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White Paper: Boiler Tube Erosion Reduction using the SHOCKSystem™ Online Detonation Cleaning™ Product View as PDF |
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| SHOCKSystem™ Reduces Wear and Tear |
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Pratt & Whitney has long been an industry leader in the development of pulse detonation propulsion technology for applications including commercial and military aircraft, missiles, and rockets. Pratt & Whitney has leveraged this detonation expertise to develop the SHOCKSystem™ Online Detonation Cleaning™ product.
One of the concerns in operating coal-fired boilers, particularly those that burn coal with high fouling tendencies, is how to effectively and efficiently clean ash deposits from heat exchanger surfaces. Conventional sootblowers involve complex moving parts, do not clean adequately in areas outside the jet’s line of sight, consume valuable steam or compressed air, and can increase tube wear by entraining ash particles at high velocity during operation.
Pratt & Whitney's SHOCKSystem™ can significantly reduce or eliminate these problems. Pulse detonation creates a controlled gaseous explosion in a confined combustor, external to the boiler. The blast wave produced from this explosion propagates outwardly throughout the local boiler interior. The blast wave expands in all directions, even around obstructions, reaching areas that are not in line-of-sight of the combustor. Since the detonation is a separate combustion process it does not require steam or high-pressure air. Finally, the blast wave itself is of short duration and accelerates entrained ash particles to relatively low velocity. This is expected to reduce boiler tube wear relative to conventional sootblowing technology.
This white paper specifically addresses tube erosion associated with conventional sootblowers in comparison with the SHOCKSystem™. Calculations show the entrained ash particle velocity from the SHOCKSystem™ blast wave to be much less than the jet impingement velocity of a conventional sootblower. Given that tube erosion depends strongly on particle impingement velocity, the SHOCKSystem™ blast wave is expected to significantly reduce tube wear rate, and therefore reduce tube leak incidents. Additionally, field experience confirms that the Online Detonation Cleaning™ process can effectively remove deposits without disturbing the tube's protective oxide surface layer. Operation of a pulse detonation boiler cleaning system in Europe since the early 1980s has resulted in a 30% reduction in the number of tube leaks compared to prior operation with conventional steam sootblowers.
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| Boiler Tube Wear Overview |
Industry data suggests that about one in every seven tube leaks in coal-fired utility boilers may be attributable to sootblower erosion (1,2). A large body of research, specifically concerning coal-fired boilers, shows that impacting particles (eg. fly ash) have the effect of removing protective oxide layers as well as metal from the tube surface. This not only erodes the tube metal directly but leaves it vulnerable to corrosion from flue gas. Additionally, this research shows that the velocity of the particles is a factor of primary importance.
Corrosion and Erosion
Boiler tube erosion (metal removal caused by particle impact) and corrosion (metal removal caused by chemical attack) have a synergistic effect. Rishel, et al. describes the combined effects of erosive wear by ash particles and corrosive action of flue gas (3). Metal removal rate reaches a maximum where both corrosion and erosion are occurring simultaneously.
| Figure 1 |  | Figure 1:
Metal removal accelerated by oxide removal by impact erosion (Wright, 1991) |
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Figure 1 illustrates that under the action of repetitive impact by fly ash (entrained in flue-gas or a sootblower jet) the protective oxide layer can be reduced in thickness or removed entirely, depending upon the severity of the erosion. This reduction increases the diffusion rate of corrosive elements through the oxide layer, resulting in a time-average corrosion rate that is dramatically increased compared to the normal flue gas corrosion rate (3,4,5). Thus corrosion and erosion together accelerate the overall metal removal rate at the tube surface, a rate that exceeds that of either erosive or corrosive effects acting independently.
Particle Impact Velocity
Even given various differences in coal properties and boiler operational settings, the rate of tube erosion has been found to be primarily dependent upon the impact velocity of fly ash entrained in flue gas and sootblower jet flow. This erosion rate is related to the cube of the velocity (6,7). Thus doubling the particle velocity can result in eight times the erosion rate (see Figure 2).
| Figure 2 |  | Figure 2:
Erosion wear of boiler tubes during sootblowing: A, weakly abrasive ash; B, moderately abrasive ash (Raask, 1985) |
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Normal flue gas velocity in the convective pass is generally limited by design in pulverized coal fired boilers to 50-60 ft/sec (15-18 m/sec) in order to keep tube metal removal rate associated with fly ash erosion within acceptable limits of nominally 3 to 4 millionths of an inch per hour (75 to 100 nm/h) (6).
However, the expanding jet from a sootblower lance mixes with approximately an equal volume of flue gas for every distance of the jet diameter it travels(6). Thus by the time the jet reaches the tubes, it consists largely of ash-laden flue gas6. Although there are variations in nozzle geometry, sootblowing media, source pressure, and range from the nozzle, the jet diffusion model proposed by Jameel, et al. estimates the jet impact velocity at the tube surface to be about 1400-2900 ft/sec (430 to 880 m/sec) with jet impact dwell time of tens of milliseconds on a given boiler tube8. This is on average forty times the normal flue-gas-imparted impact velocity.
