Published on: Jun 12, 2017
Installing a temperature sensor in a hazardous location requires more than an approved explosion proof connection head. To meet the safety requirements of FM, you must consider the complete system: including the sensor, the thermowell (if required and used) and the connection head.
Each component in the system must meet the code. For example, many thermowells are supplied with 1/2-14 NPSM threads for the sensor connection. FM dictates that to meet the requirements for explosion proof, a tapered thread is required. Most sensors are supplied with a 1/2-14 NPT thread; however, FM also specifies a minimum thread engagement between the sensor and the connection head. The thread engagement requirement makes it necessary to overcut the thermowell thread beyond the ASME definition of 1/2-14 NPT.Those manufacturers wishing to offer explosion proof assemblies must meet FM’s requirements. Once a design is constructed with the proper thread size and engagement between the head cover, head body, wire conduit and seal, sensor, lag fittings and thermowell, the manufacturer submits the design to FM for testing to ensure that the specified gas mixtures are properly contained. Once approved by FM, the manufacturer is not permitted to alter any of the approved configurations without FM approval. If changes are necessary, FM may require additional testing before approving any changes. In addition, FM performs periodic unannounced inspections of the manufacturer’s plant to ensure that the assemblies are constructed in accordance with the approved drawings.
With explosion proof assemblies, it is truly a case of ‘you get what you pay for’. Using unapproved sensors or thermowells with an approved, explosion proof connection head, for example, will not provide the protection you need for your plant and personel. Failure to consider the entire system, or failure to utilize the knowledge and resources of recognized experts in this critical area, is at best risky, and at worst dangerous and irresponsible.