This paper describes the application of the FFS/DTA methodology to assist plant asset managers in dealing with cracks that
have been detected in in-service components, particularly those in the power, petrochemical, mining and transport industries.
Several state-of-the-art structural integrity assessment procedures such as AS/NZS 3788, BS7910, R5-R6 and API-579-1/
ASME FFS-1 are described and discussed; and their application to practical situations using the principles of FFS/DTA is illustrated
through a series of selected case studies. The usefulness, effectiveness and versatility of this fracture-mechanics based
methodology for examination of in-service cracked components is amply demonstrated.
Featured in the Australasian Welding Journal Welding Research Supplement
This paper describes finite element and fracture mechanics based modelling work that provides a useful tool for evaluation of the remaining life of a high pressure (HP) steam turbine rotor that had experienced thermal fatigue cracking. An axis-symmetrical model of a HP rotor was constructed. Steam temperature, pressure and rotor speed data from startups and shut downs were used for the thermal and stress analysis. Operating history and inspection records were used to benchmark the damage experienced by the rotor.
Fracture mechanics crack growth analysis was carried out to evaluate the remaining life of the rotor under thermal cyclic loading conditions. The work confirmed that the fracture mechanics approach in conjunction with finite element modelling provides a useful tool for assessing the remaining life of high temperaturecomponents in power plants.
|Posted in: Risk Based Assessments Analytical Services Environment|
At 2:43 am on the 1st of January 2004 an explosion and fire was caused by the failure of a cold box manifold. This destroyed one gas processing train and interrupted natural gas production for approx. 6 weeks. HRL Technology was commissioned to determine the root cause of this major failure.
The fire resulted from the failure of an inlet nozzle to a cold box which is part of the refrigeration process in the Liquid Recovery Plants (LRP). The findings from the HRL study, based on analysis of samples taken from the affected area, found that the failure of the inlet nozzle was due to liquid metal embrittlement (LME) of the aluminium cold box by elemental mercury. LME is a complex metal fracture mechanism that occurs without warning. Natural gas from subterranean reservoirs in Australia and South East Asia typically contains minute levels of elemental mercury.
The natural gas purchased by consumers consists almost entirely of methane, the simplest hydrocarbon. In gas reservoirs, however, methane is typically found in mixtures with heavier hydrocarbons - such as ethane, propane, butane and pentanes - as well as water vapour, hydrogen sulphide , carbon dioxide, nitrogen and other gases. Almost all of these substances are removed from the gas stream at processing plants located near production areas. The actual practice of processing natural gas to pipeline dry gas quality levels can be quite complex, but usually involves four main processes:
Once NGLs have been removed from the natural gas stream, they are broken down into their base components and separated into different streams. The process used to accomplish this is called fractionation. Because the different NLGs have different boiling points fractionation can separate the constituents in stages by boiling off hydrocarbons one by one. The purpose of the LRP at the refinery is to recover the maximum amount of liquid hydrocarbon components from the gas feed utilising cryogenic fraction. This is accomplished by cryogenic distillation, separating ethane and heavier hydrocarbons from sales methane. Mercury is known to collect during this process.
The heat exchanger that failed is shown in Figure 1(a) and the failed nozzle is shown in Figure 1(b).
The failure investigation focussed on the failed nozzle that was relocated to the HRL laboratories in Mulgrave, Melbourne, Australia. A section from this nozzle is shown in Figure 2. All cracks and fracture surfaces were contaminated with elemental mercury and/or oxide whiskers consistent with mercury induced corrosion. Significant bulging of the manifold had occurred in the vicinity of the rupture, and there was extensive secondary cracking, particularly in areas showingthe most deformation. From chevron markings on the longitudinal fracture surface it was determined that the crack propagated in the downstream direction with further chevron markings either side of the longitudinal crack bifurcation at the flange indicating further crack growth away from the longitudinal rupture.
There was significant delamination cracking in the circumferential/longitudinal plane running from the longitudinal rupture for distances varying from 25-75 mm as shown in Figure 3(a) and significant fine branched cracking subsurface as shown in Figure 3(b) and Figure 4.
The brittle nature of the failure and observations of elemental Hg on the fracture surfaces, delamination of the aluminium nozzle material and branched intergranular cracking strongly suggested the failure had occurred through LME. Consequently an investigation was commenced into the likelihood of mercury induced LME in gas plants.
Types of Hg Attach in Aluminium Heat Exchangers
Mercury can occur in natural gas feed stock, sometimes in quantities sufficient to cause severe attack and failure of cryogenic aluminium heat exchangers and other gas conditioning and separation processes. The cooling equipment in a gas separation or dew point control process is typically an aluminium plate-fin heat exchanger, the construction of which is often an Al 3003 core with Al 5083 or 6061 headers, nozzles and piping.
