Mention the word NAMUR to a valve specialist and the first thing this person will think about are the interfaces used to connect solenoids to actuators and switch boxes or positioners to rotary pneumatic actuators.Mention NAMUR to a controls person and the first thing that will come to mind are intrinsic safety techniques.
Both are correct. A typical automated valve assembly for on/off service consists of a valve (rotary or linear travel), an actuator, position sensors, and a solenoid. An automated valve for control service adds a valve positioner but subtracts the solenoid.
The selection of the components needed in the assembly is dependent upon the service application and the area (environment) where the assembly will be installed.
Such areas, known in the process industry as Divisions or Zones, may be simplified to three categories:
- Occasionally hazardous
- Continuously hazardous
By hazardous we are referring to the risk of an explosion
occurring where an ignition-capable mixture of air with gas, vapors, or dust exists. The energy to ignite this mixture can come from different sources, including electrical or mechanically generated sparks, thermal effect, static electricity, electromagnetic radiation, or even lightning.
Electrical devices used in hazardous areas must not be capable of initiating an explosion or else be encased in an approved heavy-walled enclosure that will contain an explosion should one be caused. A device that is intrinsically safe cannot create an explosion; therefore, it prevents an explosion from occurring.
Prevention is always the preferred choice over containment when selecting hazardous location devices.
A NAMUR-compliant sensor is intrinsically safe, prevents an explosion from occurring, and can operate continuously in a hazardous location.
A NAMUR sensor is paired with a barrier that limits the electrical energy available to the sensor to non-ignition levels. The barrier provides enough power for the sensor to properly operate but not enough to cause ignition via heat generation or by the creation of a spark above safe threshold levels. When the sensor detects the valve position, the barrier amplifies the output signal and then initiates either a relay closure or an electronic signal. This output is typically fed into a PLC or similar control/supervisory system.
By using NAMUR sensors to detect valve position for process valves in hazardous locations you benefit in two ways—proven explosion prevention and the elimination of a costly unnecessary enclosure.