INSPECTIONS OF PRESSURE VESSEL
It is realized that there are numerous types of pressure vessels in use, many of which have multiple functions, varying operating pressure, complex design and construction. These may be used for containing or processing gases or liquids having none or varying degrees of corrosive or erosive qualities. The following guide to inspection is intended to provide direction as it would be difficult to adequately cover inspection procedures for all.
To make a meaningful assessment of the pressure vessel to be inspected, the inspector should determine the operating conditions and normal contents of the vessel. The inspector should also determine when the vessel was last inspected internally and make and evaluation if the interval from that time until the next scheduled inspection is adequate. If the vessel is subject to jurisdictional requirements regarding an inspection certificate; the inspector should ensure that the vessel has a current certificate.
The inspection described is not all inclusive for every vessel but include those features common to most vessels and those considered of greatest importance. Inspector must supplement this list with any additional items necessary for the particular vessel or vessel involved.
The Inspector should examine the surfaces of shells and heads carefully for possible cracks, blisters, bulges, and other evidences of deterioration, giving particular attention to the skirt and to support attachment and knuckle regions of the heads. If evidence of distortion is found, it may be necessary to make a detailed check of the actual contours or principal dimensions and compare those contours and dimensions with the original design details.
The Inspector should examine welded joints and the adjacent heat affected zones for cracks or other defects. Magnetic particle or liquid penetrant examination is a useful means for doing this. On riveted vessels, examine rivet heat, but strap, plate, and caulked edge conditions. If rivet shank corrosion is suspected, hammer testing or spot radiography at an angle to the shank axis may be useful.
The Inspector should examine the surface of all man ways, nozzles, and other openings for distortion, crack, and other defects, giving particular attention to all welding used for attaching such part and their reinforcements. Normally, weep holes in reinforcing plates should remain open to provide visual evidence of leakage as well as to prevent pressure build-up in cavity. Examine accessible. Flange face for distortion and to determine the condition of gasket-seating surfaces.
The parts of a vessel which should be inspected most carefully depend upon the type of vessel and its operating conditions. The inspector should be familiar with the operating conditions of vessel and with the causes and characteristics of potential defects and deterioration.
Careful visual examination is by far the most important and most universally accepted method of inspection. Other means which may be used to supplement visual inspection include: magnetic particle examination for cracks and other elongated discontinuities in magnetic materials; fluorescent or dye penetrant examination for disclosing cracks, porosity, or pin holes that extend to the surface of the material and for outlining other surface imperfections, especially in nonmagnetic materials; radiographic examination; ultrasonic thickness measurement and flaw detection; eddy current examination; metallographic examination; acoustic emission examination; hammer testing while not under pressure; and pressure testing.
Adequate surface preparation is important for proper visual examination and for the satisfactory application of any auxiliary procedure, such as those mentioned above. The type of surface preparation required is dependent upon the individual circumstances but may necessitate wire brushing, blasting, chipping, grinding, or a combination of these operations.
External Inspection of Pressure Vessels
- External inspection of pressure vessel is made to determine if its condition is safe for continued operation.
- On process vessels where corrosion is a major consideration and where periodic examinations are made to determine the vessel wall thickness, the Inspector should review the records of such examinations.
- The Inspector should ensure that the vessel is properly stamped or marked to comply with the applicable section of the ASME Code.
- Any leakage of gas, vapour or liquid should be investigated. Leakage coming from behind insulated coverings, supports or settings, or evidence of past leakage should be thoroughly investigated removing any covering necessary until the source is established. Leakage should not be tolerated and any action required for its correction should be taken promptly.
- The Inspector should check for adequate allowance for expansion and contraction of the vessel on its mounting such as provided by slotted bolt holes or unobstructed saddle mountings.
The Inspector should note the pressure indicated by the required gage and compare it with other gages on the same system. If the pressure gage is not mounted on the vessel itself, he should ascertain it is on the system and installed in such a manner that it correctly indicates the actual pressure in the vessel.
Pressure Relief Valves
- When practicable, the pressure relief valve(s) should be tested by raising the working pressure to the valve setting to check for operation at the set pressure. If this is not practicable and the valve is equipped with a try lever, free movement of the valve stem and disk can be checked by use of the try lever. This test should not be conducted unless the pressure in the vessel is at least 75% of the valve set pressure and the contents can be safety discharged to atmosphere or the valve discharge is piped to a safe place.
