What Went Wrong Part 3 pdf

closed, and a drain valve in between opened.. A flow through the drain valve showed that one of the isolating valves was leaking, so the drain valve was closed and a message left for the

closed, and a drain valve in between opened A flow through the drain valve showed that one of the isolating valves was leaking, so the drain valve was closed and a message left for the employees workmg the next shift, telling them to open the drain valve before work started Nothing was written on the permit-to-work The mes- sage was not passed on: the drain valve was not opened and the fitter broke the joint the wrong way, removing all the bolts The joint blew apart, and the fitter received head injuries from which he will never fully recover [37]

Cc) It is not only flamiable oils that cause accidents In another inci- dent two workers were badly scalded when removing the cover from a large valve on a hot water line, although the gauge pressure was only 9 in of water (0.33 psi or 0.023 bar) They removed all the nuts, attached the cover to a chain block, and tried to lift it To release the cover they tried to rock it The cover suddenly released itself and hot water flowed out onto the workers' legs

se of Excessive Force

A joint on an 8-in line containing a hot solvent had to be remade The two sides were %-in out of line There was a crane in the plant at the time, SO it was decided to use it to lift one of the lines slightly The lifting strap pulled against a %-in branch and broke it off (Figure 1-11)

It was not a good idea to use a crane for a job like this on a line full of process material Fortunately the leaking vapor did not ignite, although

8" Pipe Full of Hot Solvent

Joint to be Remade 3/4" Branch

Figure 1-11 A branch broke when a crane was used to move a live line

nearby water was being pumped out of an excavation At one time a diesel pump would have been used, but the use of diesel pumps had been banned only a few months before the incident

Section 2.11.1 describes an explosion caused by the failure of nuts that had been tightened with excessive force

1.5.3 Ignorance of Material Strength

(a) When a plant came back on line after a long shutdown, some of the flanges had been secured with stud bolts and nuts instead of ordi- nary bolts and nuts And some of the stud bolts were located so that more protruded on one side than on the other On some flanges, one

of the nuts was secured by only two or three threads (Figure 1-12) Nobody knows why this had been done Probably one nut was tighter than the other, and in attempting to tighten this nut, the whole stud was screwed through the second nut Whatever the rea- son, it produced a dangerous situation because the pressure on dif- ferent parts of the flange was not the same

In addition, stud bolts should not be indiscriminately mixed with ordinary bolts or used in their place They are often made of differ- ent grades of steel and produce a different tension

In the plant concerned, for the eight-bolt joints the bolts were changed one bolt at a time Four-bolt joints were secured with clamps until the next shutdown

n n

Figure 1-12 Nuts fitted incorrectly to studs

(b) There was a leak on a large fuel-gas system operating at gasholder pressure To avoid a shutdown, a wooden box was built around the leak and filled with concrete It was intended as a temporary job but was so successfbl that it lasted for many years

On other occasions, leaks have been successfully boxed in or encased in concrete But the operation can only be done at low pressures, and expert advice is needed, as shown by the following incident

There was a bad steam leak from the bonnet gasket of a 3-in steam valve at a gauge pressure of 300 psi (20 bar) An attempt to clamp the bonnet was unsuccessful, so the shift crew decided to encase the valve in a box Crew members made one 36 in long, 24

in wide, and 14 in deep out of %-in steel plate Plate of this thick- ness is strong, but the shape of the box was unsuitable for pressure and could hardly have held a gauge pressure of more than 50 psi (3 bar), even if the welds had been full penetration, which they were not (Figure 1-13)

The box was fitted with a vent and valve When the valve was closed, the box started to swell, and the valve was quickly opened

A piece of 2-in by 2-in angle iron was then welded around the

box to strengthen it The vent valve was closed A few minutes later the box exploded Fortunately the mechanic-if he deserves the title-had moved away

This did not happen in a back-street firm but in a major interna- tional company

These incidents show the need for continual vigilance We can- not assume that because we employ qualified craftsmen and gradu- ate engineers they will never carry out repairs in a foolish or unsafe manner

1.5.4 Failure to Understand How Things Work or How They Are Constructed

(a) Several spillages have occurred from power-operated valves while the actuators were being removed because the bolts holding the valve bonnets in position were removed in error Figures 1-14 and 1-15 show how two such incidents occurred The second system is particularly vulnerable because in trying to unscrew the nuts that

Figure 1-13 This steel box was quite incapable of containing a leak of steam at

a gauge pressure of 300 psi (20 bar)

Valve These bolts should

have been removed

Motor mounting Plug ipacking

Figure 1-14 Wrong nuts undone to remove valve actuator

Actuator mountinq bracket

adjustment

Figure 1-15 Wrong nuts undone to remove valve actuator

hold the actuator mounting bracket in place, the stud may unscrew out of the lower nuts This incident could be classified as due to poor design [ 101

