Management of Hot Spots in Refractory Lined High

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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

he outlet piping system of a steam me-thane reformer (SMR) plant gathers the reformed syngas from SMR catalyst tubes inside a furnace and transports the hot

syngas to the process gas boiler (PGB). For hy-drogen production, the reformed syngas typically has a temperature exceeding 870°C (1600°F) at the catalyst tube exit. One of the common outlet piping system designs is refractory lined piping where the pressure boundary can be constructed with low alloy steels such as carbon steel or Cr-Mo steels. This type of design is known as a cold outlet system. The refractory is designed with the following considerations:

1. The low alloy steel piping must stay be-low a certain temperature limit to prevent from high temperature creep and high temperature hydrogen attack (HTHA);

2. The temperature of the low alloy steel piping must also be greater than the dew point of the syngas to prevent carbonic acid induced aqueous corrosion.

Under normal conditions, such design provides adequate reliability for long term operation. However, it is not uncommon that premature degradation of the refractory occurs even during the early stage of plant operation possibly due to inferior quality or installation issues. The degra-dation can have a significant impact on the per-formance of the refractory, and as a result, hot spots can develop at the pressure boundary. De-pending on the temperature, size, quantity, and location of the hot spots, the outlet piping system may be subjected to various failure mechanisms that present a significant risk to the structural in-tegrity of the outlet system. Continuing opera-tion with the presence of hot spots must be vali-dated by fitness for service analysis. This technical paper shares a successful experience of managing hot spots developed in the outlet pip-ing of an SMR plant. Through engineering stress analysis, rigorous monitoring, inspection and ex-ternal cooling, the plant was successfully oper-ated for an extended time period until the replace-ment of the outlet piping system.

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Outlet Piping System Design and Operating Anomalies The subject outlet piping system is part of the SMR plant that produces 8.75 tons/hour (82 MMSCFD) of hydrogen. The piping system con-sists of a transfer line that connects six headers to a PGB. Each header is connected to 39 catalyst tubes inside the SMR furnace. A schematic illus-tration of the outlet piping system is shown in Figure 1.

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 and is constructed with refractory lined ASME SA-387 Grade 11 Class 2 which is a 1.25 Cr 0.5 Mo alloy steel. The B31.3 design temperature is 400°C (750°F). The metal temperature of the piping is 200°C (390°F) at the normal operating condition. The temperature limit for thermal ex-pansion design is 260°C (500°F).

During operation, numerous hot spots were de-tected, primarily at the catalyst tube to header joints but also the header to transfer line joints. Typical hot spots at the tube and header joints are shown in Figure 2. The temperature at the hot spots was up to 340°C (644°F), which is still be-low B31.3 design temperature, but significantly exceeded the normal operating temperature of 200°C (390°F) and the thermal expansion tem-perature limit.

(a) Thermal image

(b) Corresponding piping

Figure 2. Representative hot spots on header

As an immediate action, external cooling was ap-plied by blowing air on the hot spots. The objec-tive was to prevent further temperature excursion 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.

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Since the temperature was below the B31.3 de-sign temperature, the risk of imminent failure due to high temperature failure mechanisms such as creep or HTHA was insignificant. However, due to the large quantity of hot spots, and the high temperature gradient at the hot spots, concerns were raised about possible high piping stress due to uneven temperature distribution, high local-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:

1. Thermal expansion limits of the global piping system

2. Global piping stresses in accordance with ASME B31.3[2]

3. Piping thermal stresses due to hot spots 4. Local stresses at hot spots in accordance

with ASME Section VIII Division 2[3] In the Level 3 fitness for service study, piping stress and flexibility analysis of a three-dimen-sional (3D) piping model and finite element anal-ysis (FEA) of localized stresses at a hot spot were performed. The analytical calculations were per-formed by PAI Engineering.

Piping stress and flexibility analysis

A 3D piping model was established using Bent-ley AutoPIPE® that included the transfer line, six rows of outlet headers and catalyst tubes as shown in Figure 3. Refractory lining was also included in the model because of its significant

influence on the piping stiffness. The global sys-tem stresses were calculated in accordance with ASME B31.3 with considerations of a range of friction coefficients at the piping supports. The results indicated that the highest piping stresses were located at Header A due to bending of the transfer pipe from thermal expansion. The cal-culated stresses are tabulated in Table 1.

