Management of Hot Spots in Refractory

Lined High Temperature Equipment

Refractory lined equipment is typically used in high temperature processes in the chemical processing
industry. Degradation of refractory over time commonly leads to development of hot spots at the pres-
sure boundary which may present a serious risk to the structural integrity of the equipment and can
even lead to a catastrophic failure. This paper discusses a successful experience of managing hot
spots developed in refractory lined high temperature syngas piping in a steam methane reforming
(SMR) plant. Level 3 fitness for service (FFS) analysis was performed to define a series of operating
limits. Through rigorous temperature monitoring, in-situ inspection of critical dimensions and appli-
cation of external cooling, the plant has been successfully operated under these conditions for an ex-
tended time period before the piping was replaced.

K. Xu, A. Alexander, T. S. Varma, M. DeLoatch and J. Armstrong

Praxair

Introduction

T
Under normal conditions, such design provides
he outlet piping system of a steam me- adequate reliability for long term operation.
thane reformer (SMR) plant gathers the However, it is not uncommon that premature
reformed syngas from SMR catalyst tubes degradation of the refractory occurs even during
inside a furnace and transports the hot the early stage of plant operation possibly due to
syngas to the process gas boiler (PGB). For hy- inferior quality or installation issues. The degra-
drogen production, the reformed syngas typically dation can have a significant impact on the per-
has a temperature exceeding 870°C (1600°F) at formance of the refractory, and as a result, hot
the catalyst tube exit. One of the common outlet spots can develop at the pressure boundary. De-
piping system designs is refractory lined piping pending on the temperature, size, quantity, and
where the pressure boundary can be constructed location of the hot spots, the outlet piping system
with low alloy steels such as carbon steel or Cr- may be subjected to various failure mechanisms
Mo steels. This type of design is known as a cold that present a significant risk to the structural in-
outlet system. The refractory is designed with the tegrity of the outlet system. Continuing opera-
following considerations: tion with the presence of hot spots must be vali-
dated by fitness for service analysis. This
1. The low alloy steel piping must stay be- technical paper shares a successful experience of
low a certain temperature limit to prevent managing hot spots developed in the outlet pip-
from high temperature creep and high ing of an SMR plant. Through engineering stress
temperature hydrogen attack (HTHA); analysis, rigorous monitoring, inspection and ex-
2. The temperature of the low alloy steel ternal cooling, the plant was successfully oper-
piping must also be greater than the dew ated for an extended time period until the replace-
point of the syngas to prevent carbonic ment of the outlet piping system.
acid induced aqueous corrosion.

2018 99 AMMONIA TECHNICAL MANUAL

 Outlet Piping System Design and During operation, numerous hot spots were de-
Operating Anomalies tected, primarily at the catalyst tube to header
joints but also the header to transfer line joints.
The subject outlet piping system is part of the Typical hot spots at the tube and header joints are
SMR plant that produces 8.75 tons/hour (82 shown in Figure 2. The temperature at the hot
MMSCFD) of hydrogen. The piping system con- spots was up to 340°C (644°F), which is still be-
sists of a transfer line that connects six headers to low B31.3 design temperature, but significantly
a PGB. Each header is connected to 39 catalyst exceeded the normal operating temperature of
tubes inside the SMR furnace. A schematic illus- 200°C (390°F) and the thermal expansion tem-
tration of the outlet piping system is shown in perature limit.
Figure 1.

(a) Thermal image

Figure 1. Illustration of outlet piping system

The piping is anchored at the PGB and the trans-

fer line is allowed to grow away from the PGB to
accommodate thermal expansion during opera-
tion. Similarly, the headers are allowed to grow
away from the transfer line. The headers are also
designed to support the catalyst tubes.

The piping system is designed to ASME B31.3 (b) Corresponding piping

and is constructed with refractory lined ASME
SA-387 Grade 11 Class 2 which is a 1.25 Cr 0.5 Figure 2. Representative hot spots on header
Mo alloy steel. The B31.3 design temperature is
400°C (750°F). The metal temperature of the As an immediate action, external cooling was ap-
piping is 200°C (390°F) at the normal operating plied by blowing air on the hot spots. The objec-
condition. The temperature limit for thermal ex- tive was to prevent further temperature excursion
pansion design is 260°C (500°F). and to maintain the temperature at the hot spots
below the B31.3 design temperature. Thermal
imaging temperature measurement was deployed
to constantly monitor the temperature variation.

AMMONIA TECHNICAL MANUAL 100 2018

 Since the temperature was below the B31.3 de- influence on the piping stiffness. The global sys-
sign temperature, the risk of imminent failure due tem stresses were calculated in accordance with
to high temperature failure mechanisms such as ASME B31.3 with considerations of a range of
creep or HTHA was insignificant. However, due friction coefficients at the piping supports. The
to the large quantity of hot spots, and the high results indicated that the highest piping stresses
temperature gradient at the hot spots, concerns were located at Header A due to bending of the
were raised about possible high piping stress due transfer pipe from thermal expansion. The cal-
to uneven temperature distribution, high local- culated stresses are tabulated in Table 1.
ized stress at hot spots, and the ability to maintain
thermal expansion and contraction at different
plant rates. Therefore, extensive fitness for ser-
vice analyses were performed to establish the op-
erating limit of the SMR plant.

