Hydrocarbon Processing - February 2022 - 67

Heat Transfer
A. K. PANDEY, A. MODI, P. DARJI and S. SHARMA,
L&T Chiyoda, Vadodara, India
Calculate temperature at the bottom
of pipe supports and heat loss rate
Pipes transporting hot media are insulated
to reduce heat loss and maintain the
required process conditions. Shoe or trunnion
supports are used to support such
pipes at regular intervals. Since the support
is directly connected to pipe, there
will be conductive heat transfer through
the support and pipe. This creates an uninsulated
heat bridge between the pipe
and supporting structure via the support
shoe or trunnion, as shown in FIG. 1.
For high-temperature lines (≥ 400°C),
a substantial amount of heat transfer can
occur through the pipe support. As a result,
significant temperatures can be observed
on the structure surface (bottom
of shoe or trunnion). Engineering consultants
and clients should limit the pipe
support bottom temperature to < 350°C
resting on steel and < 150°C resting on
concrete to avoid overstressing/degradation
of the supporting beams.
Therefore, for high-temperature lines,
it is vital to establish a shoe height that
complies with temperature limits at the
pipe support. If the shoe height cannot
be increased adequately, a decision can
be made to put a heat isolation block at
the bottom of the pipe support.
Computational fluid dynamics (CFD)
or finite element analysis (FEA)-based
simulation software can be used to calculate
temperature at the bottom of the
pipe support. However, CFD/FEA are
labor intensive, adding to the project cost
and time delays considering the substantial
number of pipe supports in a plant.
This article provides a method to analyze
heat transfer through pipe supports
and calculate temperatures at the pipe
support bottom. Based on theoretical
basis explained in subsequent sections,
a mathematical model was developed to
FIG. 1. Pipe support geometry.
determine the temperature at the bottom
of a pipe support, as well as heat loss rate.
Calculation results are supported by results
performed using CFD software.1,2,3,4
Theoretical basis. Heat transfer occurs
from the hot pipe to the structure via the
pipe support through various modes of
heat transfer, including (FIG. 2):
a. Heat conduction between the inner
and outer layers of the pipe
b. Heat gain from the source
(i.e., pipe) or conduction
through the pipe support
c. Atmospheric cooling or convective
heat loss through the pipe support
d. Radiative heat loss through the pipe.
From the above, mechanisms (b) and (c)
are more dominant in piping scenarios.
For the discussion here, a list of unit
abbreviations is included:
t = Thickness of shoe/thickness of
trunnion support, mm
H = Height of pipe support
(shoe or trunnion) outside
pipe insulation, mm
E = Width of bottom plate
for shoe support, mm
L = Length of shoe support, mm
Tb
TH
FIG. 2. Modes of heat transfer in a pipe shoe
support.
D = Outside diameter of trunnion, mm
P = Perimeter of pipe support
(shoe or trunnion), mm
Ac
= Cross-sectional area of pipe
support that is exposed to
atmosphere/outside of
insulation, m2
= Pipe temperature, K
= Temperature at the bottom of
the pipe support, K
T∞ = Ambient temperature, K
-K
h = Convective heat transfer
coefficient for air, W/m2
Hydrocarbon Processing | FEBRUARY 2022 67
Hydrocarbon Processing - February 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - February 2022
