Acoustic Emission Testing – Data Analysis
The data obtained from an acoustic emission test of a pressure vessel are analyzed to determine the existence of flaws and their locations in the shell of the vessel. Accurate analysis of the data is highly dependent upon the expertise of the vendor performing the test. Only those vendors that can demonstrate that they have acquired appropriate knowledge and experience should be used. It is vitally important for the vendor to be able to distinguish between acoustic emissions attributable to flaws, and other “noises” detected by the acoustic sensors during the test.
The acoustic emission signals detected by each sensor during a test are generally characterized for analysis by number of counts above a threshold amplitude that determines the sensitivity of the test, amplitude, duration, energy (area under signal envelope), and rise time as illustrated in Figure 700-25. The data are then displayed on CRT monitors as illustrated in Figure 700-26.
Plots of cumulative count, and count rate vs. time are shown in Figure 700-26a and b respectively. A plot of cumulative count rate vs. test pressure is shown in Figure 700-26c. An abrupt increase in counts vs. either time or test pressure is an indication of the existence of a significant flaw. Innocuous flaws are generally characterized
by a more gradual increase in counts.
A plot of the number of counts vs. amplitude is shown in Figure 700-26d, which can be useful for distinguishing between various plastic deformation and flaw propagation mechanisms, or for separating emissions associated with flaws from background noise. Figure 700-26e is a plot of the cumulative number of counts of equal or greater amplitude, which is more useful for evaluating the severity of a flaw.
Figure 700-26f is an acoustic emission source location display, which is basically a map of the vessel with the computer location of each emission source. This map can be used to focus subsequent NDE, such as MT and UT, that are usually desirable to confirm the existence of flaws in the vessel and to determine their sizes for fitnessfor-service evaluations (see Section 750).
The location of flaws is determined by analyzing the arrival time of the same acoustic emission from a flaw at different acoustic sensors attached to the shell of the vessel. The arrival time of an acoustic emission at a sensor is dependent on the distance of the flaw from the sensor, and the speed of sound in the material. Therefore, the differences in arrival time of the same emission at different sensors can be used to locate the flaw by triangulation. A computer is used to perform this function using the raw time-of-arrival data recorded during the test, and to drive an X-Y recorder to plot the location of each flaw on a “map” of the vessel. The computer is programmed to calculate the distances between the flaw and the sensors following the curvature of the vessel shell, and not as straight lines through the air between the flaw and the sensors.
The above displays of acoustic emission data can be obtained during the test to monitor the results, and can subsequently be recreated from the recorded data for more detailed analyses.
Acoustic Emission Testing – Physical Principles
Sonic impulses are emitted by a flaw in a ductile material subjected to a gradually increasing applied stress, due to the release of strain energy in the intensified stress field associated with the flaw. The release of strain energy creates stress waves that propagate through the material at the speed of sound. These stress waves behave like sonic impulses, and are, therefore, commonly referred to as acoustic emissions. However, there may be no significant release of strain energy in a brittle material to cause acoustic emissions prior to the occurrence of fracture.
Acoustic emissions attributable to flaws in metals are usually inaudible, but they can be thought of as being similar to the familiar sounds emitted by a piece of wood as it is gradually bent to the breaking point. The sounds not only reveal that the wood is cracking, but an individual hearing the sound can also sense the direction that it is coming from to help locate the piece of wood. It is useful to draw another analogy for brittle materials, such as a glass rod that is bent to the breaking point. No warning sounds will be heard as the glass rod is bent before it breaks.
The nominal applied stress in a component of a pressure vessel manufactured from a ductile metal can be well below the yield strength of the material, but the local stress concentrations developed at flaws can be high enough to cause highly localized plastic deformation or enlargement of the flaw by microcrack propagation. Both plastic deformation and crack propagation produce acoustic emissions that can be detected by acoustic sensors to disclose the existence of the flaw.
A single sensor can usually detect acoustic emissions originating anywhere in a relatively large vessel, because attenuation of the sonic impulses in a metal shell is very low. However, several sensors are usually employed to obtain confirming indications, and, more importantly, to locate the origin of the emissions in the shell by triangulation.