Fitness for Service Analysis

The unexpected temperature excursion obviously
exceeded the original piping design scope. A
Level 3 fitness for service study as defined in
API-579[1] was conducted to investigate the
structural integrity of the outlet piping system
with the presence of hot spots and to define the
temperature limits at the hot spots. The follow-
ing factors were considered: Figure 3. 3D piping model for piping stress and
flexibility analysis
1. Thermal expansion limits of the global
piping system Table 1. Piping stress analysis results
2. Global piping stresses in accordance with Average pipe Calculated Stress Ratio
ASME B31.3[2] wall temperature µ=0 µ=0.05 µ=0.2
3. Piping thermal stresses due to hot spots 200°C (392°F) 0.60 0.63 0.78
4. Local stresses at hot spots in accordance 220°C (428°F) 0.68 0.70 0.84
with ASME Section VIII Division 2[3] 260°C (500°F) 0.84 0.89 0.96

In the Level 3 fitness for service study, piping With a very conservative friction coefficient of
stress and flexibility analysis of a three-dimen- 0.2, the piping stress was found to be below the
sional (3D) piping model and finite element anal- B31.3 allowable stress (stress ratio less than 1) at
ysis (FEA) of localized stresses at a hot spot were 260°C (500°F). Based on the calculations, a
performed. The analytical calculations were per- maximum average pipe wall temperature of
formed by PAI Engineering. 260°C (500°F) was established. The temperature
limit agreed with the design limit for thermal ex-
Piping stress and flexibility analysis pansion. This temperature served as the base
temperature for the FEA hot spots analysis.
A 3D piping model was established using Bent-
ley AutoPIPE® that included the transfer line, six Uneven average temperatures at the top and bot-
rows of outlet headers and catalyst tubes as tom of the headers were also taken into account
shown in Figure 3. Refractory lining was also in the piping stress calculation. At the observed
included in the model because of its significant overall temperature gradient, it was found that

2018 101 AMMONIA TECHNICAL MANUAL

 the effect of thermal bowing was not significant.
However, the thermal bowing may affect the ver-
tical piping supports and limit their design capa-
bility for thermal motion. Periodic inspections
were recommended to ensure the performance of
the piping support.

FEA analysis of hot spots

The impact of hot spots on localized stresses was
investigated by extensive FEA analysis. The fol-
lowing five temperature profile cases were con- (b) Case 2
sidered for hot spots at tube to header joints as
shown in Figure 4:

Case 1: Hot spot around nozzle circumference

with hot spot on adjacent nozzle at top

Case 2: Hot spot located at the side of the nozzle

edge

Case 3: Hot spot located at the top of the nozzle

edge
(c) Case 3
Case 4: Adjacent nozzles with hot spots around
the nozzle circumference

Case 5: Adjacent nozzles with hot spots at the

side edge

(d) Case 4

(a) Case 1

AMMONIA TECHNICAL MANUAL 102 2018

 (b) Case 2
Figure 5. Temperature cases at header to trans-
(e) Case 5 fer line joint in FEA analysis
Figure 4. Temperature cases at SMR tube and
header joints in FEA analysis In the FEA analysis, the lateral and vertical trans-
lational constraints were applied to one end of the
Similarly, two temperature profile cases were header and transfer line such that the piping is an-
considered for hot spots at the header to transfer chored on one end and is allowed to move axially
line joints as follows and shown in Figure 5: on the other end. The whole pipe section is al-
lowed to move radially. The refractory lining
Case 1: Hot spot near top of the transfer pipe was not included in the FEA model since the ad-
ditional loads from the refractory were already
Case 2: Hot spot near reinforcement pad weld considered in the piping analysis.

The local stresses were calculated at hot spots

with the operating stress from the internal pres-
sure. Based on the thermal imaging temperature
measurements, the maximum hot spot tempera-
ture varied from 300°C (500°F) to 482°C
(900°F). The average piping temperature was
fixed at 260°C (500°F) at the bottom of the pipe.
The calculated Von Mises equivalent stress was
compared with the allowable stress determined
based on primary stress plus secondary stress
(S ps ) in accordance with ASME Section VIII Di-
vision 2 [2].