The first incident resulted in the release of 70-100 tons of vinyl chloride There was little wind, and the cloud of vapor and mist drifted SIOWIY backward and forward After an hour, when the cloud was about 240 m across and 1.5 m deep, it ignited Some of

the vinyl chloride had entered buildings, and it exploded, destroy- ing the buildings The rest burned outside and caused several vinyl chloride tanks to burst, adding further fuel to the fire Remarkably, only one man was killed The injured included spectators who arrived to watch the fire [30]

(b) A similar accident occurred on a common type of ball valve Two

workers were asked to fit a drain line below the valve There was

not much room So they decided to remove what they thought was

a distance piece or adaptor below the valve but which was in fact the lower part of the valve body (Figure 1-16) When they had removed three bolts and loosened the fourth, it got dark, and they left the job to the next day

The valve was the drain valve on a small tank containing lique-

fied petroleum gas (LPG) The 5 tons of LPG that were in the tank

escaped over two to three hours but fortunately did not catch fire However, 2,000 people who lived near the plant were evacuated from their homes [ 1 11

Flow from Tank

(c)In canned pumps the moving part of the electric motor-the rotor-is immersed in the process liquid; there is no gland, and gland leaks cannot occur

The fixed part of the electric motor-the stator-is not immersed in the process liquid and is separated from the rotor by a stainless steel can (Figure 1-17)

If there is a hole in the can, process liquid can get into the stator compartment A pressure relief plug is therefore fitted to the com- partment and should be used before the compartment is opened for work on the stator Warning plates, reminding us to do this, are often fitted to the pumps

The stator compartment of a pump was opened up without the pressure relief plug being used There was a hole in the can This had caused a pressure buildup in the stator compartment When the cover was unbolted, it was blown off and hit a scaffold pole 2 m above On the way up, it hit a man on the knee, and the escaping process vapor caused eye irritation Persons working on the pump did not know the purpose of the plug, and the warning notice was missing

Stainless Steel Can

Figure 1-17 Canned pumps

For a more detailed diagram and description of a canned pump, see Reference 12

(d) On several occasions fitters have removed thermowells without realizing that this would result in a leak They did not realize that the thermowell-the pocket into which a thermocouple or other temperature measuring device sits-is in direct contact with the process fluid A serious fire that started this way is described in Reference 13

(e) A high-pressure reciprocating ammonia pump (known as an injec- tor) had run for 23 years without serious problems when the crank- shaft suddenly fractured, due to fatigue, and the plungers came out

of the cylinders Tko men were killed by the ammonia No one real- ized that a failure of the motion work would produce a massive release of ammonia If people had realized this they would have installed remotely operated emergency isolation valves (see Section 7.2.1) These would have greatly reduced the size of the leak but would not have acted quickly enough to prevent the fatalities [31]

1.5.5 Treating the Symptoms Instead of the Disease

The following incidents and Section 10.5.3 show what can happen if

we go on repairing faults but never ask why so many faults occur

(a) A cylinder lining on a high-pressure compressor was changed 27 times in nine years On 11 occasions it was found to be cracked, and on the other 16 occasions it showed signs of wear No one asked why it had to be changed so often Everyone just went on changing it Finally a bit of the lining got caught between the pis- ton and the cylinder head and split the cylinder

(b) While a man was unbolting some %in bolts, one of them sheared The sudden jerk caused a back strain and absence from work Dur- ing the investigation of the accident, seven bolts that had been sim- ilarly sheared on previous occasions were found nearby It was clear that the bolts sheared frequently If, instead of simply replac- ing them and carrying on, the workers had reported the failures, then a more suitable bolt material could have been found

Why did they not report the failures? If they had reported them

would anything have been done? The accident would not have occurred if the foreman or the engineer, on their plant tours, had noticed the broken bolts and asked why there were so many

(c) A line frequently choked As a result of attempts to clear the chokes

the line was hammered almost flat in several places It would have been better to have replaced the line with a larger one or with a line that had a greater fall, more gentle bends or rodding points

1.5.6 Flameproof Electrical Equipment

On many occasions detailed inspections of flameprooF electrical equipment have shown that many items were faulty For example, at one

plant a first look around indicated that nothing much was wrang A more

tlhorough inspection, paying particular attention to equipment not readily accessible and that could be examined only from a ladder, showed that

out of 121 items examined, 33 needed repair The faults included missing and loose screws, gaps too large, broken glasses, and incorrect glands Not all the faults would have made the equipment a source of ignition hut many would have done so

