Level sensing for sewage lift stations

As many lift stations are located in very inaccessible places, the overwhelming requirement is for reliability. This requires a clean sensor design with high integrity seals. Due to the nature of the effluent, the sensing element must be exposed to avoid clogging. This problem is also becoming more important due to the increase in fats, oil and grease associated with fast-food restaurants.

The use of ceramic capacitive sensing technology provides a rugged open face sensor while having the ability to achieve high accuracy, better than 0.1%, down to sewage levels of just a few inches of water. The technology also provides a very high overpressure of at least three times the rated range without any degradation of the sensor performance. This protects the sensor against damage due to overflow or back pressures. The housing of sensors such as the PMC VL2000 shown in Figure 2 is laser welded and is generally made from 316L stainless steel, although titanium is often preferred where the effluent is more corrosive.

Figure 2: VL2000 submersible open face level sensor specifically designed for sewage lift stations.

This is a very valuable feature if the sensor has to be removed and is buried in the sludge at the bottom of the tank.

The cable termination is also important, not only to provide connection to the control system and pump, but also to provide an outlet for the breather tube to the atmosphere. This is vital to ensure the correct operation of the sensor, which would otherwise be affected by changes in barometric pressures. However, this reference breather tube must be protected from ingress of moisture. There are many techniques for this, such as the use of desiccant within the termination enclosure to enhance the long-term reliability of the sensor. PMC has developed a sealed Mylar enclosure which requires zero maintenance and does not rely on the use of desiccants or consumables.

Figure 3: SW2000 sink weight (bird cage style) incorporating submersible level sensor.

Because these sensors are relatively light in weight and it is preferred to position them a few centimetres from the tank bottom, it is fairly common to use sink weights. An example of this can be seen in Figure 3. This type of sink weight is sometimes called a ‘bird cage’ and, in the case of the PMC sensor, can be removed from the sensor if necessary. In other cases, the bird cage is integral to the sensor.

Deep groundwater wells

As a spin-off to the custom-designed submersible sensors for sewage level described above, the same technology can be applied to deep wells for the monitoring of groundwater. In this case, the sensor is usually slightly smaller in diameter, about 2 cm, and more likely to be contained within a titanium housing to protect against the possibility of corrosion caused by brackish water. An example of a typical installation in Figure 4. One additional feature of these sensors is a protective nose cone. This not only avoids damage while lowering the sensor into the well, but also protects the sensor from harm due to water hammer, a phenomenon often created in close proximity to a down hole pump.

Summary

The high-integrity well-developed submersible pressure transmitters of today provide very reliable, zero maintenance, level monitoring and pump control for sewage lift stations and deep well monitoring. These hydrostatic level measuring sensors are continually monitoring the sewage level and, with the enhancements in the associated control systems, provide information related to pump performance and the general health of the facility.

Figure 4: Schematic of a deep well installation featuring the depth/level sensor and termination.