November 2022

Heat Transfer

Mechanical design challenges in high-temperature electric heaters

Electric heaters are used in the process industries as an alternative to fired heaters or process heaters—where the heating medium is either steam or any heating fluid—for some specific applications where the process duties are low but fluids are heated to high temperature.

Electric heaters are used in the process industries as an alternative to fired heaters or process heaters—where the heating medium is either steam or any heating fluid—for some specific applications where the process duties are low but fluids are heated to high temperature. Due to high-temperature service, electric heaters are exposed to very high external loads imposed by the piping system connected to the nozzles of the heater shell. Moreover, the shell size of the electric heater is small due to lower process heat duty and a high temperature difference between the heater elements and the process fluid, and are designed out of pipe material with standard pipe schedule thickness.

The initial mechanical designs of the shell and nozzle are carried out by electric heater suppliers as per specified design code based on the specified design pressure and temperatures. The thermal expansion loads from the piping system are not considered since they are made available to the heater suppliers at a later date after the finalization of the piping system around the electric heater. The issue is further aggravated if heaters are located at an elevated place where the wind load from the piping system is combined with thermal loads and further increases the primary and secondary loads on the heater shell and nozzles, increasing the chances of heater failure at the shell-to-nozzle junction.

Therefore, a careful evaluation must be done to determine the most economical external load combinations¹ to ensure minimum impact on the thickness of the shell and nozzles already calculated based on the applicable pressure design code. This is usually done by a simulation study to arrive at the correct thickness of the shell and nozzle before the piece of equipment goes for fabrication.

This article will detail a simulation study of a high-temperature vertical electric heater 6-in. NB pipe located on a tall structure (FIG. 1) and will demonstrate the impact of various combinations of external loads—namely Sustained, Thermal and Occasional—on the thickness of the shell and nozzle. The optimum combinations of these loads to limit the local stresses on the shell and nozzle junction within the allowable stresses are recommended.

Design conditions

The design and operating pressures and temperatures comprise:

• Design pressure: 0.625 MPa (g)
• Design temperature: 690°C
• Operating pressure: 0.19 MPa (g)
• Operating temperature at inlet: 130°C
• Operating temperature at outlet: 649°C.

Materials of construction comprise:

• Heater shell: SA312-TP304H
• Nozzle: SA-403-TP304H (conical fitting)/SA-312-TP304H (pipe).

External loads

Originally, the standard piping loads that are applicable for vessels were provided (TABLE 1) to the electric heater supplier as the imposed external loads on the heater nozzles. Since this electric heater is not a standard pressure vessel, is small in size and designed for very high temperature, the heater shell, nozzle and shell-to-nozzle junction failed to withstand these standard piping loads. Piping stress engineers considered heater shell and nozzle flexibilities in their stress analysis and proposed reduced loads, as indicated in TABLE 2.

Load cases

The following two load cases with various combinations of external loads were considered. While Load Case 1 considered thermal loads at design temperatures, Load Case 2 considered thermal loads at operating temperatures (i.e., inlet temperature for the inlet nozzle and outlet temperature for the outlet nozzle).

Load Case 1: At design temperature conditions:

• Sustained loads (Sustained): Dead weight (loads from component weights, fluid and internal pressure)
• Operating loads (Thermal): Sustained loads + thermal load (design)
• Occasional loads (Occasional): Sustained loads + thermal load (design) + wind load.

Load Case 2: At operating temperature conditions:

• Sustained loads (Sustained): Dead weight (loads from component weights, fluid and internal pressure)
• Operating loads (Thermal): Sustained loads + thermal load (operating)
• Occasional loads (Occasional): Sustained loads + thermal load (operating) + wind load.

Since dead weight (design)—which by definition will include the design pressure of the fluid—cannot be considered as a sustained load, dead load (design) is not considered here. As the electric heater is located on an open structure at an elevation, the wind load is considered as part of the occassional load. These load combinations are presented in TABLE 3.

Allowable stresses

The following allowable stresses were considered as per Part 5 of ASME Sec VIII Div 2 in the stress analysis:

• S = Allowable at design/operating temperature obtained from ASME Section II Part D² [3]
• S_PL = 1.5S³ [4]
• S_PS = 3Savg³ [4]
• Sa = Allowable obtained from a fatigue curve for the specified number of operating cycles.³ [4]

STRESS ANALYSIS

Stress analysis was performed at the shell-to-nozzle junctions at the inlet and outlet (referred to as shell here) and the inlet and outlet nozzles, using a proprietary finite element analysis (FEA) program^a and considering two different pipe thicknesses of the shell and nozzle, namely 40S and 80S for Load Case 1 and the thickness of 40S for Load Case 2. The results are presented in TABLES 4–12 and FIGS. 2–14.

Results and discussions

Load Case 1 for the SCH 40S thick shell: An FEA using the software^a was carried out, considering an allowable stress at a design temperature of 690°C for both the shell and inlet and outlet nozzles.

• The primary membrane stresses for Sustained loads are tabulated in TABLE 4. The calculated stresses are within the allowable limit.
• The primary membrane stresses for Thermal loads are tabulated in TABLE 5.
  • The primary membrane stresses at the inlet shell and inlet nozzle are ~357% and ~359% of allowable limits, respectively.
  • The primary membrane stresses at the outlet shell and outlet nozzle are within allowable limits.
• The primary membrane stresses, secondary membrane and bending stresses, and fatigue stresses for the Occasional load are tabulated in TABLE 6.
  • The primary membrane stresses at the inlet shell and inlet nozzle are ~427% and ~432% of allowable limits, respectively.
  • The primary membrane stresses at the outlet shell and outlet nozzle are ~129% of allowable limits.
  • The secondary membrane and bending stresses at the inlet shell and inlet nozzle are ~113% and ~114% of allowable limits, respectively.
  • The secondary membrane and bending stresses at the outlet shell and outlet nozzle are within allowable limits.
  • The fatigue stresses at the inlet and outlet shells and inlet and outlet nozzles are within allowable limits.