- Many pressure vessels contain liquids or gases which are hazardous or costly, making in service pressure relief valve testing impractical. Under these conditions the valve should be removed from service at the time of internal inspection or at frequency agreed to by the owner and the Inspector.
- When an inspection reveals a defective pressure relief valve, the vessel or vessels dependent on the valve shall be taken out of service until the valve has been repaired or replaced, unless special provisions acceptable to the Inspector are made to permit the continued operation of the vessel on a temporary basis.
- The Inspector should satisfy himself that the relieving capacity of the pressure relief valve(s) is adequate and that the required set pressure and relieving capacity are stamped on the valve body or name plate as required by the applicable section of the ASME Code. (See Appendix A, page 90, for the recommended pressure differentials between the pressure relief valve set pressure and the operating pressure of the vessel).
- The Inspector should ensure that there are no intervening stop valves between the vessel and its protective device or between the protective device and the point of discharge, unless they are in conformance with UG-135 (e) and Appendices M-5 and M-6 of Section VIII, Division 1, of the ASME Code. Any such stop valve that requires locking and sealing in the open position shall be operated only in accordance with a written procedure that has the prior acceptance of the Inspector.
- The Inspector should check the markings on rupture disks to ensure that the stamped burst pressure and temperature are correct for the intended service conditions.
- Where a rupture disk is installed between the vessel and a spring loaded safety relief valve, the space between the rupture disk and valve should be provided with a pressure gage, try cock, free vent or suitable telltale indicator in order that leakage or rupture may be detected.
- When a rupture disk is installed on outlet side of a spring loaded safety or safety relief valve, the valve is to be of such a design that it will not fail to open at its set pressure regardless of any back pressure that can accumulate between the valve and the rupture disk should be vented or drained to prevent accumulations of pressure due to small amount of leakage from the valve.
- In all cases, where rupture disks are installed in combination with safety of safety relief valves, the Inspector should refer to the requirements of Section VIII, Division 1, of the ASME Code for the proper discharge capacity rating of such combinations.
Manways, Inspection Openings and Other Closures
The Inspector should check for any distress, distortion or leakage in these areas particularly on closures that are periodically opened and closed in a process procedure. Quick actuating closures should be checked for proper working safety locks, releases, indicators or alarms.
The Inspector should check that drainage is provided where required. Where provided, and when practicable, the Inspector should have the drain opened to ensure that it functions properly.
Piping components attached to the vessel should be checked by Inspector for indications of inadequate support or provisions for expansion which may cause excessive loading on the vessel shell.
Any device or control attached to or installed for safety of the vessel should be demon-strated to the Inspector by operation or procedures and records for verification of correct and proper operation.
INTERNAL INSPECTIONS OF PRESSURE VESSELS
- The following guide gives a general recommended procedure for the internal inspection of pressure vessels. As pressure vessel types range from very simple in design, materials and service to very sophisticated and complex, there will be occasions where a more detailed examination may be required.
- Pressure vessels containing noncorrosive substances may require only an external inspection. Such vessels may not be provided with manholes or other inspection openings. In these cases the Inspector may accept non-destructive methods of examination to determine the condition of the vessel.
Modes of Deterioration and Failures.
Contaminants in fluids handled in pressure vessels may react with metals in such a way as to cause corrosion.
Stress reversals (such as cyclic loading) in parts of equipment are common, particularly at points of high secondary stress. If stresses are high and reversals frequent, failure of parts may occur because of fatigue. Fatigue failures in pressure vessel may also result from cyclic temperature and pressure changes.
Locations where metals having different thermal coefficients of expansion are joined by welding may be susceptible to thermal fatigue.
Deterioration or creep may occur if equipment is subjected to temperatures above those for which the equipment is designed. Since metals become weaker at higher temperatures, distortion may result in failure, particularly at points of stress concentration. If excessive temperatures are encountered, structural property and chemical changes in metals may also take place which may permanently weaken equipment. Since creep is dependent on time, temperature, and stress, the actual or estimated levels these quantities shall be used in any evaluations.