(a) Case 1 Figures 6 and 7 show the FEA stress analysis re-
sults for the header and transfer line respectively.
The equivalent stresses are plotted against the hot
spot temperature in five temperature profile
cases. The allowable stress (S ps ) is also plotted
as a comparison which shows a slight decrease
with the hot spot temperature and a sharp de-
crease at about 454°C (850°F). On the header,
the maximum temperature limited by the hoop

2018 103 AMMONIA TECHNICAL MANUAL

 stress is 473°C (885°F) based on B31.3 stress cal- as on the header. In Case 1, the equivalent stress
culations. (not shown in Figure 7) was very low and stayed
below the allowable stress. On the other hand,
the equivalent stress exceeded the allowable
stress when the hot spot temperature is greater
than 414°C (777°F) in Case 2. The highest stress
is located at the weld of the reinforcement pad
next to the hot spot. Therefore, Case 2 is the con-
trol case for the transfer line.

A sensitivity analysis was also performed to

study the size effect of the hot spots on the
stresses. The results indicated that the maximum
local stress is dictated by the temperature gradi-
ent from the hot spot to the rest of the pipe rather
than by the absolute temperature at the hot spots.
Based on the hot spot size measurement and FEA
Figure 6. FEA stress analysis results on hot spot analysis, the critical local stress can be repre-
at tube to header joints sented by the differential temperature over
380mm (15in) distance between the hot spot and
the bottom of the pipe.

In the actual hot spot situation, the temperature

profile closely resembled Case 2 and Case 5 on
the headers; and Case 1 on the transfer line.
Based on the FEA analysis results, a conservative
maximum differential temperature equal to
156°C (280°F) over 380mm (15in) distance at the
hot spot was established. This differential tem-
perature also agreed with the worst case scenario
on the transfer line. The external cooling was
controlled to maintain the maximum differential
temperature throughout the outlet piping system.
Figure 7. FEA stress analysis results on hot spot
at header to transfer line joint Monitoring and Inspection
In all temperature cases except Case 2 for the Once the operating temperature limit was estab-
tube to header joint, the equivalent stresses ex- lished, a rigorous monitoring and inspection pro-
ceeded the allowable stress at a lower tempera- cedure was implemented in the plant. The mon-
ture than the B31.3 temperature limit. In the itoring and inspection matrix included:
worst case (Case 4), the equivalent stress at the
hot spot exceeded S ps at 404°C (760°F) which • Twice a day temperature measurement at
represents a temperature gradient of 260°F. The hot spots.
maximum stress is located at the edge of the tube • Thermal imaging documentation of hot
branch connection to the header. Therefore, Case spots 2 - 3 times every month.
4 is the control case for the header. On the trans- • Piping support inspection every two days
fer line, the B31.3 temperature limit is the same

AMMONIA TECHNICAL MANUAL 104 2018

 • Thermal expansion gaping inspection The inspection results indicated that the average
every two days header temperature was maintained below 240°C
(464°F) in the worst case, which is lower than the
As an example, Figures 8 to 10 provide repre- 260°C (500°F) limit. The differential tempera-
sentative average header temperature measure- ture between the hot spot and the bottom of the
ment, the differential temperature measurement pipe was maintained below 125°C (224°F) in the
and thermal expansion of the headers respec- worst case, below the 156°C (280°F) limit. The
tively. The inspection frequency was increased thermal expansion was maintained at 30mm
whenever the plant experienced rapid load (1.2in) in the worst case, below 80% of the de-
changes to meet customer demand. sign thermal expansion limit. In addition, the in-
spection of piping supports showed no indica-
tions of piping lift or restrictions of thermal
motion. The outlet piping system has been suc-
cessfully managed to operate within the safe de-
sign margin with the presence of significant hot
spots.

Conclusions
Significant hot spots were detected on the refrac-
Figure 8. Header average temperature tory lined outlet piping system of an SMR plant.
Extensive fitness for service analyses were per-
formed to study the impact of hot spots on the
structural integrity of the piping system and to es-
tablish the safe operating limits. External cool-
ing was applied to control the piping tempera-
ture. Rigorous temperature monitoring and
inspection procedures were implemented to en-
sure the plant was operated within the safe limits
established by above analysis. The SMR plant
was successfully operated over a year with the
Figure 9. Header differential temperature presence of hot spots on the outlet piping system
until the refractory lined piping was replaced.

The successful experience demonstrated that the

state of the art engineering analysis, robust mon-
itoring and inspection procedures and a strong
culture of operational discipline are essential
components for safe plant operation in case an
unexpected situation arises.

Figure 10. Header thermal expansion measure-

ment

2018 105 AMMONIA TECHNICAL MANUAL

 Acknowledgments
The authors are grateful to Praxair Inc. Manage-
ment for the support and approval of publication
of this work.

References
1. API-579/ASME FFS-1, “Fitness for Ser-
vice”, 2016
2. ASME B31.3-2016, “Process Piping”,
ASME, 2016
3. ASME Boiler and Pressure Vessel Code,
Section VIII Rules for Construction of
Pressure Vessels, Division 2 Alternative
Rules, 2017

AMMONIA TECHNICAL MANUAL 106 2018