Abstract
The aim of this thesis is to identify the heat and mass transfer mechanisms governing
the drying behavior of insulation material and to advance the understanding of
their interactions during prolonged drying, with the objective of improving drying
efficiency in industrial applications and thereby reducing the risk of corrosion under
insulation (CUI). CUI arises when water is retained within insulation systems
due to ingress from the ambient environment or condensation of humid air trapped
within the system. Persistent moisture promotes corrosion of underlying metallic
components, decreasing structural integrity and operational reliability. CUI is an
external corrosion phenomenon, and many studies have attempted to apply principles
from general external corrosion research to CUI. However, these approaches
have often been less successful. Relying solely on conventional external corrosion
principles to understand and mitigate CUI, without accounting for its hidden and
less obvious behaviors, does not lead to meaningful improvements in risk-based assessment
strategies. Reducing CUI risk enhances long-term performance and sustained
operation of energy-producing systems, including power plants, oil refineries,
chemical processing facilities, and offshore oil and gas installations. Moreover, improved
insulated equipment’s drying performance reduces maintenance frequency
and associated material disposal, thereby mitigating environmental impacts.
Repeated wetting and drying cycles are known to significantly accelerate CUI progression.
Therefore, this thesis also examines the coupled heat and mass transfer
processes associated with phase change within insulation materials under temperature
gradients and relative humidity conditions representative of industrial environments.
Thus, experimental work specifically focuses on studying condensation and
evaporation, since additional liquid water requires more time for re-evaporation.
The work focuses on mineral wool insulation, a widely used material in industrial
facilities. The topic of heat and mass transfer including phase change within insulation
has been extensively investigated through both modeling and experimental
studies in the field of building physics. In this context, identifying phase change
processes within insulation materials is essential for improving building energy efficiency,
preventing mould formation, and maintaining desired indoor environmental
conditions. However, literature reviews reveal significant gaps in data on humid air
within mineral wool insulation exposed to the strong temperature gradients typical
of industrial applications, and phase change processes in these system have not
been adequately studied. This study is the first to systematically investigate the
transport of water vapor from the location of evaporation, through mineral wool
insulation, toward the cladding during alternating wetting and drying cycles. It
further demonstrates how liquid water forms on the cold cladding surface due to
condensation, and the extent to which this condensation can spread, potentially
allowing liquid water to reach the hot metal surface and re-evaporate. The present
work therefore represents a significant step forward in understanding whether water
completely escapes the system after each wet–dry cycle or partially remains
trapped within the insulation fibers. Particular attention is given to the influence
of geometry, as certain configurations of insulated assets are more prone to moisture
retention and delayed drying. We have through this work tried to answer how
different geometrical arrangements, amounts of water evaporating into the mineral
wool insulation, pre-exposure of the material to the humid air, affect moisture accumulation
and drying rates, with the aim of identifying designs and applications
that promote more effective moisture removal. We have also explored how identifying
the governing mechanisms of moisture transport and phase change contribute
to improved predictive capability of insulation drying performance using sensor
monitoring technology.
To address these issues, we designed a methodology and constructed experimental
rigs for measuring the temperature and relative humidity of the humid air within
mineral wool insulation using sensors, specifically, the specific humidity, during
evaporation and drying on the hot vertical pipe. By inducing evaporation of varying
amounts of liquid water added from the reservoir, the drying times of the
insulation material were determined. Moreover, to closely examine whether water
completely leaves the system, the rebuilt vertical rig was equipped with a weighing
scale to quantify any liquid water accumulation following the evaporation of
varying amounts of added water. In addition, an NMR technique was employed to
measure moisture content in the mineral wool sample during condensation, under
applied temperature gradient and varying relative humidity conditions of the humid
air inflow. Additional experiments were conducted using dynamic vapor sorption
(DVS) to evaluate the potential of mineral wool for water vapor adsorption and
the formation of liquid water films.
We have in this work established a first set of experimental data for drying time of
the mineral wool insulation material depending of different amounts of water evaporating
into it. We have found that amount of liquid water added, time between
subsequent water addition events, insulation system geometry (drainage opening
size or fully closed system, insulation thickness) mostly affect the length of the
insulation material drying. In terms of condensation, we have shown that condensation
area width and moisture content increase due to relative humidity increase
at the inlet to the insulation material, and that supersaturation is needed for nucleation
of droplets initialization. The DVS measurements revealed that mineral
wool exhibits low water sorption, reaching a maximum of 0.3% at 98% relative
humidity. In particular, with respect to repeated wetting and drying cycles, experimental
results demonstrate that pre-exposure of mineral wool insulation leads
to progressively longer drying times. Experiments conducted on the reconstructed
vertical rig are considerably longer, providing a time scale for all relevant mechanisms,
especially adsorption of gases to manifest, as reaching equilibrium is an
inherently slow process. In that sense, repeated wetting tests have a notable consequence:
the secondary drying time cannot be exactly the same, even when eight
months separate subsequent evaporations of the same amount of liquid water. This
indicates that in applications where wetting and drying occur repeatedly, each subsequent
wetting event results in a prolonged drying period. Such cumulative effects
are unfavorable from a corrosion perspective, as extended moisture residence time
within the insulation increases the likelihood and severity of CUI.
The results in this thesis show that there are pathways to improving monitoring
and maintenance strategies by identifying the areas in the system that are under
prolonged wetting conditions, and thus, with high risk of CUI. The findings
show that: 1) thinner insulation layers, 2) increased size of the drainage slit, 3)
lower amounts of liquid water evaporating into the insulation, 4) single wetting
(avoiding pre-exposure) of the insulation material, are resulting in lower insulation
material drying times. The adsorption of liquid water films and the presence
of condensed liquid water were identified as the primary mechanisms responsible
for the prolonged drying time of insulation materials, representing an additional
source of liquid water that requires time to re-evaporate. We have also demonstrated
monitoring relative humidity, particularly specific humidity, can reveal the
point within the mineral wool where condensation of supersaturated water vapor
begins, between the cold cladding and the pipe wall. Recognizing the dry zone
during supersaturated water vapor flow, where adsorbed water films start to form,
offers essential information for estimation of the drying time in sensor-based monitoring.
The findings from this work represent a significant advancement in maintenance
data collection, thereby improving strategies for CUI prevention. The results
demonstrate that thermodynamic properties of humid air can inform CUI risk assessments
and validate a sensor-based technique for measuring specific humidity
in mineral wool insulation to estimate drying time, an essential indicator for CUI
inspection, while offering a novel approach for detecting wet zones and guiding
targeted maintenance.