tank failures

5/21/2018 Tank Failures

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1. Title slide


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2. Quote from an excellent reference book in chemical process safety.See reference 1 for full details.


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3. Tanks come in all colors, shapes and sizes. This first slide is of theBoston Gas Liquified Natural Gas (LNG) storage tank recently dismantled in Bostonafter 20 years of a perfect record. Liquefied natural gas tanks are unique as thisdiscussion will point out. This tank has been dismantled because Boston Gas ispurchasing most of its natural gas through gas lines. This presentation will focus on

tanks holding more than 50,000 gallons. (Photograph courtesy of Boston Gas,Boston, MA)


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4. This is a water storage tank located in Portland Oregon. Severalfeatures common to tanks are shown. Note the manhole for tank entry. Also notethe welded seams. Tanks are constructed from plates of carbon steelapproximately 4 X 8 to 8 X 32 feet welded together. The plates may or may not bepre-formed to the tank diameter. Finally, note the hold down bolts along the bottom

to keep the tank in place in case of a seismic event.


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5. This is a fixed roof, cone with column support, tank for fuel oil storagelocated in the mid-western part of the U.S.A. The tank is insulated. This tank is 52feet high and about 240 ft in diameter. It can hold nearly 21 million gallons ofmaterial (500,000 barrels or 80,000 m3). Another tank common for the oil industry isthe floating roof tank. The floating roof can be either external or internal

(Photograph courtesy of Chicago Bridge and Iron Company, Oak Brook, Illinois).


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6. This is a tank farm in which several of the tanks have external floatingroofs. Floating roofs greatly reduce loss by evaporation. For fixed roof tanks,temperature cycles for a 24 hour period can lead to a large loss of material becauseof the "breathing loss" (expansion of saturated air into the surroundings duringdaylight and contraction in which fresh unsaturated air is added back into the tank

during evening hours). Another type of a significant material loss for fixed tanks is"filling loss" in which saturated air is displaced upon filling (a similar everydayexample is the filling of a automobile gas tank). Floating roof tanks significantlylower breathing and filling losses. If the true vapor pressure of the material to bestored is between 1.5 to 11.1 psia (10 to 75 kPa) the tank should be equipped with afloating roof (Ref. 3) (Photograph courtesy of Chicago Bridge and Iron Company,Oak Brook, Illinois).


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7. There are five major considerations for tank construction. The majority oftank failures can be prevented via the use of these major concerns:

1. Adequate base - The foundation must be designed by a qualified engineer andconstructed in accordance with recognized structural practice. A sub-surface investigationneeds to be conducted by a qualified soils engineer. If ground can freeze, consideration

must be given to base depth. Fault lines must be checked. Special conderation has togiven if high winds can exist. Essentially the foundation must provide a stable plane withvery limited settlement after the tank is placed into operation. Adequate drainage aroundthe foundation is a must.

2. Floor or bottom - must be thick enough to be impermeable to fluid in tank. 1/4" isstandard.

3. Wall or shell - must be thick enough to withstand the hydraulic head. 7/8" thick steel atthe bottom panels is not uncommon, 3/16" at the top. Maximum wall thickness is 1 3/4"thick and is determined by formulas like those found in API 650.

4. Material - must be suitable for the stored material with minimum corrosion. Carbon steel(0.2 to 0.25% C in iron or ASTM A36/A36M steel) meets most requirements. Specializedtanks such as LNG tanks require 9% Ni steel or aluminum as we will learn later. Rubberlining, stainless steel, aluminum, and clad steel are other materials which can be used forspecialized cases.

5. Roof - Fixed or Floating - must be strong enough to hold 25 pounds per square foot(typical U.S.A. design standard). In cold climates, the roof must be able to hold severalinches and even several feet of snow. The drainage system must be adequate for floatingroof tanks to handle the severest of rain falls so that the roof doesn't sink into the tank.


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Floating roof tanks are generally designed to handle 10" of rainfall in a 24 hour period for aroof floating above a liquid of sp. gr. of 0.70 (See API 650 Ref. 11).


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8. This view shows a schematic of an external floating roof tank. Notethe articulated roof drain system, the rim seal and bleeder unit. The vacuumbreaker plays an especially important role. Although the roof can withstand someload, only several inches water vacuum inside can cause a collapse. (Diagramcourtesy of CBI).


