Page 111 - Failure Analysis Case Studies II

case, an installation with a concentrated caustic soda tank adjacent to a concentrated ferric chloride
tank, an access bridge could not support the supply vehicles, so the tanks were always kept well
below capacity! It is understood that the tanks concerned have now been brought up to an
acceptable standard.
Despite an extensive literature search for other examples of failure in such tanks, only one
relevant example was found. References to earlier tank failures are exclusively concerned with
GRP rather than thermoplastic vessels [7], or are theoretical exercises for comparison of different
plastic tanks for fatigue resistance 181. There is a report of a test tank which failed during a second
fill of water to test the particular design calculations used [9]. The tank was under-designed with a
barrel-like structure of the kind already discussed here. Unfortunately, details of how exactly the
tank failed remains unclear, although the paper remains a good basis for the estimation of design
stresses. Designing the wall to resist the creep strain developed by hydrostatic pressure is discussed,
but without explicit mention of the need to increase the wall thickness towards the base, a
point which receives greater emphasis in DVS 2205. It is also discussed in detail, with tables of
recommended wall thickness, in a publication from Forbes Plastics Ltd [IO]. The publication
presents a good basis for design of plastic tanks, and should help to prevent future failures of the
kind discussed in this article, especially in the more stringent regulatory environment for bulk
storage of materials [l I].

Acknowledgements
The author would like to thank the insurers, Independent Insurance Ltd and loss adjusters
(Gillies Adjusting Ltd) for permission to publish the results of the investigation, and to Jim Moffatt
and Gordon Imlach of the Open University for performing mechanical and chemical tests. Richard
Black performed photomicroscopy (Figs 5-7).

References

[I] Forbes L. Plastics now set the standards for tanks. Process Industry Journal Nov/Dec 1989.
[2] Kieselbach R. Bursting of a silo. Engineering Failure Analysis 1997;449.
[3] DVS 2205 is published by the publishing arm of the German Welding Institute (Deutscher Verlag fur SchweiB-
technik, or DVS).
[4] Roark’s Formulas for stress and strain. 6th ed. 1989. p. 516.
[5] Lange’s Handbook of chemistry. 10th ed. p. 1150.
[6] Peterson RE. Stress concentration factors. 1974. Figure 128. p. 195; also in Pilkey WD Peterson’s stress con-
centration factors. 2nd ed. Chart 4.50 1997.
[7] Ezrin M. Plastics failure guide: cause and prevention. Hanser, 1996. section 10.5.2, p. 345 ff.
[8] Hertzberg RW, Manson JA. Fatigue testing. Plastics World May (1977);50-53.
[9] Forbes K, McGregor A, Turner S. Design of fluid storage tanks from polypropylene. Brit Chem Engng October
1970.
[lo] Forbes Plastics Ltd. A Guide to DVS 2205. Denver, Downham, Norfolk PE38 ODR, 1993.
[I I] Forbes L. Risk assessment of tanks. Water and Waste Treatment March (1993).