Lift stations can be difficult and costly to monitor, especially when the stations are located in remote areas.
Despite the challenge, collection system operators need to monitor lift stations to track performance and obtain real-time alarms on a long list of potential problems: pump failures, wet well overflow, energy consumption, vandalism, power failures or exceeding pump temperature or vibration specifications.
An ideal system should provide real-time alarm and flow data to operations, maintenance and collections, 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 engineering and modelers, to help them understand the variables that affect station performance.
Such a system provides time-stamped event data, trend data, station flow history, pump cycle data and historical data on pumping rates and energy efficiency. Site diagnostics are also a key element in an effective lift station monitoring strategy.
With so much valuable data required to keep lift stations in top working order, an economical solution is needed to this 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 historic data on pump activity and performance.
This system shares data with interested parties over a web server or through email and SMS messages, as each recipient prefers. Operators can generate daily, weekly or monthly reports, and share longer blocks of data with third-party software including SCADA or modeling programs.
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| A unit monitoring a lift station |
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 are sent over standard wireless networks to the information management system at the wastewater 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, which informs central operations of a problem 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 a report format or graphical presentation on a designated website. This makes the information easy to access, providing operators with the data needed to make informed decisions about troubleshooting before a malfunction leads to a flooded neighborhood or rural area. Alarm notifications can be forwarded by email or SMS, as the recipient prefers.
This application can be implemented at relatively low cost. Expenses include the price of the RTUs and 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 the frequency and duration of each pump cycle can help engineers see changes in the lift station's operability, 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.
Choose RTUs that can record and time stamp each pump's on/off event with one second resolution. Using this information, operators can compute cumulative run times and cycle times with greater accuracy, revealing issues quickly when conditions change.
Charting pump flow rate on a daily basis
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 pre-set limits
- Digital pulse inputs including rain gauge tip, flowmeter frequency and similar measurements
Installing sensors at the lift station 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 current draw, line pressure, temperature, vibration and rain levels. The RTUs can collect measurement and alarm data from installed programmable logic controllers (PLCs) as well.
Lift Station Flow Algorithm
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 magmeters, or to complex computations using the lift station algorithm to approximate the flow without a flowmeter. Most utilities forego the calculations altogether.
A lift station algorithm programmed into the Telogers Enterprise 4.0 information management system allows users to monitor lift stations effectively and find trouble spots before they become backup 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 will 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. The pump turns on when the water in the sump reaches a set level that the utility determines.
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 pumped with each operation, beyond the physical dimensions of just the sump. As the pump pulls the water out of the sump, we must also track the amount of new water that flowed 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 the station in a garden hose, but leaves the station 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 available data.
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.
The lift station algorithm can determine when an abnormal situation is in progress, revealing 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 is an immediate clue that something is amiss. A change in the pump rate 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, and calculate the flow with a high level of accuracy. This helps utilities with lift stations avoid potential pump breakdowns, averting backup situations before they happen-and replacing costly repairs and clean up with routine maintenance.
Conclusion
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 emerging.
Pumps & Systems, May 2010