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9. A tank under construction. In this photograph one can see the pre-formed steel plates being set in place and welded together. A person is standingnear the tank at the bottom to provide a size reference (Photograph courtesy ofCBI).


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10. Standards are set by a number of associations.

API 650 for welded storage tanks for petroleum storage.

API 620 for low pressure, low temperature, and cryogenic tanks (primarily for designand construction).

NFPA 59A & 58 for LNG and LPG storage tanks (primarily for location, Q.C., andtesting)

AWWA 100-84 for welded steel tanks for water storage.

UL-142 for steel tanks holding less than 50,000 gallons.


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11. Large storage tank failures are memorable events. One example isthe 1925 great molasses flood caused by the explosion of a storage tank formolasses. A wave of molasses twenty feet high knocked down several buildingsand drowned twenty-one people. Clean up took years and some Boston residentsclaim that they can still smell molasses on warm days.

In this teaching module we will concentrate on 3 more recent disasters whichdemonstrate major storage vessel failures followed by a brief discussion oncommon tank and operational failures. Many failures that have and will occur arethe result of poor or inadequate design of the auxiliary systems - relief valves,venting, purging, foundation heating systems etc. A summary handout (Table 1 inthe notes, Slide 61 in this presenation) describes major tank types and commonfailure modes. This presentation concentrates on three major disasters because ofthe significance they have made in engineering design and development.


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12. Definition of BLEVE. BLEVE stands for boiling liquid expandingvapor explosion. The origin of the word BLEVE came from the Factory MutualInsurance Company in the mid 1950's. Originally, the word was used to describe apressure vessel explosion which occurred for a vessel holding formalin (37% bywt% formaldehyde in water) above its normal boiling point. BLEVEs can occur with

any pressure vessel holding a liquid under pressure and at a temperature above itsnormal boiling point. It is the rapid phase transition in which a liquid containedabove its atmospheric boiling point is rapidly depressurized, causing a nearlyinstantaneous transition from liquid to vapor with a corresponding energy release.During the 1960's several liquid propane storage vessels failed when the levelinside dropped below an outside flame impingement point. The vessel wall failed. Alarge amount of liquid propane then vaporized (entrapping liquid particles as well)creating a huge vapor cloud. The vapor cloud then exploded. The word BLEVEwas revived to describe these catastrophes. Early fire fighting techniques withpropane storage vessels cost several lives because of lack of knowledge about theseriousness with fires around liquid propane storage vessels. Through aneducational film produced by NFPA, many fire fighters know that the word BLEVE

means precaution and evacuation first.


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13. A liquified petroleum gas (LPG) pressure vessel failure. LPG can beliquified propane, a propane/butane mixture, or liquified butane. In order to achieveliquid storage at ambient temperature, the material is stored at high pressure.Pressures can range from 30 psig for liquified butane to 150 psig for liquifiedpropane. The pressure vessels for LPG storage is often rated for pressures up to

250 psig (LPG can also be stored in low temperature tanks near ambient pressure).

By far BLEVEs are the worst storage vessel failures and the mostspectacular.


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14. A BLEVE involves several steps.

Simplified steps are:

1. First, for LPG pressure vessels, a flame from a fire must be striking the outside ofthe storage vessel. BLEVEs can also occur because of a weakness in a metal wall,corrosion, or a dent from an impact with another object. For this example we willconcentration on a LPG storage vessel BLEVE.

2. Because energy is being added to the contents, vaporization will occur and thematerial will leave the vessel via the relief valve. When the level inside the vesseldrops below the flame impingement point the metal of the vessel wall weakens.Note that failures can occur even when the vessel pressure is below the designpressure and relief valve set point.

3. A rupture occurs at this weakened point and a sudden expansion of vaporizingmaterial occurs. Some liquid is also entrained by the escaping gas in this rapiddecompression process. Exit temperature of the LPG is approximately -40oC, thuscondensing water vapor from the air. A large cloud of material forms around thevessel.

Note that a BLEVE can occur with water held in a tank at a pressure andtemperature above its normal boiling point. Many serious injuries have resultedbecause of BLEVES with water. Of course, when the liquid is a volatile flammable,the consequences are dramatic.

4. If the vapor is flammable, then the release will ignite. One very large fire ball canresult.


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15. A diagram slide to go with the description given in 15.


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16. This slide is a synopsis of one of the worst BLEVE's in terms of lost oflife and property damage on record.


