The Case for Snoring

In our last entry on Lift Station Maintenance, we discussed the need for routine testing of redundant control systems; we also covered the ways that the Lift Station Guardian (LSG) automatically audits the condition of the floats and notifies operators when preventative maintenance is needed. Now that we’re starting to catch problems before they occur, the next question is how do we mitigate the impact of failure modes and reduce the costs of operating a lift station?

A source of many headaches in lift station operation is floatable material in the wet well. A surface cap of solids in the wet well can cause problems with level sensors and float switches, create odor issues, and potentially clog pumps. Normal pump operation of a lift station merely makes this mass rise and fall with the well level. What’s worse is that the longer a surface cap is left unattended, the more unmanageable and failure-prone the problem becomes. We’ve discussed ways to detect and mitigate the problems when floatable interfere with float switches in the last entry (and we will discuss similar mitigation strategies for odor and clogging in the future), but ideally the conditions of the wet well are such that operators are not worried about these failure modes happening on a routine basis. In other words, a “healthy” lift station is one that routinely flushes its surface cap to keep the wet well clean.

But how does one “flush” the cap? Fortunately, the most robust solution also requires the least manual intervention, harnesses the equipment that already exists at most lift stations, and (when properly designed into the control structure of the station) will be the simplest for operators to interact with. This is solution is what we call Pump Snoring, a.k.a. draining the wet well to allow the pumps to break up the surface cap and pass the material down the collection system. In this entry, we will discuss how to correctly snore pumps and why it should be as routine for lift stations as teeth cleanings are for people.

How Do You Snore Pumps?

Pump Snoring is very simple: drain the wet well and run the pumps until they break suction, at which point you let them run for enough time to break up the surface material and pass it out of the wet well. You simply need to do this regularly enough such that the cap doesn’t become unmanageable, which varies depending on the lift station usage.

Operators could do this task manually, but keeping up with every lift station and doing it regularly enough to be effective is a laborious task and operators have enough on their plates. We propose that this repetitive and tedious task is best accomplished via automation.

Like with many facets of control design, the devil is in the details. For example, one must check to make sure the pumps at a particular station are capable of being unsubmerged and breaking suction for the duration and frequency of the Pump Snore. In this engineer’s experience, almost all are, but it’s best to check with your supplier. In most cases, the wet well will only need to break suction for less than a minute to thoroughly clear the surface.

Additionally, to reach the suction level of the pump, the wet well needs to drop below the low float. It is a massively common control strategy at lift stations (those controlled by LSG included) to use the low float as the “all pumps off” control indicator. For LSG sites, the Pump Snore function has a bypass method to temporarily override the low float when performing the snore. This bypass was designed in such a way that should anything go wrong in the Pump Snoring process, it will automatically default back to normal control after a preset amount of time (we’ve found 2 minutes to be a sane default).

Finally, you must ask yourself two questions of timing: how long should you snore your pumps and how frequently should you snore? Snoring should only take as long as required to sufficiently clear surface material, and it should happen on a routine basis that prevents an unmanageable cap from building up. This is very dependent on the characteristics of the lift stations and what is realistic for this form of maintenance. Bigger pumps will clear material faster, larger wet wells will have a larger surface area and require more snoring time, and some wet wells just experience a higher load of floatable material from the collection system. When it comes to frequency, some lift stations may only need to snore every other week, and when they do the wet well is, for lack of a better word, spotless afterwords. In other situations, you could be snoring several times a week, and only passing a portion of the surface cap. While this looks like fighting an uphill battle, it decreases the frequency at which manual intervention via a vac-truck is needed to clean out the wet well. The best strategy for operators when setting up such a system is to start snoring for a short period of time (30 seconds is a good starting point) once a week and adjust from there to meet the needs of the lift station. With a few rounds of adjustments, you can find a stable setup for your lift station with very little difficulty and very little risk.

The Results

snore

At one utility where this system was deployed, the cost savings that were previously used for manual intervention via vac-trucks was $26,000 per year per lift station, and 11% of total staff time was freed up with a projection of up to 30% in years to come. Users can remotely adjust the settings of the snore cycle and see how their previous snore cycles performed. In one notable case of tuning the snore system, an operator happened to notice that the wet well at one lift station was saturated with surface material. He was able to take out his phone and initiate a snore via the Specific Energy app and watch the Pump Snore perform its job.

Between Float Testing and Pump Snoring, we are starting to put together a system of layered protection built of preventative maintenance, notification by exception, and problem mitigation. In further entries, we will continue to add to these layers by asking “how can a lift station fail, and how can the control system detect or prevent it?”.