Acoustic Emission Testing (AE)
AE was developed to assess the integrity of pressure-containing equipment in the nuclear power industry. Acoustic sensors are attached to the O.D. surface of the equipment to detect and locate flaws in the material during a pressure test. AE can be a very useful NDE method to use for the in-service inspection of pressure vessels, especially because one test will cover all components of a vessel and weld joints in the shell.
Acoustic Emission Testing – Acoustic Sensors and Instrumentation
Most acoustic sensors used for acoustic emission testing of pressure vessels employ piezoelectric ceramic elements that convert the stress waves (i.e., acoustic emissions) propagating through the vessel shell into electrical impulses. These sensors are usually attached to the vessel shell with magnets or an epoxy glue. Several sensors are positioned on the shell in a geometric pattern that is based on the size and configuration of the vessel, to be certain that significant emissions from any major component of the vessel will be detected by at least two of the sensors. Multiple sensors are also required for the location of the acoustic emission sources by triangulation.
The electrical outputs of the acoustic sensors are sent separately to a preamplifier that contains a band pass filter to cut off frequencies below 50 kHz. Acoustic emissions with lower frequencies are predominantly mechanical and hydraulic “noises” (such as movement of insulation, vessel supports, internals, and flange connections), and they could seriously confuse the interpretation of emissions from flaws. Some emissions from these mechanical and hydraulic sources can have higher frequencies that coincide with those that originate at flaws, which must be taken
into consideration when analyzing the data. The filtered signal from each sensor is subsequently sent to a main amplifier, and then on to various multichannel recording and monitoring instruments.
Recording of the acoustic emission data for permanent retention is usually accomplished with a magnetic disk. The amplitude of every emission detected by each acoustic sensor is recorded on a time base. Audio and visual monitors can be provided for “real-time” observation of the test results. X-Y recorders are used to plot the acoustic emission data as a function of some test parameter, such as hydrostatic test pressure. These plots can be made during the test, but they are more commonly made immediately after the test for a more detailed analysis of the data. X-Y recorders can also be used to plot on a “map” of the vessel the locations of the flaws revealed by the acoustic emissions.
Pressure transducers are used to determine the hydrostatic test pressure, which is also recorded on the magnetic disk on a time base.
Inspection of Spheres
In general, the practices that apply to pressure vessels also apply to spheres although the techniques may vary. The tall legs necessary to support spheres should be carefully inspected to determine the condition of the fireproofing and the rain seals.
Remove cracked or spalled fireproofing and inspect the exposed steel. Earthquake or sway bracing must be tight and in good condition.
Inspect the cooling deluge rings on the upper shell, if any, for deposits of rust and sediment. Severe external corrosion can occur at these rings if the drain openings are not kept clean.
The internal surfaces can be inspected by filling the sphere with water and making the inspection from an inflatable raft. Life jackets must be worn while making this inspection. The sphere is usually filled to a point near the top and then inspected and gaged at various levels as the water is emptied.
Voice communication is difficult in a sphere due to reverberation and echoes. Do not take the portable ultrasonic instruments into the sphere on a raft. A large horseshoe magnet is available for use in the spheres to assist in holding the raft to the shell during gaging operations. All tools and equipment should be tied to the raft to prevent loss. Another method to gage the shell is the use of a magnetic ultrasonic crawler.
Reactors External Inspection
Visually inspect gasket faces if opened, ORJ gaskets and bolting material. On vessels with pressure sealed gaskets check for distortion, corrosion, and cleanliness.
a. Obtain UT thickness measurements at representative locations on shell and heads.
b. For Isomax reactors, ultrasonically shear wave top and bottom heads, headto-nozzle welds, head-to-shell welds, nozzles, shell-to-support welds. This inspection is to be performed by qualified personnel using the established procedure. All findings should be recorded on appropriate drawings for comparison with future inspections. This inspection will be performed at scheduled frequencies as necessary.
Inspect for corrosion and spalled concrete. Inspect condition of platforms, stairs, and guard rails. Deteriorated areas should be cleaned and painted.