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17. A 1979 photograph of the tank farm before the accident. Note howclose the storage vessels are. Two more 2,400 m3 spheres were added after thisphotograph! The initial accident was a leak from a broken joint or faulty packing inthe pipe line/valve due to poor maintenance, or sabotage???. This release travelledseveral hundred meters to an ignition source. Remember gaseous LPG has a

higher density than air thus it tends to move along at ground level. (Slide courtesyMr. Olle Johansson, Skandia Group, Stockholm, Sweden)


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18. An aerial view that shows the point of initial release and ignitionsource. The distance involved between release and ignition is 200 m. Ignitionsources may not be immediately apparent, however, they are always free andavailable. Most accidents follow a three-step sequence. (Slide courtesy of SkandiaGroup)

Initiation: the event that starts the accident (in this case a release from a faultyvalve?)

Propagation: the event or events that maintain or expand the accident (in this casea release of propane which formed a vapor cloud which expanded over 200 mbefore ignition, then ignition occurred, and then...)

Termination: the event or events that stop the accident or diminished it in size (inthis case consumption of combustible material and there was a large quantity!) (SeeRef 15. p 14. for more details about the accident process).


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19. On the left is an aerial photograph of the PEMEX depot and buildingsin the vicinity before the mishap. (Scale is 1:20,000)

On the right is an aerial photograph of the PEMEX depot and buildingsin the vicinity after the mishap. Note the relocation of many cylindrical vessels.

Also, only two of the 6 original spheres remained. Nearby, all of the residential areashown in the picture was affected directly by the shock from the explosion, by theheat from fireballs, or by the rain of droplets of cooled LP-Gas which set people'sclothing and surrounding buildings on fire (portion of text was from the SkandiaGroup case study). (Slide courtesy of the Skandia Group) Reference: "Bleve! TheTragedy of San Juanico," First 1985, Skandia Group International InsuranceCompany, Stockholm, Sweden.

.


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20. Fire fills the sky. The magnitude of a BLEVE is overwhelming. Notethe size in comparison to the electrical transmission tower and the water tower.(Slide courtesy of the Skandia Group).


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21. Here we see (lower left horizontal tank) a safety device workingproperly. - a relief valve. All that the firemen can do is keep the sphere and tankswet and let the contents burn out in a slow controlled manner. (courtesy of theSkandia Group).


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22. Two remaining spheres and disaster in the foreground. Thesespheres did not explode probably because they were filled to 90% capacity andconsequently contained a large amount of cooled liquid. Also the flame fronts werenot as severe near these spheres. Nevertheless, note that the supports wereweakened. (Slide courtesy of the Skandia Group).


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23. Believe it or not this cistern (21 m long and 3.5 m in diameter) set aworld's record. It flew across the sky a distance of 1200 m and up an elevation of50 m destroying a home when it landed. It holds the distance record for 20 tonbleveing cisterns. Fortunately, the family members of this household wereevacuated one hour earlier. (Slide courtesy of the Skandia Group)


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24. A photograph which shows an overview of the diaster. 15 LPGstorage vessels bleved. 500 people were killed. This sums up the violent nature ofBLEVES. (Slide courtesy of the Skandia Group).


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25. How to prevent BLEVES.

Bleveing pressure vessels take time. The minimum known time is approximately 30minutes for fixed storage vessels and 10 minutes for a tank trucks. The fire chief incharge must order an evacuation of all non- critical personnel as soon as possible.


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26. LNG is primarily liquefied methane. LNG storage tanks are nearambient pressure (1.5 to 2.0 psig). In order to achieve liquid storage, the material is

stored at low temperatures (-260oF).


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27. The initial interesting fact about LNG tanks are that they are a tankinside a tank. The outer tank is made out of carbon steel and is under a slightpressure inside of 1 to 2 psig. Inside is another open roof tank which is constructedof 9% Ni steel or aluminum. The two tank walls are separated by an annular spaceof 3 to 4 feet. This space contains insulation, usually a powdered low density

material called Perlite. Perlite is from a volcanic material which when heated atvery high temperature expands to a highly porous-low density material. It is anexcellent insulating material.


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28. This is a cutaway of a typical double-walled, flat bottomed tank and itsprincipal components. The inner tank is suitable for LNG storage at cryogenictemperatures (-260oF or -160oC). The outer tank provides a proven means ofprotection for the insulation and is vapor tight. Connections to the inner tank mustallow for differential movement because of temperature changes and hydrostatic

loads. (From A.G.A. LNG Information book Figure 3.2 p. 36).


