As a systems integrator that services refineries and chemical plants in Louisiana, we have found our customers’ biggest maintenance headaches come in the area of temperature sensor replacement. Rigid temperature sensor technology, used inside thermowells for 20-50 years, is the nightmare of every maintenance department. Problems using rigid sensors include stocking difficulties, finding suitable replacements, ordering the correct length and size, and being unable to install a replacement sensor in an existing thermowell.

Flexible temperature sensors, on the other hand, offer a universal solution for all maintenance dilemmas. A flexible sensor fits nearly everywhere, can be cut to the correct length and reduces the number of spare parts a plant has to keep on hand.

Rigid sensor challenges
A standard rigid temperature sensor, made by virtually every sensor manufacturer in the world, consists of a sensor element—thermocouple (T/C) or Resistance Temperature Device (RTD)—protected inside a rigid stainless steel shaft in a 2” sensitive area, forming what most users know as a “fixed length sensor” (Fig. 1). These fixed length sensors are either spring loaded (for use with thermowells), welded to a hex nipple for a fixed immersion length into a process or sealed with epoxy, exposing the sensor leads for external measurement connections.

Typically, the T/C or RTD element is embedded inside the bottom two inches of a stainless steel tube, which is then filled with mineral insulated powder (MGO) and sealed with epoxy to prevent moisture penetration. The rigid sensor assembly fits into the thermowell beneath the connection head. The wires from the sensor are then terminated in the enclosed head and connected to extension wires using

The first problem posed by rigid sensors is the difficulty involved in replacing a faulty sensor. Typically, a maintenance technician has to remove the enclosure cap, disconnect the wires from the transmitter or terminal block, disassemble the union, conduit and fittings attached to the transmitter and thermowell, and then move them out of the way before he or she can pull the rigid sensor out of the thermowell. Depending on the age of the installation,...(Read whole article)