CWB Level 2 Questions and Answers Already Passed

CWB Level 2 Questions and Answers Already Passed What is the description of damage of a brittle fracture? Sudden rapid fracture under stress (residual or applied) where material exhibits little or no evidence of ductility or plastic deformation What type of Steel are of primary concern with brittle fractures? Carbon and low alloy steels. 400 series SS are also susceptible What has a significant influence on toughness and resistance to brittle fractures? Steel cleanliness and grain size In most cases brittle fractures occur only at what temperature? Below the charpy impact transition temperature.(or ductile-to-brittle transition temperature) The point at which the toughness of the material drops off sharply What kind of equipment or units can be affected for brittle fractures? *Can occur during ambient temperature hydro testing due to high stresses and low toughness at the testing temperature *Most processes run at elevated temperature so the main concern is during startup, shutdown, or hydro/tightening testing. What is the appearance of a brittle fracture? Typically appear straight, non branching, and largely devoid of any associated plastic deformation. (Although fine shear lips may be found along free adge of the fracture or localized necking around neck of crack. What is thermal fatigue a result of ? Cyclic Stress caused by variations in temperature. In thermal fatigue damage occurs as cracking, where may this occur? Can occur anywhere in a metallic component where relative movement or differential expansion is constrained. Particularly under repeated thermal cycling. What type of material can thermal fatigue happen on? All material of construction. What are a couple but not all critical factors for thermal fatigue? *Key factors affecting thermal fatigue are the magnitude of the temperature swing and the frequency (number of cycles) *Damage is also prompted by rapid changes in surface temperature that results in thermal gradient through the thickness or along the length of a component. What are some affected units or equipment of thermal fatigue? *Major problem in coke drum shells *another example is the mix point of hot/cold streams such as hydrogen mix points in hydroprocessing units. *locations where condensate comes in contact with steam systems such as de-superheating or attemporating equipment What is the appearance of a thermal fatigue crack? *Usually initiates on the surface. Generally wide and often filled with oxides due to elevated temperature exposure. *May occur as single or multiple cracks *usually propagate transverse to the stress and are usually dagger shaped, transgranular, and oxide filled. *Can also sometimes be axial, or circumferential or both at the same location. How is thermal fatigue best prevented? Through design and operation to minimize thermal stresses and thermal cycling. Methods of prevention include but not all: Control rate of heating and cooling during startup and shut down Design that incorporates reduction of stress concentrators, blend grinding of weld profiles & smooth transitions should be used. What are 3 methods of inspection for thermal fatigue? *Visual Inspection *MT *PT Define Erosion? Accelerated mechanical removal of surface material as a result of relative movement between, or impact from solids, liquids, vapors or any combination thereof. Define Erosion-Corrosion? description of the damage that occurs when corrosion contributes to erosion by removing protective films or scales, or by exposing the metal surface to further corrosion under the combined action of erosion and corrosion. What kind of units/equipment are affected by Erosion, Erosion-Corrosion? All types of equipment tat are exposed to moving fluid and/or catalyst are subject to erosion, erosion-corrosion. What is the appearance of damage for Erosion, Erosion-Corrosion? Localized loss in thickness in the form of pits, grooves, gullies, waves, rounded holes or valleys. These often exhibit a directional pattern. What are some ways of prevention/Mitigation for Erosion, Corrosion-Erosion? -Improvement in design. Involving *shape, geometry, material selection -Improvement of resistance achieved through increasing substrate hardness using harder alloys, hard-facing or surface-hardening treatments. -Heat ex changers utilize impingement plates and occasionally tube ferrules to minimize erosion What is used to detect refractory loss on stream when erosion is suspected? IR scan What are 3 ways to inspect an area of erosion Visual UT checks RT to check extent of metal loss What is the description of Mechanical Fatigue? Mechanical fatigue is a form of degradation that occurs when a component is exposed to cyclical stresses for an extended period, often resulting in sudden unexpected failure. What type of material is subject to Mechanical fatigue? All engineering alloys are subject to fatigue cracking What are the predominant factors in determining the fatigue resistance of a component? Geometry, Stress Level, Number of cycles and material properties ( strength, hardness, micro-structure) With mechanical fatigue where do fatigue cracks usually initiate? Surface of mechanical notches, stress raisers under cyclic loading, Weld joints, tool markings, grinding marks, corrosion areas, key holes on drive shafts With what 3 types of material does fatigue fractures decrease with stress amplitude until an endurance limit is reached? Regardless of the number of cycles Titanium, Carbon Steel, and Low Alloy Steel. What are some affected units/equipment from mechanical fatigue? Thermal Cycling; *Equipment that cycles daily like coke drums *Equipment that is on continuous standby but sees intermittent service such as auxiliary boilers. *Quench nozzle connections that sees significant temperature deltas during operation like a water washing system. Mechancial Loading ; *Pressure swing Absorbers on hydrogen purification units *Rotating shafts on centrifugal pumps and compressors that have stress concentrations