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1/429 CHEMICALPROCESSING.COM NOVEMBER 2010
STORAGE TANKS play a vital role at chemicalplants, oil reneries and other facilities, and repre-
sent major investments. Yet, despite all-too-frequent
incidents of wrinkled roofs, collapsed walls andother failures, many people who design, construct,
operate and maintain low-pressure storage tanks
dont appreciate how frail they are.
In his popular book What Went Wrong? Case
Histories of Process Plant Disasters [1], Trevor Kletz
starts the chapter on storage tanks by saying: No
item of equipment is involved in more accidents than
the storage tank, probably because storage tanks are
fragile and easily damaged by slight overpressure
and vacuum. Fortunately, the majority of accidents
involving tanks do not cause injury, but they do
cause damage, loss of material and interruption of
production.
Dramatic tank failures feature in about a quarter
of the case histories presented in the Beacon [2], a
free Internet publication of the Center for Chemi-
cal Process Safety (CCPS) of the American Institute
of Chemical Engineers. e incidents covered stem
from causes such as overlling, vacuum damage and
explosions resulting from ammable mixtures in the
vapor space.
So, here well look at factors that can compro-
mise such vessels and how to address them.
TANK CLASSIFICATIONS
Tanks come in a variety of designs. Some low-
pressure storage tanks, sometimes ca lled at-
mospheric storage tanks, are built to American
Petroleum Institute (API) 650 specications.
Many large API-650 tanks have a at to slightly
coned roof and appear similar to a can of tuna sh
or soup. However, a can of tuna sh or soup will
withstand many times more pressure than a typi-
cal low-pressure tank [3]. e weight of the roof
often limits the pressure rating of low-pressure
tanks. (Some low-pressure tanks have a oating
roof but we wont discuss these here.) Another
popular choice is the API-620 tank, which is lim-
ited by code to 15 psig. Its top resembles a pued
cupcake. ere also are small shop-built tanks,
transportation vessels, plastic tanks and an array
of non-code vessels.
In addition, a wide range of pressure vesselsare designed and fabricated to Section VIII of
the American Society of Mechanical Engineers
(ASME) Pressure Vessel Code. Such pressure
vessels come in a variety of shapes the more
popular ones are like a fat stra ight sausage or a
sphere. e ASME Code covers vessels with inter-
nal design pressures f rom 15 psig to 3,000 psig.
e largest low-pressure storage tanks often
are the most fragile and most vulnerable to failure
from both over-pressure and vacuum [3]. Basically
thats because a slight pressure over a very large
area creates a large force.
Treat Tankswith CareA variety of easily avoided problems can
cause vessel failure
By Roy E. Sanders, chemical process safety consultant
8/3/2019 Treat Tanks With Care
2/4NOVEMBER 2010 CHEMICALPROCESSING.COM 30
SOME COMMON PROBLEMS
Simple operational situations often cause tank failures.
Accidental overlling, impeding exiting vent ow, and
not allowing in-breathing as a tank is being pumped out
are cardinal sins.
Overlling. A recent CParticle, Dont Underesti-
mate Overllings Risks, www.ChemicalProcessing.
com/articles/2010/143.html, focused specically on the
hazards posed. It cited three major industrial accidents
resulting from overlling including a massive re in
December 2005 at the Bunceeld Oil Storage Depot in
Hertfordshire, England. e tank that caused the inci-
dent had an independent high-level alarm and interlock
but the system didnt work. e September 2006 Beacon
provided details on that disaster and related that overll-
ing has contributed to a number of serious incidents in
the chemical and oil industries in recent years.
Tanks must be engineered to provide protection
via alarms and high-high level interlocks against
overlling of hazardous materials and the resulting
spillage.Over-pressure and under-pressure. Its crucial to main-
tain the integrity of tank venting systems. Otherwise,
catastrophic damage may result.
Over-pressure caused a sudden drastic failure at
the base of a 12-ft-dia., 24-ft-high ber-glass acid tank
(Figure 1). e tank was equipped with a separate vent
line, an overow line and a vacuum breaker.
As a safety precaution when repairing an under-
ground sewer line that would receive acid if the tank
overowed, supervision had the overow line blinded
and instructed operators to run the vessel well below the
overow line. e thought was that the vent line wassized suciently for lling purposes.
Unfortunately, a blind from a previous job had been
left within the vent line and wasnt detected. As opera-
tors started lling the ber-glass tank, the inerts had no
place to go and the tank was pressurized to destruction.
Fortunately, no one was injured [4].
Under-pressure led to a well-maintained low-
pressure 20-ft-dia., about-30-ft-tall carbon-steel solvent
tank with -in. walls ending up as scrap metal after
improvements to the vent system. To reduce emis-
sions, vent recovery compressors and more-sophisticated
instrumentation were replacing old-style conservation
vents.
While the rst batch of material after the tank
conversion was being pumped out, the roof and two
courses of vertical walls were sucked in due to the lack of
the nitrogen padding and the vacuum protection system
being inadvertently isolated by a small block valve.
A simple hinged vent lid had served well for decades.
e lid was replaced with a much more complex system
involving a vent compressor to recover the vapors and
a nearly zero leakage pressure/vacuum device. e
REFERENCES1. Kletz, Trevor A., What Went Wrong? Case Histories of Process Plant Disasters, p. 97, 5th ed., Gulf
Publishing, Burlington, Mass. (2009). Similar details are also found in all earlier editions.
2. Beacon, a free single-page monthly publication of the CCPS comes in many different languages.
To subscribe, go to: http://www.aiche.org/CCPS/Publications/Beacon/index.aspx.
