Monitoring of transient thermal stresses in pressure components of steam boilers using an innovative technique for measuring the fluid temperature Magdalena Jaremkiewicz a, * , Piotr Dzierwa a , Dawid Taler b , Jan Taler a a Institute of Thermal Power Engineering, Cracow University of Technology, Al. Jana Pawla II 37, 31-864 Cracow, Poland b Department of Thermal Processes, Air Protection, and Waste Utilization, Cracow University of Technology, ul. Warszawska 24, 31-155 Cracow, Poland article info Article history: Received 31 October 2018 Received in revised form 21 December 2018 Accepted 8 March 2019 Available online 9 March 2019 Keywords: Fluid temperature measurement New design thermometer Dynamic error Steam superheater Steam transient temperature abstract High thermal loads of the thick-walled components arise during the start-up and shutdown in the thermal power units, both classical and nuclear. The thermal stresses should be determined on-line during start-up of the power unit, to avoid reducing the lifetime of the so-called critical pressure components. It is necessary to know the time variations in fluid temperature, and heat transfer coeffi- cient on the internal surface of the pressure components to determine the transient distribution of temperature and thermal stresses in the critical components of boilers. The thermal stress can only be calculated correctly if the temperature of the flowing fluid is accurately measured. Unfortunately, massive industrial thermometers used in power units are not able to measure the transient temperature of the fluid with sufficient accuracy due to their high thermal inertia. The goal of the paper is to present the use of the new measurement technique to determine the unsteady temperature of the superheated steam. The temperature of superheated steam was measured after the second stage of superheater using the proposed measuring technique and conventional industrial thermometers that are used today. The tests carried out showed very clearly the advantage of the proposed method over conventional thermometers. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction One of the parameters most difficult to measure in fluid flow is the measurement of transient temperature. Significant errors in the measurement of transient fluid temperatures result from the high inertia of the sensor and, above all, it's casing. Another factor complicating the determination of the transient fluid temperature is the problem with determining the time variations of the heat transfer coefficient on the outside surface of a thermowell. The value of the heat transfer coefficient is strongly dependent on other flow parameters, i.e. fluid temperature and mass flow. Most studies focus on steady-state fluid temperature measure- ments. The time constant of the thermometer is determined only for the case of a stepwise change in the temperature of the fluid. The thermometer time constant is usually determined for air and water because the time constant highly depends on the heat transfer coefficient on the outer surface of the thermometer. The heat transfer coefficient for air is much less than for water. The dynamic error in the measurement of fluid temperature is often determined for stepwise changes in fluid temperature. Thermometer housings are shields used to protect temperature sensors installed inside [1,2]. Thick-walled thermometer housings are located inside pipelines or pressure vessels perpendicular to their internal surface (Fig. 1). The thermometer casing is very massive although the pressure is not too high. The diameter d of the inner opening in the housing is 14 mm or 16 mm. The length of the thermometer L is from 207 to 287 mm, and the height of the thermometer housing H is between 190 mm and 270 mm. The outer diameter of the thermometer housing in its upper part is 32 mm. The thermometer thermowell is affected by a high bending moment if the fluid flows at high velocity. Karman's vortices formed behind the thermometer can cause its vibrations and in consequence the housing damage. The wires forming the thermocouple are insulated with ceramic beads. There is a large air gap between the measuring tip of the * Corresponding author. E-mail address: mjaremkiewicz@pk.edu.pl (M. Jaremkiewicz). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy https://doi.org/10.1016/j.energy.2019.03.049 0360-5442/© 2019 Elsevier Ltd. All rights reserved. Energy 175 (2019) 139e150