Document 9ar5kLRKOO7NYRa3Ve6Vw2LD

VTivY Ca4^*c INDUSTRIES PPG INDUSTRIES, INC. CHEMICAL DIVISION U.S. P. 0. BOX 1000 LAKE CHARLES, LA. 70602 January 24, 1983 Mr. Bruce J. Brocka Personnel & Safety Manager CertainTeed Corporation P. O. Box 253 Sulphur, LA 70663 Dear Bruce: At the Louisiana Loss Prevention Meeting of last week, you asked what was the standard inspection interval for vessels which store vinyl chloride. I mentioned that we do inspect the external surfaces of our vinyl chloride storate tank on an annual basis. Our internal inspections of storage tanks have not been on a scheduled routine basis as we do for say liquid chlorine storage (once every 3 years). In the vinyl chloride processing unit, we annually inspect our vessels both internally and externally. Per your request, I tried to find some published guidelines on recommended internal inspection frequencies. PPG's Ben Reynolds indicated you had probably already received B. F. Goodrich recommendations (after their 700 ton leak in Australia) on inspections. As I understand it, BFG recommends an internal on storage tanks once each 3 years. The only published source of information with actual internal inspections listed is an excellent British text entitled "Loss Prevention in the Process Industries" by Dr. Frank Lees. In chapter 19, it states that Imperial Chemical Industries would inspect such a vessel as a vinyl chloride storage tank either once each 6 years or more probably once each 12 years. I've attached the appropriate pages from Dr. Lees book which cover both vessel inspection frequency and testing methods. Please call me if I can further help. Sincerely, R^y E. Sanders Supt. of Loss Prevention & Industrial Engineering cac CTL020801 Pressure vessel mspecimn met the requirements. The condition of the metal surface should be inspected for defects which may give rise to crack growth. A check should be made on assemblies which may be subject to wear and on stress raisers such as sharp angles, changes of section and attachments such as nozzles or lugs. The presence of dissimilar metals and stray electric currents which may give rise to corrosion should be considered. The suitability of joints, gaskets and seals should be checked; so should that of the lagging and protective finishes. The adequacies of the vents and drains on the vessel should be confirmed and the means of access reviewed. Other aspects of inspection, such as tests during commissioning and pressure and leak testing, are considered in Sections 19.1 and 19.5, respectively. 19.2.6 Inspection register and records It is essential to keep a register of the equipment to be inspected and records of the results of the inspections. The precise contents of the register depend on the item concerned but in general should include (1) identification number, order number and drawing number; (2) specification, design parameters, process fluids; (3) inspection/test reports during manufacture; (4) inspection category, interval, method: (5) special features relevant to deterioration, failures: (6) materials/parts list; (7) date of entry into service. Such a register is equally appropriate to pressure vessels, protective devices, rotating machinery or instrumentation. The specific information requirements given in the ICl Pressure Vessel Inspection Code for the registra tion of pressure vessels are described in Section 19.3. For a pressure vessel there should be full infor mation on inspections/tests during its manufacture, including reports on tests on the material of construc tion, radiographs or other records of tests on welds, and reports on pressure and leak tests. Reports of the regular inspections/tests on the equipment should be prepared and filed in the register. These reports should generally cover the results of the main inspection/test and of any additional monitoring and tests, any deterioration or failure detected and repairs carried out, and any modifications or changes of duty. Again the requirements given in the ICI Pressure Vessel Inspection Code for pressure vessel reports are described in Section 19.3. 19.3 PRESSURE VESSEL INSPECTION The inspection of pressure vessels and their protective devices is crucial to the maintenance of the integrity of the pressure system and is the main activity of the inspection authority. An account of pressure vessel inspection is given in Inspection of Chemical Plant by Pilborough (1971). In the U.K. relevant codes are the Registration and Periodic Inspection of Pressure Vessels Code by 1C1 (lCl/RoSPA 1975 IS/107) and the Pressure Vessel Inspection Code by the IP (1976), and in the U.S.A. the Guide to the Inspection of Refinery Equipment by the API (1962- ). The description of pressure vessel inspection given below is based primarily on the 1C1 Pressure Vessel Inspection Code. 19.3.1 Definition It is necessary first to define the equipment which is to be brought within the scope of the pressure vessel inspection system. The ICI Pressure Vessel Inspection Code gives the following definition of pressure vessels falling within the code: `A pressure vessel is a closed vessel of any capacity consisting of one or more independent chambers, any or each of which is or may be sub jected to