What Went Wrong Part 8 pdf

c After a crack developed in a 22-in.-diameter steam main, operating at a gauge pressure of 250 psi 17 bar and a temperature of 365°C the main was checked against the design drawings..

Pipe and Vessel Failures 183

in Section 7.3.2 When flange leaks are likely, or their consequences seri- ous, flanges should be left uninsulated [ 141

Dead-ends in domestic water systems can provide sites for the growth o-F the bacteria that cause Legionnaires' disease [ 151

Some vertical drain lines in a building were no longer needed so they were disconnected and capped but left connected EO the horizontal main drain below The caps were fixed with tape but were not made watertight

as there was no way it seemed, that water could get into them Fifteen years later a choke developed in the main drain, water backed up into the disused legs and dripped into an electrical switch box All power was lost and some of the switch gear was damaged beyond repair [23]

9 1.2 Poor Support

Pipes have often failed because their support was insufficient and they were free to vibrate On other occasions they failed because their support was too rigid and they were not free to expand

(a) Many small-diameter pipes have failed by fatigue because they were free to vibrate Supports for these pipes are usually mn on-site, and it

is not apparent until startup that the supports are inadequate It is very easy for the startup team, busy with other matters to ignore the vibrating pipes until the team has more time to attend to them Then the team gets so used to them that it does not notice them

Vibration and failure are particularly liable to occur when a small-diaineter pipe carries a heavy overhung weight Within 30 minutes of the start of a new compressor, a pressure gauge fell off for this reason [24.]

When equipment receives impulses at its own natural frequency

of vibration, excessive vibration (resonance) occurs, and this can lead to rapid failure A control valve was fitted with a new spindle with slightly different dimensions This changed its natural fre- quency of vibration to that of the impulses of the liquid passing through it (the frequency of rotation of the pump times the number

of passages in the impeller) The spindle failed after three months Even al small change in the size of spindle is a modification [24] (b) A near' failure of a pipe is illustrated in Figure 9-4 An expansion bend on a high-temperature line was provided with a temporary

Weld A

Figure 9-4 A construction support on an expansion bend was left in position

support to make construction easier The support was then left in position Fortunately, while the plant was coming onstream, some- one noticed it and asked what it was for

(c) After a crack developed in a 22-in.-diameter steam main, operating

at a gauge pressure of 250 psi (17 bar) and a temperature of 365°C

the main was checked against the design drawings Many of the sup- ports were faulty Here's an example from four successive supports:

1 On No 1 the spring was fully compressed

2 No 2 was not fitted

3 No 3 was in position but not attached to the pipe

4 No 4 was attached, but the nuts on the end of the support rod

Piping with a 12-in diameter and larger is usually tailored for the particular duty There is a smaller factor of safety than with smaller sizes With these large pipes, it is even more important than with smaller ones that the finished pipework is closely inspected, to confirm that the construction team has followed the designer's instructions

(d) A pipe was welded to a steel support, which was bolted to a con- crete pier A second similar support was located 2 m away The pipe survived normal operating conditions But when it got excep- tionally hot, a segment of the pipe was torn out The fracture extended almost completely around the weld The bolts anchoring the support to the concrete pier were bent

This incident was reported in the safety bulletin of another com- pany The staff members dismissed the incident "Our design pro- cedures," they said, "would prevent it happening." A little later it did happen A reflux line was fixed rigidly to brackets welded to the shell of a distillation column At startup the differential expan- sion of the hot column and the cold line tore one of the brackets were slack

Pipe and Vessel Failwres 185

from the column Flammable vapor leaked out but fortunately did not catch fire

(e) A 10-in pipe cawing oil at 300°C was fitted with a %-in branch

on its underside The branch was located 5 in from a girder on which the pipe rested When the pipe was brought into use, the expansion was sufficient to bring the branch into contact with the girder and knock it off Calculations showed that the branch would move more than 6 in