Reactors Internal Inspection
1. Inspection Ports
Obtain measurements at corrosion reference points on reactor shell. Inspect for corrosion or scale at these locations. Loose scale on the bottom head is probably a combination of scale and catalyst. This scale can cause corrosion and must be removed.
2. Basket Tray
Inspect for basket screen deterioration and plugging. Report percentages of plugging. Plugged screens can be cleaned by hydroblasting. Thin or corroded screens should be replaced.
3. Quench Flex Hoses
Look for broken bonding or severely distorted hoses. If hoses are distorted and bonding is bunched up, replacement will probably be required. Hoses may require hydrotesting to determine soundness. Hydrotest up to 250 psig.
4. Catalyst Support Screens
Inspect for corroded or holed-through areas often caused by hot spots, where the catalyst can migrate through the screen. Do not overlook internal manway cover screen. Inspect screens for plugging, and report the percentage of plug-ging. In most cases plugged screens can be cleaned by hydroblasting. Inspect space cloth where screens have corroded through; any thinning might necessitate replacement. Loss of aluminized coating on screens will reduce screen life to approximately 1 year, and screens so affected should be replaced. Obtain a screen sample for a bend and brittleness check.
5. Tray Support Beams
Visually inspect for any distortion or cracked welds. Inspect to determine if each tray (bed) is properly supported. Slight bowing upward was noted on one reactor which was caused from reverse flow through the reactor.
6. Tray Sections
Inspect for cleanliness and condition.
7. Thermowell Bundles
Inspect for external iron sulfide scaling and corrosion. Obtain O.D. measurements. Several thermowells have required replacement because of severe external loss.
Pressure test with helium or nitrogen. If pressure drops, leaks can be located with a Delcon ultrasonic translator (noise detector).
8. Quench Lines
Note if they are in position; failure can be expected at threaded connections below support tiers. This has occurred at Richmond.
9. Catalyst Dump Pipe (in sections)
Inspection to be made after removal from the reactor. Look for corrosion, wear, and distortion. Any leak can cause bypassing around catalyst beds, and may require unloading the reactor to repair.
10. Inner Bottle
Inspect wall for scale, pitting and cracks. Any corrosion or excessive scaling should be reported.
Inspect for condition.
Pressure Vessels Inspection
1. External Corrosion
Horizontal drums resting on concrete support saddles frequently corrode externally in the area of the saddle. Inspect the water seal at the edge of the concrete If evidence of corrosion exists, it might be advisable to lift the vessel off the support or cut away the concrete for more complete inspection. Look for corrosion on the external surface of vessels where salt water drips on them from overhead coolers or condensers.
Rock, pall rings, or Raschig rings, etc., used to provide large surface area and good mixing in some vessels should be inspected for excessive breakage and fouling. Fouling or plugging of the bed can cause undesirable channeling of the flow through the vessel. Corrosion is usually more severe in a vessel in the area of the packing.
3. Demister Pads
Demisters fabricated from wire mesh pads are installed in vessels to prevent a carryover of liquids. Inspect carefully to see that the supports, ties, and retainers are securely fastened and that they cannot be bypassed. Severe fouling or plugging of a demister pad can cause it to be blown out of position.
4. Impingement Plates
Impingement or wear plates in vessels should be inspected for corrosion or erosion, and the attachment welds or bolting checked to prevent loss of the plate. Inspect the shell of the vessel carefully for corrosion adjacent to the impingement plate to be sure that the plate is large enough to cover all of the affected areas.
5. Nonmetallic Lining Materials
Vessels in highly corrosive service may be internally protected by many materials. Among those commonly used are glass, various grades of fiberglass, rubber, plastic and numerous paint-like products.
Glass linings are highly effective to protect against unusually severe corrosion, but are subject to damage by impact or localized temperature changes. Inspection usually consists of a careful search for cracks or chipped areas. Cracks, pinholes, or gas pockets in the glass can result in a leak in the vessel.
