Modern Engineering Thermodynamics

24 CHAPTER 1: The Beginning
THERMODYNAMIC CASE STUDIES
Case study 1.1. The Anatomy of an Accident
A testing company is commissioned to build a facility to spin test the
impellers for large centrifugal compressors. A test impeller is spun in a
vacuum inside a thick-walled spin chamber until it bursts. The impeller
is driven by a small air turbine with a shaft that enters through the
top of the spin chamber, and the walls of the spin chamber are lined
with thick lead bricks to absorb the rotational kinetic energy of the
pieces of the impeller when it bursts (Figure 1.17).
During the initial test run, a 600. lbm, 30.0-inch diameter stainless
steel impeller is to be spun until it bursts as part of the acceptance test
of the facility. At about 1 AM, the rotor reaches 14,000 rpm and bursts.
The people conducting the test are located in a room adjacent to and
one floor above the test chamber room. The burst makes a single
“thud” noise, typical of bursting rotors, but when the operators go to
the test room they find the corridor and stairwell full of dust and debris.
Some of the lead bricks from inside the spin chamber are found in
the hallway, and one brick penetrated the test room wall and ended
up in the kitchen of a neighborhood house. The 3000 lbm spin chamber
cover had been blown up through the ceiling and fell back down
(Figure 1.18). The entry door into the test room was blown into an
adjacent parking lot, and the test room had extensive damage from
flying lead bricks and pieces of the impeller penetrating the walls.
The cover was bolted to the chamber with 24 1-inch diameter bolts
that could resist a total force of 1.90 × 10 6 lbf. It is concluded that
the accident was caused by the impact of the impeller fragments
extruding the lead bricks vertically in the spin chamber, ultimately
exerting a force on the cover of about 2.10 × 10 6 lbf. The spin
chamber contained all the radial burst forces, and only 5% of the
rotational kinetic energy escaped the chamber via the lead extruded
against the cover and forcing it off.
The moment of inertia of the impeller is measured and found to be
542 lbm·ft 2 , and the rotational kinetic energy of the impeller at
the point where it burst is
Rotational KE = 1 2 Iω2 = 7:80 × 10 6 ft.lbf = 10,000 Btu = 10:6MJ
which is equivalent to the explosive power of about 7.20 lb of TNT
(or about 60 hand grenades).
Drive turbine
Removable cover
Test rotor
Containment
vessel
Lead bricks
Vacuum
FIGURE 1.17
Case study 1 illustration.
FIGURE 1.18
Case study 1, damage to centrifuge.