Design Considerations for Temperature Transmitter Installations
- keith grimes
- Jul 9
- 2 min read
Temperature transmitters and local temperature indicators are used throughout various industries to monitor temperatures in pipes, pressure vessels, and storage tanks. A temperature instrument assembly consists of a thermowell that is inserted into the process, protecting the temperature element, the temperature element itself, and a local indicator or transmitter for remote monitoring.
Temperature elements are removed and replaced through their mounting flange or threaded branch fitting. The length of the temperature element must be pulled out of the thermowell, requiring a "pull clearance" equal to the length of the temperature element opposite the thermowell.
When laying out piping near temperature transmitters, ensure sufficient clearance is left to remove the temperature element.
Temperature instruments should have sufficient clearance around them to be removed without requiring the dismantling of piping or equipment.
The length of the temperature element varies based on the diameter of the pipe, vessel, or tank to which it is affixed. Always check with the instrument engineer regarding the length of the temperature element, also known as the "U" dimension.
Thermowells should be installed on piping systems at 90-degree elbow fittings pointed into the flow for more accurate temperature measurements. When installing thermowells directly on piping, the nominal pipe size should be 4" minimum. Use reducers to increase small-bore pipe to 4" NPS.
In the picture above, the designer should have skewed the orientation of the temperature transmitter nozzles on the pressure vessels away from the centerline of the pipe. The piping riser connected to the top nozzle of the pressure vessel had the same centerline orientation as the transmitter, and there was no clearance to remove the temperature element without dismantling the piping. This layout was identical on all five of the vessels. Five piping spools of varying complexity had to be modified in the field, resulting in schedule delays of several weeks and change orders totaling several thousand dollars.
Pro Tip: Piping designers, place a volume inside your 3D model that represents the required pull clearance of the instrument. You can then see the clash with the piping and pull clearance in 3D model reviews or automatically identify it during clash detection.
