Harold L. Schmeilski
Illinois Division of Boiler and Pressure Vessel Safety, D. R.
Summary: The following article is a part of National Board
Classic Series and was reprinted in the January 1986 National Board BULLETIN. Permission to reprint was granted by the Illinois Division of Boiler and
Pressure Vessel Safety, D. R. Gallup, Superintendent. (6 printed pages)
This article describes the cause of failure of a monoethandamine (MEA)
absorber vessel that ruptured in the state of Illinois in 1984, resulting in 17
fatalities and property damage in excess of $100 million.
The ruptured vessel was designed in accordance with The American Society of
Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section VIII
rules. The vessel was constructed of 1 inch thick SA516 Gr 70 steel plates
rolled and welded with full penetration submerged arc joints, without postweld
heat treatment. The cylindrical vessel measures 81/2 feet in diameter with
hemispherical ends comprising an overall height of 55 feet. Operating
conditions were 200 psig internal pressure containing largely propane and
hydrogen sulfide at 100¡F. An internal system distributed monoethanolamine
(MEA) through the vessel for the purpose of removing hydrogen sulfide from the
VESSEL OPERATING HISTORY
The vessel went into operation in 1969. Soon after start-up, hydrogen blisters
were observed to be forming in the bottom two courses of the cylindrical vessel
wall. Metallurgical analysis showed laminations to be present in the steel.
In 1974, due to the large blister area found in the second course, a full
circumferential ring 8 feet high was replaced in field by inserting a preformed
ring in three equal circumferential segments. The welding was accomplished by
the shielded metal arc process ("stick welding") without preheating or postweld
The ASME Code does not require preheating or postweld heat treatment for SA516
Gr 70 steel 1 inch thick or less. However, this steel is slightly air
hardenable during welding, depending on the welding process, position and
procedure employed. This material is classified as a P1, Group 2 material
according to ASME Code Section IX.
The vessel was operated under the owner/user option of the Illinois Boiler and
Pressure Vessel Safety Act and received a certification inspection
approximately every two years. Continuing corrosion problems in the lower end
of the vessel resulted in the installation of an internal Monel liner in 1976
covering the bottom head and most of the first ring, stopping short of the
replaced ring. Periodic internal inspections were mainly visual with wall
thickness determinations made by an ultrasonic thickness gauge.
Just prior to the rupture, an operator noted a horizontal crack about 6 inches
long spewing a plume of gas. While attempting to close off the main inlet
valve, the operator noted the crack had increased in length to about 2 feet. As
the operator was evacuating the area and as the firemen were arriving, the
vessel ruptured releasing a large quantity of flammable gas which ignited
shortly thereafter creating a large fireball and the ensuing of deaths and
damage. The separation occurred along the lower girth weld joint made during
the 1974 repair. The upper portion of the vessel was propelled 3500 feet by the
thrust of the escaping gas.
The fracture surfaces exhibited the presence of four major prerupture cracks in
the heat affected zone (HAZ) of the lower girth field repair weld. The cracks
originated on the inside surface and had progressed nearly through the wall
over a period of time. The largest precrack was located in the same area as the
prerupture leak reported by the operator. In total, the four cracks encompassed
a circumferential length of about 9 feet (33.7% of circumference). The
remainder of the fracture exhibited a fast running brittle separation.
Microscopic examination of various cross sections through the failed weld joint
area showed the cracking originated in a hard microstructure in the HAZ and
progressed in a manner characteristic of hydrogen related damage in hard steels
(see figures above). The HAZ exhibited hardness of up to 45 HRC (Hardness
Rockwell "C") (450 Brinell), equivalent to a tensile strength of over 200,000
psi in the region of weld cracking. By comparison, the base metal had a
hardness value of less than 20 HRC (229 BHN [Brinell Hardness Number], 110,000
psi tensile strength). The following sections discuss technical factors
contributing to in-service cracking of weld joints under such conditions.
Welding procedures adopted must take into account not only the minimum
requirements of ASME Code Section IX and the appropriate design section, but
must also be suitable for the specific service conditions likely to be
encountered. Stress corrosion cracking, hydrogen embrittlement and corrosion
fatigue are typical of material/environment interactions that are not fully
accounted for in the ASME Code design rules. Appreciation of such potential
problems is left to the process designer, vessel designer, owner, contractor or
inspector. Reliance on only the ASME Code rules is not enough to assure safety
of vessels operating in many corrosive environments.
