Chapter 6 STEP System Force Main Velocity Evaluation

Transcription

1 Chapter 6 STEP System Force Main Velocity Evaluation \\ \TOC.doc

2 CHAPTER 6 STEP System Force Main Velocity Evaluation 6.1 INTRODUCTION The City s existing STEP (septic tank effluent pump) pumps currently discharge relatively solids-free wastewater into the STEP system pipe network. Household septic tanks allow settleable solids such as grit, rocks and organic matter to settle out, forming a layer of sludge in the bottom of the tank, which must be periodically removed. Due to the high costs associated with pumping out existing septic tanks and lift station debris tanks in areas that are served by STEP systems, the City is interested in investigating the feasibility of replacing the existing STEP pumps with grinder pumps. Replacing the existing STEP pumps would theoretically eliminate the continuing need to pump out each existing septic tank. However, wastewater discharged into the STEP force main system using grinder pumps would contain a much higher solids content than wastewater discharged from the existing STEP pumps. Introducing high solids wastewater into the STEP system pipe network will increase the potential for unwanted sediment deposition to occur in pipe segments with low flow velocities. Sediment deposition would be most prevalent in the larger diameter STEP piping where velocities are below scour potential, at locations where STEP force mains were constructed with a positive gradient and at low points, or sags in the system. Flow velocities were analyzed for STEP force main piping 6- through 16-inches in diameter to identify locations where sediment deposition is likely to occur. The existing STEP system is discussed in more detail in Chapter 5 and the operation and maintenance elements of the STEP system are presented in Chapter STEP SYSTEM CONSTRUCTION Construction drawings of the STEP force main system were reviewed. The drawings show that the STEP piping was installed in a manner similar to traditional force main piping. Pipe material is typically of PVC construction and was generally installed at a depth of 3-feet, following the contours of the existing ground or roadway. Many of the force mains are quite long and were constructed with thrust blocking at horizontal bends and air release valves installed at the high points. The 3-foot burial depth resulted in force main profiles that show a series of high points and low points as it follows the ground contours. 6-1

3 The system was constructed as a pressure system. However, some of the downhill gradient pipes, particularly where there are long sections of downhill gradient, flow in an open channel (non-pressurized) condition. 6.3 STEP FORCE MAIN VELOCITY EVALUATION For the purposes of this evaluation and because any change out of STEP pumps to grinder pumps, if it occurs at all, would take place over a period of time looking forward, STEP system flows were calculated using 2010 population values. Due to the likelihood that velocities in the large diameter STEP force mains may be quite low, inflow and infiltration estimates were included to provide as realistic an estimate as possible. Velocity values were determined for average daily flow (ADF) conditions and for peak hour flow (PHF) conditions. Commercial and residential populations were obtained from the hydraulic model s year 2010 database for those service areas that are tributary to the large diameter STEP force mains. Average daily flows were calculated in a spreadsheet model by summing residential flows, commercial flows and I/I contributions as follows: ADF = (Residential population x 67 gpcd) + (Commercial population x 31 gped) + (service area acreage x 500 gpad) Peak hour flows were then determined by peaking the sewage flows by a factor of 2.5 and adding the I/I contribution as follows: PHF = 2.5 x [(Residential population x 67 gpcd) + (Commercial population x 31 gped)] + (service area acreage x 500 gpad) The flow calculations described above are consistent with the wastewater characterization described in Chapter 4 and with the hydraulic model s flow calculations described in Chapter 7. The extent of the STEP force main system evaluated is shown in Figure 6-1. Wastewater enters the large diameter STEP force mains in several ways: entering as discharge from individual STEP connections, as discharge from STEP neighborhoods and as discharge from the high capacity STEP/submersible lift stations. Flow rates within the force mains can vary substantially and are affected by the complex interaction of individual STEP pumps and the cycling on and off of the lift stations. Force main velocities under ADF and PHF conditions were calculated for two scenarios. The first scenario assumed that flow rates were defined solely by the peak hour flow calculations shown above. The effect of pumped flows discharged from lift stations was ignored. The second scenario assumed the operation of the lift stations was taken into account. This scenario assumed a condition where all of the lift stations were operating in unison at design capacity. 6-2