dur to changes in radii and key ways *Components like small diameter piping that may see vibration from adjacent equipment and/or wind. *High pressure drop control valves or steam reducing stations can cause serious vibration problems in connecting piping. What is the appearance of mechanical fatigue damage? The signature mark for fatigue failure is a clam shell type fingerprint. What can you do to prevent mechanical fatigue? *Best defense against fatigue cracking is good design. *Selecting a metal with a design fatigue life sufficient for its intended cyclic service *Allowing for generous radius along edges and corners How do you inspect for mechanical fatigue cracks? NDE techniques such as PT,MT, and SWUT How would you inspect small diameter piping to detect oscillation or other cyclical movement that could lead to cracking? VT Define Galvanic Corrosion? Form of corrosion that can occur at the junction of dissimilar metals when they are joined together in a suitable electrolyte, such as a moist or aqueous environment, or soil containing moisture. What kind of material is affected by galvanic corrosion? All Metals with the exception of most noble metals What are 3 critical factors that must be met for galvanic corrosion? 1)Presence of an electrolyte, a fluid that can conduct a current. Moisture or a separate water phase is usually required for a solution to have enough conductivity. 2) 2 different materials or alloys known as the anode and the cathode, in contact with an electrolyte. 3) An electrical connection must exist between the anode and the cathode. What units/equipment does galvanic corrosion affect? *Galvanic Corrosion can occur in any unit where there is a conductive fluid and alloys are coupled. *Heat exchanges are susceptible if the tube material is different from the tube sheet and/or baffles. Usually in salt water cooling systems *Buried pipelines, electrical transmission support towers and ship hulls are typical locations for galvanic corrosion What is the appearance or Morphology of Damage with Galvanic Corrosion? *Damage occurs where 2 materials are joined at welded or bolted connections. *The more active material can suffer generalized loss in thickness or may have the appearance of a crevice, groove or pitting corrosion, depending on the driving force, conductivity and relative anodic/cathodic areas ratio. What is the best prevention for galvanic corrosion? Best method is through design *Differing alloys should be in intimate contact in conductive environments unless the anode/cathode surface area ratio is favorable. *Coatings can be helpful *With piping specially designed electric insulating bolt sleeves and gaskets can eliminate the electrical connection. How can you inspect galvanic corrosion? Visual inspection UT thickness gauging are very effective methods. Describe Corrosion Under Insulation (CUI)? Corrosion in piping, pressure vessels, and structual components resulting from water trapped under insulation or fireproofing What kind of material can be affected from Corrosion Under Insulation? Carbon Steel, Low alloy steels, 300 Series SS, and duples stainless steels What are some critical factors for Corrosion Under Insulation? It affects externally insulated piping and equipment and those that are in intermittent service or operate between: 10F(-12C) and 350F(175C) for carbon and low alloy 140F(60C) and 400F(205C) for austenitic stainless steels and duplex stainless steels. Corrosion rates will increase with increasing metal temperature up to the point where the water evaporates quickly (ex. Corrosion becomes more severe at metal temperatures between boiling point 212F(100C) and 350F(121C), where water is less likely to vaporize and insulation stays wet longer. Insulating material that hold moisture (wick) is a problem Equipment that operates below water dew point creates condense water on metal surfaces Cyclic thermal operation or intermittent service can increase corrosion Plants located in areas with high rain fall Environments that provide airborne containments such as chlorides, stack emissions (SO2), cooling tower drift What kind of equipment is affected by corrosion under insulation? All equipment is susceptible to CUI What are some examples where corrosion under insulation can occur? Can be found on equipment with damaged insulation, vapor barriers, weatherproofing or mastic, or protrusions through the insulation or at insulation termination points such as flanges. Piping or equipment with damaged/leaking steam tracing. Localized damage to coating or paint systems Where water/moisture will naturally collect. Deadlegs (vents, drains, and other similar items) Pipehangers Caulking that has hardened, has separated, or is missing What is the appearance of corrosion under insulation? Carbon and low alloy steels are subject to localized pitting corrosion and loss of thickness 300 Series SS are subject to stress corrosion cracking is chloride is present 300 Series SS and Duplex SS are subject to pitting and localized corrosion After insulation is removed from carbon and low alloy steels CUI often appears as loose flaky scale covering corroded components. What is some prevention for corrosion under insulation? Best achieved by appropriate paints/coatings and maintaining insulation/sealing/vapor barriers to prevent moisture ingress. Flamed-Sprayed alluminum coatings have been used for carbon steels Using aluminum foil wrap on stainless steel piping under insulation Closed-cell foam glass materials will hold less water against vessel/pipe wall What are some ways to inspect corrosion under insulation? Should be structured and systematic approach starting with prediction, then looking at more invasive procedures. Inspectors should consider areas that are most susceptible such as *Partial stripping of insulation fr visual exam *UT for thickness *Real-time Xray for small bore piping *Neutron backscatter techniques for identifing wet insulation *Deep penetrating eddy-current inspection *IR thermography *Guided wave UT