An Introduction to Comparative Measurements

This article presents the new non-invasive temperature sensor from ABB for non-invasive temperature measurement. It also gives examples for applications and accuracies that can be achieved.

The TSP341-N surface-mounted temperature sensor enables reliable and very precise temperature measurements without needing to intervene in the process by taking into account the ambient conditions.

This also means that the system safety is significantly increased. Due to quick and easy surface mounting and by eliminating the thermowell and the need to open the process, costs can be substantially minimized.

Furthermore, the sensor can also be retrofitted at any time or even temporarily installed for additional measurements and this increases the system flexibility.

Temperature sensor for non-invasive temperature sensor

Classic Temperature Measurement in the Measuring Medium

Almost no chemical process can do without temperature measurement. Mostly the purpose of this is to ensure system safety, increase process efficiency and secure product quality.

Classically, temperature measurement in process technology is undertaken by inserting a temperature sensor with a thermowell directly into a medium, which is commonly located in pipping or a vessel. Measuring media are comprised of substances in gaseous, liquid or pasty state and, often, are a combination of these states. Additionally, they can contain solid particles. A measuring medium can be abrasive and also chemically aggressive. It may flow at high speed or be at a standstill.

The situation outlined in the classical temperature measurement setup demonstrates that contact with the measuring medium can cause a broad range of problems. These need to be mastered in order to create a reliable and, most importantly, safe system operation and this is costly [1].

Costs are incurred as early as the planning and design of a system for openings in vessels and piping, through which the temperature sensor is introduced into the measuring medium. At this stage, flanges in addition to structural reinforcements are needed and these must satisfy incredibly strict safety requirements.

Classic installation of temperature sensors in piping.

Robust thermowells for demanding applications.

Thermowells need to be designed for the characteristics of the selected medium in order to protect the temperature sensor from mechanical and chemical stress. A special challenge is created by abrasive dust or sand, which move through the piping at high speed. These thermowells must be inspected regularly and replaced when necessary because chemically aggressive and abrasive media can cause critical removal of the thermowell material. Special thermowells create additional high costs. [1]

At least a partial standstill of the system, and usually also a complete emptying of the system, is required for the inspection and, when necessary, exchange of thermowells. This is also the case if additional measuring points need to be installed. The use of thermowells in a pipeline can also increase the cleaning cost, for example if regular cleaning using so-called “pigs” is not feasible.

Classic Temperature Measurement in the Measuring Medium

As well as increased costs, aspects of safety must also be considered. A thermowell that is placed in flowing media can start to vibrate because of vortex formation and, in extreme cases it can break. This can have major consequences for the system and also for the entire environment.

A serious accident occurred in 1995 at the Monju Nuclear Power Plant, the only ‘fast breeder’ nuclear power plant in Japan. It happened after vortex formation caused a thermowell to fracture and the large quantities of leaked sodium led to excessive heat generation after a chemical reaction and this melted steel parts of the system.

Vortex formation in the area of a thermowell in the flowing measuring medium.

As a result, standards and norms, for example ASME PTC 19.3 TW 2010 and TW2016, for the stability of thermowells have become increasingly restrictive over time. This in turn has increased the costs of maintenance and exchange.

If the process temperature could be measured in a non-invasive and reliable manner outside the process, these safety issues and cost factors can be eliminated.

The new surface-mounted temperature sensor from ABB allows, in many cases, for process temperatures to be determined with sufficient repeatability and accuracy for the specific application.

References

1. “Industrial temperature measurement, basics and practice”. ABB Automation Products GmbH (2013)

This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.

For more information on this source, please visit ABB Measurement & Analytics.