High temperature monitoring the height of condensed water in steam pipes Yoseph Bar-Cohen, Shyh-Shiuh Lih, Mircea Badescu, Xiaoqi Bao, Stewart Sherrit, Scott Widholm, Patrick Ostlund, and Julian Blosiu Jet Propulsion Laboratory/California Institute of Technology, MS 67-119, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, 818-354-2610, yosi@jpl.nasa.gov , Web: http://ndeaa.jpl.nasa.gov Abstract An in-service health monitoring system is needed for steam pipes to track through their wall the condensation of water. The system is required to measure the height of the condensed water inside the pipe while operating at temperatures that are as high as 250 o C. The system needs to be able to make real time measurements while accounting for the effects of cavitation and wavy water surface. For this purpose, ultrasonic wave in pulse-echo configuration was used and reflected signals were acquired and auto-correlated to remove noise from the data and determine the water height. Transmitting and receiving the waves is done by piezoelectric transducers having Curie temperature that is significantly higher than 250 o C. Measurements were made at temperatures as high as 250 o C and have shown the feasibility of the test method. This manuscript reports the results of this feasibility study. Keywords: High Temperatures (HT), HT piezoelectric transducers, Fluid Height Monitoring, health monitoring, monitoring steam condensation, Sensors 1.0 Introduction Steam pipe systems are used in various major cities including Manhattan and are operated as a district heating system carrying steam from central power stations under the streets to support heating, cooling, or supply power to high rise buildings and businesses. Health monitoring of such systems is critical to assure their safe operation. Excessive rise in the level of water condensation inside the steam pipes is a source of concern due to the possible excitation of water hammer effects that may lead to serious consequences including damaged vents, traps, regulators and piping. The water hammer effect is caused by accumulation of condensed water that is trapped in horizontal portions of the steam pipes. In this study, the authors sought to develop and demonstrate the feasibility of using an ultrasonic based technique of monitoring the condensate height that sustains the harsh environments of the steam pipe system (<250 o C). Using pulse- echo and HT transducer, the feasibility was demonstrated with a good accuracy [Bar-Cohen et al., 2010a, 2010b, and 2010c]. Making nondestructive measurements of the water height level through a pipe wall may be feasible only by an ultrasonic method. For this purpose, there is a need to be able to measure with good accuracy a parameter that is related to the height of the water. Three techniques were considered: Pulse-Echo, Pitch-Catch and Acoustic Emission. The first two have the highest potential where the time-of-flight of the wave reflections from the top surface of the water is measured and the height is calculated using the wave velocity. For first order analysis, it is reasonable to assume that the speed of sound in water at elevated temperatures is close to the one at room temperature. In the case of Pulse-Echo, the transducer is connected to both the transmitter (function generator), which leads to generating elastic waves, and the receiver amplifies the reflected waves that are converted to electric signals. Alternatively, in the Pitch-Catch arrangement, the pair of transducers is physically separated. In this case, one transducer generates the waves and the reflected signals are received by the second transducer. Of the two methods, Pulse-Echo is more capable since there is no reliance on receiving the reflection at a specific angle. However, numerous reflections are received and they require an effective signal processing technique to identify the reflections from the top and bottom surfaces of the condensed water. Several issues needed to be accounted for, including strong reflections from the interface of the steel pipe, the effect of the pipe curvature, and the wave losses due to scattering from a non-flat surface of the pipe, there is an issue of interference of the multiple reflections within the pipe bottom wall; the pipe-transducer interface; turbulence in the condensed water; potential sediments in the bottom of the pipe inner surface along the path of the wave; and the multiple reflections inside the condensed water. An alternative method that was also considered is based on measuring emitted shock waves from cavitation that is potentially formed in the condensed water. This acoustic-emission method assumes that the greater the condensation Proceedings of the SPIE Smart Structures and Materials/NDE Symposium, San Diego, CA, March 7-10, 2011 © Copyright 2011 Caltech