Cs> On many occasions pipe hangers have failed in the early stages of a

fire, and the collapse of the pipes they were supporting has added to the fire Critical pipes should therefore be supported from below (g) An extension was added to a 30-year-old pipebridge that carried pipes containing flammable liquids and gases To avoid welding, the extension was joined to the old bridge by bolting Rust was removed from the joining surfaces, and the extension was painted Water penetrated the crack between the old and new paint and pro- duced rust As rust is more voluminous than the steel from which it

is formed the rust forced the two parts of the pipebridge apart-a phenomenon known as rust-jacking (see Section 16.3) Some of the bolts failed, and a steam main fractured Fortunately, the liquid and gas lines only sagged [ 161

(h) Eleven pipelines, 2-8 in (50-200 mm) in diameter, containing hydrocarbon liquids and gases, were supported on brackets of the type shown in Figure 9-5 (a), 2.1 m tall and 6 m apart The pipes were fixed to two of the brackets and rested on the others The pipe run passed through a tank farm, and the wind flow through the gaps between the tanks caused the upright part of the supports to incline 2" from the vertical This was noticed when the pipe run

was inspected, but no one regarded it as serious

As the result of a power failure, the flow through many of the pipes suddenly stopped, and the surge caused the angle of inclination

to increase to 6" The tops of the supports were now 5 in (125 mm) out of line The supports were now unstable Eleven hours after the power failure and three hours after the flows had been restored, the pipe pun collapsed over a length of 23 m; 14 tons of gasoline were

spilled Three hours later a further length collapsed The pipe sup- ports were replaced by the type shown in Figure 9-5 (b)

A better arrangement is shown in Figure 9-7 The dimensions are cho- sen so that the water injection pipe can be removed for inspection However, this system is not foolproof One system of this design was assembled with the injection pipe pointing upstream instead of down- stream This increased corrosion

As discussed in Section 3.2.1, equipment should be designed so that it

is difficult or impossible to assemble it incorrectly or so that the incoirect assembly is immediately apparent

9.1.4 Bellows

Bellows (expansion joints) are a good example of equipment that is intolerant of poor installation or departure from design conditions They

Pipe and Vessel Fail&fres 187

Figure 9-6 Water injection-a poor arrangement

Figure 9-7 Water injection-a better arrangement

should therefore be avoided on lines carrying hazardous materials This can be done by building expansion loops into the pipelines

The most spectacular bellows failure of all time (Flixborough) was described in Section 2.4 Figure 9-8 illustrates a near-failure

