2018 Spring/Summer Training Schedule
Join us in Deer Park Utah, USA in May or Sophia-Antipolis, France in June for HRSG & Combined Cycle training. Don't miss our Early-bird discounts! See our training page for more details.
Recent Projects: RCA of Repeat Tube Failures
TETRA Engineering was recently contacted by a client in Asia concerning Superheater tube failures in a supercritical coal boiler and asked to conduct a Root Cause Analysis (RCA). A senior Tetra engineer visited the plant to gather information on the history of the failures and relevant data to be used for the RCA. The failures were occurring in one specific region of the boiler (in one instance not long after tube replacement), on hanger tubes concentrated at the right-hand side of the unit. Observations made on the tube failures included
- The gas temperature at the right-hand side of the boiler was often over 100°C higher than the left
- Metallurgical analysis of failed samples revealed fish-mouth failures caused by short term overheating
- Failures occured following increase from 60% load to full load
After the elimination of other possibilities, it was hypothesized that the tubes were overheating due to temporary loss of steam flow which, when combined with the higher gas temperatures at the RHS, led to a concentration of failures. Following this, the operating conditions were modelled using Power Plant Simulator and Designer (PPSD) to evaluate the necessary conditions for these failures to occur.
It was shown that the under low loads, the steam may have been condensing at the bottom of the investigated hanger tubes due to low flue gas temperatures in this region (it entered the tubes only just above saturation). This led to blockage of the tube and restricted steam flow. When load increases, and gas temperatures increase, the tubes overheat – particularly at the RHS due to flue gas temperature skew.
Technical Tidbits from the HRSG Inspection Planning Guide
Thermal overstress is generally associated with the high temperature sections of the HRSG (HP Superheater and Reheater). Flow starvation in a tube relative to its neighbors results in the tube metal temperature approaching the gas temperature. In that case, tube-to-tube differential temperatures may exceed 500°F (260°C). The enormous thermal stress that results exceed material yield stress and cause permanent bowing…