Mechanisms of Mercury Attach
The mercury can degrade aluminium materials via three basic mechanisms:
Amalgamation is the process by which mercury forms liquid solutions with various metals, primarily Al, Au, Ag and Zn. Of these, Aluminium has engineering significance. The Aluminium is generally prevented from contact with mercury by the Al2O3 protective surface oxide. While the oxide on Aluminium is not homogeneous and contains numerous defects, in general mercury lacks the ability (due to high surface tension) to diffuse through these microscopic cracks and defects to reach the underlying metal. However, this inability to penetrate the protective oxide layer may be mitigated by thermal or mechanical stresses, by abrasion and by some chemical environments.
Once mercury has breached the aluminium oxide and come in contact with the underlying metal, the rate of amalgamation depends, largely on the metallurgy and microstructural condition. It is observed, for example, that mercury amalgamates selectively with weldments and more rapidly with some alloys. Amalgamation generally results in surface etching and pitting as long as no water vapour is present. It can result in the loss of mechanical strength in weldments.
Amalgam corrosion is the combined action of mercury and moisture on susceptible materials, primarily Al, Cu and Sn. The difference between the mode of attack and simple amalgamation is that the corrosion process requires water and propagates with miniscule amounts of mercury. The reaction is:-
Hg + Al Hg(Al) amalgam (1)
Hg(Al) + 6H2O Al2O3 . 3H2O + 3H2 + Hg (2)
Hg + Al Hg(Al) (3)
The amalgam corrosion process does not consume the mercury and hence is self-propagating so long as mercury is in contact with aluminium metal and water is available. If sufficient moisture and mercury are present, aluminium structural components can be penetrated fairly rapidly. The rate of attack is mass transfer limited but does not proceed as rapidly as liquid metal embrittlement (LME). The amalgam corrosion reaction proceeds more rapidly with some aluminium alloys, but all are susceptible to some extent.
The amalgam corrosion was unable to occur if the aluminium was not wetted by the mercury and the reaction required the presence of moisture. Nelson acknowledges that mercury can cause catastrophic attack of aluminium in the presence of 'free' water, however, suggests that in cryogenic heat exchangers the presence of water is extremely unusual, and has not been a factor in any reported leak occurrence.
Liquid Metal Embrittlement (LME)
Liquid metal embrittlement by mercury differs from amalgamation and amalgam corrosion in that LME produces rapid brittle fracture, affects a broader range of materials (Al, Ni-Cu, Brasses, Cu alloys, Sn alloys, and some stainless steels) and is generally much more severe than other embrittling processes such as hydrogen-embrittlement or stress-corrosion cracking. Once cracks have initiated, very rapid sub-critical cracking can occur even at low stresses. Cracking occurs preferentially along grain boundaries for the Al:Hg couple (and for many other couples), but transgranular (cleavage-like) fractures can also occur. Liquid metals are drawn into growing cracks so that the crack tip is always in contact with embrittling metal atoms. The rate-controlling process for cracking is still being debated, but the rate of flow of liquid within cracks may control the rate of cracking in some circumstances
Thin films of liquid metal are left behind the advancing crack tip and, hence, fracture surfaces are covered with a film of liquid mercury. For the Al:Hg system, 'de-wetting' can occur so that small globules of mercury are present on fracture surfaces. The presence of mercury on fracture surfaces can also result in the growth of oxide whiskers after fracture, a phenomenon peculiar to Al (and to a lesser extent Mg) alloys and described in the section on amalgam corrosion.
Breaching the Aluminium Protective Oxide
From the above discussion it is quite clear that for LME cracks to initiate there must be intimate contact between liquid and solid metals, with no intervening oxide films to prevent wetting and adsorption. Al alloys are covered by a thin, protective oxide film, and surfaces can be covered by liquid mercury indefinitely without any reaction until the oxide is damaged. Oxide films can be broken by mechanical processes e.g. by scribing or abrasion, by chemical processes, e.g. corrosion, or by plastic deformation of the aluminium resulting in slip steps at the surface.
Gordon favours the abrasion on the surface by hard particles in the gas or liquid streams. He suggests that as the gas entering cryogenic equipment usually consists of mainly methane, CO2 and hydrogen, it is oxygen-free. Hence the reducing atmosphere of the gas stream may prevent any reformation of the protective oxide layer once damaged from hard particles in the gas stream has occurred. Others suggest that the differential thermal expansion between the aluminium substrate and the alumina oxide being a factor of around 3 could cause the oxide to crack when the heat exchangers is warmed.
Whichever mechanism is operative in this situation the brittle nature of the cracking, the intergranular nature of crack propagation and delamination, and the presence of Hg on the fracture surface supports the Hg induced LME failure mechanism.
LME of aluminium alloys requires:
The conclusion that the aluminium heat exchanger failed due to mercury induced LME is based on the following:
Sincere thanks go to Dr Stan Lynch (DSTO now retired) for his invaluable assistance in this investigation.
Those who would like to read more about this failure are directed to 'The interaction of mercury and aluminium in heat exchangers in a natural gas plants', R. Coade, D. Coldham, May 2006 · International Journal of Pressure Vessels and Piping.
W Gordon. 'Assessing the susceptibility of an LNG Plant to Mercury-induced Attack', 14th International Corrosion Congress Proceedings, Cape Town, South Africa, Oct 1999.