A large distillation column was made in two halves, connected by a

42-in vapor line containing a bellows During a shutdown this line was

Figure 9-8 A large bellows between the two halves of a distillation column

steamed Immediately afterward someone noticed that one end of the bel-

lows was 7 in higher than the other, although it was designed for a maxi-

mum difference of 3 in Someone then found that the design contractor

had designed the line for normal operation But the design contractor had

not considered conditions that might be developed during abnormal pro-

cedures, such as startup and shutdown

9.1.5 Water Hammer

Water hammer (also known as hydraulic shock) occurs in two distinct

ways: when the flow of liquid in a pipeline is suddenly stopped, for exam-

ple, by quickly closing a valve [13] and when slugs of liquid in a gas line

are set into motion by movement of gas or condensation of vapor The lat-

ter occurs when condensate is allowed to accumulate in a steam main,

because the traps are too few or out of order or in the wrong place High-

pressure mains have been ruptured, as in the following incident

(a) A 10-in.-diameter steam main operating at a gauge pressure of 600

psi (40 bar) suddenly ruptured, injuring several workers

The incident occurred soon after the main had been brought back

into use after a turnaround It was up to pressure, but there was no

flow along it The steam trap was leaking and had been isolated An

attempt was made to get rid of condensate through the bypass

valve But steam entered the condensate header, and the line was

isolated, as shown in Figure 9-9 Condensate then accumulated in

the steam main

Faulty Steam Trap

Figure 9-9 Arrangement of valves on steam main that was broken by

hammer

water

Pipe and Vessel Failures 189

When a flow was started along the steam main by opening a %-

in valve leading to a consuming unit, the movement of the conden-

sate fractured the main 161

(b) Figure 9-10 shows how another steam main-this time one operating

at a gauge pressure of 20 psi (1.4 bar)-was burst by water hammer

Two drain points were choked and one isolated In addition the

change in diameter of the main provided an opportunity for con-

densate to accumulate The main should have been constructed so

that the bottom was straight and so the change in diameter took

place at the top

(c) An operator went down into a pit to open a steam valve that was

rarely operated and had been closed for nine months Attempts to

open the valve with a reach rod, 8 m long, had been unsuccessful

The pit was recognized as a confined space, and so the atmosphere

was tested, the operator wore a rescue harness, and a stand-by man

was on duty outside The steam main was up to pressure on both

sides of the valve, and the gauge pressure was 120 psi (8.3 bar) on

the upstream side, 115 psi (7.9 bar) on the downstream side There

was a steam trap on the downstream side of the valve but not on the

upstream side, and as the valve was on the lowest part of the sys-

tem, about 5 tons of cold condensate had accummulated on the

Drain points choked (Steam trap bypasses not shown)

Figure 9-10 Arrangement of drains on steam main that was broken by water

hammer

The operator took about one to two minutes to open the valve halfway; very soon afterward, there was a loud bang as a 6-in cast- iron valve on a branch (unused and blanked) failed as a result of water hammer The operator was able to climb out of the pit but later died from his burns, which covered 65% of his body [17] Figure 9- 11 explains the mechanism

Water Hammer in Pit

This frame illustrates the valve lineup prior to the accident About 1,500 gal of 55°F

condensate had collected upstream of valve MSS-25, which was located at the dead-end

of an 800-ft pipe and was the lowest point in the system

X O t ) f t s t e m line, containing

j Y F condensate

6-in wIw

As valve MSS 25 was opened, the

water mixed with the steam on the

downstream side of the valve

As the water and steam interacted,

the turbulence sealed off a pocket of

steam, which quickly condensed,

lowering the pressure in the pocket

and creating a void

Figure 9-11 Condensate collected in a steam main A valve was opened quick-

ly Sudden movement of the condensate fractured another valve The figure explains how this occurred

Pipe and Vessel Failures I91

The accident would not have occurred (or would have been less serious) if

0 Cast iron had not been used It is brittle and therefore not a suit- able material of construction for steam valves, which are always liable to be affected by water hammer

* There was a steam trap upstream of the valve

* The valve had been located in a more accessible place

*The operator had taken longer to open the valve On previous occasions operators had taken several hours or even longer, but there were no written instructions, and the operator on duty had not been trained or instructed

* The operating team as a whole had been aware of the well-known hazards of water hammer in steam mains

For another failure due to water hammer see Section 10.5.3

9.1.6 Miscellaneous Pipe Failures

(a) Many failures “nave occurred because old pipes were reused For example, a hole 6 in long and 2 in wide appeared on a 3-in pipe carrying flammable gas under pressure The pipe had previously been used on a corrosive/erosive duty, and its condition was not checked before reuse

In another case a 4%in.-diameter pipe carrying a mixture of hydrogen and hydrocarbons at a gauge pressure of 3,600 psi (250 bar) and a temperature of 350-400°C burst producing a jet of flame longer than 30 m (Figure 9-12) Fortunately the pipe was located high up, and no one was injured

The grade of steel used should have been satisfactory for the operating conditions Investigation showed however, that the pipe had previously been used on another plant for 12 years at 500°C It

had used up a lot of its creep life

Old pipes should never be reused unless their history is known in detail and tests show they are suitable (see Section 9.2.1 h)

(b) Many failures have occurred because the wrong grade of steel was used for a pipeline The correct grade is usually specified but the wrong grade is delivered to the site or selected from the pipe store

k

a

1

”‘

Figure Y-12 An old pipe was reused and failed by creep

The most spectacular failure of this sort occurred when the exit pipe from a high-pressure ammonia converter was constructed

from carbon steel instead of 1!4% Cr, 0.5% Mo Hydrogen attack occurred, and a hole appeared at a bend The hydrogen leaked out, and the reaction forces pushed the converter over