internal pressure greater or less than atmospheric. This definition includes (a) Vessels not normally subject to internal pres sure but in which internal pressure would occur in the event of a failure of any part, maioperanon, isolation or other circumstances, for example, the failure of an internal steam coil in a vessel not adequately vented to prevent a pressure rise. (b) Vessels in which vacuum can develop, for example, steam receivers, steam condensers and other vessels containing condensable gases and vapours.' For other equipment the code states 'Consideration should also be given to the regis tration and routine inspection of other items which although not normally classified as pressure vessels, may give rise to hazardous situations, e.g. fired equipment, large diameter piping, expansion bellows, and pressurized equipment subjected to erosive or corrosive conditions. Where cross country pipelines are employed special arrange ments will be necessary for their routine examina tion.' The code defines a protective device as `Any automatic device which protects equipment from over-pressure, over-temperature, over-filling, corrosion, explosion or other hazardous conditions: including atmospheric vents, lutes, flame traps, deadweight flaps, some non-return--^valves,-relief valves, fusible plugs, bursting discs and instrumental trip systems, many of which incorporate an advance alarm indication.' CTL020802 -rt 5 686 Plum < omrmvwi'MJ'JU on*f /n'p < tu>n The code also lists equipment which, subject to approval, may be excluded from the requirementsofthe code itself, although it may still require some degree of inspection. Some important items in this list are vessels subject to static head only or to applied pressures not exceeding +0.07 bar (1 lbf/in") and not subject to corrosive risks; parts of prime movers and of machines driven by them: protective devices whose failure does not result in a hazard. The IP Pressure Vessel Inspection Code defines a pressure vessel as either to include in addition specifications for special test procedures and records of such tests. The IP Pressure Vessel Inspection Code gives broadly similar requirements. 19.3.3 Classification It is convenient to classify pressure vessels according to whether or not they are covered by statutory re quirements for inspection. The ICI Pressure Vessel Inspection Code gives the following classification of pressure vessels; '(i) A closed vessel of not less than 0.1 m3 capacity Class A Pressure vessels for which periodic inspec consisting of one or more independent cham tion is a statutory requirement or is required bers any or each of which may be subject to an by regulation (e.g. local authority regu internal pressure greater than 70 mbar gauge lations). or less than 930 mbar absolute or Class B Pressure vessels not in Class A and certain *(ii) A closed vessel wherein P x V > 1 where P other equipment. is the internal pressure in bars and V is the enclosed volume in cubic metres.' The schedules of Class A and Class B pressure vessels and of protective devices listed in the code are `Excluded are vessels subject to pressure generated given in Tables 19.7-19.9, respectively. solely by static head and storage tanks designed and The IP Pressure Vessel Inspection Code also classi constructed to Standards and codes such as fies equipment as Class A and Class B according to BS 2654, BS 2594 and API RP 620.' whether or not there is a statutory inspection require ment, so that the classification given in this code is 19.3.2 Registration similar to that of the ICI Code. Before any equipment or device covered by the system is brought into service it should be registered. 19.3.4 Inspection categories It should be a requirement for such registration that On the basis of the classification of equipment it is the equipment has full documentation. The ICI possible to specify inspection categories which define Pressure Vessel Inspection Code states the interval between inspections. For Class A pressure vessels the inspection intervals `For equipment the file shall include its are specified by the statutory or other requirements. identification number, the order number, the draw The ICI Pressure Vessel Inspection Code states: `It is ing reference, specifications, materials lists, inspec almost impossible to obtain any relaxation from the tion reports during manufacture, test certificates responsible authorities.' relating to the material and tests on the completed The inspection intervals for Class A vessels are given equipment. It shall also contain details of the in Table 19.7. design conditions, the process fluids, of the expected For Class B pressure vessels there is more flexibility. operating conditions, and the date of entering The ICI Code defines three inspection grades. Grades service, in addition the file shall include a note of Z, Y and X, according to the inspection interval. Grade any special kinds of deterioration--e.g. stress Z having the shortest interval. corrosion--to which the equipment may be subject.' A vessel which it is recognized may deteriorate in service and for which there is little information on The register should give the equipment classification ' which-to predict behaviour