Figure 7.2 outlines how informational, standard & critical IOW limits/targets might interact
with different levels of communications (notifications, alerts & alarms) for controlling
elevating temperatures on furnace tubes.
Several high temperature damage mechanisms are possible in furnace tubes. In general,
long term creep life and corrosion from temperature dependant damage mechanisms are
the primary concerns. However, when operating at temperatures significantly higher than
the design, failure can occur rapidly due to overpressure from the significant loss in material
strength, i.e. short-term overheat and stress rupture. For example, furnace tubes that
normally operate at 900 °F (482 °C) and have an API 530 design temperature of 950 °F (510
°C) would have an increasingly shortened service life if operated beyond this design
temperature. Using the principals from Figure 7.2, a more specific example of how IOWs
may be employed to establish furnace tube temperature limits to avoid premature rupture
or unplanned replacement of the tubes is presented below;

Informational IOWs – Inspection, Corrosion and Process Engineering personnel
(SME’s) would be responsible for tracking and trending furnace tube temperatures
operating below the design temperature, <950 °F (510 °C). Limits or target
operating ranges may be set and/or notifications sent to inform the SME if the
temperatures exceed a normal operating range of 900 °F (482 °C).

Standard IOW Limit - The initial standard limit for furnace tubes is frequently set at
the API 530 design metal temperature (100,000 hour design life). This standard
limit may be adjusted based on an engineering analysis from detailed knowledge of
the time dependent damage mechanisms (creep and corrosion), and the estimated
remaining life. For this example the standard limit is set at 950 °F (510 °C). An alert
(or an alarm) is used to notify SME’s and Operations when this temperature is
exceeded. Operators would be directed to adjust furnace controls to get the tube
temperature back to below 950 °F (510 °C) within a preset amount of time.

Critical IOW Limit – The critical limit set at a temperature prior to the point when
failure is imminent due to loss of strength with some amount of safety factor. For
this example, a critical temperature limit of 1025 °F (552 °C) was selected. An alarm
point is set for the board operator that alarms when this temperature is exceeded
and the operator is directed to take immediate actions to regain control or even
shut down the furnace to avoid failure.
This example shows how there may be more than one IOW limit for the same process
parameter (in this case furnace tube temperature), for tracking/trending or to gain control
prior to reaching a critical IOW limit. In addition, there may be more than one predefined
response, depending upon the degree of exceedance of the process parameter limit. In this
example, all three levels of IOWs were set to show a progression of failure risk and
commensurate communication and response activity initiating with the SME’s, then
Operations and ultimately the Board Operator to correct the increasing temperature.
Imminent Failure
IOW
Potential Issue
Example Response to
Exceedances
Typical Range for
Critical IOW’s,
Based on Short
Term Failure
Short term overheating and
stress rupture due to loss
of tensile strength
Alarm is set for the Board
Operator whom responds
immediately with
predetermined actions to
gain control of the
operation or shut down the
furnace
Typical Range for
Standard IOW’s,
Based on Remaining
Life Assessment
Temperature range for
operation where some
damage mechanisms may
begin to shorten the
equipment life, such as
creep, sulfidation,
oxidation, carburization,
etc.
Alarms and/or Alerts are
set for Operations which
may or may not have a
predetermined action and
timing to implement.
Otherwise the situation
may be reviewed with
SME’s and a plan
developed to bring the
operation back into control
and/or adjust the
inspection plan and
conduct remaining life
calculations as appropriate
Typical Range for
Long term reliability of
equipment out to the
design life. Typically the
SME would be responsible
for tracking and trending
this information and
adjusting the inspection
and test plans, as necessary
The SME (e.g. Corrosion /
Materials Eng.) is typically
responsible for tracking the
furnace tube temperatures
and recommending
inspection and testing
activities such as routine IR
examinations, tube
strapping, UT
measurements, etc.
Informational
IOW’s Based on
Design Life
Figure 7.2 Example of how IOW’s might be set for furnace tube temperatures