Page 95 - Failure Analysis Case Studies II

80
- OUTSIDE WALL -

I I
I PIPEWORK t I
#l t COMPRESSOR
1 IEQUIPMENT
1
. ---e

CAUSTIC
JET

/'SHORT
?/./ PANEL
..-
0
SIGHT
TUBE

BUND WALL

PARTITION I
WALL
Fig. 1. Sketch plan of the caustic soda tank (diameter ca 2.7 m), showing four vertical welds (W1-4) and their position
relative to the bund wall. Three of the four plastic panels had a contour length of ca 2.5 my the fourth being shorter, at
about 1.2 m. An internal heating element lay below the critical crack, but had apparently never been used. The sight
and overflow tubes led to the top of the tank.

The jet of caustic soda could not be contained safely, since the fracture had occurred about 1.5
m from the floor of the tank (which itself was free standing on the factory floor), the jet projecting
the liquid stream over the adjacent bund wall (Fig. 1). Since the initial fracture was only relatively
small, the jet stream continued for several hours under hydrostatic pressure from the tank (which
had only been filled that same morning by tanker) before the bund wall contained the liquid. The
unconfined fluid spilled out over nearby equipment, and subsequently flowed into adjacent prem-
ises, which used specialised printing equipment. Severe damage ensued owing to the highly corrosive
nature of the concentrated caustic soda, a fluid which degrades much organic and inorganic
material very rapidly. Escape into local water streams could also have created a further hazard to
animal and plant life as well as a potential danger to water supplies in the area, but was collected
and safely disposed by the Fire Brigade and others. The tank apparently met the stringent
conditions imposed by the only public standard available, the German Code of Practice DVS 2205
[3]. An English translation of the document only became available some time after onset of the
investigation, and is discussed fully in Part I1 of this paper.
Preliminary analysis of the failure indicated that the critical fracture might have been caused by
weld defects (blow holes), so that a programme of tests was undertaken on the weld material as