Page 94 - Failure Analysis Case Studies II

Failure Analysis Case Studies N
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 79

Catastrophic failure of a polypropylene tank Part I:
primary investigation

P.R. Lewis", G.W. Weidmann
Department of Materials Engineering, Faculty of Technology, The Open University, Walton Hall.
Milton Keynes MK7 6AA, U.K.
Received 19 October 1998; accepted 4 November 1998

Abstract
The creep rupture of a new polypropylene tank used for holding caustic soda solution is described from
the primary evidence. The immediate cause of fracture was a pinhole in the centre of a weld, which initiated
a brittle crack. The crack grew slowly during each loading until it penetrated the wall shortly after the fourth
filling. The design of the structure fell short of the required dam- rather than barrel-like form needed for
resisting hydrostatic pressure. 0 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Tank; Creep rupture; Weld; Polypropylene

1. Introduction

Polypropylene is a popular material for applications where toughness, low density and environ-
mental resistance are needed. It has been used for many years for constructing storage tanks (e.g.
silos for storing bulk powders) and as a lining for large GRP tanks (e.g. for storage of corrosive
fluids) [l]. However, good design demands that allowance must be made for the maximum stresses
to which such products can be exposed in service. The example described in this case history offers
some apposite lessons in using polymeric materials for highly stressed products, especially where
failure can threaten life and property. The failure bears distant similarity with the failure of a steel
silo reported earlier [2].
On 23 August 1994, a large storage tank containing concentrated caustic soda (NaOH) fractured
suddenly along one of its welded seams, a jet of liquid shooting across the factory. The incident
occurred without any prior warning of failure, and in a tank which had only recently been installed
(6 March 1994) at a factory used to manufacture dairy detergent. The last full loading was only
the fourth since installation. The tank was made of polypropylene (PP) panels welded together.

*Corresponding author. Tel.: 01908 653271; fax: 01908 653858.

Reprinted from Engineering Failure Analysis 6 (4), 197-214 (1999)