Optical 3D Laser scan for Penstock Corrosion Damage Assessment for Hydropower Plant

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1 More info about this article: Optical 3D Laser scan for Penstock Corrosion Damage Assessment for Hydropower Plant G. Kishore kumar and M. Janardhana Materials Technology Division, NDT Group Central Power Research Institute, Bangalore Abstract In India the sources of power generations are from thermal power plantsof 65%, hydroelectric power plants of 22%, nuclear power plants of 3% and rest by10% from other alternate sources like solar, wind, biomass etc. The hydro power plants are renewable in nature and many of these plants are installed way back 1940 to 1960 and most of theseplant equipment are undergone deterioration due to various reasons viz. corrosion, wear and tear. Penstock is considered to be one of the major equipmentoperating in hydro power plant and it is criticalin nature, where it is used to conduit the water from reservoir to power station turbine. Corrosion of penstock is a major issue to be assessedin order eliminate the water leakage & damages to the weld joints.the water flow through penstock becomes extremely important to ensure hydraulic efficiency and overall plant structureincluding inhabitations safety. Condition assessment of penstockto detect corrosion and to estimate the remaining shell wall thickness is important parameter to ascertain the hoop stress of conduit pipe. Presently conventional method such as ultrasonic thickness measurement is employed to know theexact remaining wall thickness. As this is of a spot inspection type, which needs proper surface preparation and provideonly the thickness of the spot on the penstock shell surface in the corroded region. Estimating the corrosion spreadon the total circumferential /longitudinalarea using conventional ultrasonic testing method is of cumbersome processand time consumption, further it is difficult to measure the exact remaining wall thickness on the corroded region. CPRI used advanced In-situ NDT methodof Optical 3D Laser scan to assess the exact quantum of external corrosion spread on the surface of penstock including thickness loss and corrosion depth. In this paper discus the evaluation of corrosion area and determining themetal losson the penstock shell corrosion flaws spaced longitudinally and closed transversely. Key words : Hydro power, Penstock, Lase scan and Corrosion.

2 1.0 INTRODUCTION The hydro electrical power plant require large amount of water, which is storedin reservoir and operates with high fluid flow rate. A penstock, as a part of hydro-power plant, is the pressure conduit between the open water upstream and the turbine. The most desirable penstock is steel,due to the fact that steel pipe possesses many desirable qualities of any goodconduit: strength and toughness, durability and long service life, economy of installation andmaintenance, permanent high-carrying capacity, ductility and adaptability, reliability and resilience,watertight joints. The steel penstocks have been considered as very high pressure conduits, usually of large diameter, and operating with frequent surges during the normal condition. Penstocks may also be subject to pulsations of varying frequency and amplitudes transmitted from the turbine or pump. Based on these conditions, penstocks have been designed to standards with an allowable design stress at normal conditions of 2/3 of yield or 1/3 of tensile strength. Therefore, a penstock should be as hydraulically efficient as practical to conserve available head and structurally safe to prevent failure. The penstock which can be exposed to high stresses is susceptible to failure. The consequences of unexpected failure can be catastrophic, producing a great risk in service which would result in loss of life and property.the failures of penstocks are not frequent; the mechanical damages observed during service, fatigue, welding imperfections, corrosion defects and environment effect are referred as most important causes. The initial study by the author wanted to investigate corrosion and achieve to understanding varying severities of corrosion taking place at hydro power station. It is hoped that the subject of the corrosion types and possible consequences of corrosion and its evaluation will be discussed in this paper for the penstocks which are installed above ground. 2.0 CORROSION MECHANISM The hydro plant are faced with aging assets and limited resources, a more proactive approach is needed to ensure continued safe and reliable operation. The pen stock system continues to becoming increasingly important to consider corrosion and its effects on the long term sustainability of the energy systems infrastructure. Corrosion is the deterioration of a material as a result of its interaction with its surroundings. Furthermore, corrosion processes not only influence the chemical properties of a metal or metal alloy, but they also generate changes in their physical properties and mechanical behaviours.