M Pinnel & J Bennett, 'Voluminous Oxidation of Aluminium by Continuous Distribution in a Wetting Mercury Film', J of Mat Sci, 7, 1972, p1016
L Bruce & G Wise, "Comments on - Voluminous Oxidation of Aluminium by Continuous Distribution in a Wetting Mercury Film', J Mat Sci, 9, 1974, p335
D R Nelson, 'Mercury Attack of Brazed Aluminium Heat Exchangers in Cryogenic Gas Service', Proceeding of the 73rd GPA Annual Convention, New Orleans, March 1994, pp178-183
M.O. Speidel, p. 302, The Theory of Stress Corrosion Cracking, NATO, 1971, and
J.A.Feeney and M.J. Blackburn, p. 389, ibid
|Posted in: Case Study Reliability Efficiency Sustainability Power Plants Heat exchange Analytical Services Environment|
HRL Technology Group's plant performance team has extensive experience in thermal power plant performance testing and optimization across Australia and South East Asia including Indonesia, the Philippines and Hong Kong. We combine high-tech calibrated field testing equipment, feasibility studies, laboratory and pilot-scale facilities and the latest desktop modelling techniques to investigate and optimize process efficiency and deliver tested solutions which improve heat rates and electrical output, improve plant flexibility including low load operation, reduce off-loading and deliver on-going financial and environmental benefits.
We apply our expertise in both large and small industrial scale boilers raising steam and generating power and also in other plants operating at high temperature and/or pressure. Examples of industrial equipment that can be tested includes boilers, mills, rotary air heaters, furnaces, rotary kilns, smelters, dust collection equipment, fans and turbines.
In a recent project a team of HRLTG engineers and technicians performed a comprehensive testing program over one month in a power station in Indonesia. The testing involved turbine heat rate testing and determination of boiler thermal efficiency.
The testing and subsequent data evaluation and power plant modelling was performed to evaluate the impact of a fuel change on plant operation and efficiency, to assist in establishing the Power Purchase Contract for the plant, to determine future fuel usage and provide information for an upcoming turbine upgrade. The program was very successful and HRLTG has been requested to perform similar work programs for other Indonesian power plants.
|Posted in: Efficiency Power Plants|
HRL Technology has undertaken risk based inspection at power stations throughout South East Asia and Australia, to understand critical component with life limiting issues and/or recommendation of extension of inspection interval where required in accordance with the local statutory regulations. In many instances the RBI leads to identification of emerging issues and an opportunity to develop a targeted asset management programs, identifying required works and/or options for improvement, and developing budgetary estimates and programs for such activity.
With limited inspection budget, it is often necessary to optimise inspection schedules and resources through risk based approach and the processes are used to better define the plant inspection and NDT requirements to feed into the maintenance program over the life of the station or to prepare for replacements where directed by the process. Most importantly, the assessments are undertaken to ensure the safe use of at risk component to continue operation economically and reliably until a scheduled outage.
As power stations are approaching their end of design life, power station asset managers are responsible for the continued safe operation of plant while maximising plant availability with minimum cost. Our independent risk based inspection are equipped to identify the life limiting component in a power plant for current and future operation
The opportunities are:
Risk based inspection can be used to:
RBI provides the following benefits:
Our assessments are performed using current industry methods based on AS Standards, ASME, R5/R6, API 579 and British Standards
|Posted in: Reliability Efficiency Power Plants|
It seems like the year has just flown since February this year when I joined HRL as CEO, and it is important, at this time, to thank you, our customers and friends for your ongoing support during 2016. We look forward to working togther in 2017.
At HRL, our team comes to work every day driven to solve our customers' most difficult technical challenges, deliver innovative solutions, create real value and transform commercial performance. Our experts take deep personal pride in sha
king hands with a customer inviting them back to work on a new project because of the sustained value created last time. We are proud to have some of Australia and South-East Asia's largest energy, resources, and industrial companies as regular customers.
Over time, HRL has assembled and continues to build a genuinely unique blend of advanced laboratories and field testing, materials, science, digital and engineering capability stemming from our roots as the R&D group within the State Electricity Commission Victoria.
Our mission today is to dramatically improve the reliability, efficiency, and sustainability of our customers' assets and processes, creating operational competitive advantage which can make the difference between exceeding performance targets or not. We are also leading forensic metallurgical failure investigators, often providing expert witness services in State and Federal courts. In galvanising our brand, we are also re-invigorating how we partner with customers - we call this the HRL Blueprint.
It's a tried and trusted, yet differentiated approach, applying all our years of experience plus new thinking to tailor a relationship and outcome best-suited to the customer's situation. As your business identifies the technical challenges it needs to solve to sustain and drive growth, the team at HRL looks forward to partnering with you to deliver the innovation you need to win in the market.
From all of us at HRL the very best wishes for a wonderful holiday season, and a healthy and successful year ahead.
From where we sit, it's an exciting time in the world with major advances being made at the intersection of multiple technical and commercial disciplines. In an intensely competitive market, there are huge opportunities for companies who can successfully innovate.
Best wishes for a safe and enjoyable festive season.
|Posted in: CEO message|