WASTE SYSTEMS
only to be robust, but also easy to fit, test, and maintain, without the need for specialist apparatus, or operators with a PhD in computer science.
The majority of practical systems work by detecting water/sewage levels that are outside normal parameters. Float switches have been used, but with limited success, as FOG (fats, oils, and grease), rags, and congealed pulped cardboard from maceration, can lead them to get stuck in the ‘down’ position, and hence fail to report. In addition, float switches can be difficult to set up, and require a certain amount of free travel to operate; otherwise they can easily get snagged or hit the underside of the cover before they trigger.
Dual redundant BDT-based blockage detection
Where we have had a great deal of success is with what we call the bulk dielectric principle. This is where we use a transducer that automatically measures the change in the electrical permittivity of its surrounding media as it transitions from air to water, a technology that we abbreviated to BDT (Bulk Dielectric Transducer), and which, when allied with a radio transmitter head, becomes a very powerful and effective first line of defence in the battle against blockages. The BDT principle also has the very useful trait, since, unlike a float switch, it will ‘fail safe’ (false positive) if covered in rags or sodden debris.
Blocked stack detection Detecting blockages in stacks is much more of an art, and does require a degree of mechanical intervention. Experience
A typical BMS gateway with air gapped relay contacts.
has shown that ultrasonics can be used, but they are difficult to set up, can generate false alarms with scale build-up, and are very difficult to verify/test without deliberately blocking the stack. Further, they need to be coupled using an acoustic grease, and held very securely in place, as the slightest airgap or misalignment can cause the system to fail, ‘false negative’.
Three methods proven to work In practice there are three methods that have been proven to work, each with its own merits. These include static head pressure, conductivity, and air back pressure.
Air hose and nipple for pressure monitoring system Static head pressure is best measured using an atmospherically compensated pressure transducer that is attached to the stack or a lateral via a sealed air nipple. As soon as level backs up the stack beyond the connection point, the alarm message is generated. Typically, this results in a minimum detection head requirement of circa 1 ft (30 cm) above the point of connection of the detector. Reliable detection does require the entire assembly to be both fully air and watertight, plus freely vented to atmosphere, and not reliant on an air admittance valve. Regular inspection is recommended, as is the use of clear air hoses. The latter permits scale/debris build-up, which could prevent detection.
Stack monitoring system using an air pump
A stack monitoring system utilising an air pump.
42 Health Estate Journal April 2020
The back-pressure technique is more active, and involves a small air pump proactively forcing air into the pipe via a length of smallbore hose, typically 1/4 inch in diameter (6 mm). This technique can be highly effective in awkward
locations where space is limited, and is probably the only viable solution that can cater for small waste pipes of below 2 inches (50 mm), in particular those often associated with showers, urinals, and sinks. As with the pressure transducer system, the pipes need to be freely vented to atmosphere. In practice the minimum head for detection is 2 ft (30-60 cm). However, this can be compensated for by feeding the hose down the pipe/stack to a lower level, enabling much earlier detection to be made.
Conductivity-based blocked stack detection system The third of three aforementioned solutions is conductivity. Numerous
A conductivity-based blocked stack detection system installed in a hot stack.
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