Surfaces Of Heat Exchanger Tubes Research Articles

Long-term effects of a chemical decontamination procedure on the corrosion state of the heat exchanger tubes of steam generators

Our previous studies have revealed that a ”hybrid” structure of the amorphous and crystalline phases is formed in the outermost surface region of the austenitic stainless steel tubes of steam generators (SGs) as an undesired consequence of the industrial application of the AP-CITROX (AP: alkaline permanganate; CITROX: citric and oxalic acid) decontamination technology. The formation of this mobile oxide-layer increased the amount of the corrosion products in the primary circuit significantly, resulting in magnetite deposition on fuel assemblies. Owing to the fact that there is no investigation method available for the in-situ monitoring of the inner surfaces of heat exchanger tubes, a research project based on sampling as well as on ex-situ electrochemical and surface analytical measurements was elaborated. Within the frame of this project, comprehensive investigation of the general corrosion state and metallographic features of 36 stainless steel specimens, cut out from various locations of the 21 steam generators of the Paks NPP in the time period of 2000-2007 has been performed. The present work gives a brief overview on the general corrosion state of the heat exchanger tubes of SGs, concerning the long-term effects of the AP-CITROX procedure on the chemical composition and structure of the protective oxide-layer.

Comprehensive investigation of the corrosion state and surface properties of the stainless steel tubes of steam generators

Evaluating the water chemistry in the primary circuit and the effect of chemical econtamination
 of the heat exchanger tubes performed by the AP-CITROX (AP: alkaline permanganate; CITROX: citric and oxalic acid) procedure at Paks NPP (Hungary), a project dealing with the comprehensive investigation of the general corrosion state of the steam generators (SGs) has been initiated. Owing to the fact that there is no investigation method available for the in-situ monitoring of the inner surfaces of heat exchanger tubes, a research program based on sampling as well as on ex-situ electrochemical (voltammetric) and surface analytical measurements (SEM-EDX, CEMS, XRD, XPS) was developed and elaborated. In the time period of 2000-2008 - within the frame of the above project - 45 stainless steel specimens, cut out from various locations of the steam generators of the Paks NPP were investigated. Based on the measured corrosion characteristics (corrosion rate, thickness and chemical composition of the protective oxide-layer) it was found that these parameters are strongly dependent on the decontamination history of steam generators. The present work gives a brief overview on the general corrosion state of the heat exchanger tubes of SGs, concerning the long-term effects of the AP-CITROX procedure on the chemical composition and structure of the protective oxide-layer.

Protection against Corrosion and Restoration of Heat Exchanger Surfaces during Repair and Operation

This article describes a physical model of the application of a protective coating as well as an approach to mathematical simulation of thermal processes accompanying the application of a polymer coating on the surface of heat exchanger tubes, which allows us to estimate the temperature fields as a function of the material and thickness of heat exchanger tubes and the composition of the polymer coating.

Revealing and Assessing the Deposit Amount on Heat-Exchanger Tubes of NPP Steam Generators According to the Operational Data of Routine Eddy Current Monitoring

One of the measures aimed at preventing unscheduled shutdowns of reactor units at nuclear power plants owing to inter-circuit leaks in steam generators consists in monitoring the formation of deposits on the surfaces of heat exchangers. The azimuthal and axial extension of the electrically conductive layer in the deposits, as well as its thickness, can be used to judge the intensity of corrosion. In addition, assessing the contamination of the heat exchanger tubes from the side of the second circuit is important to prevent the accumulation of deposits on the heat exchange surfaces and to plan measures for their removal in the course of chemical washing. Monitoring conductive deposits is based on the existence of a quantitative relationship between the energy parameters of a low-frequency signal of the axial eddy-current transducer on the absolute channel and the volume of the conductive layer in the deposit. The results of studies aimed at revealing and numerically assessing the thickness (volume) of deposits on the outer surface of heat-exchanger tubes in steam generators at nuclear power plants with VVER-440 and VVER-1000 reactor units are presented. For this purpose, operational data based on standard eddy-current monitoring have been used to assess the condition of the metal in heat-exchanger tubes obtained using the PIRATE software package. At the first stage, characteristic diagnostic parameters of eddy-current signal readouts from electrically conductive deposits on heat-exchanger tubes have been determined. Criteria were then formulated according to which readouts corresponding to the sites with deposits can be automatically detected. The next stage consisted in determining the parameters of the eddy-current signal readouts that make it possible to estimate the thickness of the deposits. An algorithm has been developed for constructing a calibration curve for assessing the volume and average thickness of electrically conductive deposits on the surface of heat-exchanger tubes.