is classified as Grade Z. and inspection categories. It should specify the initial If experience shows that deterioration is slow, the and subsequent inspection intervals, the type of vessel may be reclassified as Grade Y. The Grade Z inspection to be done and, where appropriate, the inspection intervals given in Table 19.8 are 2-6 years. inspection acceptance standards. Grade Y applies to a vessel which is not expected to To this initial information should be added other deteriorate significantly but on which there is insuffi relevant information generated during the life of the cient information to justify assigning it to Grade X. equipment. This includes reports of periodic inspec Again it may be reclassified as Grade X, if experience tions and tests, reports offailures, failure examinations shows that deterioration is insignificant. On the other and repairs; records of corrosion and other condition hand, if unexpected deterioration is found, then it monitoring; details of modifications or changes of may be necessary to reclassify the vessel as Grade Z. duty. Table 19.8 gives Grade Y inspection intervals as 4-6 The registration of protective devices should be years. along similar lines, but it is necessary with some devices Where there is a group of similar vessels of Grade CTL020803 F 6S* Table 19.7 Inspection categories for Class A pressure vessels (Imperial Chemical Industries Ltd -- ICI RoSPA 1975 IS 107, hem Vessel Initial period Subsequent periods Inspection required 1 Steam boilers or other vessels that are defined within the meaning of the Factories Act as steam boilers including waste heat boilers: also economizers and steam superheaters Large installations, less than 21 years since first commissioned 14 months 26 months Examination Small steam boilers and large boilers after 21 years since first commissioned 14 months 14 months Examination 2 Steam receivers 26 months Examination 3 Air receivers 26 months I Clean and examination Air receivers (solid drawn) 4 years ] and/or test 4 Gasholders (water sealed) greater j 2 years external f 2 years external Examination than 5000 ft1 (140 m3) capacity |20 years internal j 10 years internal 5 Containers for the transport of compressed and liquefied gases 6 Vessels for the underground Will vary according to by-laws'*' Examination and lest storage of petrol, benzole, etc. 7 Transport containers for goods other <> than compressed or liquefied gases (dL Orifioal table refers to test of code (H Reference should be made to local authorities. Table 19.8 Inspection categories for Class B pressure vessels (Imperial Chemical Industries Ltd--ICI/RoSPA 1975 IS/107) U> K> Item 1 5 6 7 8 Vessel Initial period (y) Maximum period (y) ZYX Steam containers, i.e. vessels which exhaust through a permanent outlet to the atmosphere or into a space where the pressure does not exceed atmospheric pressure or approximately that pressure Compressed or liquefied gas containers (not used for transport) Process vessels (including fired heaters and vacuum vessels) Gasholders (water sealed) less than 140m3 (5000 ft3) capacity. Waterless of any capacity 6 6 2 f 2 (external) | 20 (internal) Static storage tanks with `under the roof or over the liquid pressure: (a) less than 0.07 bar (1 Ibf/in2 g) (b) greater than 0.07 bar (1 lhf/in2g) Transport containers for dangerous goods other than compressed or liquefied gases Intercoolers, aftercoolers, condensers, damping and other similar vessels associated with prime movers and machines driven by them (inspection grading to be assessed as for process vessels) Selected pipework in known erosive/corrosive service or high hazard. Includes selected expansion bellows and other critical fittings 6 2 <> 2 2 66 12 66 12 24 12 f 2 (external) ] 10 (internal after first ^ internal inspection) 66 24 12 12 24 12 24 12 (), Original table refers to teat of code. \J CTL020804 i i ? i 6&8 VhnM t <>nttni\\tt>nt>hj iithi , it>m Table 19.9 Inspection ealetfurirs for proti-clitr devices I Imperial t hemical Industries Ltd--I Cl RoSPA 1975 IS 107) Item Device Routtnc examination Maximum micnul lor thorough examination, overhaul and test i All types, when fined to ... Class A vessels 2 Relief valves fitted lo Class B vessels Only in special cases 2 years 3 Bursting discs (on Class B Check that they are in operable 2 years or not greater than that vessels) condition and have not for the vessel being protected, ruptured, at intervals whichever period is shorter depending on process conditions 4 Atmospheric venis or lutes Visual examination at intervals 2 years or not greater than that depending on process for the vessel being protected, conditions whichever period is shorter 5 Instrumented trip systems Check on operability, 2 years 1, 2 or 4 weekly or as required. The Competent Person will witness these tests at his discretion (a! Original labic refers io ten of code. Y, the code allows a system of sampling, so that not In the IP Pressure Vessel Inspection Code the every vessel need be inspected during the Grade Y inspection intervals for Class A equipment are similar inspection interval. to those in the ICI Code. For Class B equipment the Classification as Grade X is allowed if a vessel has inspection categories are somewhat different. Inspec been examined on successive occasions and has shown tion Grades I, II and III are defined in a rather similar no deterioration over a period of 6 years or if the vessel way to Grades Z, Y and X in the ICI Code, but there I is new and is essentially similar in duty and construc is also a Grade 0 into which, with some exceptions, all i tion to an existing Grade X vessel. The Grade X equipment is put until it has received its first inspection. inspection interval is 12 years, as shown in Table 19.8. The inspection intervals for Class B non-statutory The code does not permit an inspection interval longer equipment given in the IP Code are shown in Table than 12 years. 