Many companies now insist that if use of the wrong grade of steel can affect the integrity of the plant, all steel must be checked for composition before use This applies to flanges, bolts, welding rods, etc., as well as the raw pipe Steel can be analyzed easily with

a spectrographic analyzer Other failures caused by the use of the wrong construction material are described in Section 16.1

(c) Several pipe failures have occurred because reinforcement pads have been welded to pipe walls, to strengthen them near a support

or branch, and the spaces between the pads and the walls were not vented For example, a flare main collapsed, fortunately while it was being stress-relieved

Pipe and Vessel Failures 193

Pipe reinforcement pads can be vented by intermittent rather than continuous welding, or a %-in or %-in hole can be drilled in the pad

(d) Corrosion-internal or external-often causes leak-before-break failures but not always

A line carrying liquefied butylene at a gauge pressure of about

30 psi (2 bar) passed through a pit where some valves were located The pit was full of water, contaminated with some acid The pipe corroded, and a small leak occurred The line was emptied for repair by flushing with water at a gauge pressure of 110 psi (7.5

bar) The line was designed to withstand this pressure However in

its corroded state it could not do so, and the bonnet was blown off a

valve The operator isolated the water This allowed butylene to flow out of the hole in the pipe Twenty minutes later the butylene exploded, causing extensive damage [ 7 ]

(e) A 1-in screwed nipple and valve blew out of an oil line operating

at 350°C The plant was covered by an oil mist, which ignited 15 minutes later The nipple had been installed about 20 years earlier, during construction, to facilitate pressure testing It was not shown

on any drawing and its existence was not known to the operating team members If they had known it was there, they would have replaced it with a welded plug

Similar incidents are described in Section 7.1.5

(f) Not all pipe failures are due to inadequacies in design or construc- tion (for example, the one described in Section 1.5.2)

A near-failure was also due to poor maintenance practice A

portable, handheld compressed-air grinder was left resting in the space between two live lines The switch had been left in the On position So when the air compressor was started, the grinder start-

ed to turn It ground away part of a line carrying liquefied gases Fortunately the grinder was noticed and removed before it had ground right through the line, but it reduced the wall thickness from 0.28 in to 0.21 in

(g) Figure 9-13 shows the pipework on the top of a reactor When the pipework was cold, any liquid in the branch leading to the rupture disc drained out; when it was hot, it remained in the branch, where

it caused corrosion and cracking [ 181

12mm differential vertical expansion

Pipe and Vessel Failures 195

gasoline pipe ruptured and 700 tons of gasoline spread across a strip of swamp The incident received little publicity, but it seems that, as at Bhopal and Mexico City, a shanty town had been allowed to grow up around the pipeline on stilts over the swamp The cause of the failure is not known, but the pipeline was said to have been brought up to pressure

in error, and it was also stated that these was no way of monitoring the pressure in the pipeline [ 101

9.2 PRESSURE VESSEL FAILURES

Failures of pressure vessels are very rare Many of those that have been reported occurred during pressure test or wese cracks detected during rou- tine examination Major failures leading to serious leaks are hard to find Low-pressure storage tanks are much more fragile than pressure ves- sEls They are therefore more easily damaged Some failures are described in, Chapter 5

A few vessel failures and near-failures are described next-to show that they can occur Failures of vessels as a result of exposure to fire are described in Section 8.1

9.2.1 Failures (and Near-Failures) Preventable by Better Design or

Construction

These are very infrequent

(a) A leak of gas occurred through the weep hole in a multiwall vessel

in an ammonia plant The plant stayed on line but the leak was watched to see that it did not worsen Ten days later the vessel dis- integrated causing extensive damage

The multiwall vessel was made from an inner shell and 11 layers

of wrapping, each drilled with a weep hole The disintegration was attributed to excessive stresses near a nozzle These had not been recognized when the vessel was designed

