bulletin - The National Board of Boiler and Pressure Vessel Inspectors

FEATURE
28
by George Galanes, P.E.,
Midwest Generation EME, LLC
PPressure testing is a practical method used by
industry to determine the leak tightness of pressureretaining
items; this is fact. Over the years, industry has
expanded the hydrostatic (pressure) testing concept
specified by most construction codes or standards, using
this approach to verify leak tightness and, to some extent,
ensure structural integrity of inservice pressure-retaining
equipment. This reliance of pressure testing for integrity
is built on fiction.This article outlines some of the common
facts and fiction of pressure testing inservice components.
Hydrostatic testing was originally conceived and used
by early boiler and pressure vessel manufacturers to
verify that a component, after it had been designed and
fabricated, could withstand a level of pressure to prove
the design in a safe manner.Water was selected as the
working fluid for pressure testing because of its
abundance and, more important, because it is relatively
incompressible. Since the water cannot be compressed,
pressure can be safely applied in all directions, as there
is no storage of potential energy (as would occur with
compressible fluids, like air). Thus, if the pressureretaining
component failed during testing, the likelihood
of explosion or a shock wave from sudden decompression
would be averted with water as a fluid medium.
Pressure Testing for
Leak Tightness
Over the years, as pressure-retaining items were placed
into service, it became practice that pressure testing with
treated water could ensure leak tightness. This approach
is acceptable for most inservice components, provided
NATIONAl BOARD BUllETIN/FAll 2006
Pressure Testing:
Fact and
Fiction
the test pressure does not exceed working pressure, and
the metal temperature of the component is above the
ductile-to-brittle transition temperature (DBTT).
The ductile-to-brittle transition temperature (DBTT) is the
metal temperature at which the fracture behavior of the
steel changes from ductile to brittle. This is of concern
when pressure testing because if the metal temperature
during a hydrostatic test is colder than the DBTT, any
areas of local stress concentration containing flaws or
cracks can suddenly fail in a brittle manner. Performing
a hydrostatic test to provide a second “proof test” of an
inservice item with poor fracture toughness (i.e., a DBTT
above the testing metal temperature) could result in
brittle fracture at locations of stress concentration. This
concern is something that inspectors and owner-users
need to understand completely before conducting
inservice pressure testing of aged components.
Pressure Testing for
Structural Integrity
Two structural “benefits” of hydrostatic testing are
commonly reported. The first is that pressure testing of a
newly fabricated object at or above the working pressure
results in a redistribution of stresses at locations of stress
concentration. Such areas of stress concentration include
nozzle openings or other appurtenances. The other
benefit reported is that the testing acts as a “proof testing”
of the design.
During normal component fabrication, the materials are
subject to forming stresses, including those caused by