Figure 3. 3D piping model for piping stress and flexibility analysis Table 1. Piping stress analysis results

Average pipe wall temperature

Calculated Stress Ratio µ=0 µ=0.05 µ=0.2

200°C (392°F) 0.60 0.63 0.78 220°C (428°F) 0.68 0.70 0.84 260°C (500°F) 0.84 0.89 0.96

With a very conservative friction coefficient of 0.2, the piping stress was found to be below the B31.3 allowable stress (stress ratio less than 1) at 260°C (500°F). Based on the calculations, a maximum average pipe wall temperature of 260°C (500°F) was established. The temperature limit agreed with the design limit for thermal ex-pansion. This temperature served as the base temperature for the FEA hot spots analysis. Uneven average temperatures at the top and bot-tom of the headers were also taken into account in the piping stress calculation. At the observed overall temperature gradient, it was found that


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-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 Case 4: Adjacent nozzles with hot spots around

the nozzle circumference Case 5: Adjacent nozzles with hot spots at the

side edge

(a) Case 1

(b) Case 2

(c) Case 3

(d) Case 4


(e) Case 5

Figure 4. Temperature cases at SMR tube and header joints in FEA analysis Similarly, two temperature profile cases were considered for hot spots at the header to transfer line joints as follows and shown in Figure 5: Case 1: Hot spot near top of the transfer pipe Case 2: Hot spot near reinforcement pad weld

(a) Case 1

(b) Case 2

Figure 5. Temperature cases at header to trans-fer line joint in FEA analysis In the FEA analysis, the lateral and vertical trans-lational constraints were applied to one end of the header and transfer line such that the piping is an-chored on one end and is allowed to move axially on the other end. The whole pipe section is al-lowed to move radially. The refractory lining was not included in the FEA model since the ad-ditional loads from the refractory were already 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 (Sps) in accordance with ASME Section VIII Di-vision 2 [2]. 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 (Sps) 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


stress is 473°C (885°F) based on B31.3 stress cal-culations.

Figure 6. FEA stress analysis results on hot spot at tube to header joints

Figure 7. FEA stress analysis results on hot spot at header to transfer line joint In all temperature cases except Case 2 for the tube to header joint, the equivalent stresses ex-ceeded the allowable stress at a lower tempera-ture than the B31.3 temperature limit. In the worst case (Case 4), the equivalent stress at the hot spot exceeded Sps at 404°C (760°F) which represents a temperature gradient of 260°F. The maximum stress is located at the edge of the tube branch connection to the header. Therefore, Case 4 is the control case for the header. On the trans-fer line, the B31.3 temperature limit is the same

as on the header. In Case 1, the equivalent stress (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 analysis, the critical local stress can be repre-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.

Monitoring and Inspection Once the operating temperature limit was estab-lished, a rigorous monitoring and inspection pro-cedure was implemented in the plant. The mon-itoring and inspection matrix included:

• Twice a day temperature measurement at hot spots.

• Thermal imaging documentation of hot spots 2 - 3 times every month.

• Piping support inspection every two days


• Thermal expansion gaping inspection every two days

As an example, Figures 8 to 10 provide repre-sentative average header temperature measure-ment, the differential temperature measurement and thermal expansion of the headers respec-tively. The inspection frequency was increased whenever the plant experienced rapid load changes to meet customer demand.

Figure 8. Header average temperature

Figure 9. Header differential temperature

Figure 10. Header thermal expansion measure-ment

The inspection results indicated that the average header temperature was maintained below 240°C (464°F) in the worst case, which is lower than the 260°C (500°F) limit. The differential tempera-ture between the hot spot and the bottom of the pipe was maintained below 125°C (224°F) in the worst case, below the 156°C (280°F) limit. The thermal expansion was maintained at 30mm (1.2in) in the worst case, below 80% of the de-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-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 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.


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


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Management of Hot Spots in Refractory Lined High