Investigation of passive residual heat removal system for VVERs: Effects of finned type heat exchanger tubes

After nuclear reactor shutdown, decay heat generation continues in the reactor core and it has the possibility to cause a severe accident. This residual heat has to be removed from the core sufficiently. In Water-Water Energetic Reactors (WWER) or VVERs, passive residual heat removal system is designed to remove decay heat after reactor shutdown. This system does not need any power operation, since ambient air is used as the cooling fluid in heat exchanger. The steam that is generated in the steam generators due to the heat released in the reactor core condenses and rejects its heat to the ambient air within this heat exchanger. The condensed liquid is returned back to the steam generator. The motion of the cooling medium occurs due to natural circulation. In the presented study, the original geometric design of passive residual heat removal system of VVER type nuclear power plants is introduced. The performance of this system is investigated. In the previous study, geometric design parameters for this system was determined for plain tubes of heat exchanger. Heat exchanger tubes were positioned as a helical in the previous study. With the knowledge of enhancing heat transfer using finned surfaces, geometrical design is modified in presented study. Circular type fin structures (having rectangular cross-section) are added to the outer surface of heat exchanger tubes. In the present heat exchanger model, tubes are positioned as a staggered bank. As the fin parameters, fin thickness, fin radius and total fin number are investigated in this study. Results are compared with the one obtained with plane tube (in tube bank configuration). This study shows that adding finned surfaces enhances heat transfer mechanism and also reduces dimensions of heat exchanger unit. This study also shows that the 2% amount of nominal reactor power can be removed by this design.

Cathodal polarization plus weighing to quickly evaluate scale inhibitors

It is more reasonable to evaluate scale inhibitors from examining the scale deposited on the surface of heat exchanger tubes than from testing the bulk precipitations in industrial circulating cooling water. In this contribution, a facile cathodal polarization plus weighing has been put forward for fast assessing the common scale inhibitors. Through the short-time cathodal polarization and followed weighing of the mass of scale, the antiscale efficiencies of scale inhibitors were obtained quickly, typically with the order of antiscale efficiencies being PBTCA ≈ HEDP ≫ PAA. Since the tested scale was the one deposited on working electrodes made of heat exchanger tubes, the new method in this study could reflect actually the antiscale performance of the used scale inhibitors. Therefore the simple operation and reasonable evaluation would endow this route the wide applications in quickly selecting the right scale inhibitors for the water consumers.

Investigations of Ash Fouling with Cattle Wastes as Reburn Fuel in a Small-Scale Boiler Burner under Transient Conditions

Fouling behavior under reburn conditions was investigated with cattle wastes (termed as feedlot biomass [FB]) and coal as reburn fuels under a transient condition and short-time operation. A small-scale (30 kW or 100,000 Btu/hr) boiler burner research facility was used for the reburn experiments. The fuels considered for these experiments were natural gas (NG) for the ashless case, pure coal, pure FB, and blends of coal and FB. Two parameters that were used to characterize the ash “fouling” were (1) the overall heat-transfer coefficient (OHTC) when burning NG and solid fuels as reburn fuels, and (2) the combustible loss through ash deposited on the surfaces of heat exchanger tubes and the bottom ash in the ash port. A new methodology is presented for determining ash-fouling behavior under transient conditions. Results on the OHTCs for solid reburn fuels are compared with the OHTCs for NG. It was found that the growth of the layer of ash depositions over longer periods typically lowers OHTC, and the increased concentration of ash in gas phase promotes radiation in high-temperature zones during initial periods while decreasing the heat transfer in low-temperature zones. The ash analyses indicated that the bottom ash in the ash port contained a smaller percentage of combustibles with a higher FB percentage in the fuels, indicating better performance compared with coal because small particles in FB burn faster and the FB has higher volatile matter on a dry ash-free basis promoting more burn out.

石炭灰の付着挙動に及ぼす液相成分の影響

In order to prevent adhesion of molten ash particles on the surface of heat exchanger tubes in coal combustion boilers, surface treatment of the tubes, using a thermal spray coating technique, is conducted in this study. As one of the important mechanisms of ash adhesion relates to liquid-bridge force between the tube surface and the molten ash particles, contact angle of the liquid-phase on the surface for the simulated molten ash particles was measured, varying types of coating metal species. The ash deposition experiments on the coated tubes were also conducted, using a vertical furnace under the high-temperature condition. The result showed that the two values of the wettability and the slagging factor of the ash particles deposited could evaluate the ash adhesion characteristics. The wettablity could be controlled by changing types of the coating metal species. The amount of ash deposition increased with an increase of the slagging factor. Especially, sodium and potassium compounds were concentrated in the position apart from 60 degrees from the stagnation point. These results suggest that the surface treatment of the tubes, using a thermal spray coating technique, can control the ash deposition behaviors.

