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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.

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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 Sanders.Roy@Suddenlink.Net.

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Figure 2. There is no substitute for an internal inspection tohelp ensure mechanical integrity.