Page 88 - Failure Analysis Case Studies II

Failure Analysis Case Studies 11
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 73

CREEP FAILURE OF A SPRAY DRIER

P. CARTER
Advanced Engineering and Testing Services, CSIR, Private Bag X28, Auckland Park 2006,
Republic of South Africa
(Received 3 February 1998)

Abstract-NDT, design calculations and metallurgical analysis were performed on specimens from a collapsed
spray drier. Failure modes initially regarded as possible were: corrosion leading to reduced sections and loss
of strength, fatigue and fracture, and creep. The calculations pointed to creep, and no positive metallurgical
or physical evidence was discovered to support any of the hypotheses. However, the compression stresses
implied that creep deformation could have occurred without inducing discernible creep damage. It was
concluded that buckling and collapse of the structure was due to excessive creep deformation. 0 1998 Elsevier
Science Ltd. All rights reserved.
Keywords: Creep, buckling, overheating, process-plant failure, stress concentrations.

1. BACKGROUND
A spray drier which had been in service for nearly 20 years at the Western Platinum Mine
metallurgical plant, collapsed on a quiet day while operating normally. The spray drier consisted of
a cylindrical shell some 15 m in height and 5.5 m in diameter, supported vertically on four 5 m steel
columns. Combustion gas controlled at 550°C from a chain-grate stoker entered an annular chamber
encircling the base of the shell. The gas entered the cylinder from a number of ports on the inside
of the annulus; it then travelled up the cylinder, drawn by an induced draught fan, in order to dry
a slurry falling from the top of the drier in a counterflow arrangement. The dry product was collected
from a cone at the bottom of the cylinder. The drier was lagged and clad from top to bottom to
conserve energy.
Figures 1-3 show views of the collapsed drier. Braced columns, the lagged and clad annulus and
shell, and the bottom cone, are all visible.
These figures show the remarkable nature of the collapse, with the column and cone moving
down axisymmetrically until the weight was supported by the cone on the ground.
The aim of the investigation was to explain the failure and to make recommendations to ensure
that it was not repeated on the two remaining driers, which had seen some 7 years' service.

2. INSPECTION

Ultrasonic NDT was performed on columns and some areas of the annulus and shell on the
remaining two driers. Attempts to measure the temperature of the insulated skin of the annulus of
these driers were made with limited success. A probe inserted into the lagging against the outer
annulus shell indicated temperatures in the range 330°C-360"C. This was felt to be unrealistic, due
to the fact that the plate had gas at 550°C on one side and 250 mm of fibre glass lagging on the
other side. Where possible, thickness checks were made on the failed drier, and sections of shells
and columns were removed for metallurgical analysis.
These investigations all gave negative results, that is, no significant corrosion was observed, and
both columns and shell material were consistent with Grade 430 mild steel without any deterioration
in properties. No evidence of fatigue and fracture was found and in particular, no physical evidence
of creep damage was found.
Reprinted from Engineering Failure Analysis 5 (Z), 143-147 (1998)