Measurement of the Piping Elbow Integrity Using the Fiber-Optics Technique

This paper deals with the development of a non-destructive conditioning monitoring technique for low alloy steel piping elbows. The objective is to develop a technique, which enables on-line monitoring of the flow induced wall thinning of the elbows. The technique involves use of optic interferometry sensors for the measurement of a flow-induced vibration. The special feature of this system is that it provides a non-destuctive method and enables measurement of the very small displacements caused by wall-thinning. The system configuration and the proposed methodology to be employed related to this developmental work has been discussed. The data and information collected on the vibration characteristics of the piping elbows using fiber optic accelerometers has been analyzed. The application feasibility of the proposed fiber optic interferometer in condition monitoring for nuclear power plant has also evaluated. The results obtained so far are very encouraging with respect to the measurement of wall-thinning in the piping elbows. Introduction Pressure vessels and heat exchangers such as steam generators in nuclear power plants have very sophisticated piping systems operating in a very aggressive erosion/corrosion environment of a turbulent flow with a high temperature and pressure. These adverse operating environments make a piping system very vulnerable to accelerated wear and degradation. The condition monitoring of the tube bundles in a steam generator and piping elbows in a secondary system in nuclear power plants is one of the most interesting issues for nuclear inspection and maintenance activities.[1] Several methods to measure the wall-thinning effect of a pressure tube have been developed. One is the electro-chemical approach such as a pH sensor, and an electro-chemical corrosion potential sensor.[2] But these sensors require the penetration of the pressure piping to contact the measured fluid, therefore, are not a non-destructive method. The second is the ultrasonic method, which is widely used in the nondestructive technology. Even though the ultrasonic transmitter has a good capability to measure the wall thickness of pressurized piping under a single phase fluid within the piping, it is difficult to measure the wall thickness when two phase and turbulent fluid exists in the pressurized piping.[3] The last is a passive method which is to measure the changes of the physical phenomena of the passive components such as elastic waves, and pipe or tube vibrations due to fluid flow. The measurement of vibration generally requires the determination of the displacement of a surface as a function of time. The measurement is to determine the amplitude and frequency content of the vibration. In general, measurements are not required to respond to vibrations over the range of 1 Hz to 1 MHz with amplitudes ranging from a few millimeters to a sub-angstrom. Many of the standard means of monitoring vibration utilize accelerometers, which are contact devices and have limited resonance bandwidths. If the vibrating target can be incorporated into an interferometer, this Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 750-755 doi:10.4028/www.scientific.net/KEM.270-273.750 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:37:57) Title of Publication (to be inserted by the publisher) optical technique may be possible to use as a non-destructing and non-loading technique to monitor a vibration. If the combination of lasers and fiber optics is used, no direct line of sight is required, in order for the monitored surface to be incorporated into an interferometer. The fiber itself offers to measure ‘round corners’ and to give a small measurement region. The primary techniques of optical measurement are shadow moiré interferometry [4], holographic interferometry [5], speckle interferometry[6,7], and Doppler effect[8]. The development of these methods can have good qualitative observation of full field vibration in principle. There are some disadvantages in these methods, such as lack of high frequency vibration measurement devices, quantitative analysis of interferogram consuming too much time, or optical device being too complicated to be used practically. In this article, a single-point, real time vibration measurement technique based on the idea of traditional Febry-Perot interferometry is presented. The fiber optic accelerator which has been used in our test was developed by Park et.al. to monitor the gas pipe line integrity against third-party damage[9].The measured vibration frequency range is a wideband with 100Hz to 10kHz. It depends on the weight of the vibrating material and distance between the reflecting surface and fiber sensor. This study evaluates that a fiber optic accelerometer can be used as a condition monitoring system in the secondary side of nuclear power plants. FAC Test Loop In a Pressurized Water Reactor secondary side, the moisture separator and reheater (MSR) drains were selected as a first application area of the Flow Accelerated Corrosion (FAC) monitoring system. MSR drains with single-phase flow have a high susceptibility to FAC, and its temperature and pressure are relatively low as compared with feedwater or blow-down. The maximum temperature of the MSR drains is 232 o C and the maximum pressure is 30 atm.[1] Another concern of condition monitoring is the steam generator tube vibration. A steam generator in a pressurized water reactor has the U-shape tubes which has vulnerability against the flow induced vibration. To study the vibration characteristics of the piping elbows and the steam generator U-tube, a test loop has been developed. The developed test loop is a scale down loop which has been developed to conduct a lesser scale test series on a piping elbow as a prelude to that on the tube bundles and secondary piping elbows in a steam generator and a secondary system in nuclear power plants. An accelerated erosion/corrosion environment has been implemented and monitored in the condition monitoring test series on a piping elbow. A schematic of the FAC test loop is shown in Fig.1. The test system is composed of a test solution tank and supply system, high temperature/high pressure circulation loop, cooling and pressure controlling system, and a test specimen instrumented with various sensors. Sensors used for condition monitoring of the FAC phenomenon can be divided into three categories: electrochemical sensors, ultrasonic sensor, and vibration sensors. Electrochemical sensors include EREP and AUEN, which are used for measuring the electro-chemical potential of the specimen and the pH of the solution. Thermodynamic variables such as temperature and pressure can be easily measured through thermocouples and pressure transducers. Ultrasonic sensors are used for in-situ the measurement of the piping wall thickness Fig. 1. Schematic of the FAC Test Loop. Key Engineering Materials Vols. 270-273 751 Title of Publication (to be inserted by the publisher) changes. Vibration sensors include optical sensors and a capacitance accelerometer. A. fiber optic accelerometer has been installed on the piping elbow within the rectangular box as shown in Fig.1. Measuring System The schematic diagram of the optical fiber accelerometer measurement using the Febry-Perot interferometry is shown in Fig. 2. The reflective surface is illuminated by a laser beam, the reflected beam from the reflective surface returns to the semi-reflective surface of the optical fiber core. Reflecting beam at the optical fiber core repeatly illuminates the reflective surface. Beam intensity depends on the reflection and transparent coefficient of the reflecting surface of the fiber optic, and the reflection coefficient of the reflective surface, etc. The final beam intensity( R H ) can be represented as