Sound Advice

The measurement of level and flow is crucial to effective and efficient waste water treatment. Ultrasonic technology promises that and some more.

When it comes to flow and level measurement in waste water treatment, ultrasonic is the way to go. The single phrase that sums up the benefits of ultrasonic is non-contact. You have a choice in measuring sewage level, by being in contact with it or not being in contact.

Level devices can be continuous like ultrasonic and radar or point-level like floats and hydrostatic. Contact measurement devices like floats, bubbler and hydrostatic systems are vulnerable to clogging and other problems caused by suspended solids, harsh corrosives, grease and silt in the effluent, and need to be cleaned and replaced often. The higher maintenance costs and repairs associated with contacting devices can exceed any initial savings. Moreover, there is also a high element of risk to personnel going down into wet wells for maintenance of contact equipment.

Ultrasonic meters are amenable to quicker installation and configuration. They can also configured on site. The unit acts as an ultrasonic tape measure. Once the empty distance is programmed in, the unit will base all its calculation from this reference. The empty distance could be from the transducer face to the bottom of the well or wherever you want the end point to be. The unit can be commissioned, whether the well has sewage in or not. Hydrostatic devices have to be calibrated in the factory. So if you order them wrong, you are stuck.

As the core purpose of level measurement is one of efficiently controlling a process using pumps and valves, ultrasonic level meters are a smart choice because they can provide the information you need to monitor and control pumps effectively. Most ultrasonic meters come equipped with relays for simple pump control that facilitate, for instance, economy pumping by scheduling pumping for off peak periods to save on energy costs; monitor pump performance including pump status, efficiency and total pumped volume so that failing or under-performing pumps can be repaired, replaced or used less. Where a plant has pumps of varying capacities, one can program the controller to turn the pumps on and off depending on flow. Utilities want to engage the high capacity, high energy-consuming pumps only when needed.

Advanced controllers offer a variety of pump control routines and subroutines such as fixed duty assist and fixed duty backup; alternate duty assist and alternate duty backup; service ratio duty assist and service ratio duty backup. These routines manage pumps efficiently to save energy. This information enables plants to optimise operations with pump control by level rate of change, service ratios to balance pump run times, pump start delays to reduce power surges, pump run-on or valve flushing to clear sediments. The end result is a reduction in scum line build-up and more effective routine maintenance. The range of control options is extensive with relays and current outputs as standard, but digital communications is becoming also more common. Advanced systems also enable monitoring of flow trends. By doing that across the entire network, plants can identify and address current and future problems. For example, one of the big challenges over time is that of specifications to meet future population demand. It is common for pumps to be under or over-specified. Trend flow monitoring identifies increases in demand for future planning and engineering.

So how should one go about selecting ultrasonic level measurement systems? Usually, customers are concerned mainly about accuracy and reliability. Measurement devices today are more or less meeting the user expectations on accuracy and reliability fronts. They are expected to work first time out of the box and still be working a year later in the field. The twist in the tale is their perceived measurement reliability over time.

Measurement Reliability:

The fact of the matter is that the world inside a wet well is anything but ideal with various influences that affect the return signal. There are internal structures such as pump cases, benching, guide rails, ladders, braces and struts. The process will have filling and emptying flows that obstruct parts of the well and cause turbulence or foam on the water surface. Ambient noise can be a negating influence and so can temperature. The challenge is to ensure reliable and accurate level measurement under such real life conditions.

Transducer Placement for Liquid Applications:

An ideal situation would be one where the level measuring product can be mounted clear of all obstructions. However, this is often not possible, and even otherwise, the ideal state could be disturbed once the filling and agitation begins.

An alternative system with built in signal-processing software, will not only automatically learn an installation but continue to adapt its internal map as things change. The intelligence is derived from a set of algorithms that analyses echo profiles (a digital picture of the reflected echo) and learns to ignore obstructions and false echoes.

In sump applications where submersion shield is a standard protection for the sensor face, one can look at the signature of the echo from the air within the shield. This information can be used to turn on additional high-high level alarms.

Turbulence or foam on the water surface can result in echoes being directed everywhere except back to the receiver. In such a situation, many level measuring products quickly revert to “lost echo mode” and fall back to various modes of ‘hold last' reading or simple error output. Sonic intelligence can determine if the surface is agitated or covered in foam by analysing the statistics of the fluctuating signals which manage to return. With the knowledge of how the level was moving before and also which pumps or valves are open, it can continue to track the level even with intermittent measurements.

- David A Hewitt, TM Nagarajan