19.10. These inspection periods apply to the U.K. i Inspection intervals for protective devices on Class only. A vessels are subject to statutory requirements as Equipment in Grade III is subject to review at the shown in Table 19.9. There are many other protective periods given, in order to check that the conditions I devices and it is not appropriate to give inspection which led to that classification are still applicable. intervals for all these, although the table gives guidance on a few principal devices. In addition, the code states 19.3.5 Inspection procedures the two general principles that a protective device Inspection should be carried out by a competent should be examined at an interval not exceeding that person within the inspection interval specified for the used for the equipment which it protects and that the equipment. inspection interval between examination/test for a The ICI Pressure Vessel Inspection Code describes protective device should not exceed 2 years. the objectives of inspection as follows: Table 19.10 Inspection grades for Clas B pressure vessels (Institute of Petroleum--IP 1976 Pressure Vessel Inspection Code) Equipment Grade 0 Inspection period (months) Grade I Grade 11 -Grade III Review Process pressure vessels and process vacuum vessels Pressure storage vessels Heat exchangers Protective safety devices 24 36 72 108 72 60 60 90 120 90 24 36 72 108 72 24 36 60 -- -- i >***JvfjJ V'i'v*',.h CTL020805 i % < * t * f Pressure i essel inspection 6*9 `The objective of vessel inspection is 10 detect any The code requires pressure testing after any modifi deterioration such as corrosion, cracking or distor cation or repair which may alTect the strength of the tion indicating possible weaknesses that may vessel and allows it at discretion as an alternative a/Tect the continued safe operation of the vessel. method ofinspecnon where visual or other methods of Primarily inspection shall be visual, but it shall examination are inadequate. be supplemented by other techniques and measure If it is found that the vessel has deteriorated or has a ment when necessary to determine the extent of any defect, it may be necessary to modify or repair it. The losses of thickness, pitting, cracking, etc. Resorting repair is normally carried out according to the code to a pressure test as the sole method of periodic by which the vessel was originally designed. inspection is an inferior alternative to examination In other cases it may be necessary to downrate the and should be used only when examination is not vessel so that it can be operated only at a lower pres practicable or the findings of the visual examination sure. If this is done, all necessary changes should be are inconclusive." made to the protective devices and to the documenta tion for the vessel, for the protective devices and for The inspection should also include examination of the plant operation. protective devices. There are various preparatory measures which need 19.3.6 Defects and failures to be taken before an inspection is carried out. The Some of the features which may be revealed by an inspector should brief himself on the history of the inspection are vessel and, where applicable, on that of other similar vessels. The vessel is normally taken out of service, isolated, emptied and cleaned. The surfaces of the vessel are then exposed and prepared for inspection. This may involve such measures as removal of lagging from the outside of the vessel or cleaning of internal surfaces. These acti vi ties should be covered by appropriate procedures and permit-to-work systems as described in Chapter (1) internal corrosion; (2) surface defects; (3) weld defects; (4) wear defects; (5) deposits and debris; (6) high stress situations; (7) inadequate drainage; (8) external corrosion. 