3. Safe Tank Farms and (Un)Loading Operations, BP Process Safety Series, BP Safety Group,
Sunbury-on-Thames, U.K. (2008).
4. Sanders, Roy E., Chemical Process Safety: Learning from Case Histor ies, p. 108, 3rd ed., Elsevier
Butterworth-Heinemann, Burlington, Mass. (2005).
5. Sanders, Roy, Human Factors: Case Histories of Improperly Managed Changes in Chemical
Plants, p. 150, ProcessSafetyProgress(Fall 1996).
Tilting Tank
Figure 1. Over-pressure due to left-in-place blind causedbase of tank to give way.
8/3/2019 Treat Tanks With Care
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operators were well trained on the new compressor but
not on the new piping arrangement. Worse yet, closure
of a single small-diameter impulse valve rendered all
the well-conceived improvements worthless. Oops, a
$100,000 mistake [5].Plant designers must strive to develop user-friendly
piping, layout and control schemes, and must clearly la-
bel equipment safety systems to reduce opportunities for
failure. Venting systems should ensure proper protection
during all phases of operations.
Tank venting systems mustnt be altered or tam-
pered with without a management-of-change review.
AN OLD STORY
None of this is new. In an ICI Safety Newsletterpub-
lished in the 1970s, Kletz predicted a storage tank would
be sucked in each year. Experienced process safetypeople hear of such situations every so often.
CCPS has pointed out tanks vulnerability to
vacuum in two issues of the Beacon. Vivid photos of
failed tanks demonstrated the importance of maintain-
ing proper vacuum protection.
e February 2002 Beacon, titled A Little Noth-
ing Can Really be Deating VACUUM is a Power-
ful Force!, showed a rail car sucked in and a tank that
collapsed while being painted. One main message was:
Whenever vacuum relief systems are removed, covered,
modied, etc., special precautions are needed to prevent
an incident.
Vacuum Hazards Collapsed Tanks in the
February 2007 Beacon stressed three key points:
1. Well-intentioned people can easily block vents.
2. Never cover or block the atmospheric vent of an
operating tank.
3. Routinely check for plugging of vents on tanks in
fouling service.
OTHER FUNDAMENTALS
Always keep in mind the following points about low-
pressure tank layout and design: Tank spacing and layout are critical. Various prop-
erty insurance publications and pamphlets from
the National Fire Protection Association (NFPA)
oer some guidance about the proper spacing of
storage tanks, especially those that contain am-
mable or toxic liquids.
Tanks containing incompatible chemicals
shouldnt be allowed within the same diking
systems.
Fire-protection features, including static
electricity dissipation, vapor space inerting,
protective foam generators, water spray and
dike designs, demand professional handling.
Venting systems not only must be well designed
but also must be inspected and maintained dur-
ing the life of the equipment. Tamper-proof vent
designs are ideal.
Tank bottoms should be sloped and associated
piping should be laid out to facilitate complete
drainage. Tanks should be checked for internaland external corrosion.
Local conditions, such as the possibility of ood-
ing or hurricanes, which can aect low-pressure
storage tanks should be considered.
Ongoing corrosion monitoring is essential. e BP
booklet [3] contains a number of photos and brief
descriptions of tank failures from corrosion.
RELATED CONTENT ONCHEMICALPROCESSING.COM
Dont Underestimate Overllings Risks, www.
ChemicalProcessing.com/articles/2010/143.html
Bhopal Leaves a Lasting Legacy, www.Chemical-Processing.com/articles/2009/238.html
Its Time to Tank Complacency, www.Chemical-
Processing.com/articles/2006/028.html
8/3/2019 Treat Tanks With Care
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Many helpful references are available. ese
include API-650, API-620, API-510 Pressure Ves-
sel Inspection Code Maintenance, Inspection,Rating and Alteration, API-653 Tank Inspec-
tion, Repair, Alteration, and Reconstruction,
NFPA codes and Reference 3.
VESSEL INSPECTION
To quote from the BP booklet [3], Most tanks are made
of carbon steel, which can corrode when exposed to air
and water. Over time, uncontrolled rusting can weaken
or destroy the components of a tank, resulting in holes
or possible structural failures, and release of stored prod-
ucts into the environment.
Eective timely inspections can drastically reducefailures from corrosion.
ree dierent approaches to tank inspections are
widely used.
A periodic visual inspection by operators is the
rst line of defense. is type of routine monitoring
focuses on evidence of seepage or leakage, tank set-
tling, bulging or signicant corrosion.
In-service inspections generally are less frequent
than operator reviews and typically are performed by
certied inspectors. Such checks often start ve years
after commissioning, with frequency adjusted accord-
ing to tank history, the risk involved and the corrosion
rate. ese most often involve taking ultrasonic thick-
ness readings at key locations.
Periodic internal inspections after the tank is
drained and washed are a must (Figure 2). ese can
identify components that have shifted, localized pit-
ting, etc., that may not be apparent from an external
inspection. Typically internal inspections take place
at a frequency between annually and once every ten
years. e exact frequency is best determined by the
corrosive nature of the uid, including its trace com-
ponents, and the past history of similar equipment onthe site.
DONT TAKE TANKS FOR GRANTED
Tanks can and do hold large inventories of a wide
variety of raw materials, intermediates and nished
products safely for decades. However, if a tank and its
accessories are poorly designed, abused by operations
or deprived of eective inspection and basic mainte-
nance, bad things can happen.
ROY E. SANDERS is a chemical process safety consultant based
in Lake Charles, La. E-mail him at [email protected].
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