The report on the incident states: “Our reading of the literature led us to believe that as long as the leaking gas could be relieved through the weep holes it would be safe to operate the equipment

We called a number of knowledgeable people and discussed the safety issue with them Consensus at the time supported our con- clusion But after the explosion, these was some dispute over

exactly what was said and what was meant Knowing what we know now, there can be no other course in the future than to shut down operations in the event of a leak from a weep hole under similar circumstances." [SI

(b) An ammonia plant vessel disintegrated as the result of low-cycle fatigue-the result of repeated temperature and pressure cycles [9] (c) An internal ball float in a propane storage sphere came loose When the tank was overfilled, the ball lodged in the short pipe leading to the relief valve, in which it formed an exact fit When the sphere warmed up, the rise in pressure caused its diameter (14 m) to increase by 0.15 m (6 in.) The increase in diameter was noticed when someone found that the access stairway had broken loose

A similar incident occurred in a steam drum in which steam was separated from hot condensate On this occasion, the operator noticed that the pressure had risen above the set-point of the relief valve and tripped the plant [ 191

If you use ball-float level indicators, compare the size of the balls with those of the branches on the top of the vessel If a loose ball could lodge in one of the branches, protect the branch with a metal cage, or use another type of level indicator

(d) Several vessels have failed, fortunately during pressure testing, adjacent to internal support rings that were welded to the vessel Expert advice is needed if such features are installed

(e) N-butane boils at 0°C and iso-butane at -12°C When the air tem- perature is below 0°C and a vessel containing butane is being emp- tied it is possible to create a partial vacuum and suck in the vessel; this has occurred on several occasions Vessels used for storing butane and other liquefied gases with boiling points close to O"C,

e.g., butadiene, should be designed to withstand a vacuum If an existing tank cannot be modified then warm butane can be recy- cled, or the butane can be spiked with propane (but the pressure may then be too high in warm weather and the relief valve may lift) (f)Although I have said that pressure vessel failures are rare, this is not true if vessels are not designed to recognized standards Daven- port [ 111 has described several liquefied petroleum gas (LPG) ves- sel failures that were due to poor construction In the UK in 1984,

no one knew who made 30% of the LPG tanks in use, when, or to what standard [ 121

Pipe and Vessel Failures 197

(g) The catastrophic failure of a 34-m3 vessel storing liquid carbon dioxide killed three people injured eight, caused $20 million dam- age, and lost three months' production [25] There were failures by all concerned

= The vessel was leased from a supplier of carbon dioxide and the user company did not check that it conformed to the company's usual standards

The supplier modified the vessel, but the workmanship was poor

A weld was weak, as it was not full penetration, and brittle because the weld surface, cut with a torch was not ground before welding

0 As the result of a heater failure, the temperature of the vessel, designed for -29°C fell to -60°C by evaporative cooling (see Section 10.5.2); at this temperature carbon steel becomes brittle, and cracking may have started

0 Five weeks later the heater failed again this time in the On posi- tion, and the pressure in the tank rose The two relief valves failed

to open because they were fixed to the side of the vessel and con- nected to the vapor space at the top by an internal line (Figure 9- 14)-a most unusual arrangement presumably adopted so that one nozzle could be used for filling, venting (during filling), and relief As a result the relief valve was cooled by the liquid in the

vessel and became blocked by ice from condensed atmospheric moisture There was no drain hole in the tailpipe (see Section 102.4) The vessel burst most of the bits ending up in a nearby river from which they were salvaged

After the accident a search disclosed 11 other failures that had occurred but had received little or no publicity [26] If they had been publicized, this incident could have been avoided The com- pany concerned withdrew all its carbon dioxide vessels that could not withstand low temperatures and replaced them with stainless steel ones The company found that it could manage with 75% fewer vessels than it had used before (see Section 21.2.1)

At least two of the other 11 failures occurred because the plates from which the vessels were made did not get the correct post- welding heat treatment Once a vessel has been constructed, it is not easy to check that it has had the correct heat treatment The