At subfreezing temperatures, water and some chemicals handled in pressure vessels may freeze and cause failure. Carbon and low alloy steels may be susceptible to brittle failure at ambient temperatures. A number of failures have been attributed to brittle fracture of steels which were exposed to temperatures below their transition temperature and which were exposed to pressure greater than 20% of the required hydrostatic test pressure. However, most brittle fractures have occurred on the first application of a particular stress level (that is, the first hydrotest or overload). Therefore, in addition to excessive operating conditions below the transition temperature, the potential for a brittle failure shall also be evaluated when rehydrotesting or pneumatic testing equipment or when adding any other additional loadings. Special attention should be given to low alloy steels [especially 2- ¼ percent – Cr-1 percent – Mo) because they are prone to temper embrittlement. (temper embrittlement is defined as a loss of ductility and notch toughness due to postweld heat treatment or high temperature service, above 700oF (371oC)].
Other forms of deterioration include but may not be limited to graphitization, high temperature hydrogen attack, carbide precipitation, intergranular attack, and embrittlement. Deterioration may also be caused by mechanical forces such as thermal shock, cyclic temperature changes, vibration, pressure surges, excessive temperature, external loading and faulty material and fabrication.
Corrosion is one of the most common conditions found in pressure vessels. Where active or excessive corrosion is found, remedial action should be taken.
The Inspector should thoroughly examine the vessel for the following types of corrosion:
- Pitting – Shallow, isolated, scattered pitting over a small area does not substantially weaken the vessel. It could, however, eventually cause leakage and, if possible, steps should be taken to eliminate the cause or treat the condition by protective coating.
- Line Corrosion – This is a condition where pits are connected, or nearly connected, to each other in a narrow band or line. Line corrosion frequently occurs in the area of intersection of the support skirt and the bottom of the vessel, or liquid-vapor interface.
- General Corrosion – This is corrosion which covers a considerable area of the vessel. When this occurs, consideration must be given to the safe working pressure of the vessel which is directly related to the remaining material thickness.
Every effort must be made to determine the remaining thickness of sound base material by using NDE methods such as ultrasonic testing, or by drilling. Maximum allowable working pressure should be based on the findings.
- Grooving – This type of corrosion is a form of metal deterioration caused by localized corrosion, and may be accelerated by stress concentration. Grooving may be found adjacent to riveted lap joints or welds and on flanged surfaces, particularly the flanges of unstayed heads.
- Galvanic Corrosion – Two dissimilar metals in contact with each other and with an electrolyte (i.e., film of water containing dissolved oxygen, nitrogen and carbon dioxide) constitute an electrolyte cell, and the electric current flowing through the circuit may cause rapid corrosion of the less noble metal (the one having the greater electrode potential). This corrosion mechanism is most active when there are large differences between the electrode potentials of the two metals, but galvanic corrosion may also exist with relatively minor changes of alloy composition (i.e., between a weld metal and the base metal). Natural (i.e., an oxide coating on aluminium) or protective coatings may inhibit galvanic corrosion, but in most instances the metals or alloy must be selected on the basis of intrinsic resistance to corrosion. In boilers and pressure vessels the effects of galvanic corrosion are most noticeable at rivets, welds or at flanged and bolted connections.
The Inspector should note any action caused by abrasives and high velocity streams which may erode the surfaces of the metal.
Dents. Dents in a vessel are deformations caused by their coming in contact with a blunt object in such a way that the thickness of metal is not materially impaired. In some cases, a dent can be repaired by mechanically pushing out the deformation.
Distortion. Vessel shall be examined for visual indications of distortion. If any distortion is suspected or observed, the overall dimensions of the vessels shall be checked to determine the extent and seriousness of the distortion.
Cuts or gouges. Cut or gouges are caused by contact with sharp objects in such a way as to cut into the metal decreasing the wall thickness. Cuts or gouges can cause high stress concentrations and, depending upon the extent of the defect, it may be necessary to repair the area by welding or patching. Blend grinding may be a useful method of eliminating some minor types of cuts or gouges.
Preparation And Safety Precautions For Internal Inspection
When an internal inspection is to be performed, the owner or user shall prepare the pressure vessel as deemed necessary by the Inspector. Usually this includes the following:
- When the vessel normally operates at high temperature, the vessel should be allowed to cool at a rate to avoid damage to the vessel.
- The vessel should be drained of all liquid and should be purged of any toxic or flammable gases or other contaminants that were contained in the vessel. Mechanical ventilation using a fresh air blower or fan shall be started after the purging operation and maintained until all pockets of “dead air” which may contain toxic, flammable, or inert gas eliminated.