High-performance polymer coatings for carbon steel heat exchanger tubes in geothermal environments

The most critical issue in developing thermal conductive coatings for the interior surfaces of heat exchanger tubes made from mild carbon steel (MCS), which are used in geothermal power plants at temperatures ranging from 110° to 89°C, is the deposition of scales. These scales, induced by the brine, chemically adhere to the coating surfaces. One of the major factors governing the formation of a strong interfacial bond at interfaces between the coatings and scales was the brine-promoted hydrothermal oxidation of the coatings. In seeking coating unsusceptible to hydrothermal oxidation, two semi-crystalline thermoplastic polymers, polyphenylenesulfide (PPS) and polytetrafluoroethylene (PTFE)-blended PPS, were applied as interior surface coatings to the zinc phosphated MCS tubes. The PPS coating surfaces suffered some oxidation caused by their chemical affinity for FeCl2 in geothermal brine. FeCl2-promoted oxidation of PPS surfaces not only incorporated more oxygen into them, generating a sulfide → sulfone → sulfonic acid conformational transformation within the PPS, but also caused the disintegration of PPS, yielding fragmental polychloroaryl compound and ferrous sulfate (FeSO4) derivatives. The FeSO4 reaction product formed at the interfaces between the scale and PPS coating was soluble in water, so that the coatings could be easily removed by highly pressurized water. The oxidation of PPS was considerably inhibited by blending PTFE into it, forming coating surface unsusceptible to hydrothermal oxidation reactions with hot brine. The major reason for such inhibition of oxidation was the formation of a chemically inert PTFE layer segregated from the PPS layer at the outermost surface site of the coating. Hence, the scale easily flaked off from the PTFE-blended PPS coating surfaces. This characteristic of surface was similar to that of the stainless steel surfaces. Nevertheless, both PPS and PTFE-blended PPS coatings can be classified as scale-free coatings.

A novel approach to enhance heat and mass transfer in adsorption heat pumps using the zeolite–water pair

A mathematical model is developed for a novel arrangement proposed in order to cope with the drawbacks originating from the inefficient heat and mass transfer in adsorption heat pumps. The zeolite, used as the adsorbent, is envisioned to be synthesized on the surfaces of heat exchanger tubes as continuous coatings, which was also achieved experimentally. Simplifications regarding the boundary conditions are made by using the scaling method and a finite difference scheme is employed for the solution of the resulting differential equations to determine the variation of the temperature of the adsorbent as well as the concentration of the adsorbate within the adsorber. The typical restrictions of adsorption heat pumps are observed to be removed to a great extent and the duration of a single cycle is shortened significantly.

Simulation of the process of scaling of tube heat exchangers

A semiempirical model of the process of deposit formation on the inner surface of heat exchanger tubes is obtained. By experimental investigation, constants of the scaling process that close this model are obtained for carbonate-type deposits.

4917176 Device for cleaning inner surfaces of heat exchanger tubes : Toshimasa Shimada, Katsuhei Tanemura, Yasuo Fujitani, Katsumoto Otake, Shigeo Oda, Yoshio Sumiya, Chofu, Japan assigned to Hitachi Ltd; The Tokyo Electric Power Co Inc

4917176 Device for cleaning inner surfaces of heat exchanger tubes : Toshimasa Shimada, Katsuhei Tanemura, Yasuo Fujitani, Katsumoto Otake, Shigeo Oda, Yoshio Sumiya, Chofu, Japan assigned to Hitachi Ltd; The Tokyo Electric Power Co Inc

数種の不純物を含むヘリウム中におけるハステロイXの高温すきま腐食

Interaction between a Ni-base heat-resistant alloy and simulated HTR primary coolant environment, which contained controlled concentrations of both oxidizing and carburizing impurities, was examined with emphasis placed on the reactions inside narrow crevice gaps. A test method of providing artificial crevice using a Mo container was developed to obtain quantitative results with reasonable reproducibility. The test environment was helium of low oxidizing potential with trace amount of gaseous impurities: H2, H2O, CO2 and CH4. The state of oxidation on the inner surfaces of the crevice was examined by measuring the change in concentration of Cr on the specimen surface using EPMA. Significant difference in the chenge of the surface was seen along with the distance from the crevice entrance. There was a preferential consumption of oxidizing species in the outer part of the crevice due to the formation of continuous oxide film, leading to porous or discontinuous oxide film formation, and the resultant extensive carburizations in the inner part of the crevice. Vaporization was noted far inside the crevice where oxide film was scarecely formed. The zone of continuous oxide film formation was found to extend into the crevice as a function of the 1/4 power of exposure time, and its reaching distance in a given time was proportional to the square root of opening of crevice gap. These relations were consistent with a model with an assumption that the diffusion of reactants through the gas was a rate determining step. The model based on the results obtained will be applied to the crevices with the similar geometrical effects such as the internals of fatigue and creep cracks and also actual crevice formed on the surface of heat exchanger tubes.