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29. The Cleveland LNG disaster was quite significant. This was the firstlarge scale commercial plant in the world. No further commercial LNG tanks wereconstructed in the United States after this accident for a period of 20 years. Theinner tank itself was 42 feet high and 70 feet in diameter (1.2 MM gallon or 4,600m3 capacity). The outer tank was 51 feet high and 76 feet in diameter. Rock wool

was used for insulation in the annular space. Approximately 120 people died and1500 people were left homeless because of the resultant explosions which occurredafter the tank failed and spilled it contents into the nearby streets and sewers.


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30. Composite of NY times headlines related to the accident. Note theheadlines: PANIC IS WIDESPREAD! Red Cross is involved. 1,500 homeless. FBIreported to be studying the possibility of sabotage. Later the investigation ruled thatthere was no evidence of sabotage.


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31. Aerial view of LS & R Plant and No. 2 works, East Ohio Gas Co., andadjoining property damaged by fire October 20,1944, looking north. Note the upperright quadrant. Nearly all of the neighboring factory complex was destroyed. In thelower quadrant one sees that the blocks of 3 to 4 story bricked store fronts andoverhead apartments were completely wiped out. That street is East 61st Street.

Near the middle of this photograph running diagonally towards the bottom rightquadrant is East 62d street. LNG probably flowed into the storm sewer underneaththis street. Eventually the mixture exploded, cracking up the street for 3 blocks fromthe original tank whose location is marked on the photograph. (Photograph reprintedfrom Bureau of Mines Report of Investigation #3867, Ref 7.)


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32. Elliot et al. (Ref 7.) in a Bureau of Mines Report carefully developedthe evidence for and against several possible causes. Perhaps the most plausiblereason for the failure is that 3.5 percent nickel steel used for the inner shell may nothave been suitable for the tank. Since the writing of the report, it has beenestablished in many research laboratories that 3.5 percent nickel steel has low

impact resistance at the service temperature of LNG. (from Ref 6.). Impactresistance is related to brittle failure something we will discuss in the next casestudy in further detail.


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33. Today LNG tank failures are non-existent. As of 1988, 189 fielderected tanks were in service world wide. The inner wall material is typically 9%nickel steel (or aluminum) whose strength properties and ductility (impact strength)actually increases with lower temperature near the cryogenic temperatures of LNG.

Also, some tanks are constructed with aluminum walls. There have been very few

leaks and no ruptures in 28 years since these tanks have been used. The TokyoElectric Power Company Inc. has developed an in-tank black and white televisioncamera with a separate light source for the monitoring of the interior of an LNG tankwhile it is in operation. The price is a bit prohibitive presently over $1,000,000.


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34. Another significant tank failure was because of brittle failure. In thiscase, a large inland liquid spill resulted. This spill affected 3 states and 2,000,000people.


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35. The day was January 2nd, 1988. A large above ground tank (48 feethigh, 120 feet in diameter, 4.0 MM gallon or 15,000 m3 capacity) was being filledwith diesel fuel. This was the first time that the tank was being filled to nearcapacity since it's relocation (disassembled, moved in pieces, and reassembled)from a site in Cleveland, Ohio to a terminal located in Floreffe, Pennsylvania. The

tank was within 2" of being at its fill capacity when at 5:02 PM a loud noise soundinglike thunder was heard coming from the area near the tank. An operator observedthe roof collapse, and a cloud of mist started to form. A kid playing street hockeythought that the sound lasted a period of 3 to 4 seconds. Finally, Mrs. AlvaRogerson who lived across the street heard a rumble for a period of 30 secondsfollowed by the observation of a wave of foam splashing over the top of Route 837.


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36. Unfortunately, the wave of foam observed by Mrs. Rogerson was partof a 3.9 million gallon diesel fuel spill. 0.75 million gallons of diesel fuel reached theMonongahela river which flows towards Pittsburgh into the Ohio river. The Ohioriver flows into the Mississippi. 10 states and approximately 2,000 miles ofriverways exist between the spill and the gulf of Mexico. A tremendous national

disaster was in the making and heroic measures by local, county, state, and federalofficials prevented the catastrophe from causing more pervasive consequences (ref.9 p 1). A round-the-clock clean-up effort was undertaken and 80% of the fuel wasrecovered (that still left 511,000 gallons unaccounted for!). The West AlleghenyMunicipal Authority was the most seriously affected by the spill. West Alleghenycustomers in the Robinson township (10,000 people) went without water service fora period of 8 days. Other water companies affected included the Midland BoroughWater Authority and the Western Pennsylvania Water Co. Fortunately, no peopledied because of this failure.


