Sensing a Solution
Hydrostatic pressure sensors offer dependable level measurement
The treatment of complex industrial wastewater systems laden with items, such as ferric chloride, ferric sulfate, hydraulic oil, animal fats and antibiotics, involves brine treatment and removal of solids and oils, as well as biodegradable organics. During waste processing, liquid level measurements are needed at raw and finished water tanks to quantify liquid volume and overflow controls to prevent pollution. The repeatability and stability of the readings are very important in order to ensure accurate measurements for closed-loop control systems.
The Value of Level Sensors
Level sensors are used extensively for the processing of wastewater in industrial wastewater treatment. Due to the size of tanks and floatation of oils and waste, hydrostatic pressure sensors (also called submersible pressure sensors) with venting to atmosphere are widely used to indicate the water level.
In these applications, the pressure sensor is dropped to the bottom of the tank and used in the differential pressure (dP) mode known as hydrostatic measurement with respect to atmosphere. Fully submerged in the tank, the dP sensor measures wet media at the bottom of the tank and the “dry” air from the atmosphere (outside the tank via a vent tube embedded in the power and signal cable), providing a linear output as a function of liquid height.
Hydrostatic sensors offer an alternative to radar and ultrasonics in a wide range of liquids and environments such as temperature, dust and icing. Radars and ultrasonic sensors do not work reliably in shallow plastic chemical tanks and require modifications to be made to the narrow top opening (typically 2 in.) of the shallow tanks, which compromises the integrity of the tank. Hydrostatic sensors not only are more reliable, but they do not require modifications.
All-Metallic Submersible Level Sensor
Depending on the depth of the tank, submersible level sensors can measure from a few feet to hundreds of feet of water level and provide a continuous linear output proportional to the depth. Figure 1 shows a typical pressure sensor sitting at the bottom of the tank measuring the height of the water column. The cable, including the copper wires and plastic tube, provides the power and venting needed for the pressure sensor to function properly. Without venting, however, the sensor would be subjected to barometric changes and temperature gradients within the tank, leading to inaccuracies in depth measurements. While this type of sensor can be used in freshwater and diluted brine tanks, it cannot be used in ferric chloride and chlorine-rich water because it will corrode due to severe pitting.
The pressure-sensing element, made from solid Teflon (polytetrafluoroethylene), moves in a linear manner to applied pressure on the diaphragm. The diaphragm is carefully designed and fused to the Kynar (polyvinylidene difluoride) body via a proprietary selective bonding method. The signal conditioning electronics, housed inside the sensor, amplify the signal to provide voltage—such as 1-5 VDC, or a 4-20 mA current output signal—that is linear and proportional to the applied pressure as a function of depth. The electronics are protected from reverse polarity, electromagnetic interference, electrostatic discharge and fast electrical transients. To ensure full sealing against water intrusion, the electronics are encapsulated in corrosion-resistant epoxy resin.
Finally, the sensor is tested and calibrated to provide a thermally stable output from 32°F to 130°F against ambient temperature conditions. The media temperature can range from -4°F to 185°F.
Non-Metallic Submersible Level Sensor
When injected into the water tanks, some chemicals can attack metals and cause failure due to severe corrosion. While the overall pH level can be well balanced, the initial injection of chemicals can cause the sensor to be exposed to an acidic or basic shock, depending on the location of the sensor relative to the chemical additive. To overcome this, there is a strong need to use submersible sensors constructed from chemical resistance plastics such as polyvinylidene difluoride and polytetrafluoroethylene. These two polymers are well known for their chemical resistance properties and are widely used in the chemical industry.
A water treatment plant in Arizona found hydrostatic sensors to be more reliable and cost-effective than the radar and ultrasonic measurement systems originally used in an application involving the injection of chemicals in the treatment plant. The chemicals, stored in a chemical tote, are corrosive in nature and attach to metals. The plant initially used a costly non-contact radar level measurement system that provided unstable readings. It was replaced by an ultrasonic level detection system that required the top tank opening to be modified to position the sensor. After modifications, the sensor only worked for a few days before chemical fumes attached to the sensor sealing to cause unit failure.
Ultimately, a Kynar-Teflon level sensor utilizing a hydrostatic measurement principle was implemented and has been operating reliably after one year of use. The plant is now standardizing this technology for all wastewater and oil field injection chemical tanks.