Scott Driscoll is a product development engineer at Sentrinsic, Inc., in Atlanta, Ga. He has an M.S.M.E. degree from Georgia Tech in controls. Driscoll can be reached at email@example.com.
When considering level sensors, end users should evaluate the whole system cost because many sensors require an additional solenoid and, in some cases, a separate programmable logic controller (PLC). Installation requirements for all the separate components should also be taken into account. Unfortunately, since many level sensing solutions are in contact with the pumping liquid, they suffer from corrosion or clogging. Even non-contact sensors can be blocked by debris or dislodged.
Figure 1. Air monitoring controllers monitor the pump through the air line without contacting the pumped fluid.
Air Monitoring Controllers
Another category of controllers that avoids the problems of level detection devices is air monitoring controllers. Air monitoring controllers work similarly to load-monitoring controllers on electric motors, turning the pump off when the air input indicates that the pump is running dry.
Some controllers in this category rely only on mechanical switches, while others use electronic controls. The mechanical controllers typically detect a change in pressure or flow to determine when a pump is running dry. Because a dry-running pump faces little to no resistance, it operates faster and uses more air than when transferring liquid.
While mechanical controllers have the benefit of operating without power, they can be unreliable when plant conditions change, such as the input or pump discharge pressure.
Air monitoring controllers with electronics can employ more sophisticated logic to compensate for changing environments, and also better detect when a pump is truly running dry.
Some of these solutions use pressure sensors to listen to the pump like a stethoscope to detect the individual stroke rate, which typically changes much more than the air input rate.
Ignoring short-term changes (false positives), restarting on timers and interfacing with PLCs are other benefits.
Image 1. AODD pump in a factory transfer application with an air monitoring controller.
Dry-running AODD pumps are a costly problem that can easily go unnoticed, wasting significant energy, money and maintenance resources. Since many people are unaware of the high cost, dry-running pumps are a common occurrence in many factories. Fortunately, many solutions are available that provide a fast return on investment, ranging from simple maintenance procedural changes to controllers that automatically turn off pumps when they are not needed.