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37. The tank was tossed aside as it collapsed. Relocating itself 120 ft from itsoriginal base. The next slide is a picture of the remaining tank. Surprisingly, theforce of the oil spill itself only wiped out one cinder block building. However, severalearthen dikes whose purpose is to contain a spill were washed out.


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38. A photograph of the collapsed tank. Remember that this tank was 48feet high. In this picture it is barely 2 times higher than the man walking by.


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39. The story begins with the need to replace a tank which requiredextensive repairs. The terminal decided to replace the tank with a tank located inCleveland on a site which was being sold. This tank was 48 feet high and 120 feetin diameter with a 96,000 barrel capacity. The tank was used for storage of #6 oilor asphalt. Surprisingly, the date of original construction could not be established.

The name plate on the manhole cover read 1934. But other records indicated 1940.


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40. This slide shows a sketch of the tank looking northwest showing thefracture path. The fracture is suspected to have started at the first tier of platesnear a T weld.


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41. "The collapse could have been and should have been averted. Both theexistence of the flaw and the tendency of the tank steel to react brittlely under normalregional climatic and service conditions were discoverable through the application of goodengineering, construction, and inspection practices and by compliance with applicableindustry and governmental standards." Ref. 9 p iii.

Unfortunately, the tank was not reconstructed according to current accepted engineering

practice.1. - Cutting of the old tank over the welds - An engineer specified that the old tank be cut atthe welds. The engineering company who dismantled the tank cut above the old welds. Infact during the process a cherry picker overturned striking several of the tanks plates.These were sledge hammered back into round. This, however, did not contribute to thefailure.

2. - Failure to get a fire marshall permit - A fire marshall's permit was never issued becauseof lack of communication. The engineer in charge called the fire marshall's office and hewas told over the telephone that the permit was in the mail. No physical permit, however,was ever issued. Once again, however, this did not have a bearing on the failure.

3. - The foundation for the tank failed compaction tests - As the foundation for the tank wasbeing built several rain storms occurred which delayed construction. Compaction testswere done at different levels and several failed. The failed compaction tests were blamedon weather conditions and the spots were rerolled. However, no retesting of the spotsoccurred as the foundation was built up. Also a concrete ringwall foundation wasrecommended. Crushed stone was utilized. (It should be noted that crushed stone isallowed by API 650 and the foundation had no impact on the initial failure).

4. - Radiograph X-ray test did not follow a schematic - Unfortunately, this counted.Radiographic tests were done on 39 welds. 22 welds were defective. However, closerinspection showed that these defects were related to the old welds. Nor, was theradiographic tests charted on a schematic so that one could trace back where theradiographic results were from on the tank. API 650 requires 160 radiographs. Thisemphasizes the need for complete inspections and tests vs. samples.


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42. The radiographic defects were related to the old welds. This slide liststhe main defects found. Slide 43 is a photograph which demonstrates thesedefects.

Inclusion of slag.

Lack of penetration.Lack of fusion.

or excessive porosity was present.


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43. This slide goes along with slide 42 to demonstrate different types ofweld defects. All of these defects were present in Tank 1338 and lack of penetrationat one point may have been the cause of the initial flaw where the brittle failureoccurred.


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44. Full hydrostatic testing was omitted. Recommendations of both API650 6th and 7th editions require full hydrostatic testing before filling the tank with ahydrocarbon. Full hydrostatic testing means that the tank is filled with water to it'srated capacity. The water is held in the tank for a period of time and leaks aresearched for. A purchase order was issued for 3,440,000 gallons of water

($4884.38). Final billing by the water company was for $802.22 worth of water(500,000 gallons). Only a partial hydrotest was completed in which the water levelin tank was 5 feet and 15/16 inches. Remember that the tank was 48 feet high.


