The Issue: Frequent failure of a critical pipe disrupting Reliability, Availability, Performance and Safety
Solution: Design Innovation –> DOM (Design Out Maintenance)
Component: Outlet pipe from a Furnance
Function: Carries hot (1000 degrees Centigrade) oxidised metal (calcine) from the Furnance.
Dimension: The diameter of the pipe is 350 mm at the inlet and 250 mm at the outlet. The length of the pipe is around 6500 mm.
MOC (material of construction): SS310 or SS321.
Failure Mode: Leakage, Accelerated Wear, Cracking (SCC)
MTBF (Mean Time Between Failures): Low, unacceptable.
Difficulty in maintenance: High and involves high degree of safety risk and uncertainty.
Type of DOM/Design Innovation: Third Type — Increase resilience (Note: First Type — Change System Structure: Second Type — Change interactions)
Goal of Problem Solving/Design Innovation: To determine a suitable material for the application that increases MTBF
Primary Analysis: Enveloped Constraint Analysis
The edges of the constraint envelope are the following:
1) Force/Momentum: High considering that the pipe handles a heavy flow of oxidised material. Resistance to the flow of material is high considering high flow of oxidised, irregular shaped material generating high frictional force.
2) Reaction: Material structure changes under the influence of high temperature
3) Environment: Halide. Induces corrosion and stress corrosion cracking of stainless steel (METALS and alloys which are resistant to corrosion usually depend for their resistance on the formation and maintenance of thin films (commonly between 5 and 200 Å thick) of passivating oxide. The breakdown of such films by “aggressive” anions such as chloride/halide (except fluoride), at sufficiently positive anode potential and at sufficiently high temperature, is often responsible for the failure of such alloys, because it usually leads to serious pitting of the bared metal.)
4) Time: Due to friction, heat and halides the frequency of the the failure mode is quite high resulting in high maintenance cost and effort — endangering safety of equipment and personnel.
5) Temperature: very high — induces failures in presence of halides and abrasive wear
6) Lubrication/Wear: High wear rate due to depletion of the thin film of passivating oxide, accelerated by temperature and the abrasive nature of the material handled.
7) Surface/Shape: Hydraulic equivalent diameter of the pipe is important so that no chocking or jamming is experienced. If jamming occurs the degradation leading to failure mode is accelerated.
8) Material: Present material is unsuitable for the application.
Pattern of Failure Mode: Predominantly Wear-out (time dependent) but also has the features of Early (frequent) and Random (degree of uncertainty is high).
Probability of Survival of the component working within the above conditions: Extremely low (<10%)
Consequences: High (5) — Safety of human beings and equipment along with production losses
Warning effect: None (so can’t be subjected to condition monitoring)
Points: Important points of this case: 2, 3, 5,
Surfaces: 2 -> 1: 2-> 5: 3 -> 4: 3 -> 6: 3 ->5
3D (Type of Problem/Insight): Multiple Causes: Multiple Effects
DOM choices of MOC:
a) 321 SS (UNS S32100) is a titanium stabilized austenitic stainless steel that features improved resistance to intergranular corrosion. This grade is suitable for high-temperature applications up to 1500°F (815°C), where the addition of titanium stabilizes the material against chromium carbide formation.
b) SS310 is a highly alloyed austenitic stainless steel designed for elevated-temperature service. The high Cr and Ni contents enable this alloy to resist oxidation in continuous service at temperatures up to 1200°C provided reducing sulfur gases are not present.
c) INCONEL® nickel-chromium alloy 625 (UNS N06625/W.Nr. 2.4856) is used for its high strength, excellent fabricability (including joining), and out- standing corrosion resistance. Service temperatures range from cryogenic to 1800°F (982°C).
d) Hastelloy C-276 maintains oxidation resistance at temperatures up to 1100 ºC and resistance to pitting, corrosion, and cracking at temperatures up to 1040 ºC .
Final Solution (increase resilience): Hastelloy C-276. Reason: It fulfils all the constraints and condition.
Expected Enhanced Probability: 90% (from 10%)
Future Work: a) Estimate the life of the new MOC b) Plan for replacement based on estimated life supported by assessment of condition (CBM methodology to be devised)