During air purging and ventilation of vessels involved with flammable gases, the concentration of vapour in air may pass through the flammable range before a safe atmosphere is obtained. All necessary precautions shall be taken to insure that there are no sources of ignition present during these operations.
- Inspection plugs and covers should be removed as required by the Inspector to allow a through examination of internal surface.
- The vessel should be sufficiently cleaned to allow visual inspection of all internal and external exposed base material surface.
- Any required pressure gage should be removed for testing if the Inspector has no other information to assess its accuracy.
- The pressure relief valve should be removed for testing if the Inspector has reason to doubt its effectiveness or has no other information to assess its proper operation.
- When a vessel is connected in a system where there is the presence of liquids or gases, the vessel shall be isolated by closing, tagging and padlocking stop valves. When toxic or flammable materials are involved, additional safety may require removing pipe sections or blanking off pipelines before any entry is made to the vessel. The means of isolating the vessel to be inspected shall be acceptable to the Inspector.
- In the case of a rotating type vessel or one with movable internal parts, additional precautions such as removal of fuses, locking of controls and blocking of movable parts should be taken prior to the entry of the vessel.
- Prior to entering a vessel that contained toxic or flammable gases or an inert atmosphere, the vessel atmosphere should be tested by a person qualified to determine the integrity of the vessel atmosphere using the appropriate detector type instruments.
A test for oxygen content must be carried out in all cases of vessel entry, regardless of previous contents or prior preparation of the vessel. The Inspector shall not be permitted to enter or to remain unless the oxygen content is between 19 and 23% by volume. Ventilation should continue if the oxygen contents are outside of these limits.
Personal protective clothing that is appropriate for the conditions inside the vessel should be worm if conditions warrant their use. Also respiratory equipment and a lifeline should be available if deemed necessary.
A responsible person outside the vessel shall keep continuous visual and voice contact with the Inspector inside the vessel and should be in position to respond to any unusual behaviour.
- If a vessel has not been properly prepared for an internal inspection, the Inspector should decline to make the inspection.
Insulation and Lining
It is not normally necessary to remove insulating or lining material for an inspection unless defects or deterioration is suspected or commonly found in vessels of the particular type or use being inspected. Where there is evidence of leakage showing on the coverage or through the lining, it should be removed to the extent necessary to make a complete investigation.
The Inspector shall get as close as practicable to the parts of the vessel, internal and external, in order to make the best possible examination. For lighting, a flashlight should be used in preference to an extension light. When a portable extension light is used in confined spaces, it should not be operated at more than 12 volts.
- When required by the Inspector, the accuracy of pressure gages which are necessary for the safe operation of the vessel should be verified by comparing the readings with a standard test gage or a dead weight tester.
- The location of a steam pressure gage should be noted by the Inspector to determine whether it is exposed to high temperature from an external source or to internal heat due to lack of protection by a proper siphon or trap. The Inspector should check that provisions are made for blowing out the pipe leading to the steam gage.
Pressure Relief Valves
- The Inspector should check that all valves are suitably marked to meet the requirements of the applicable ASME Code section and service conditions. If required by the Inspector, the valve shall be removed and tested to his satisfaction.
- If there is a procedure in effect to remove the valve at a set frequency for testing, the records of such tests should be checked at each inspection.
- The Inspector shall assure himself that all flow passages are free of foreign material or other obstructions.
The Inspector should check that the makings on rupture disks meet the requirements of the applicable ASME Code section and service conditions. The Inspector should assure himself that the pipe connections to and from disk are not obstructed.
As required by the Inspector, attachments of legs, saddles, skirts or other supports should be examined for distortion or cracks at welds.
Manholes and Other Openings
- Manholes, reinforcing plates, nozzles or other connections flanged or screwed into a pressure vessel should be thoroughly examined by the Inspector for cracks, deformation or other defects. Bolts and nuts should be checked for corrosion or defects.
- Wherever possible an inspection should be made from the inside of the pressure vessel to determine the condition of pipe connections at the vessel.
- On threaded connections the Inspector should ensure that an adequate number of threads are engaged.
- All openings leading to any external fittings or controls should be examined as thoroughly as possible by the Inspector to ensure they are free from obstructions.
Any special closures including those on autoclaves, normally termed quick actuating (quick opening) closures which are used frequently in the operation of a pressure vessel, should be checked by the Inspector for adequacy and wear. A check should also be made for cracks at areas of high stress concentrations.
