Temperature imbalance in a superheater header

Superheater outlet header
Component:

Final superheater outlet header in a European coal-fired unit, fabricated from P22 and operating at 540°C, 15MPa

Objectives:

  • To determine the remaining life of this component
  • To demonstrate a consistent life assessment route, that might be applied company-wide to plant built to different design codes
  • To compare various temperature measurement and estimation methods

Approach:

Following the phased-approach:

  • Phase 1: - Initial code-based calculations
  • Phase 2: - Visual inspection, conventional NDE and metallographic examination
  • Phase 3: - Installation of conventional thermocouples and off-line diffusion-couple sensors, application of hardness and oxide-thickness based temperature estimation method.  Recalculation of header life using quantitative inspection results
Schematic cross-section of header showing measured / estimated metal temperatures and inferred steam temperature distribution.

Results:

  • Header shown to be life-expired according to inverse-design calculations
  • Comparison of various code formulae and materials standards showed no significant variation in calculated stresses and lives
  • Adequate life demonstrated on the basis of initial inspection results
  • Apparent differences between temperature measurements reconciled through realisation that a temperature imbalance existed between the tube rows

Benefits:

  • Consistent life assessment route established
  • Adequate header life demonstrated for continued operation
  • Thermal imbalance identified, leading to loss of unit efficiency and increased stresses in the header
  • Potential design modifications proposed to reduce this problem
  • Refined FE-based life assessment proposed to address effect of the additional thermal stresses

Published:

Jarvis, P., Jones, G.T., Batista, A. and Araujo, C.L.de,
"Life assessment using codes and standard material properties in SP249 Carregado case study"
SPRINT/KBS Dissemination Workshops of MPA Seminar, Stuttgart, Germany, 6-7th October 1994

Plant Integrity