Extreme care must be taken during the entry and inspection of glass lined vessels to protect the glass from impact damage. In addition, the vessel shell and nozzles must be protected from external blows or heating. No welding, flame cutting, or hammer-testing is permitted, and the vessel should be so stenciled.
Temperature limitations may be imposed for steaming, washing, and placing the vessel in service to avoid thermal expansion strains.
All other types of protective coatings should be carefully inspected to be sure that the bond to the shell is tight and for surface deterioration. Fiberglass, plastic, and rubber linings sometimes pull loose from the shell, permitting a corrosive attack on the vessel. Loose pieces of lining material may plug off the outlet of the vessel and cause a shutdown. It is advisable to take a few thickness readings on the coatings of all lined vessels while performing an internal inspection. Most of these coatings will deteriorate at elevated temperatures.
Distillation Columns Internal Inspection
Entry of a column can be made only after the approved, signed entry tags are in place on the column. Safety rules in effect at the vessel must be observed. Inspectors must not work alone in vessels but always in pairs to provide maximum safety. Do not attempt an internal inspection until the vessel has been adequately cleaned to permit a satisfactory inspection. If you cannot see it, you cannot inspect it.
2. Shell and Heads
Inspect the shell and heads for defects. Look particularly behind downcomer plates, on the shell opposite nozzles at points of impingement, under nozzles for rundown attack, at the liquid level on the trays, and at shell welds. Corrosion can appear in many forms, some of which are not readily apparent. A smooth general loss can look like the original surface. Observe for pitting in isolated areas. Measure the depth of the corrosion, if possible, with a depth micrometer and ultrasonically gage the remaining thickness of the shell or heads where corrosion is severe. Measure all fixed gage points using a depth micrometer, or ultrasonically gage at established locations.
3. Column Internals
Inspect all bubble cap trays for out-of-level, and for leaks and holes that could affect the liquid seal on the trays. Check the weirs at the edge of the trays to be sure they will maintain the proper level. Note the condition of the internal tray manways and the gasket surfaces on tray and cover.
Examine all tray support members for mechanical defects or corrosion. If corrosion is a problem, measurements should be obtained to establish corrosion rates.
Bubble caps, chimneys, bolts and holding members should be tight and in position. If they are loose enough to rattle, the tray will leak excessively. Note the condition of the caps. Check for corrosion.
Visually inspect and hammer-test all internal piping and spargers. Check spray holes in reflux headers to see that they are not plugged. Inspect the tray and shell carefully in the reflux area as corrosion is frequently severe in this area.
Inspect the reboiler baffle for leaks or holes, and ensure that the baffle manway cover fits tightly and is properly gasketed. Check that the vortex strainer is securely in place.
a. Strip Lining
Special corrosion resistant linings may be installed in columns or vessels where corrosion rates are excessively high. These linings are usually of light gage strips of alloy material welded to the shell. Where lining has been installed it should be carefully inspected for cracks or corrosion. Where lining failures have occurred, representative sections should be removed for gaging of the shell.
Carefully check the condition of all shell and head lining for full protection. If there is any evidence of leakage or corrosion deposits between the lining and vessel wall, request removal of a section of the lining. Recommend abrasive blasting the exposed vessel wall and inspect for corrosion. Record the extent of corrosion observed. The lining must be maintained in such condition as to prevent any circulation of stock between it and the
Inspect lined nozzles for bulges, collapse of the liner seams, and for torn or cracked weld seams.
Whenever lining is removed, inspect the metal surface of the column, particularly at the welds, connections, riveted seams, and vacuum bracing for corrosion.
In case of new lining installation or lining repairs, abrasive blast and inspect all column welds and joints before lining, and any which are exposed during repair. Inspect all new installations of lining for workmanship and condition of welds before returning the column to service.
Some columns may be protected from severe corrosion by the use of alloyclad materials. Cladding is a thin alloy sheet, factory bonded integrally to the carbon steel plate.
In those corrosive services where cladding is used, the cladding must be carefully inspected, since penetration to the carbon steel usually results in vessel failure in a relatively short time. Such penetration is sometimes evidenced by rust stains on the cladding.