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45. The cause of tank failure was a brittle failure. Materials have two mainproperties; strength and toughness. Strength is the ability of a material towithstand a force or force per unit area under constant conditions. Toughness isthe ability of a material to absorb energy. A dish is an example of high strength andlow toughness. Under constant force the dish doesn't easily break; however, just

drop it to create a sudden impact and it breaks. Strength failures occur under staticconditions. On the other hand toughness failures are often associated with impacts,but they can occur under non-impact situations and brittle failure is such anexample. The dropping dish suffers a brittle failure. It is an instantaneous event.The material cracks instead of deforms.

On this particular day, there was a convergence of three separate but necessaryfactors to cause brittle failure. The full tank created high stresses in the first coursetank wall. The cold weather contributed to the increasing probability of brittle failurebecause toughness is inversely related to temperature. Finally, latent flaws existedthroughout the old tank at old welds and the tank wall itself. Several tests can beused to quantify resistance to brittle fracture. The Charpy V-notch test is the mostnoted and the first course tank wall material measured 5.9 ft-lbs at 40oF. 13 ft-lbs is

considered a minimum value for tank wall material.It should be noted that tank standards did not have material toughnessrequirements until API 650 7th Edition, 1980. The original tank wall material datedto the 1930's or 40's.


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46. These photographs show the suspected defect where brittle failureoccurred. The photographs showed a lack of weld penetration. A gap of nearly a1/2 wall thickness appears to be the point where the brittle failure started.


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47. One recommendation can be drawn from this case. Reconstruction ofold tanks must follow current design practices (API 650).


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48. Fortunately, the large tank failures are few and far between. However,tank failures of much smaller magnitude and operational errors do occur more oftenthan admitted. Let us spend some time looking at common failures and how toavoid these.


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49. Overfilling: an operational error. This simple slide demonstrates thatstorage tanks are not built to hold liquid above the ring wall. In fact, some accidentshave occurred where the tank roof was lifted off of the tank wall because ofoverfilling. (Slide courtesy of Roy E. Sanders, PPG Industries)


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50. Overfilling - A few ways

1. Lack of attention is self explanatory: If gas stations didn't have automatic shut offnozzles, I wonder how many of us would overfill the gasoline tank in our cars?

2. Wrong setting of valves: Somehow a valve gets left open that should have been closed.Material flows unnoticed into a tank. Often tanks are at different levels within plants. Anyplant located along a steep river bank may face this situation. One time in a paper mill avalve was inadvertently left open between a tank storing 5 tons of a clay-water coating atthe coating prep building and a storage tank within the paper mill located in the basement.Since the coating prep building was located on a side of a hill and the papermill buildingalong the river bank, the difference between elevation of the two tanks was about 50 feet.One night material flowed between the two tanks over a period of 18 hours. In the morninga 5 ton clay-water coating spill was found in the basement of the paper mill.

3. Error in level indicators: Often the experienced operator will know when the indicator iswrong and issue a work order to correct it. Other times the operator may say, "well thelevel indicator has never been correct. I know when the tank is full." Guess what? Theoperator doesn't know all of the time.

4. Change in shift duty: A change in shift can mean a missing link in communication. Onecrew may be filling a tank and inadvertently forget to tell the crew coming in that Tank 61 isbeing filled. The second crew begins a routine shift assuming everything is under control.Sure enough Tank 61 overfills and warning alarms suddenly change what would have beena routine shift.


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51. Some solutions to overfilling problems include: On large tanks usuallytwo independent methods of level checks are used. Redundant backup is used foralarms and often the alarms may be of different types. Interlocks can preventvalves from being left opened after a transfer. In order for the next processing stepto proceed, a valve to or from a storage tank has to be closed. Routine

maintenance is a must; level indicators can fail. Sometimes level indicators are setto a certain specific gravity. If a lower specific gravity material is added to a tank afalse reading will result. Operators must be forewarned. Use check valves toprevent back flow. Finally, communications. You will hear a lot about the need forgood communications within the plant environment.


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52. Over-pressurization

Flat cone fixed roofs above tanks are generally designed to withstand only a fewinches water of pressure. Our lung pressure is about 25 inches water pressure. Ifgiven the time (probably years) and correct piping (check valves etc.), you could

blow the roof off of a 120' diameter tank. The failure usually occurs at the roof/wallweld. Both liquid and gas can cause overpressurization. For example, many tankshave N2 atmospheres above a hydrocarbon to prevent explosive mixtures. Anitrogen regulator can fail and suddenly a rush of high pressure nitrogen surges intothe tank. The relief valve which is poorly maintained may fail. Guess what? Thetank fails because of overpressurization.


