Computing Flow Through Wastewater Lift Stations
Advanced monitoring solutions can help prevent pump issues
Lift stations—the remote pumping facilities that move wastewater from lower to higher elevations—can be difficult and costly to monitor, especially when they are located in remote areas.
Despite this challenge, collection system operators need to monitor lift stations in order to track performance and obtain real-time alarms on a long list of potential problems: pump failures, wet well overflows, energy consumption, vandalism, power failures or exceeding of pump temperature or vibration, for example.
An ideal system should provide real-time alarm and flow data to operations, maintenance and collection so operators can ensure that the pumps are functioning properly and address issues as soon as they arise. In addition, an effective monitoring system should provide historic trend data to engineers and modelers, to help them understand the variables that affect station performance.
Such a system provides time-stamp event data, trend data, station flow history, pump cycle data and historical data on pumping rates and energy efficiency. Site diagnostics are also key elements in an effective lift station monitoring strategy.
With so much valuable data required to keep lift stations in top working order, it is time for an economical solution to the monitoring problem. Water utilities can indeed obtain a cost-effective remote monitoring system that collects information on station operation and provides real-time alarms and historical data on pump activity and performance.
This system shares data with interested parties over a Web server or through e-mail and SMS messages, as each recipient prefers. Operators can generate daily, weekly or monthly reports and share data with third-party software, including SCADA or modeling programs.
Remote telemetry units collect and send data
This lift station monitoring solution uses remote telemetry units (RTUs), which collect station data automatically at intervals set by the operator. The collected data is sent over standard wireless networks to the information management system at the water utility’s central server.
The best RTUs for this application allow users to choose the communications technology each RTU employs to send the data to the host server: packet-switched cellular, landline telephone, radio or Ethernet. The RTUs should transmit the data either at regular intervals or on detection of a station alarm, thus informing central operations of a problem at the moment the issue begins.
When the data reaches the information management system on the host server, it can be made available to operators in report format or in a graphical presentation on a designated website. This makes the information easy to access, providing the data operators the ability to make informed decisions about troubleshooting before a malfunction leads to a flooded neighborhood or rural area.
This application can be implemented at relatively low cost. Expenses include the price of the RTUs and the installation of dedicated telephone landlines, if required. (The use of cellular phone technology is significantly less expensive.) Choose RTUs that run on long-lasting batteries to limit routine site visits to once a year or less.
Event and trend data
Pump run times are key parameters for water utilities. Knowing how often pumps cycle and the duration of each cycle can help engineers see changes in the lift station’s operability, thus alerting utilities to potential wastewater increases or lift station malfunctions.
The existing SCADA methodology for monitoring pump run times polls the pump’s on/off intervals once per minute or even less frequently, limiting the usefulness of the information it gathers. A one-minute variance at the beginning and end of every cycle can skew the results significantly, misleading operators into a false sense of security about the cycle’s regularity.
Instead, choose RTUs that can record the time stamp of each pump’s on/off event within one second. With this information, operators can compute cumulative run times and cycle times with greater accuracy, bringing issues to light quickly when conditions change.
Other useful measurements
The best RTUs monitor three types of inputs:
• Analog signals, including wet well level, temperature and vibration
• Digital alarm inputs when generators fail or water levels exceed preset limits
• Digital pulse inputs, including rain gauge tip, flowmeter frequency and similar measurements.
Installing sensors at lift stations for these inputs will maximize the usefulness of information gathered by the RTUs. Consider adding sensors for wet well level, pump on/off status, pump use of power current, line pressure, temperature, vibration and rain levels, and even a PLC interface to collect measurement and alarm data from installed programmable logic controllers.
Lift station algorithm provides real information
Water utilities in hilly terrain know the challenges of monitoring their lift stations—especially those with forced mains—for potential overloads and breakdown situations. Flow measurement methods have been restricted to the use of expensive mag motors, or to complex computations using the lift station algorithm to approximate the flow without a flowmeter. Most utilities forego the calculations altogether.
With this in mind, we programmed the lift station algorithm into the new release of the Telogers Enterprise 4.0 information management system, allowing users to monitor their lift stations effectively and find trouble spots before they become flood events. This permits the system to gather information about the volume of water traveling through the sump and the intervals at which the pump turns on and off, and to calculate the flow with a high level of accuracy.
The algorithm works for stations with one or two pumps, but as most lift stations use a two-pump system, we should look at this configuration.
Under normal conditions, the two pumps in a station will alternate their operation: One pump runs first and the other pump runs the next time. Each pump turns on when the water in the sump reaches a set level, determined by the utility.
Remote telemetry units can monitor these pumps under normal operating conditions, sending the data to the central server. Using this data, operators know how long it takes each pump to empty the sump of a standard volume of water. By monitoring the pump activity and knowing the volume of the sump, it is now possible to compute the flow through the lift station.
Here is the tricky part: While a pump is running, additional flow enters the sump, resulting in an increase in volume being pumped with each operation beyond the physical dimensions of the sump. As the pump pulls the water out of the sump, we must also track the amount of new water that flows into the sump while the pump is running. The actual inflow amount is unknown, but we have developed a way to estimate this additional volume in a reliable manner.
Think of this as if water enters a station through a garden hose, but it leaves through a fire hose. We know that the total outflow is equal to the total inflow, but neither rate is known in advance. Thus, both rates must be computed from the data we do have.
Using these parameters—the time it takes for the sump to fill, the time it takes for the pump to empty the sump and the volume of the sump—the typical inflow rate can be determined based on observations of the pump’s behavior over a period of days. Knowing the inflow rate, sump volume and pump run time, the system can determine the outflow, or how many gallons per minute the pump can lift from the sump. This is the pumping rate. Once we know the pumping rate of each pump at the station, we can compute the total flow through the station under any condition—even if both pumps run simultaneously.
The lift station algorithm can determine when an abnormal situation is occurring, bringing to light any problems with the pump while it is working in a stress situation. For example, the pumps in a lift station are usually identical, so if one is running more frequently than the other, it's an immediate clue that something is amiss. If the pumping rate changes, that is a signal that a pump may need service.
Through the RTUs, the information management system can gather data about the volume of water traveling through the sump and the intervals at which the pump turns on and off to calculate the flow with a high level of accuracy. This helps utilities with lift stations avoid potential pump breakdowns, averting overflow situations before they happen and replacing costly repairs and cleanup with routine maintenance.
Using cost-effective tools and the advanced technology provided by today’s information management systems, water utilities can save time and money by monitoring lift stations more regularly. The information gathered from lift stations on a second-by-second basis can change the way municipalities maintain these stations, trading costly emergency repairs for predictable maintenance cycles and problem-solving within minutes of an issue coming to light.