Document GEOq2r53VMb0vxB8KOe1OQj4

BOILERS AND UNFIRED PRESSURE VESSELS below the working pressure Creat care must be used because a vessel may fail un der test and shatter Detecting cracks and measuring thickness Pressure vessels used to process gases or oily materials may have very small leaks which will not show under hydrostatic tests To detect them, a small amount of ammonia can be released inside the vessel and compressed air is then applied until a maximum pressure of 50 per cent of the working pressure is attained A swab soaked in hydrochloric (munatic) acid is passed over all seams and other suspect areas Leakage will be indicated by a white vapor (ammonium chlonde) formed by contact of escaping ammonia and the acid Usmg a burning sulfur stick is also effective A change in the flame indicates ammonia Radiography is especially good for End ing cracks For example, ammonia tanks used for agriculture are subject to corro sion and may stress crack If care is not taken to catch tanks which become defec tive, there may be some surprised--and seriously hurt--fanners Also, there are a number of applications where it is vital to check the thickness of an unfired pressure vessel without damag ing it There are a number of manufac turers who make instruments for doing this Some are listed under "Gauges or Gages Thickness" ui Thomas Register, and include Sperry Products Division of Auto mation Industries, Inc , Danbury, Conn , and Magnaflux Corporation, subsidiary of General Mills, Chicago Other instruments employ ultrasonic and various electronic-ray producing and measuring means Usmg these instruments, a qualified op erator can determine the thickness of metal to within 2 or 3 per cent Some, like the Magnaflux test, will disclose cracks which extend or are slightly below the surface The use of a radiograph will then be necessary to determine bow deep the cracks are One large boiler insurance company has developed a lacquer method of detecting hairbne cracks that cannot be seen by the naked eye When the head of a pressure vessel is suspected, it is cleaned and given a coat of clear lacquer After the lacquer hardens a hydro test is applied The weak spots, hairline cracks, or fatigue stress cracks will show up as the head expands and cracks the lacquer The more modern dye penetrant tests, when made by compe tent inspectors according to manufacturers* instructions, are considered quite reliable At each inspection of such pressure ves sels as vulcamzers, digesters, and auto claves which have removable cover plates, heads or doors, the holding bolts, cover plate bolts, slots and retaining rings should he checked for wear, and hammer tested for soundness Since these parts are badly abused in service and receive considerable wear it is advisable to replace them annual ly They are comparatively inexpensive The width of the slot should be gauged and a careful check should he made of the retaining rings and cover plates, as cracks are set up by the stress of improper ad justment or dosing of the door plate If cracks cannot be satisfactorily re paired, the vessel should be condemned Operator training and supervision It is important that employees working with pressure vessels, particularly with those which are used in chemical processes, be thoroughly trained both in roufaDe duty and emergency procedure Supervision, too, should be competent and alert A new em ployee being trained as an operator or help er should have the entire process explained to him, the hazards mvolved, and just how his operation affects the entire process In some plants a check list is used to make certain that no step has been over looked in the processing cycle The opera tor or his helper records on a card the information obtained from recording ap paratus and thermometers, and the time and frequency at which he operates the valves to each pressure vessel After each com plete processing cycle, the operator initials the card check list (shown in Fig 16-6) If the contents of a pressure vessel are being discharged to a vessel which the op erator cannot see, or one run by another operator, a system of whistle, bell, or light signals should be installed The tune that these signals are given and action is taken should be noted on the check hst so that the wrong valve will not be opened or closed The plant operating supervisor should in struct operators of vessels with cover plates or removable doors how to tighten the bolts 16-18