- When there is doubt as to the extent of a defect or detrimental condition found in a pressure vessel, the Inspector may require a pressure test. A pressure test normally need not be made as part of periodic inspection. However, a test shall be made when unusual, hard to evaluate forms of deterioration possibly affecting the safety of a vessel are disclosed by inspection and also after certain repairs.
- To determine tightness, the test pressure need be no greater than the set pressure of the pressure relief valve having the lowest setting.
- The pressure test should not exceed 1-1/2 times the maximum allowable working pressure adjusted for temperature. When the original test pressure includes consideration of corrosion allowance, the test pressure may be further adjusted based upon the remaining corrosion allowance.
- During a pressure test, where test pressure will exceed the set pressure of the pressure relief valve having the lowest settings, the pressure relief valve or valves should be removed or each valve disk be held down by means of a test clamp and not by applying additional load to the valve spring by turning the compression screw.
- The temperature of the water used to apply a hydrostatic test should not be less than 60oF (15.6oC) unless the owner provides information on the toughness characteristics of the vessel material to indicate the acceptability of a lower test temperature.
The temperature is not to be more than 120oF (49oC) unless the owner specifies the requirement for a higher test temperature. If the test is conducted at 1-1/2 times the MAWP and the owner specifies a temperature higher than 120oF (49oC) for close examination.
- When contamination of the vessel contents by any other medium is prohibited or when hydrostatic test is not possible, other testing media may be used providing the precautionary requirements of the applicable section of the ASME Code are followed. In such cases, there shall be agreement as to the testing procedure between the owner and the Inspector.
The Inspector should review any pressure vessel log, record of maintenance, corrosion rate record or any other examination results. The Inspector should consult with the owner or user regarding repairs made, if any, since the last internal inspection. The Inspector should review the records of such repairs for compliance with applicable requirements.
The Inspector should discuss with the owner or user, any defects or deficiencies in condition, maintenance practices or misuse of the pressure vessel and, if necessary, recommended corrective action. All repairs should be carried out in accordance with the requirements of Chapter III of this Code.
External Inspection of Water Heaters Supplying Potable Hot Water for Purpose Other Than Space Heating
Upon entering the hot water room or area, the Inspector should observe the general upkeep of the unit the immediate surrounding are. Obvious external leakage of the heater or its fittings should be evaluated and repaired as necessary.
The installed thermometer when required by construction Code is the only external device which indicates internal operation. A defective thermometer should be promptly replaced.
Safety Relief Valves
The freedom of operation of safety relief valves or officially rated pressure temperature relief valves should be tested by means of the try lever. The Inspector shall witness this test. In the event that the valves is inoperative, has inadequate capacity or improper set pressure, the unit must be taken out of service until the unsafe condition is corrected and the valve replaced or repaired.
Mounting Safety Relief Valves
The Inspector should determine that the safety relief valve pressure temperature valve is properly mounted in accordance with the Code of construction, with no valves of any type between the unit and the valve.
Drainage of Safety Relief Valve and Discharge Piping
- Safety relief valves shall discharge safety
- Discharge piping, if used, must be run full size from the valve discharge opening to its point of safe discharge.
- When the disc is below the outlet of the valve, provisions for adequate drainage of the discharge line must be provided.
Water Supply Connections
- The water supply to the water heater must be independent of other opening or connections.
- When water pressure to the water heater exceeds 75% of the maximum allowable working, pressure of the until, a pressure reducing valve must be provided.
Expansion Tank Systems
- The expansion tank, if installed, shall be constructed in accordance with Section VIII, division 1 for new installations.
- The expansion tank maximum allowable working pressure may not be less than the maximum allowable working pressure of the water heater.
- A drain must be installed at or near the expansion tank.
- A stop valve must be installed between the expansion tank and the water heater.
Operating stop valve should be installed on the water heater inlet and outlet sides so that the unit may be isolated when necessary.
Bottom Drain Valve
- The bottom drain valve shall be tested to insure proper operation. The Inspector shall witness this test.
- Bottom drains shall discharge safety.
- Discharge piping, if installed, shall run full size to a safe point of discharge.
If practical, the Inspector shall witness the operation of all controls. If any control is found defective or inoperative, the water heater shall be taken out of service until the deficiency is corrected. Control devices which show evidence of tampering should be considered suspect of improper operation.