21. The inspection is then carried out. On the examina Some of the types of internal corrosion which occur tion of vessels the IC1 Code states in process plant were described in Chapter 12. Corro sion may reduce not only the thickness of the metal `The purpose of examination is to record fully but also its strength. Methods of detection include the state of the vessel at the time of inspection. This visual examination, thickness measurement including implies that a record should be made of any unusual ultrasonics, and corrosion monitoring. Changes to the or notable features whether or not they are con base material such as graphitization or hydrogen sidered significant to the operation of the vessel at embrittlement may be detected by metallurgical that time; thus any change in condition between examination. subsequent inspections can be reliably traced and Surface defects such as cracks or pitting arising rates of deterioration assessed. from such causes as corrosion, erosion or crack `Particular attention should be paid both to any growth may be detected by visual examination or by features of the vessel that are known to have methods such as magnetic particle or dye penetrant given rise to problems in previous service or in techniques. other vessels, and to regions which may be par The avoidance of surface defects is particularly ticularly at risk from either process or design important where there is a risk of fatigue failure, since considerations.' a rough surface can greatly reduce the fatigue strength All fittings on the vessel which affect its safe operation should also be examined/tested. of a steel. These methods of crack detection may also be used to find weld defects. The latter include weld deterio The 1C1 Code also places on the inspector a ration, which may arise due to the roughness of the ' responsibility to satisfy himself that the protective weld, or weld decay, which is intergranular corrosion | devices are examined/tested according to the specified and occurs in austenitic stainless steels. intervals and procedures. Cracks may be particularly serious as they can grow In a few -instances it is not practicable to enter a and lead to failure. It is often possible to remove a vessel or to carry out a hydraulic pressure test on it. crack from a weld by chipping it out and depositing ` Entry into a vessel may be difficult, for example, due fresh weld metal. Small cracks may sometimes be to toxic, flammable or corrosive substances. For such stopped by drilling small holes at the extremity, which vessels the code requires that a written case be made reduces the stress concentrations. ^ stating the reasons for dispensing with internal exami- Wear on moving parts is normally found by visual ; nation and the alternative means to be employed. examination and may result in failure if not rectified. H CTL020806 690 PL/nl timtniiwiitntnu tint/ tn\pei Ittm The build-up of deposits of materials and/or debris can create a hazard if equipment such as pipes and valves, or devices such as vents, pressure relief valves or instruments become blocked. Such deposits are revealed by visual examination. There are certain undesirable features which as far as practicable are eliminated in the original design of the equipment but which may be reintroduced by plant modifications. High stress situations may have been created at vessel connections or at lugs. There may be elements which are under high stress due to the limitation of thermal expansion or contraction. Similarly, modifications may have been made which impede complete drainage. Visual examination may detea these facts. External corrosion of the plant is detected by visual examination. It can be severe and can lead to serious loss of metal thickness. mques of Son-Destructite Testing by Hogarth and Blitz (1960) and in Principles ami Practice of Son- Destructive Testing by Lamble (1962) and by Pil- borough (1971). Some non-destructi testing and monitoring methods are Visual inspection horoscopes Dye penetrant methods Stress wave emission monitoring Radiography Holography X-rays Electrical measurements y-rays Ultrasonics resistivity capacitance Magnetic methods magnetic particles magnetic prints eddy currents These methods are n described in more detail. 19.3.7 On-line monitoring Some of the non-destructive testing and condition monitoring techniques described in Sections 19.4 and 19.6 arc applicable to the on-line monitoring of pressure vessels. The 1CI Pressure Vessel Inspection Code allows that where by reason of process conditions a vessel has been given an inspection interval less than the maxi mum for its grade, on-line monitoring methods may permit the interval to be increased to the maximum value. 19.4.1 Visual examination Visual examination is the original and most widely used method of inspection and is non-destructive. It is effective in delecting surface defects ranging from cracks to corroded areas, and defective assemblies. The range of visual examination can be extended by the use of aids such as periscopes or horoscopes. Boroscopes are available with lens or fibre optics. They are used to examine the condition of internal components such as the blades of turbines and com pressors. BS 5500 requires only visual examination for Cate gory 3 pressure vessels. 