Works Departments will very often have this flow information. The average daily flow combined with observations of peaks and low flow times will help determine primary pumping system parameters.
Tips and Hints for Temporary Diversion Systems
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 much surcharge in the manhole is allowable?
The allowable surcharge in a specific manhole will vary. Once the manhole is plugged it’s important to determine how high the level in that specific manhole can reach before impacting properties or environments upstream of the pumping system. The ideal suction depth for Dri-Prime pumps is under 20 feet. Over 20 feet is considered a critical suction depth and special consideration must be given to the specific Net Positive Suction Head of the pumps used in the system.
Don’t forget the height from ground level to the eye of the impeller when determining suction depth. Often the distance from the ground to the impeller is overlooked. If a pump is mounted on a highway trailer the additional height from the ground to the eye of the impeller (center of suction connection) is approximately four (4) feet.
How far from the manhole can you place your pumps?
Determine the horizontal distance from the suction point to allow for additional suction hose. This horizontal distance is significant in determining friction loss as it will add DISTANCE to the fluid travelling through the system which will then be subject to additional friction loss. The additional suction distance will increase the time that the pump needs to self-prime as there is more air to evacuate with the additional line added.
How far is the discharge manhole?
Again, DISTANCE is a fundamental design component, as it 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 feet it may be necessary to up-size the discharge line in order to reduce friction loss.
Will the system run constantly?
Is the system going to run over-night or 24/7? A bypass must always be watched and monitored. A fueling program may have to be used, while the situation may require qualified people to run and maintain the pumps.
Is noise an issue? (Critically Silenced)
If noise is an issue then Critically Silent pumps can be used to minimize the noise, as opposed to open pumps where the engine and pump are exposed to the environment. Critically Silent units are designed for 69 dBA at 30 feet (9 meters). That sound level, as a rough estimate 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, if any?
A bypass system will have to reach a certain TDH (Total Dynamic Head) to pump to a physical location. Once pumped to a certain location the fluid may only have to exit the end of the pipe and be influenced by gravity (down into a receiving manhole) or open ended discharge. Sometimes, however, if pumping into a pressurized system, it’s important to also ADD the additional pressure to the system TDH. This is done by multiplying the pressure requirement in psi by 2.31 and adding it to the system TDH.
Is traffic control required to protect the pumping system and work area?
A pumping system must be protected from traffic. Special road ramps might have to be used to allow traffic to pass over the system while concrete barriers 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. Any number factors can lead to a pump clogging or a line getting damaged. Some measure of redundancy must be built into the system if performance is continuous. An example would be three similar pumps in a system that only requires two. The third is connected into the line and would operate automatically by float switch if it sensed that the level it was set at was breached. This establishes the pump as intrinsic to the system rather than just a mechanical standby.
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.
Operating and maintaining the bypass system
Understand the operating speed of the pumps. A pump supplier will instruct the operating RPMs of the pumps and the estimated fuel consumption per hour. The RPM of the pump as recommended by a pump technician will meet the duty point at which the system must run in order to meet the demands of the system bypass.
Fuel consumption data and refueling schedule.
Generally Dri-Prime pumps are able to pump 24 hours at full load per full tank of fuel. Pump technicians can estimate fuel consumption by the RPM of the engine in order to meet the System Duty Point. If a system must run continuously for a period of time a refueling program must be put in place to keep the system running.
Are automatic controls required (floats or pressure transducers)?
Automatic floats are available to turn on and off pump(s). Prime Guard Controllers are available on all Dri-Prime model pumps. In some cases the system must work off of a pressure sensor rather than floats! Prime Guard Controllers can accommodate pressure transducers as well as floats. Suction pits or manholes can get very congested and turbulent. Often level transducers are used rather than floats. These level transducers are installed inside a perforated tube to allow the level to enter the tube while the tube protects the level detection device.
Pumps must be serviced every 250 hours.
Diesel pumps must have the engine oil and filters changed every 250 hours to ensure continuous performance. These services usually involve the entire system being assessed by a trained pump mechanic to make sure the pumps are running as designed. They can also confirm that there are no “changing conditions” that could affect the pumping system performance.
Are back-up provisions in place while service is being done?
It’s important to build in time for the system to be serviced. Can the pump be turned off for one hour? If not, then it’s vital to build in redundancy to allow for servicing so a pump can be taken off line while it is being serviced without affecting the bypass performance.
If pumps become clogged with items like sanitary wipes, heavy solids around baskets, etc. have provisions been made for back-up suction lines for quick change out in order to clean dirty baskets?
This is a common problem in sanitary pumping. The simple fix is having a standby suction line complete with suction basket ready to attach in case the primary one gets clogged. Indicators of clogging include: the level starts to rise in the suction manhole or the pump begins to cavitate (or shake) because the suction basket is clogged and the pump impeller is not receiving enough liquid.
Decontaminating and Dismantling the System
Have arrangements been made for clean water to flush bypass lines and pumps?
Make arrangements for equipment chlorination and system sanitizing. Transportation of soiled equipment is illegal and therefore the system must be cleaned into the existing sanitary sewer.
*Whenever introducing hypochlorite solutions into municipal systems great caution must be taken to ensure that the system is not shocked. In other words, de-chlorination chemicals may have to be introduced to match the hypochlorite discharge.
Sanitary Sewer Bypass – Emergency Response, Mobilization to Address a Blocked Sanitary Sewer Bypass Pumping Requirement
Faced with a sanitary sewer damaged by nearby construction activities, a municipality in the GTA requested that the Aquatech team implement a full bypass by the following day, as the forecast warned of a significant rainfall event. Having conducted a full site review, Aquatech undertook a number of measures, beginning with having the manhole taper tops removed and risers extended to the surface to accommodate suction and discharge lines. The pumping system was offloaded on the site and jersey barriers were installed, as the location bordered a six-lane road. The staff worked for the next 24 hours to make sure the pumping system was in place as the anticipated rainfall event occurred. As part of its responsibilities, Aquatech oversaw the pump watch, system maintenance, refuelling and requisite operation over the next three months while the sewer was repaired. Upon completion of the project all items were cleaned and sanitized prior to the removal of the equipment and re-instatement of the site.
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