Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

6.8 Afterword

The investigators concluded that the design of the tank was faulty, having been made like a barrel rather than a dam. Failure was inevitable, and explains why failure occurred so early in the tank's life. Because the walls were exposed to excessive stresses, it was inevitable the tank would fail quickly. The stress would seek out the weakest welds in the most exposed panel, and failure was not caused by faulty welding at all. That is not to say the welding process could not be improved, for example, by annealing panels and carefully controlling bending before final welding.

But the failure should never have occurred in the first place because there was a standard for such thermoplastic tanks, DVS 2205 published by the German Welding Institute. Only partial translations were published at the time the tank was designed, but the design philosophy was perfectly clear. The design procedure is described in Paper 5, and makes allowance – using safety factors, or derating factors – for holding dangerous chemicals, pinholes in welds and so on. The manufacturer of the tank, had hired an engineer to check and approve the design. The engineer performed misguided calculations, leaving the insurers to shoulder the considerable expenses of the clear-up.

So, having explained the failure, what were the consequences of the investigation? As is usual, several investigators had been instructed by loss adjusters acting for the two injured parties. The investigation described here was carried out on behalf of the tank manufacturer, and because liability was accepted, the role of the other investigators was limited. They did however, agree with the general thrust of the analysis.

A more serious problem – which arises whenever design defects are discovered – was the state of other tanks made to the same design. How many had been built and installed? What fluids were they holding? For how long had they been installed?

6.8.1 Inspection of other tanks

A range of tanks built the same way had been installed, but as it turned out, only relatively recently. The original investigator was asked by the manufacturer to inspect these installations, where tanks were holding fruit juices, soap solution and ferric chloride (FeCl3), an acidic fluid used for water treatment.

SAQ 22

Using the given data, estimate the stress on the centre of the lower panel of some storage tanks when completely full with fluid of a given specific gravity. The tanks below are built like the tank introduced in Section 6.2, and are made from 12 mm thick polypropylene sheet.

  • (a) A 40 m3 tank holds detergent solution of specific gravity 1.2, has a height of fluid above the critical panel of 1.265 m, and has a radius of 1.91 m.

  • (b) A 25 m3 tank holds dilute detergent of specific gravity 1.16, has a height to the critical panel of 2.025 m, and has a radius of 1.6 m.

  • (c) A 25 m3 tank holds ferric chloride solution of specific gravity 1.5, has a height of fluid 1.8 m, and has a radius of 1.65 m.

Which tank is most likely to fail? What particular features would the investigator be looking for during inspection of that tank? What special methods might they use to help in this task? Assume specific gravity is equal to the density in units of gm cm−3.

Answer

The hydrostatic pressures in the different tanks will be given by Equation (9), where the densities of the contents can be inserted to estimate the pressure. The hoop stress σH at the centre panel can then be estimated using Equation (7), which relates hoop stress directly to the internal pressure exerted by the contents. The data can be used now to give answers for (a),(b),and (c).

(a) Detergent

So

(b) Dilute detergent

So

(c) Ferric chloride solution

So

So the last tank is the most seriously stressed in the centre of the lower panel, and should clearly be examined first. It may also be observed that the hoop stress is actually above that estimated for the tank containing caustic soda in the case study, so the matter is urgent. All welds need examining, preferably using white dusting powder to show any cracks in the key welds.

Although the tanks were relatively new, few were found with microcracks, essentially because none of the tanks had been filled to capacity. The cracks in the welds that were found were far from critical, being only millimetres in size. The national economy at the time was depressed, and the companies concerned had been on short-time work. Some of the other tanks inspected were much smaller than that at Warrington, so had not been stressed to the same extent. The most serious potential problem was found at a steel-wire works in the Midlands, where there were two adjacent tanks (Figure 78). One held caustic soda, the other ferric chloride. If the two tanks had split, the two chemicals would have reacted together with the evolution of heat, possibly causing a fire. The tanks were in a confined space, close to manned equipment, so the consequences of failure would have been serious. The bund walls had only been designed to accept the contents from a floor leakage, so would have been no help in a crisis like the Warrington failure.

No microcracks were found, however, because the tanks had never been more than half-full since installation. The factory was set in a narrow gorge and the only access to the tank building was over a small bridge incapable of carrying a full tanker. Needless to say, this and other tanks had to be replaced by tanks of the correct design.

Figure 78: Potential hazard from adjacent tanks, one containing ferric chloride, the other sodium hydroxide

6.8.2 Paint tank failure

Storage tanks continue to fail however, despite the publicity given by Papers 4 and 5. One failure tha occurred in 1998 involved a paint tank. It had been designed with a sloping floor, as shown in Figure 79. This internal cone was essentially to allow all the contents to be mixed within the tank and then drained away for further processing. It failed during the first time it was used, collapsing in a misshapen heap on the factory floor, letting the contents spill on the floor.

The weakness of the design is that the mass of the contents is taken by a single weld (arrowed in Figure 79), with no support at all for the inner cone. The weld had peeled away, failing progressively, so that almost all the weld had failed. The design was again at fault, with the absence of support recommended by the DVS 2205 standard, such as a steel frame resting on the ground, and bearing the weight of the cone and contents. Fortunately, the design was a one-off, being made by a small company for a specific contract, and had not been repeated elsewhere.

Figure 79: Paint storage tank with unsupported base, putting the load of the contents onto the weld between the sloping floor and the upper part of the tank
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