19.4 NON-DESTRUCTIVE TESTING Non-destructive testing techniques are used to detect defects in equipment such as pressure vessels and pipe work during both its construction and its operational life. Equipment is tested by non-destructive methods during fabrication. Pressure vessel codes such as BS 5500 lay down detailed non-destructive testing requirements. These are minimum requirements. For some pressure equipment it may be necessary to carry out more extensive testing. This is particularly the case where equipment has been fabricated on site. It may be necessary to carry out further non destructive testing of some equipments during con struction and commissioning. Thecondition ofthe equipment during the operation of the plant is checked by non-destruaive testing, particularly, but not exclusively, during plant shut down. Once the plant is operational, it becomes difficult to obtain access for visual examination of some parts of the equipment, e.g. internal surface of a refrigerated storage tank, so that non-destruaive testing techniques which can be used externally become very attractive. Accounts of non-destructive testing are given in Non-Destructive Testing by Hinsley (19591, in Tech- 19.4.2 Radiography An important non-destructive testing method is X-ray radiography. This is used during fabrication to detea internal defects principally in welds but also in parent material. Radiographic examination is covered by BS 2600: 1962 General recommendations for the radiographic examination of fusion welded butt joints in steel and in various other British Standards, some of which are given in Appendix 4. BS 5500 requires non-destructive testing by radi ography and/or ultrasonics for Category 1 and 2 pressure vessels. Details are given below. The X-rays used are produced by an X-ray machine which is relatively immobile and expensive. X-rays arc recorded on a photographic plate. In X-ray radiography it is important that there be a high quality image. Standards generally call for a technique which is capable of recording deviations from the normal ofnot more than 2 % of the maximum thickness of the item under test and specify the use of image quality indicators, or penetrameters. Image indicators are dealt with in BS 3971. The interpretation of X-ray radiographs is a skilled matter and contains a considerable subjective element. The first step is to identify the type of defect and the CTL020807 -fr: k- x Sstructll t` ICMlrUJ f>9l second to quantify it, if quantification is appropriate. Below are listed some of the principal defects in welds. Terminology and symbols for weld defects are given in BS 499 and BS 2597. Planar defects cracks lack of fusion (side, root, interrun) lack of root penetration Cavities porosity (isolated, localized, linear) wormholes (isolated, aligned) crater pipes surface cavities Slag inclusions Other solid inclusions tungsten copper Once a defect is identified, it is necessary to decide whether it is sufficiently serious to require rejection. This has again tended to be a subjective matter and has therefore caused considerable difficulty. In conse quence, much work has been done to develop accep tance standards. BS 5500 contains a list of defects similar to that given above, together with the corres ponding acceptance levels. X-ray radiography is used to detect internal defects in parent metal such as pressure vessel walls and pipe work and in castings as well as in welds. >'-rays are also used for radiography. They are produced by a radioactive isotope source such as Cobalt 60. Equipment using y-rays is portable and relatively cheap. It is used particularly for radiography of castings. The method is to arrange the castings in a circle with the y-ray source in the centre and with photographic plates behind the castings. Isotope sources have a relatively large area emitting radiation (usually about 2-6 mm diameter) and therefore tend not to give such good definition as X-rays. If a permanent record is not required. X-rays may be used with a fluorescent screen instead of a radiographic plate. X-ray fluoroscopy is widely used for rapid inspection of components on conveyor belts to detect coarse defects and unwanted objects. Another technique for rapid inspection is nucleonic gauging of thickness of materials passing a /1-ray source. Ultrasonic waves are generated by a transmitter and detected by a receiver and are usually displayed on an oscilloscope or on a recorder. Two basic techniques in ultrasonics arc (1) trans mission and (2) reflection. In the transmission method the ultrasonic waves arc generated by a transmitter on one side of the item under test and are detected by a receiver on the other. The method is shown in Figure 19.1(a). The reflection method generally uses pulsed waves which are generated by a transmitter and detected by a receiver on the same side of the item. One way in which the method may be applied is shown in Figure 19.1(b), which is known as the A scan. Another application is the B scan, illustrated in Figure 19.1(c). Both these methods give the depth of the defect. A third method is the C scan, shown in Figure 19.1(d), which does not show the depth. Ultrasonics is also used during operational inspec tions to detect weld defects and to measure thickness. -epr Defect Receiver probe (a) Transmitter probe . Receiver probe -------- & y/ '.'