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5 Although the second scenario accounts for the operation of lift stations, it is not likely that all lift stations and all individual STEP pumps will be in operation at the same time. However as it turned out, the first scenario developed the highest calculated force main velocities. Since one of the objectives of this evaluation is to determine if the additional solids may cause problems in the system in the form of excessive settleable solids, it was desirable to establish theoretical peak velocities that would occur in the force mains. Peak velocity conditions determine whether settled solids can be re-suspended or if permanent settling of the solids is likely to occur. Permanent settling of solids suggests that mechanical removal of the solids may be required. As a result of this objective, flow calculations are based on the first scenario, which produces the higher theoretical velocities. An assumption was made that if the highest theoretical velocities are not sufficient to re-suspend and transport solids; then the actual velocities, which are lower, are even less likely to do so. The peak hour and average daily flow velocities based on the first scenario are shown on Figure 6-1. It is evident that force main velocities, even under peak hour conditions remain quite low, generally below scour velocities. The force main system was constructed with several long sections of pipe installed with an upward gradient. In order to transport solids that have settled out, the frictional force provided by velocity must be sufficient to both re-suspend the particles and to overcome gravitational forces as they are being pushed up the invert of the inclined pipe. It is often not possible to maintain constant scouring velocities in sewers since minimum flows often approach zero. Nonetheless, sewer conduits, whether they operate under open channel or pressure flow conditions should be self-cleansing frequently enough to resuspend solids that have settled out during low flow periods. A velocity of 2 fps is traditionally accepted as the minimum velocity required to keep settleable solids in suspension. This can vary depending on the size and density of the particles. The forces necessary to re-suspend settled particles and transport them up an inclined pipe were not determined; however it is likely that velocities well above 2 fps would be required. Figure 6-1 shows that peak hour velocities range between 0.5 fps and 4.1 fps. The flow rates required to produce a velocity of 2 fps in various pipe diameters similar to the STEP force mains are shown in Table 6.1. Table 6.1 Flow Rates Required to Produce a Velocity of 2 feet per Second (Full Pipe Flow) Diameter (in.) Required Flow Rate (gpm) ,

6 The Washington State Department of Ecology s Criteria for Sewage Works Design (often referred to as the Orange Book) does not define minimum velocity criteria for STEP pipelines; however pig ports must be installed at the end of each line and at critical line size changes. Lacey s STEP system meets these criteria. The criteria also states that the minimum velocities for grinder pump pipelines shall be 2 fps. The Lacey system does not appear to meet this criterion. The maximum allowed velocity in gravity and pressure sewers is not defined as each situation is unique. However, it can be argued that the optimal range of velocities in sewers is between 2 fps and 10 fps. Below this range, solids begin to settle out and when velocities over this range are present, problems such as high power requirements and pipeline degradation often begin to occur. Table 6.2 shows the flow rates that produce velocities between 2 fps and 10 fps in pipes sized 6- though 16-inches in diameter. Since nearly all of the collection system is pumped through one or more lift stations, each with a known pumping rate, City staff can use this information to identify pipelines with low velocities that have a high potential for sediment deposition. 6-5

7 Table 6.2 Flow Rates that Produce Velocities Between 2 and 10 feet per Second (Full Pipe Flow) Diameter (inches) gpm Velocity (feet per second) CONCLUSION In summary, converting the existing STEP pumps to grinder pumps in the larger lift stations will add an increased solids/sediment load into the force main system. This condition would be exacerbated if the existing debris tanks currently installed upstream of the inlet to some of the lift stations are removed from service. Conversion to grinder pumps would convert the force main system into operating as grinder pump pipelines. Average day flow velocities in the large diameter STEP force mains are all below 2 fps and are generally not high enough to keep sediments in suspension. Peak hour flows in portions of the force main system do exceed 2 fps; however this occurs infrequently and may not be sufficient to re-suspend sediments throughout the system. This does not necessarily preclude converting to grinder pumps; although the likely outcome of doing so will be higher maintenance costs. More frequent pipe cleaning in the form of pigging or high-velocity flushing may be required. 6-6