INSPECTIONS OF PRESSURE VESSEL
INTRODUCTION
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.
General Inspection
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.
Inspection Techniques
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
Introduction
- 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.
Pressure Gage
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.
Rupture Disks
- 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.
Drains
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
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.
Safety Controls
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
INTRODUCTION
- 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
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.
Erosion
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.
Lighting
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.
Pressure Gages
- 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.
Rupture Disks
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.
Supports
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.
Special Closures
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.
Pressure Test
- 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.
Record Review
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.
Conclusions
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
General
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.
Inspection Procedures
Thermometer
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.
Stop Valves
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.
Controls
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.