Due to the failure of the shell and nozzles as noted above for the Thermal and Occasional load conditions, it was decided to repeat the analysis for higher shell thickness (SCH 80S) to check whether the increase in thickness coulc bring the induced stresses within allowable limits.

For Load Case 1 for the SCH 80S thick shell: The stress analysis was carried out considering allowable stresses at a design temperature of 690°C for both the shell and inlet and outlet nozzles.

• The primary membrane stresses for Sustained loads are tabulated in TABLE 7, and the calculated stresses are within the allowable limit.
• The primary membrane stresses for Thermal loads are tabulated in TABLE 8.
  • The primary membrane stresses at inlet the shell and inlet nozzle are ~234% and ~239% of allowable limits, respectievly.
  • The primary membrane stresses at the outlet shell and outlet nozzle are within allowable limits.
• The primary membrane stresses, secondary membrane and bending stresses, and fatigue stresses for Occasional load are tabulated in TABLE 9.
  • The primary membrane stresses at the inlet shell and inlet nozzle are ~282% and ~286% of allowable limits, respectively.
  • The primary membrane stresses at the outlet shell and outlet nozzle are within allowable limits.
  • The secondary membrane and bending stresses at the inlet and outlet shells and nozzles are within allowable limits.
  • The fatigue stresses at the inlet and outlet shells and inlet and outlet nozzles are within allowable limits.

The shell and nozzle failed with the SCH 80S thickness, as well. Any further increase in shell thickness to bring the induced stress within allowable limits could not be considred since the heater supplier informed that any increase in pipe schedule above 80S will result in the shell’s inside diameter becoming smaller than the bundle diameter.

For Load Case 2 for the SCH 40S thick shell: The stress analysis was carried out considering the allowable stress at an operating temperature of 130°C at the inlet and 649°C at the outlet for both shell and nozzles.

• The primary membrane stresses for Sustained loads are tabulated in TABLE 10. The calculated stresses are within the allowable limits.
• The primary membrane stresses for Thermal loads are tabulated in TABLE 11. The calculated stresses at the inlet and outlet shells, and the inlet and outlet nozzles are within the allowable limits.
• The primary membrane, primary + secondary + bending stresses and fatigue stresses for Occasional loads are tabulated in TABLE 12. The calculated stresses at the inlet and outlet shells, and the inlet and outlet nozzles are within the allowable limits.

Since the induced stresses in the shell and inlet and outlet nozzles are wthin allowable limits for all the three load combinations (i.e., Sustained, Thermal and Occasional), a further increase in thickness from 40S to 80S is not required.

Load Case 1, where the Thermal load is considered at design temperature, results in stresses exceeding the allowable limits with thickness higher than the standard thickness (40S) of pipe shell. An increase in shell thickness of more than 80S is infeasible since that results in the inner diameter of the shell becoming smaller than the outside diameter of the heater bundle, thereby making the insertion of the heater bundle inside the shell impossible.

Load Case 2, where the Thermal load is considered at operating temperature, is a more practical scenario compared to Load Case 1 since the heater nozzles and shell will be under operating conditions most of the time—this is particularly true in combination with the wind load, which by definition is considered an Occasional load. This load combination also enabled the adoption of a standard pipe wall thickness by keeping the induced stresses within the allowable limits.

Takeaways and recommendations

With cost-competitive and schedule-driven projects, detailed engineering contractors (DECs) are expected to specify all technical requirements correctly and completely in the requisition document to avoid changes at a later stage that may affect the project cost and schedule. To avoid post-order changes, DECs should not over-specify the external loads (e.g., specifying standard piping loads for vessels and exchangers for electric heaters). Instead, DECs should evaluate various external loading combinations and specify the most feasible load combination.

As explained above, a simulation study with appropriate load combinations, considering operating temperatures instead of design temperatures, can help avoid increased hardware costs, and (in this case) increase the thickness of the electric heater shell during the project’s detailed design stage, preventing both cost and schedule overruns.

To avoid late changes in the supplier design with a conservative load combination, it is recommended to consider thermal loads at operating conditions—the more likely scenario—rather than at design conditions, which tend to be conservative and may result in a late change in form and an increase in the thickness of the shell and/or nozzles. HP

NOTE

^a Paulin Research Group, Codeware Nozzle PRO v15.0 Build 16

NOMENCLATURE

P_L = Primary local membrane stress
P_b = Primary bending stress
Q = Secondary membrane + bending stress
F = Peak fatigue stress
S = Allowable stress at design/operating temperature
S_PL = Allowable primary local membrane stress
S_PS = Allowable primary + secondary membrane and bending stress
S_a = Allowable peak fatigue stress
S_avg = Average allowable stress between design/operating temperature and ambient temperature

LITERATURE CITED

1. Pramanik, R., “Challenges in design and engineering of electric heaters,” Chemical Engineering World, September 2017.
2. ASME Boiler and Pressure Vessel Code Section II, “Materials,” 2019.
3. ASME Boiler and Pressure Vessel Code Section VIII Division 2, “Rules for construction of pressure vessels,” 2019.

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