'Why were there so many faults'? Before this inspection, there had been

no regular inspections Many electricians did not understand why flame- proof equipment was used and what would happen if it was badly main- tained Spare screws and screwdrivers of the special types used were not

i n stock, so there was no way of replacing those lost

Regular inspections were set up Electricians were trained in the rea- sons why flameproof equipment is used, and spares were stocked In addition, it was found that in many cases flameproof equipment was not really necessary Division (Zone) 2 equipment-cheaper to buy and easi-

er to maintain-could be used instead

1.5.7 Botching

Section 1.5.3 (a) described a botched job Here are two more

(a) A pressure vessel was fitted with a quick-opening lid, 10 in diame-

ter, secured by four eye-bolts (Figure 1-18) They had to be replaced, as the threads were corroded Instead of replacing the whole eye-bolt, a well-meaning person decided to save time by simply cutting the eyes off the bolts and welding new studs onto them As soon as the vessel was pressurized (with compressed air) the new studs, which had been made brittle by the welding, failed, and the lid flew off Fortunately a short length of chain restrained

it, and it did not fly very far [38] (See Sections 13.5 and 17.1 for the hazards of quick-opening lids.)

Figure 1-18 Instead of replacing the eye-bolts, new studs were welded in place

of the threaded portions They were made brittle by the heat and failed in use Fortunately the chain prevented the lid from going into orbit

(b)A screwdriver was left in the steering column of a truck after the truck was serviced The truck and semitrailer crashed, and the ser- vicing company had to pay $250,000 in damages To quote from the report, “Workplaces need to be as rigorous as the aviation and medical industries in ensuring that all tools are accounted for when servicing is completed” [39]

1.5.8 Who Should Decide How to Carry Out a Repair?

The following report raises two interesting questions:

* Who decides how a maintenance job should be carried out?

= How should we clear chokes in small bore lines?

A sample point on the suction line of two water pumps became choked and a maintenance worker was asked to clear it He was not told how to do so-craftsmen dislike people from other departments telling them how to do their jobs-but the operators assumed he would use water under pressure or a rod Instead he used compressed air at a gauge pressure of 115 psi (8 bar), and a pocket of air caused the pumps to lose siiction (Figure 1- 19)

The results were not serious The pumps supplied water to cool the hot gases leaving an incinerator: when the water flow stopped, a high-tem- perature trip shut down the burner The incinerator was new, was still undergoing tests, and the job had not been done before The water was recycled, and ash in it probably caused the choke [40]

Emergency Spray

(Fire Water)

pump #1 Sample Line

Figure 1-19 Simplified drawing of incinerator quench recirculation system

(Illiistmtion courtesy of the U.S Department of Energy.)

According to the report, the maintenance worker should have been given more detailed instructions But, as it also points out, some skills are skills of the craft; we should be able to assume that craftsmen are aware of them and should not need to give them detailed instructions on each and every occasion We should not need to tell them for example, how to break a joint every time they are asked to do so (but see Section 1.5.1) Where do we draw the line?

Craftsmen (and operators) ought to be taught, as part of their safety training, that compressed gases should not be used to clear chokes There

is a lot of energy in a compressed gas, and it can accelerate a plug to great speed, putting it into orbit if there is an open end or breaking a pipeline if the plug hits a bend (see Section 17.2)

This incident shows how much we can learn from a simple event if we treat it as a learning experience and do not say, “No one was hurt, and there was no damage, so let’s forget about it.”

1.6 A PERSONAL NOTE

The recommendations described in this Chapter go further than some companies consider necessary For example, companies may put Do Not Operate notices on valves instead of locks, or to save time, they may turn

a blind eye to occasional shortcuts Nevertheless, bitter experience has convinced me that the recommendations are necessary

In 1968, after 16 of years experience in production, I was transferred

to a new position in safety It was an unusual move at the time for some- one with my background, but five deaths from three serious fires in three years, two of them the results of poor preparation for maintenance, con- vinced the senior management that more resources should be devoted to safety and that it could no longer be left to nontechnical people and elderly foremen Since then I have read scores of reports about other accidents that happened because of this cause Some were serious; others were near-misses

When I retired from industry one of my first tasks was to sort the many accident reports I had collected The thickest folder by far was one labeled Preparation For Maintenance Some of the incidents from that folder, together with more recent ones, are described above

If you decide my recommendations are not right for your organization, please do not ignore the accidents I have described Check that your pro-

cedures will prevent them, or they will happen again

REFERENCES

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July-Sept 1980, p 15

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207

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1996, p 9

p 52

33 Loss Prevention Bulletin, No 107, Oct 1992, p 17

34 Operating Experierice Weekly Siimrnary, No 96-44, Office of

Nuclear and Facility Safety, U.S Dept of Energy, Washington, D.C.,

1996, p 2

35 Occupatiorzal Snfeg a i d Health Obseiver; Vol 3 Nos 7 and 8, U S

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29 Safe0 Management, South Africa, Sept 1995, p 2 Though reported

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