The weld HAZ contains potentially crack susceptible metallurgical structure,
hardness variations and residual stresses that can promote various types of
unexpected service induced cracking depending on the chemical environment and
operating temperature. Industry experience has shown that steel having a
hardness of 22 HRC maximum is resistant to cracking even under severe exposure
conditions where hydrogen can be absorbed by the steel. At hardness levels
above 22 HRC, steel becomes less resistant to hydrogen induced cracking and
other environmental effects. At high hardness (above about 40 HRC), steel
becomes quite susceptible to cracking in the presence of hydrogen.
In potentially critical environments, the weld joint properties must be
carefully controlled. Weld HAZ hardness is a function of the cooling rate after
welding. Preheating to at least several hundred degrees and maintaining an
interpass temperature during welding can warm the joint area sufficiently to
prevent rapid cooling after welding. Carbon content and alloy composition will
dictate the appropriate temperature. Rapid cooling of even mild steel can
result in unacceptably high HAZ hardness for service in aggressive chemical
Postweld heat treating (PWHT) is often necessary in critical weld joints to
temper (soften) or stress relieve weld joints in rugged duty or aggressive
chemical environments. Higher carbon steels and more alloyed steels are nearly
always given PWHT. Even when not specifically called for in ASME Code Section
IX, preheating or PWHT may be necessary. In hydrogen environments, avoiding
formation of a hard HAZ is crucial. Other corrosive environments present
The specific weld procedure employed must be developed by individuals with
pertinent knowledge of the ASME Code (which should be viewed as the minimum
guideline) as well as material behavior expertise in aggressive environments.
There are many specific ways that corrosion may contribute to unexpected
failures. Often, corrosion problems are handled simply by making the component
thicker (a corrosion allowance). This is appropriate so long as the corrosive
conditions are known, the vessel is periodically inspected and if the corrosion
is not highly localized. Corrosion fatigue, pitting, stress corrosion and
hydrogen attack are examples of metal/environment problems that cannot be
adequately handled by a corrosion allowance and superficial inspection methods
Hydrogen-assisted cracking and stress corrosion cracking will not always be
readily apparent. Carefully preparing the surface for visual examination, along
with other techniques such as dye penetrant, magnetic particle, or shear wave
ultrasonic inspection methods, may be required to detect such defects.
Corrosion-enhanced damage is often associated with welds, nozzles, or areas of
unstable environmental conditions; places where either the environment, stress,
or metallurgical condition may abruptly change.
High pressure hydrogen or acidic environments can introduce damaging levels of
hydrogen into steel, particularly hard steels or hard HAZs. The mechanism of
hydrogen evolution and penetration is illustrated above. The absorbed hydrogen
atoms are attracted to high stress regions in the structure, such as crack-like
defects. The combination of hard steel and absorbed hydrogen leads to the
development of cracks. Once inside the steel, these hydrogen atoms also migrate
to inclusions or laminations and create hydrogen fissures and blisters.
Hydrogen sulfide, cyanide and arsenic, even in trace deposits, are examples of
materials that greatly increase the amount of hydrogen that becomes absorbed by
steel. Therefore, under acidic corrosive conditions, particularly those
environments that also contain hydrogen sulfide, cyanide or arsenic, hydrogen
damage can be severe. Weld HAZ hardness must be carefully controlled under
these circumstances, regardless of whether or not the ASME Code or the National
Board Inspection Code specifically address the subject.
Welding procedures, repair methods, and inspection procedures must include
careful consideration of potential failure modes in corrosive environments. If
pressure vessels or allied components are operating in an aggressive
environment, special steps should be taken to assure that individuals with
pertinent expertise are involved in the planning and review stages of design,
inspections and repairs. When distress signals are present, take the time to
evaluate the cause and determine what special precautions are necessary.
The problems of in-service cracking of weld zones can be minimized by attention
to the important factors summarized below.
Preheat or postweld heat treat weld joints that may develop a hard HAZ when
corrosive conditions are met.
Inspect weld HAZs for cracks by a suitable NDE method if hard HAZs are
Field repair welds are likely to have hard HAZs unless proper preheat or PWHT
Small welds on thick members and arc strikes are examples of conditions
resulting in rapid heating and cooling and are likely areas for trouble.
Shop welds made according to the ASME Code may also crack in service under
severely corrosive conditions.
Preheating field weld joints will help drive off the dissolved hydrogen that
has been picked up by the steel in service.
Be particularly cautious when inspecting critical components in unfamiliar
corrosive service, especially when prior history reveals problems and when
field repairs have been made.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.