Author: Ray Hardee, P.E.
Engineers working on any project that involves more than a cursory amount of digging must consider how to manage groundwater. Whether we’re talking about construction site preparation or pipe trenching going down tens of feet, or mining operations going down hundreds of feet, water from runoff or seepage has to be removed from the work area and in many cases must be treated in some way before being discharged back to the surrounding environment. In this article, we’ll explore some of the challenges associated with water removal from work sites.
While dewatering systems are typically relatively simple in structure, they highlight some of the less obvious complexities associated with piping systems. Let’s consider a site with two drainage sumps as shown in Figure 1. There are two pump elements (West Pump and East Pump) providing hydraulic energy to the system, two control elements (SV-W and SV-E) which are used to maintain the quality of the process, and the process elements (tanks and piping) which deliver the process, which in our case is removing water from the work site to a discharge tank in an elevated location (100 feet above the site where the pumps are located).
When we model the case where one pump is operating and discharging to atmosphere above the surface of the receiving tank, we see our selected pump can move 182 gpm running at a pump efficiency of around 44%. This operating point, however, is far away from the BEP and likely to cause significant vibration and lead to reduced pump life.
However, if we add an elbow and a length of pipe to discharge below the surface of the receiving tank, the flow rate increases to 260 gpm and efficiency goes to over 49% (see Figure 2). This will yield benefits in operating costs, and, since the pump is now operating much closer to the Best Efficiency Point (BEP), there will be significantly less vibration and cavitation than what would have been seen with the discharge above the receiving tank. Many dewatering operations overlook the benefit associated with the siphon effect at the discharge and incur higher operational and maintenance costs that significantly outweigh the cost of an additional length of pipe.
Depending on the seepage rate, the dewatering pump may operate continually or may be turned off and on based on the water level in the sump. This represents another control in the system and highlights a key point: control methods range from quite sophisticated to very simple depending on the application. In this case, it may be a simple on-off float switch, or even a manual switch that is driven by the operator following a procedure to visually check and flip the switch when the sump gets to a specified level.
Let’s take a look at what happens when the pump is first turned on using system simulation. Initially the pump must fill up the pipe, which means initially the static head is significantly lower than when the pipe is full. Figure 3 shows the pump’s head and flow as a function of time as the pipe is being filled. Initially the pump is trying to run beyond runout (shaded area) and precise flow rates can’t be predicted, however it is clear the pump is running well above the BEP flow rate and will experience serious vibration and cavitation as the pipe fills. Based on the length and diameter of the pipe in our example, this will take about 120 seconds, but with larger pipes or greater distances this can last for several minutes and can result in greater amounts of damage and reduced pump life.
Control elements need to be employed to prevent this from happening. Again, control elements vary in level of complexity; in this case one viable solution could be a back pressure regulator. Using system modeling software we can determine that a back pressure regulator set at 48 psi will allow the pump to run very close to its best efficiency point no matter how much fluid is in the pipe.
Alternatively, if the start up control method is having an operator start the pump manually when the sump is getting full, the start-up process could include starting with valve SV-W opened a smaller amount, and after a set time manually open it to the fully-charged operating condition. This wouldn’t hold the operating point near the BEP for the entire time, but would reduce the severity of the vibration during the initial filling of the pipe. Using our system model we can determine the best valve position and fill time by evaluating the static head on the pump over time. In our example case, setting the valve at 50% open at start up, then opening the valve to 90% after 90 seconds was determined to be an optimum process involving only one adjustment and minimal operator attention (see Figure 4).
Regardless of the control method, we see that in this system, like in every piping system, the pump, process, and control elements all work interdependently to determine the total system behavior. Instrumentation and controls such as level indicators, float switches, regulators, or manual valves are an integral part of the system, ensuring it operates within the range of acceptable conditions, preventing overflow, spillage, and/or equipment damage.
Lastly, it is common in dewatering situations to have multiple sumps feeding into a common header, and the interaction of multiple pumps is something that must be considered. In our simple model, the addition of the East Pump has only a small negative effect on the performance of the West Pump, pushing the operating point away from the BEP. In systems with multiple sumps, wellpoints, or deep wells, multiple source elevations and pumping into common headers can have significant effects, including reversing flows and pump damage as various pumps are brought on- and off-line. A full system analysis looking at different scenarios will lead the engineer to a greater understanding of the interactions between the pump elements, determine the most critical areas within the process elements, and find the most efficient locations and controls to operate the system in all situations.
As we have seen, even a simple dewatering system has all the elements of more complex piping systems – pump elements, process elements, and control elements – as well as all the same problems large systems experience. Taking steps to mitigate these problems, even simple steps such as extending discharge piping or adding a well placed manual valve, can have a positive effect on uptime and reducing operating costs.