Mukesh Gautam
Bharat Vyakaranam
L. Eric Smith, PhD
ONLINE HEAT EXCHANGER TUBE GUIDED WAVE SENSOR
A relatively standard once-through shell and tube heat exchangers typically consist of a cylindrical shell with a 2 to 20 cm thick flat tube-sheet on either end of the cylinder. Two plenums are formed at the ends of the cylinder by the hemispherical shell-ends. These ends are half-sphere caps containing one or more nozzles that allow fluid to be introduced or extracted from the end plenums and they are typically designated as the inlet or outlet plenums with inlet or outlet nozzle depending on whether fluid is entering or exiting the heat exchanger. These two plenums are joined by hundreds to thousands of tubes (typically 1 to 3 cm diameter with 1 to 2mm wall thickness) that are seal-welded to the tube-sheets. The plenums and the tube inside volumes are connected as a single volume that can be filled with fluid at temperature T1. The shell volume between the two tube sheets and on the outside of the tubes may be filled with fluid at temperature T2. This allows heat to flow across the tube wall without the two fluids mixing. The same basic heat exchanger approach is achieved with a single divided domed cylinder where the tubes are formed in a U-shape extending from the inlet quarter-sphere plenum to the outlet quarter sphere plenum. This configuration is designated as a U-Bend heat exchanger. For light water nuclear reactor heat exchangers, the T1 and T2 temperatures are nominally 370 oC and 750 oC and oC respectively., A clamp-on high temperature piezoelectric sensor mounted near the union of the tube to the tube-sheet is envisioned to generate an axial and a torsional/axial guided wave ultrasonic signal that will travel the full length of the tube. When such an ultrasonic signal reaches the opposite tube end, the signal is reflected back and may be sensed by the same signal-generating piezoelectric sensor or a similar receiving piezoelectric sensor. If corrosion or crack anomalies occur in the tube, part of the signal will be reflected and will be detected by the receiving sensor at an earlier point in time than the reflected signal from the tube end. The expectation (based on experience and literature using sensors mounted to the tube ID and based on larger diameter and flat-plate sensors) is that anomalies originating from the tube ID or OD can be detected before they reach a 100% through-wall breach and ideally before exceeding 50% through wall., Normally a material such as PZT (lead zirconate titanate) is used for piezoelectric sensors. PZT however does not work well above 300oC. Other piezoelectric materials like lithium niobate (LiNbO3), lithium tantalite (NiTaO3), aluminum nitride (AlN), and other materials that have a lower piezoelectric coefficient but still perform up to and above 600-800 oC. The goal is not to get the strongest signal from the sensor - since the damage detection is simply based on detection of a signal above the noise floor of an indication corresponding to degradation at a point in time of interest, these lower sensitivity higher temperature materials can be used., The logistics of bringing signal and power to and from guided wave sensors mounted to the tube ID are complicated by not only the high temperature but also by the high flow forces associated with fluid flow in the end-plenums and through the tube IDs. Sensors mounted to the tube OD are still subject to high temperatures but typically the lower temperature T2s are on the tube ODs plus the flow forces are minimal in the stagnant area near the intersection of the tube and tube sheet. The wires must still be managed by a protective corrosion resistant structure and routed to or through the shell wall. This management is characterized as an embedded sensor. Moreover, the sensor is considered an embedded sensor because it must be installed as the heat exchanger is being fabricated. Spacing between the tubes may allow some periphery tubes to be instrumented after completing the tube/tube-sheet assembly but tubes away from the periphery are inaccessible after all tubes are installed. The primary invention considers that the sensor signals will be brought through the tube bundle near the tubsheet or within the tubesheet to a commercial grade qualified cable penetration (commercially available) through the heat-exchanger shell to a multiplexing instrument located away from the heat using high temperature ( ceramic or tungsten or other high-temperature insulation) cabling. The cable penetration challenge may also be mitigated with high temperature electronics that may reduce or eliminate conducting penetrations through the shell however this is not the focus of this invention disclosure., The specific innovative features of the invention are:, A high temperature sensor that can be clamp-mounted to the heat exchanger tube OD as an embedded sensor during the time of manufacturing., The location of the sensor near the stagnant flow region of the tube to tube-sheet interface has no influence on the heat exchanger performance and minimizes flow forces on the cable conduits., The sensor system provides advanced warning of tube degradation prior to a full through-wall breach thereby allowing an orderly shutdown for further assessment, repair/removal from service, or replacement of the heat exchanger before any fluid exchange / leak occurs., With corroborative data, this technique could justify extended inspection intervals or mandate shutdown and inspection only if the guided wave monitor showed an indication.
Sai Pushpak Nandanoori
Konstantinos (Kostas) Oikonomou
Quan Nguyen
Di Wu
Di Wu is an electrical engineer at PNNL, serving the optimization and control group as chief engineer and team lead.