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53. This slide is a diagram to illustrate the point about how little pressure8" water gauge pressure is.


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54. The solution to overpressurization is properly designed relief valves,vents, and vent systems.


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55. Vacuum is probably the most common way a tank is damaged. Theways in which it occurs are legion. Great ingenuity is sometimes required. First off,less negative pressure is required than positive pressure to cause damage. Watergauge pressure of -2.5 inches, the height of a cup of tea, is just enough vacuum tosuck in many tanks. (Ref. 1 p 73.)


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56. This photograph, courtesy of Roy E. Sanders, PPG Industries, is apicture of a tank sucked in. This tank was 8 ft (2.2 m) in diameter and 8 ft (2.2 m)high and held hydrochloric acid for pH control. During a filling of the tank, itoverflowed and the vent line worked as designed. Then, the alert truck driverresponded and abruptly closed the delivery valve. Next, unexpectedly a partial

vacuum was created by a siphoning action of the overflowing liquid and the tankwas sucked in and totally destroyed (Ref. 17. p 9).


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57. As mentioned above many methods exist in which a tank can besucked in. Here three are mentioned along with solutions.

1. Vents can plug: Birds have a habit of building nests in the most unusual places.In fact, birds like tanks. Monomers can polymerized in vent lines as well as

polymers building up in non flow relief lines. Routine maintenance and field checksprevent vents and relief valves from plugging.

2. Vents are covered: Occasionally vents are purposely covered for maintenance.Alternative venting must be arranged.

3. A sudden change in temperature: A rain storm can do more damage thanimagined, especially if a crew has just finished steam cleaning a tank. The rapidchange in temperature can cause the steam to condense and the tank to suck in ifadequate ventilation (vacuum breaker valves) is not provided. (It has happened seeRef.1.)

When empty, tanks are large cavernous vessels which require breathing during the

day's normal swing of temperature.Prevention includes routine visual checks of vents and/or the use of vacuumbreakers. Never cover a vent without proper authorization.


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58. There are many additional concepts which must be considered whendesigning tanks. A few additional concepts are listed in this slide.


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59. Let's finally conclude

Remember that tanks are fragile. An egg shell can withstand more pressure andvacuum on a per unit mass basis. If we approach tanks with this concept, fewertank failures will occur because we are aware of the care that must be taken with

handling of eggs and tanks. Routine maintenance is a must as well as periodicinspections. Remember to follow the standards. These standards were written bypracticing engineers who have probably already experienced a catastrophe or twoand know what precautions must be made. Finally, in terms of tank failures I hopethat you will disprove the following famous line:

"History repeats itself"


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60. This package included contribution from several international experts instorage tanks. I am indebted to cooperation from insurance representives for use of theMexico City disaster (Olle Johansson of Skandia Group) and for my initial education in tankfailures from Bob Nelson (IRI in Hartford Ct). The diesel fuel tank disaster material camefrom an excellent report by the Tank Collapse Task Force. This task force was a group ofinter-agency and inter-disciplinary team assembled by Governor Casey of theCommonwealth of Pennsylvania. The task force was headed by Mr. Keith Welks, ChiefCouncil for DER with the Commonwealth of Pennsylvania. He served as a reviewer of thismodule as well as Mr. Mike Heilman who also served on the task force. I also acknowledgeCBI company for several photographs used in the beginning of this module. The principalengineer of CBI, Mr. Taavi Kaups, educated me on tank construction and design. In termsof LNG tanks, I acknowledge the assistance and encouragement of Armand Santacroce ofDistrigas and to Frank Bonomi and Frank Arricale of Boston Gas who allowed me theopportunity to walk through the Boston Gas tank during dismantling. Roy E. Sanders ofPPG Industries, who is having a book published containing many case histories of tankfailures, is acknowledged for encouragement and direction. Finally Trevor Kletz isacknowledged. Although I did not meet him, he is one fine engineer. All undergraduatestudents should consider reading his books before beginning practice because of theirclarity, brevity, and succinctness. You will become an excellent engineer and manager byavoiding situations which lead to the cases Dr. Kletz discusses.

The Northeastern Unversity media laboratory (Mr. Terry Beadle) is acknowledged for

assistance in the preparation of many word slides and photographs from references.Finally, I also acknowledge my loving wife and my two sons who had to suffer lost of"quality time" while I worked on the project.