^T/ Defect (b) Transminer/receiver probe _______ *_________________ 4 Defect (c) Echo from defect base CRT display 19.43 Ultrasonics Ultrasonics is another important non-destructive testing technique. This too is used during fabrication to detect internal defects in parent metal and in welds and to measure thickness. Ultrasonic testing is dealt with in BS 3923:1968-- Methods for ultrasonic examination of welds. BS 5500 generally gives ultrasonics as an acceptable alternative to radiography for Categories 1 and 2 pressure vessels. Transmitter/receiver probe W) CRT display Figure 19.1 Some ultrasonic testing methods: (a) transmission method: (b) reflection method--A scan: (c) reflection method--B scan; (d) reflection method--C scan CTL020808 J i rc 6^2 I'iuiit ti>niitii\\uinmu wt,/ in\/ui H'"* 19.4.4 Magnetic panicle methods Magnetic panicle methods are applicable to ferro magnetic materials and are used to detect surface and some subsurface defects. BS 5500 gives magnetic particle methods as an acceptable method for the testing of welds for surface defects. Magnetic particle testing is treated in BS 4397:1969 Methods for magnetic particle testing of welds and in several other British Standards, some of which are listed in Appendix 4. There are vanous methods of magnetic particle testing, but the basic principle is to magnetize the item and coat it with a dry powder of iron or iron oxide particles, or, more usually, a wet powder or `ink' of particles suspended in kerosene. The particles con centrate at places where there is no continuous mag netic path such as cracks. Defects may then be detected visually. As a further aid to visual detection, a fluorescent substance may be added to the particles and an ultraviolet lamp used. Defects show up best when they are at right angles to the lines of magnetic flux and it is usual to test the component on two different planes. It is often necessary to demagnetize the item, which is usually done by subjecting it to a magnetic field in the reverse direction to that of the initial one and then gradually reducing the field strength. 19.4.5 Eddy current methods Magnetic particle methods are applicable only to ferromagnetic materials. An alternative which can be used for the detection of defects in ferromagnetic and other conducting materials is eddy current testing. Eddy current testing is dealt with in BS 3889:1965 Methods for non-destructive testing of pipes and tubes. In eddy current testing the item is subjected to an alternating magnetic field which induces eddy currents in it. These induced currents are measured and defects are detected from variations in the currents. 19.4.7 Other methods There is a growing number of other non-destructive testing methods such as stress wave emission analysis, holography and electrical resistance methods. In some cases these techniques are useful for inspection mainly during fabrication, but in others their appli cation is to operational inspection. Some of the latter are considered in Section 19.6 on condition monitor ing. 19.4.8 Acceptance standards It is difficult to overemphasize the importance of acceptance standards. Unless the increasing number of techniques which are capable of detecting defects is matched by the development of standards which define levels of defect below which no action is neces sary, there is an obvious danger of unnecessarily frequent rejection. M uch work has been done on this problem by bodies such as the Welding Institute and the American Welding Society, and there is an increasing amount of guidance available. As already mentioned, BS 5500 lists acceptance levels for certain defects. 19.4.9 Non-destructive testing of welds The distinction between the different categories of pressure vessel is largely based on the extent of non destructive testing of welds. For vessels in Category 1 BS 5500 requires for examination of internal flaws `The full length of all full penetration butt welds including the welds of forged butt welded nozzles shall be examined by radiographic and/or ultrasonic methods. Unless otherwise agreed the full length of all other welds (e.g. nozzles and branches! in or on pressure parts shall be examined by ultrasonic and/ or radiographic methods where the thickness of the thickest part to be welded exceeds that given in table 5.6.4.1.1.' For examination of surface flaws the standard states 19.4.6 Dye penetrant methods Dye penetrant methods are used to detect defects such as cracks. BS 5500 gives dye penetrant methods as an accept able method of testing of welds for surface defects and requires this method rather than magnetic particle testing for austenitic steels. Dye penetrant methods are covered in BS 4416:1969 Method for penetrant testing of welded or brazed joints in metals. The original dye penetrant method was to immerse the item in a heated mixture of paraflin and oil, dry it and clean it, and then dust it with chalk. On cooling the oil seeps out of any cracks in the article and stains the chalk. More modem methods use variations on this such as special white developer and red dye. `The full length ofall welds other than full penetra tion butt welds shall be examined by magnetic particle or penetrant methods. Full penetration butt welds shall be examined by these methods when agreed between the manufacturer, the purchaser and the Inspecting Authority.' For vessels in Category 2 the standard requires `At least 10% of the length of all full penetration butt welds shall be examined by radiographic and/or ultrasonic methods. Such examination shall include each intersection of longitudinal and circumferen tial seams. For each longitudinal and circumferen tial seam there shall be at least one radiograph or, where ultrasonic testing