Contractors rely on trash-handling pumps to bypass sewage during line repairs, pump station maintenance, and during new station installations. Pumps and pumping systems have a critical role in assuring sewage continues to flow while necessary upgrades are made to the existing infrastructure.
Sizing the Bypass Pumping Operation
Step 1: Find out the size of the sewer line that must be bypassed. Once the size is determined, it is used calculate the maximum flow the line can hold. This also aids in selecting the correct mechanical plug to temporarily block the line so the corresponding section of the line can be repaired.
Step 2: Determine the flow. The location of the project determines the amount of flow to bypass the sewage line repair. If the project is in an industrial or commercial area, flow should be observed during production processes to determine peak times. Traditionally, peak flows occur from 6 to 9 a.m. (when people are getting ready for work) and from 6 to 9 p.m. (when people return from work and are having dinner). During non-peak times, lines will receive lower flow levels. It is important that when applying a system, it be designed to handle this flow adjustment in an efficient manner. Over sizing the pump(s) can result in inefficient pumping and often leads to other problems. The pumps should not be repeatedly cycling for short bursts of activity, and draining the suction chamber. It is best to combine pumps to cover both the peak and low flow periods. It is also important to accommodate added flow that might be brought on by a storm event.
Step 3: Determine the depth of the suction manhole. One of the most important elements in any pumping application is the lift. The lift for sewage bypass pumping operations is determined from measuring the distance from the top of the liquid in the manhole to the center of the impeller. This distance will determine the type of pump needed for the bypass pumping job. Self-priming pumps or priming assisted pumps can employ a suction lift of 25 ft (at sea level). If a larger lift is needed, electric or hydraulic submersible pumps must be used.
Step 4: Determine how much surcharge is allowable in the manhole. This amount will vary depending on that specific manhole. After the mechanical plug has been inserted into the line, the next step is to determine how high the level can get before impacting properties upstream of the pumping system. The ideal suction depth for self-priming pumps is under 20 ft. Anything more than 20 ft is a critical suction depth and requires special consideration to the Net Positive Suction Head (NPSH) of the pump(s) used. When calculating the overall suction lift, it is important not to overlook the added height of a pump mounted on a highway trailer. Adding 4 ft to the lift of this set-up is a safe guess.
Step 5: Calculate the distance from the discharge of the pump to the receiving manhole. Distance is a vital element in determining friction loss while the fluid travels from the pump to the receiving manhole. When the line is more than 1,000 ft away, it may be necessary to increase the discharge line size in order to reduce the friction loss.
Step 6: Determine how far away from the manhole the pump(s) should be positioned. Calculate the distance from the suction of the pump to allow for additional suction hose. This distance is used to determine friction loss as the sewage travels through the system. This added distance will increase amount of time the pump will need to prime. This occurs because more air will need to be evacuated from the additional line.
Step 7: Determine the velocity and slope of the sewer line. It is important to know the grade of slope in the line as this will affect the feet per second (velocity) the sewage is flowing. The velocity in the sewer line should be at least 2 1/2 ft per second. This keeps solids in the line from settling and ultimately causing a clog in the line.
Depending on the location of the bypass project, it may be important to minimize environmental noise emitted by the pump. In this situation, an enclosed silent pump is ideal. Instead of having the pump and engine exposed, an enclosure reduces the noise to an acceptable level — usually around 71 dBA from 23 ft away.
Step 8: Determine the pressure of the system. It is important to calculate the TDH (total dynamic head) of the specific bypass system. This is decided by adding the vertical distance of the pipe and the friction loss of the pipe. The discharge pipe can either be pumped into an open discharge or it can be pumped into a pressurized system. If it is a pressurized system, this additional pressure must be added to the TDH. This is calculated by multiplying the pressure in PSI by 2.31 and then adding it to the TDH.
Specifying Pumps for the Bypass Pumping Operation
What pumps are required? Now that the flow and the pressure of the bypass operation have been determined, the proper pump(s) can be selected with the help of the pump manufacturer. A recommended operating RPM for the pump(s) is often specified at this time to meet the demands of the system bypass.
Most pumps are able to pump 24 hours at full load per full tank of fuel. Fuel consumption can be estimated by the RPM of the engine. If the bypass pump will be run overnight or 24/7, a fueling program will need instated to ensure that the pump has adequate fuel to operate around the clock.
Automatic floats can be used to turn the pump(s) on and off during varying flow levels. If there is a lot of turbulence in the manhole, submersible transducers can be mounted in the line instead of using floats.
It is important to conduct routine maintenance on pumps. Diesel driven pumps require service to the engine oil and filters every 250 hours of run time. This keeps them operating at optimum efficiencies. Clearances between the impeller and wearplate will also need checked to ensure that the pump is operating properly. Seals, gaskets and o-rings can be evaluated at this time as well. Strainers are often used in bypass operations to eliminate problems with clogging.
If the pump impeller does not receive an adequate flow of liquid, it will start to cavitate. If cavitation occurs or the level rises in the manhole, it indicates a problem with clogging. With either situation, the strainer should be evaluated for blockage. If the strainer is clear of debris, the pump may need to be shut down and evaluated for blockage in the eye of the impeller.
After the bypass pumping project has been completed, it is important to have arrangements made to flush the bypass lines and pumps as transportation of contaminated equipment is illegal.
Tim Cline is manager of construction sales for Gorman-Rupp, headquartered in Mansfield, Ohio.