The velocity imparted to fly ash by the SHOCKSystem™ blast wave depends upon the SHOCKSystem™ configuration, boiler temperature, and range from the nozzle. Nevertheless, gas dynamic theory and field testing estimate the peak gas velocity following the blast wave to be 170 to 900 ft/sec (50 to 275 m/sec), with an effective dwell time of about 1.5 milliseconds on a given boiler tube. These representative velocity and dwell time estimates are summarized in Table 1.
| Erosion Mechanism | Velocity | Time Duration |
| Flash ash erosion | 50 – 60 ft/sec | Continuous |
| Gas Accelerated by SHOCKSystem blast wave | 170 – 900 ft/sec | .0015 Seconds/Cycle |
| Conventional Sootblower Jet | 1400 - 2900 ft/sec | ~ 0.020 Seconds/Cycle | | Table 1: Relative flow velocity and dwell time of flue gas, gas accelerated by SHOCKSystem blast wave, and the conventional sootblower jet. |
Considering the strong influence that velocity has on erosion wear, the significant reduction of particle impact velocity associated with the SHOCKSystem™ blast wave suggests that a dramatic reduction in tube erosion (and associated tube leaks) can be expected with implementation of the SHOCKSystem&trade in lieu of conventional sootblowing technology.
The SHOCKSystem™ while in operation also produces a highly turbulent "blowdown" jet that maintains a velocity in excess of 3000 ft/sec (1000 m/sec) extending several feet from the nozzle. However most SHOCKSystem™ installations direct this blowdown jet away from boiler structure to prevent wear or damage of tubes. Occasionally some situations require use of this jet to augment removal of tough deposits. In those circumstances, conventional tube shield protection measures may be employed locally to minimize this effect. The blowdown jet is a side effect of the detonation process and not the primary cleaning mechanism; as such, measures are taken to minimize jet interaction with boiler structure.
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| Online Detonation Cleaning™ Field Experience |
Cleaning operations in the lower backpass of utility boilers have demonstrated that the SHOCKSystem&trade blast wave removes platenization (back-spacing) and gas lane pluggage while not removing or eroding away the more tenacious ash sublayer from the boiler tube surface. If the ash sublayer is not removed, then the underlying tube oxide is not removed either; therefore the SHOCKSystem™ blast wave is anticipated to produce a negligible contribution to tube wear.
| Figure 3: Before SHOCKSystem™ blast |  | | Figure 3: After SHOCKSystem™ blast |  | Figure 3:
Online before/after pictures of economizer platenization at the 3rd and 4th tube rows removed by the SHOCKSystem™ at an 800 MWe unit |
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Long-term SHOCKSystem™ erosion data from the field is still being generated. However, an equivalent pulse detonation-based technology has been in continuous operation in two 110 MWe boilers in a power generation facility in Bosnia and Herzegovina since the early 1980s. Following implementation, boiler statistics substantiated that application of the blast wave cleaning technique in lieu of conventional sootblowers reduced the number of tube bursts by more than 30%9.
Summary
Pratt & Whitney’s heritage stems from 80 years of innovative engineering excellence. In keeping with this tradition, Pratt & Whitney puts its products and services through a rigorous review process to guarantee that product quality and functionality exceed customer expectations. The SHOCKSystem™ Online Detonation Cleaning™ product, as an alternative to conventional boiler cleaning technology, bears the potential for significantly reducing the occurrence of tube leaks. Calculations show the ash particle impact velocity from the SHOCKSystem™ blast wave to be much less than the jet impingement velocity of a conventional sootblower. Given that tube erosion depends strongly on particle impingement velocity, the SHOCKSystem™ blast wave is expected to significantly reduce tube wear rate. Field experience also confirms that the Online Detonation Cleaning™ process blast wave can effectively remove deposits without disturbing the tube's protective oxide surface layer. Although long-term SHOCKSystem™ erosion data is still being generated, an equivalent pulse-detonation system in operation since the early 1980s has resulted in a 30% reduction in the number of tube leaks in two boilers at a power generation facility in Europe.
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References
1. TVA Fossil Power Group Boiler Reliability Program Presentation, 2005.
2. Patrick, J., R. Oldani, D. Von Benren, "Benchmarking Boiler Tube Failures – Part 2", Power Magazine. Vol 149, No 9 (Nov/Dec 2005): 55-59.
3. Rishel, D.M., F.S. Pettit, N. Birks, "Some Principal Mechanisms in the Simultaneous Erosion and Corrosion Attack of Metals at High Temperature", Proceedings of the Conference on Corrosion-Erosion Wear of Materials at Elevated Temperatures. Ed. A.V. Levy. Houston: National Association of Corrosion Engineers, 1991. pp. 16.1-23.
4. Kosel, Thomas H., "Solid Particle Erosion", ASM Handbook Vol 18. ASM International, USA, 1992. (199-213).
5. Wright, I.G., V.K. Sethi, N. Nagarajan, "An Approach to Describing the Simultaneous Erosion and High-Temperature Oxidation of Alloys", Journal of Engineering for Gas Turbines and Power. Vol 113, No 4 (1991) 616-620.
6. Raask, Erich, Erosion Wear in Coal Utilization. Washington: Hemisphere Publishing Corp., 1988.
7. Raask, Erich, Mineral Impurities in Coal Combustion. Washington: Hemisphere Publishing Corp., 1985.
8. Jameel, M.I., D.E. Cormack, H.N. Tran, "Sootblower Optimization – Part 1. Fundamental Hydrodynamics of a Sootblower Nozzle and Jet", TAPPI Proceedings. 1993 Engineering Conference, 1993. 683-691.
9. Hanjalic, K., I. Smajevic, "Detonation-Wave Technique for On-Load Deposit Removal from Surfaces Exposed to Fouling: Part II – Full-Scale Application", Journal of Engineering for Gas Turbines and Power. Vol 116 (1994): 231-236. |
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