Lift stations are an essential component to municipal water and wastewater systems. Without them, many urban centers throughout the world would simply not exist. So keeping them in operation and performing properly is absolutely vital.
Lift stations that aren’t performing properly could spell disaster, especially if an overflow occurs. In addition to the unsanitary conditions and property damage an overflow creates, there is a hefty fine imposed by the EPA (think tens of thousands of dollars per day). Therefore, monitoring the water level in the station is critical. Level sensors can be used to trip alarms or turn on/off pumps automatically if an alarm condition exists.
There are a variety of instruments available to use for lift station level monitoring. The ultrasonic level sensor is a great choice since it doesn’t come in contact with the liquid and is easy to maintain. It also costs less than other non-contact level sensors and is simple to install.
However, like any level sensing technology, there are conditions to look out for and precautions that you should take to ensure that the sensor performs optimally. Fortunately, ultrasonic sensors can be adjusted to overcome many of these conditions.
So here are 5 tips to using an ultrasonic level sensor in a lift station:
1. Give yourself plenty of sensing range
This tip actually goes for all ultrasonic sensor applications but it is still important to mention here.
Ultrasonic level sensors come in a variety of sensing ranges. For example, here at APG, we have 3 main ranges we offer: 4 – 79 inches, 1 – 25 feet, and 1.5 to 50 feet. The rule of thumb is to get a model with at least twice as much sensing range as the distance to the target. So if the bottom of a lift station is about 25 feet from where you are mounting the sensor, we would recommend that you purchase a 1.5 to 50 ft. ultrasonic level sensor.
We make this recommendation because this increases the likelihood of the unit sensing a return echo. The larger the range an ultrasonic sensor has, the larger the transducer. The larger the transducer, the more energy is being emitted from the unit. The more energy emitted in the first wave, the stronger the returning wave.
Having a stronger return signal in a lift station is especially important. Quite often, lift stations have surface turbulence, foam, and other conditions that may hamper a sound wave. So you can never have too much firepower.
2. Watch out for foam
A small presence of foam is not a big deal. However, if a particular station has a lot of foam (a complete layer of foam a few inches thick) we do not recommend an ultrasonic sensor.
As you may already know, ultrasonic sensors rely on sound waves echoing off of their targets. Sound echoes best off of hard smooth targets (think of yelling in a rock canyon). Foam is not hard, nor is it very smooth. Foam will absorb a sound wave rather than reflect it (don’t try yelling at foam to get an echo, it won’t work). In applications with foam, ultrasonic sensors will often have erratic readings or lose an echo completely and go to a zero or full-scale output.
Again, if a lift station is located next to a laundromat or has a lot of foam, please consider a different level sensor for that particular station.
3. Watch out for pipes and other obstructions
A great example of a hard target is a pipe or other metal fixture you might find in a lift station. However, you probably don’t want your sensor detecting pipes for you. To avoid such a tragedy (we call them false readings), make sure the sensor has a clear line of sight down to the level of the water.
Be aware that sound waves spread out as they travel away from the sensor face. So your line of sight isn’t really a line; it is a cone. To help you determine if an object falls inside of this area, the manufacturer of the ultrasonic sensor typically provides you with a beam angle spec. With this figure and a little bit of math you will be able to determine the area of the beam based on the distance travelled from the sensor face. If an object falls within this area, you should consider moving the mounting location of the sensor.
If the pipe or fixture is near the edge of your sensing area, you may also be able to trim the sensitivity of the sensor just enough to effectively block it out. However, lowering the sensitivity too much could cause a loss of readings.
4. Consider using a stilling well
As mentioned before, lift stations often have surface turbulence. With water constantly flowing in, and pumps turning on and off, there is almost always some wave action in a lift station.
Typically, a little bit of surface turbulence won’t be a big deal for well-built ultrasonic sensors. However, it is good practice to install a stilling well if surface turbulence is present. It is better to be safe than sorry.
You can learn more about stilling wells and how to install them by clicking here.
5. Have a backup plan
This last tip isn’t just for ultrasonic sensors. Having a backup level system or alarm to go with any primary level sensor is absolutely necessary and is often a requirement.
For most lift station applications, we typically recommend using an ultrasonic sensor and then a float switch for an emergency shut off or high level alarm. It is also not uncommon for an operator to install at least two continuous level sensors and then a point level switch for a high alarm (i.e. ultrasonic sensor, submersible pressure sensor, and float switch).
It may seem redundant but it is these redundancies that might just save you from a disaster.
Please feel free to contact us if you have any questions on lift station level monitoring or other level sensing applications. We are here to help you.