Further material:

As I developed this module, I gained more and more information. One discussion was withMr. Taavi Kaups of Chicago Bridge and Iron who graciously reviewed this module andmade several helpful comments. One suggestion was to distinguish fail modes for differenttypes of tanks. The following abbreviated table covers some of the discussion we had.You may copy this to use as a hand out during the presentation.


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Summary of Large Storage Vessel Types and Possible Failure Modes

Fixed Floating

Low Cryogenic LPG Roof Tank Roof Tank

Temperature Storage Tank Pressure Vessel Applications:

#6 Fuel Oil

Gasolines

Liq.

NH3 LNG LPG Water Napthas

Butane, LPG O2, N2 Temperature: Ambient Ambient +5to -50oC -50 to -200oC Ambient Pressure: ---------atmospheric------

----- ~14 kPag (2 psig) ----- ~1.7 MPa (250 psi) Standards: AWWA D100-84, H2O API 650

API 620 NFPA 59 A, LNG NFPA 58 API 650, Hydrocarbons

Appendix R API 620 Appendix Q

Possible Failure Modes1:Foundation: ---settlement after construction---

---heating system fails in foundation system--- structural members shift

floating roof jams bottom buckles leaks, roof caves in loss of product shell and bottom are stressed, leaks


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Tank floor: --Corrosion due to H2O or a corrosive component--

Product leakage to the surroundings Tank shell: ---high winds--- ---high winds--- ---moist. penetrates ---cryogenic contacts ---flame impact---

shell buckles shell buckles outer insulation--- outer tank wall--- BLEVE

---vacuum vent clog--- iffloating roof low increased boil off & outer CS wall cracks

---presence of H2S--- shell buckles

possible corrosion insulation packing shifts metallic stress corrosion

Tank roof: ---hurricane--- ---hurricane---

roof plates lift off floating roof lifted ---vents clog or plug--- ---

roof drains clog--- suck in or over- floating roof sinks pressurize tank ---overfilling--- roof lifts off wall

Other: ---Lightening--- fire hazard if tank is not properly grounded

1Note: (---Mode---) with possible consequence listed directly below


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References

1. Kletz, T.A. "What Went Wrong? Case Histories of Process Plant Disasters,"1985,Gulf Publishing Company, Houston, TX.

2. "Fixed Roof Tanks for Liquids," CBI Bulletin No. 33001990, Chicago Bridge and IronCompany, Oak Brook, Illinois.

3. "Evaporation Loss Prevention," CBI Bulletin No. 5331980, Chicago Bridge and IronCompany, Oak Brook, Illinois.

4. "Horton Floating Roof Tanks," CBI Bulletin No. 32001990, Chicago Bridge and IronCompany, Oak Brook, Illinois.

5. "Bleve! The Tragedy of San Juanico," First1985, Skandia Group InternationalInsurance Company, Stockholm, Sweden.

6. "LNG Information Book 1981," 1981,American Gas Association,Arlington, Virginia22209.

7. Elliott, M.A., Seibel, C.W., Brown, F.W., Artz, R.T., and Berger, L.B., "Report on theInvestigation of the Fire at the Liquefaction, Storage , and Regasification Plant of the East Ohio GasCompany, Cleveland, Ohio, October 20, 1944," 1945 U.S. Bureau of Mines, R.I. 3867, Pittsburgh, PA

8. Mounce S. W., "Nine percent Nickel - 28 Years of Reliable Service in LiquefiedNatural Gas Containment," Nickel Development Institute Technical Series No. 10 030, Toronto,

Ontario, Canada M5C 2E3.9. "Report of the Investigation into the Collapse of Tank 1338"1988, Tank CollapseTask Force c/o the Office of Chief Counsel 301 Chestnut St. Suite 400, Harrisburg, PA 17101.

10. "Investigation into the Ashland Oil Storage Tank Collapse on January 2, 1988," 1988U.S. Department of Commerce National Bureau of Standards, NBSIR 88-3792, Gaithersburg, MD20899.

11. "API Standard 650, Welded Steel Tanks for Oil Storage, Eighth Edition, November1988," 1988American Petroleum Institute 1220 L Street, N.W., Washington, D.C. 20005.


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12. "NFPA 58 Production, Storage, and Handling of Liquefied Petroleum Gas (LPG) 1990Edition," 1990 National Fire Protection Association, 1 Battermarch Park, Quincy, MASS 02269-9101.

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