The remaining thickness of nonmagnetic claddings can be measured nondestructively by the use of the coating gage or the cladding gage. Cladding thickness can also be measured with a depth micrometer by grinding through the carbon steel. The interface between the carbon steel and the cladding material is found by using copper sulfate to plate copper on the steel. This requires welding up the test area, and is not recommended if measurements can be obtained with the coating gage.
c. Weld Overlay
Check weld overlay areas for signs of corrosion, cracking or leaks. Use copper sulfate solution to check the integrity of the alloy overlay if corrosion/erosion is indicated.
6. Hydrostatic Testing
After repairs have been made to the shell or heads of a column, it may be decided to hydrostatically test the vessel for strength or tightness. Engineering instructions define the procedure for such tests to provide safety and to prevent damage to the equipment.
Be sure that the test pump is equipped with a suitable pressure gage and a safety valve that is properly set and tagged. All air must be bled from the column and the pressure raised at the pump or the column top to the pressure stipulated by the engineer. All pressure-containing parts, particularly at welds, nozzles, reinforcing pad weep holes, as well as at the repaired area are then carefully inspected for drips or leaks or visible signs of weakness. At the conclusion of the test, the inspector should check to see that the vessel is properly vented to prevent a vacuum and the collapse of the vessel as the water is emptied.
A field hydrostatic test is seldom applied to a whole column due to the great weight of the water, which the foundation might not be designed to withstand. For nozzle replacement or repairs it is sometimes advisable to test the area affected only. This is usually done by applying a welding pipe cap over the inside opening of the nozzle, then pressuring up the nozzle-to-shell weld. Make sure that the pipe cap is removed after the test is completed.
Distillation Columns External Inspection Prior to Entry
1. Support and Foundations
Distillation columns are usually supported by a cylindrical skirt which usually s fireproofed inside and outside with concrete, brick, or other insulating material.
Inspect the fireproofing material carefully for evidence of cracking, spalling, or defective seal at the top edge. Water which penetrates the insulation can cause severe corrosion of the support. Remove small sections of the insulation, if necessary, to permit inspection.
Check the anchor bolts and nuts for corrosion or evidence of failure, and observe the condition of the concrete around the bolts. Cracking of the concrete around the bolts usually indicates corrosion is taking place.
Inspect all foundation support rings, brackets, or lugs for corrosion or indication of distortion or settling. If settling is noted, the equipment should be checked for plumb.
2. Ladders, Platforms, Handrails and Davits
External structures attached to the column for the purpose of servicing the column and providing entry should be inspected with the column. Check ladder supports and clips. Look for missing bolts, broken or weakened handrails, loose toe-boards or defective floor plates. Davits used for lifting materials should be sturdy and not deformed from misuse or overloading. Davits must be load tested. Angle supports where welded to columns are frequently found corroded under the insulation where water collects. Remove the necessary insulation to permit inspection for this condition.
Note the general condition and effectiveness of the insulation on the shell and heads. Openings around manways, nozzles, and brackets should be sealed to prevent the entry of water. All insulation ties or retainer strips should be intact and tight. Evidence of weathering or cracking of the weathercoat should be noted, and repairs made if necessary.
4. External Corrosion
Inspect the external surfaces of the column where exposed or where corrosion is indicated. Note particularly welds, brackets, nozzles, breaks in the insulation, or where water has been allowed to run on the shell, support legs, or skirt. Check the weep holes in the reinforcing pads for evidence of leakage. Gage the areas where corrosion is found and establish the severity, extent, and rate of corrosion.
5. Lines, Instrument Leads and Conduit
Visually inspect and hammer-test all small piping, including instrument piping, level gage connections, vent and drain fittings, and sample connections. Note the condition of all conduit attached to the column or platforms for lighting, instrument, and thermocouple leads. Look for exposed wires, missing cover plates, broken fittings, and improper support.
Inspect the gasket faces on all flanges that might be opened, including the manway nozzles and covers. An adequate inspection cannot be made if the gasket is still in place. Remove it or have it removed. Hammer-test all small nozzles. Inspect the manway cover hinge pins for binding. A frozen pin can prevent tight closure of the cover.