How to select, apply process sensors
When specifying process sensors, several common factors need to be considered to avoid installation and application problems during implementation. These factors include its intended use, the operating environment, mounting options, installation, calibration, start-up and commissioning, operation and maintenance. All of these factors should be considered during design and will have a material effect on the final form and function of the sensor and connected or controlled equipment. Doing a thorough evaluation of these factors will avoid rework or replacing the sensor in the field, which also will introduce additional cost and possible delays.
Process sensor environments
Operating environments widely vary. For example, in water treatment applications, environments are usually wet, dirty, corrosive and hazardous. Industrial environments offer some of the same environments, but also introduce materials such as metal dust and chips or fibrous flyings that can foul or damage the sensor. The goal is providing a suitable, safe enclosure that will operate in harsh environments without creating a hazard in and of itself.
A case in point is installations in corrosive or hazardous locations. In the former case, the sensor housing must be designed to withstand the action of corrosive gases or liquids; in the latter, it is matter of preventing the materials from entering and compromising the sensor.
Sensor housings are often rated using the NEMA enclosure rating system or the Ingress Protection (IP) rating system. NEMA 4X and NEMA 7-10 are used for corrosion-resistant and explosion-proof housing, respectively. There is some correlation between the two rating systems, there is a wealth of information online regarding both.
In considering the environmental factors, using intrinsically-safe sensors and systems in hazardous locations should be considered whenever possible. Intrinsically-safe sensors use low current and voltage to limit the ability of arcing and sparking to ignite flammable materials. Intrinsically safe barriers can also be used to limit current and voltage to safe levels.
Sensor mounting options
There are many mounting options, and they will normally conform to standard interface methods to the process. Open tank process monitoring requires no explanation other than the sensor location should be carefully selected to allow optimum monitoring of the required parameter.
The sensor also should be mounted in a way to allow regular inspection, maintenance and calibration regardless of the parameter being measured. Non-standard mountings or mountings requiring special or proprietary tools are an invitation to neglect and will affect process measurement and control.
Most sensors do offer standard attachment options, which will easily integrate into process piping, tanks or vessels. It is important to ensure the sensor is mounted in an easily accessible location so staff can perform regular maintenance and avoid process upsets from a bad data.
One often overlooked issue is the method of cable attachment. In harsh or hazardous environments, a potted cable, which is connected permanently into the sensor body either mechanically or with a potting compound such as an epoxy or filler. This prevents the ingress of dirt or hazardous materials, which may damage the sensor or create arcing and sparking.
Plugs and jacks also can be used to connect cables to a sensor, but this requires a clean environment, such as in a laboratory. The use of this method allows an easy swap-out of the sensor in case of failure. In the former case, the entire sensor and cable assembly must be replaced, which may require extensive rewiring.
Daniel E. Capano is senior project manager, Gannett Fleming Engineers and Architects, and on the Control Engineering Editorial Advisory Board. Edited by Chris Vavra, associate editor, Control Engineering, CFE Media and Technology, firstname.lastname@example.org.
KEYWORDS: process control, process sensors
Process sensor operating environments widely vary and need to be considered.
A thorough evaluation will help engineers avoid costly delays.
Cable attachments are often overlooked with process sensors.
What is your biggest consideration when choosing a process sensor?