3 The steel penstock surface exposed to rough weather conditions and without any protection i.e., metallic plating orpainting depleted amounts to corrosion. If we stop source of oxygen in contact with the steel, the corrosion stops, the perfect coating deprives the surface of oxygen and steel will not rust (oxidized). 2.1 External Corrosion Inspection Primarily the external inspection of pen stock is carried out visual inspection for appearance, defects on the wall, weld joints, cracks, exterior scaling, abnormal circumstances and deformation. The carbon steel pen stock is susceptible to all forms of localized corrosion, including pitting, crevice corrosion, and microbiologically influenced corrosion. The corrosion can be broadly classified: (1) General corrosion and (2) Localised corrosion General corrosion The general corrosion rate is mostly influenced by exterior condition and oxygen content. The perfectly coated penstocks are protected from corrosion as long as the coating is intact. However, all coating will have some service life due to variation in atmospheric condition and its exposure. The coating life is often less than the desired plant life, so the portions of penstock uncovered (voids in the coating) with time occur general corrosion. The corrosion initiation will be delayed during the time when coating is intact, the rate of corrosion is not a discrete value, but represented typically by distribution in a normal or longitudinal distribution Localized corrosion The localized corrosion is significantly expressed as pitting, scaling corrosion, pinning and microbiological in nature. If any internal coating can reduce the probability of localized corrosion and also initiate corrosion to propagate. The probability of localized corrosion is low during continuous flow and further decrease the corrosion cell initiate and formation.however, localized corrosion is usually far less significant than general corrosion when compared to the distribution rates of general corrosion.

4 3.0 CORROSION INSPECTION AND EVALUATION Inspection of corroded components generally follows with visual inspection and nondestructive testing. The visual inspection provides the information about nature of corrosion and type of corrosion. The type of corrosion that occur in penstock are (1) Pitting corrosion, and (2) General corrosion. Assessment of pitting corrosion as per standards provided pin hole assessment by gauges with different in depth, but where as on the penstock corrosion the nature of corrosion observed is not exactly the pitting corrosion having multiple pin holes. The corrosion occurred like the external surface corrosion leads ununiformed metal loss on the surface. Detection of the over surface corrosion by cleaning and using conventional Ultrasonic testing is off spot inspection and adjacent area of metal loss assessment is cumbersome process and time consuming along with more measuring machinery and man power. Moreover it is a spot inspection which is unsatisfactory or inadequate. When penstock corrosion spread is large location and area, the assessment of such above 1000 area is a difficult task to inspect with UT thickness gauge for proper measurement of corrosion. The present technical paper is a case study to solve this problem; instead of using the conventional NDT tools, CPRI has adopted advanced In-situ NDT method of Optical 3D Laser scan to assess the exact quantum of external corrosion spread on the surface of penstock including thickness loss and corrosion depth. 4.0 METHODOLOGY The methodology for the case study of penstock with corroded area on the surface using laser scanner of Creaform: Identify the corrosion area Surface preparation Scan the object with 3D laser device Data acquisition of the object Analyse of data in pipecheck software Results The 3D laser scanning is a non-destructing-testing (NDT) technology used to accurately measure surface defects, metal loss or geometrical deformations with a very high

5 accuracy. The surface geometry of an object is acquired in 3D and will be used to take a wide range of measurements. A laser cross is projected on the object and a set of two cameras records the laser lines curvatures following the surface shape in real-time, as illustrated in Figure 1. The Creaform Handyscan3D technology has many advantages such as the ability to cover 100% of the scanned surface providing a huge amount of data points which increases the confidence in the results. In the case study of penstock, the 3D laser method is mainly used for external corrosion, mechanical damage, pipeovality and profiling. Figure 1: The scanner position itself by triangulation, cameras read the laser cross deformations as the scanner moves 4.1 Pipe surface preparation The penstock surface has prepared locally prior to the scan. The 3D laser scanner is an optical tool so the surface to be inspected must be visually accessible. Dirt, scale and rust must be removed by sandblasting the penstock surface or using another similar cleaning technique. The scanner will be able to scan the surface in any condition but cleaning it ensures meaningful results, as with any other NDT technique. 4.2 Scanner setup The 3D scanner has been calibrated before on-site inspection to ensure guarantee the accuracy is within tolerance. 4.3 Data acquisition The damaged area is recorded with the scanner. It takes less than 10 minutes per square meter at 2mm resolution to perform a full (100%) coverage with accuracy up to 40 microns. The scanner accuracy is a fixed value by scanner design but resolution is defined in