is specified, at least a 200 mm length shall be examined. At least 10% of the length CTL020809 Hrcwyrc anti uua Ustmu of all other full penetration butt welds shall be the expected design pressure is a safe working examined by radiographic and'or ultrasonic pressure. methods. In addition to these tests, which are carried out just At least 10% of the length of all welds other than main longitudinal and circumferential seams shall be examined by magnetic particle or penetrant methods. `In addition to the above when openings occur in. or within 12 mm of, welded seams, such seams shall be examined each side of the opening for a distance of not less than the diameter of the opening." before or after installation, there are periodic routine tests which are carried out during the operational life of the vessel. Methods of testing are (1) pressure tests--(a) hydraulic tests, (b( pneumatic tests, and (c) combined hydraulic/pneumatic tests; and (2) vacuum tests. The preferred method of pressure testing is the hydraulic rather than the pneumatic method, because in the latter the energy available is large and any failure The standard also specifies the further non-destructive testing required if defects are found. For vessels in Category 3 the standard states during the test is likely to be highly explosive. This point is discussed in Chapter 17. Thus BS 5500 states that pneumatic testing should only be carried out `Either on vessels of such design `Non-destructive testing for internal flaws is not required. However, insofar as magnetic particle or penetrant methods are aids to visual examination, which is required, they may be used subject to agreement between the manufacturer and the purchaser, or the Inspecting Authority.' and construction that it is not practicable for them to be filled with liquid, or on vessels for use on processes that cannot tolerate trace liquids and where the remo val of such liquids is impracticable', and only in consultation with the Inspecting Authority. Where hydraulic testing with water is not used, the reasons are generally that the vessel and the structure 19.5 PRESSURE AND LEAK TESTING cannot withstand the weight of water, that the water may be difficult to remove completely or that it may Pressure testing and pressure leak testing are poten freeze. tially hazardous operations and it is essential to The standard test pressure for a hydraulic, pneu observe the appropriate precautions in carrying them matic or hydraulic/pneumatic test given in BS 5500 is out. The equipment to be tested should always be given as thorough a visual inspection as practicable before Pi = 1.25 Pfj SM ~ c) (19.5.1) any test is done. where c is the corrosion allowance: fm the nominal design stress at test temperature: f the nominal design 19.5.1 Pressure testing stress under design temperature based on short term Pressure testing is carried out to check that a pressure properties; p the design pressure; p, the test pressure; vessel can be safely opierated at the design pressure. and r the nominal thickness of section under consider As already described, the pressure testing of pressure ation. Various qualifications to this test pressure are vessels is in some instances a statutory requirement, given in the standard. while in others it may be required by an inspecting The procedure for a standard hydraulic test is authority. broadly as follows. In such cases it is a general requirement that the The test fluid should normally be water, but other pressure test must be carried out in the presence of the liquids may be utilized instead if necessary. If another appropriate inspector. liquid is used, any appropriate precautions should be Pressure testing is covered in BS 5500 and the ICI observed. The ICI Code states that the liquid should Pressure Vessel Inspection Code, and is discussed by be well below its boiling point and, if flammable, Pilborough (1971). Safety precautions in pressure should have a flash point above 45C. testing are given in these sources and in GS/4 Safety in pressure testing by the HSE (1977). Typies of test include (1) acceptance tests--(a) If water is used with austenitic stainless steel, it is essential to control the chloride and alkali content The ICI Code specifies the use of demineralized or standard test, and (b) proof test; and (2) routine tests. clean condensate with a chloride content less than A standard test for acceptance is carried out on a 1 ppm. pressure vessel where the required thickness of all The test is normally carried out at about ambient pressure parts can be calculated. The object of the temperature. The hazard of brittle fracture should be test is to confirm that the calculated design pressure considered and, if necessary, the test temperature is a safe working pressure. should be chosen so that it exceeds the ductile/brittle If it is not possible to calculate the strength of a transition temperature. Also a temperature in excess vessel satisfactorily, a proof test for acceptance may of 7C is recommended in order to avoid the risk of be conducted to establish a design pressure. In this freezing. case the purpose of the test is to determine whether There are various precautions which should be ** CTL02081Q