6 the software setup. Therefore, accuracy and resolution are not related when using a 3D scanner. 5.0 RESULTS AND DISCUSSION The 3D laser scanning was done for HPP1_A1_S3A and HPP1_A1_S10Aof High pressure pipe of penstock for external corrosion and mechanical damage inspection conducted on the various corrosion areas. The corrosion area on the surface of the pipe is cleaned and dimensions are measured accurately. The corroded area is determined based on longitudinal length and maximum depth, external corrosion imperfections shall be accepted regardless of the length of corroded area provided that the maximum depth of such imperfections is 10% less than the normal wall thickness of the pipe. The coincident of cracks and external corrosion imperfections are acceptable if the external depth does not exceed the thickness of corrosion allowance for the material. Corroded area concentrated with welded pipe and material exhibit brittle fracture is considered to be defects. Corroded areas not located in dents and have depth greater than 10% and including 80% of normal wall thickness is acceptable. If the longitudinal length of the corroded area does not exceed the maximum allowable extent as specified in ASTM B31G and Maximum operating pressure is equal or less than the failure pressure of the pipe containing the corroded area. The corroded areas that are not allowed as per ASTM B31G are considered to be defects, unless these defects are assed by other engineering assessment. These assessments to consider of the original design, service history and loading, service conditions of corrosive attack, corrosion formation and growth, corrosion shape and dimension, material properties like fracture toughness. The penstock containing defects shall be repaired by acceptable repair methods (permanent repairs preceded by temporary repairs) which include corrosion abatement effectiveness, inspection interval and continuous evaluation.

7 Case Study -1 Pipe name : HPP1_A1_S3A Number of features found 7 Scan resolution 1.0 mm Nominal outside diameter mm Pipe wall thickness mm Analyzed surface Outer surface Pit gauge parameters: Center 76.2 mm Extension mm Inspection parameters: Worst Case Profile resolution : 25.4 mm Interaction parameters: Axial criteria mm Circ. criteria mm Critical factor 10% Threshold 5% Method Fit to shape Filter None Deepest point on pipe: Max. depth mm Max. depth Axial pos mm Max. depth Circ. pos Total volume of material loss mm³ Flow Stress parameters: SMYS MPa Material Plain carbon steel Temperature 27.0 C 423.6MPa S ut S yt S flow B31G (Met- 1) 221.5MPa MPa Design factor MAOP MPa MOP 1.90 MPa Features summary: (MAOP = MPa ) (MOP = MPa ) Eff. area B31G Modif. B31G Feature ID Axial start Circ. start Pressure Pressure Pressure Max. depth Axial end Circ. end FP / MOP FP / MOP FP / MOP % rem. wall mm MPa MPa MPa Mm& % Feature Feature Feature

8 Feature Feature Feature Feature Case Study -2 Pipe name : HPP1_A1_S10A Number of features 7 Scan resolution 1.0 mm Nominal outside diameter mm Pipe wall thickness 14.7 mm Analyzed surface Outer surface Pit gauge parameters: Center 76.2 mm Extension mm Inspection parameters: Worst Case Profile resolution : 25.4 mm Interaction parameters: Flow Stress parameters: Axial criteria mm SMYS MPa Circ. criteria mm Material : Plain carbon steel Critical factor 10% Temperature 27.0 C Threshold 5% S ut 423.6MPa Method Fit to shape S yt 221.5MPa Filter None S flow B31G (Met- 1) MPa Deepest point on pipe: Design factor Max. depth 4.67mm MAOP MPa Max. depth Axial pos mm MOP 1.90 MPa Max. depth Circ. pos Total volume of material loss mm³ Features summary: (MAOP = MPa ) (MOP = MPa ) Eff. area B31G Modif. B31G Feature ID Axial start Circ. start Pressure Pressure Pressure Max. depth Axial end Circ. end FP / MOP FP / MOP FP / MOP % rem. wall mm MPa MPa MPa Mm& % Feature

9 Feature Feature Feature Feature Feature Feature CONCLUSIONS 1. The 3D laser scan technic can be good solution for the penstock inspection of corroded pipe pictographically with high accuracy, reliable data. 2. The laser scanner solution discussed in this paper is a better when compared to traditional NDT techniques for penstock corrosion and mechanical damage. 3. To ensure continued safe and reliable penstock operation when assets are aging and limited resources; it is important to assess the corrosion condition and maintain the same. 4. The complexity or imperfection of some of the evaluation process, the laser scanner approach has been discussed to understand the condition of the penstock needed to make decision on continued operation. 5. The penstock structure assessment it is important to consider corrosion inspection and evaluating the hoop stress and also to address other factors such as pipe thickness, material loss, area of corrosion for a safe and reliable operation. 7.0 ACKNOWLEDGEMENT The authors would like to thank authorities of CPRI for encouragement in this work and permission to present this paper. 8.0 REFERENCES 1) ASME B31G, Manual for Determining the Remaining Strength of Corroded Pipelines, American Society of Mechanical Engineer, 2009

10 2) Pierre-Hugues Allard, įpipeline External ωorrosion Analysis using a γd Laser Scanner, 18th WωNDT, April 1θ , Durban, South Africa. 3) Martin Fingerhut, Application of Laser Profilometry for Fitness-for-Service Assessment on Pressure Vessel External ωorrosion, IPω β01β, September β4-28, Calgary, Canada.