Remaining life assessment – RemLife™

A feature of two-shift operation is that units, once they are running at close to design output, are expected to respond rapidly to load changes on the system, thus ensuring that the grid maintains the specified frequency and voltage.

The rigorous demands imposed by load varying operations in competitive markets, asset managers need to quantify the incremental costs of cycling generating units to ensure profitability and to better select, run, and upgrade production units. Furthermore, technical, operations, and maintenance staff need to understand, plan, and react to the complex nature and interaction of equipment, processes, and failure modes brought on by cycling.

The ability to break down complex plant into simple components enables RemLife to be utilised effectively in various other process industries where creep, low cycle fatigue and creep-fatigue may occur. This can include components such as refinery heaters, reactors, reformers, coker’s and many more.

The Software is designed to breach the gap between “back of the envelope” calculations for base rupture and fully detailed finite element analysis for creep-rupture and combined creepfatigue analysis. It enables a quick screening approach to see which plant components are at risk due to their past and planned future operating regime as well as a detailed approach for components which are determined to be at risk. Future operational regimes can be rapidly assessed and their impact on the component calculated.

The RemLife software uses a combination of methodologies to calculate both the base rupture damage as well as the cyclic damage due to start-ups and shutdowns. This is achieved under the guidelines of the British Energy-R5/R6 assessment code, based on time fraction, ductility exhaustion, and strain energy density exhaustion. A number of international design standards are used for the calculations of primary and secondary stresses. These are based on the use of analytical expressions for the determination of thermal stress, using an analytical expression for metastable thermal stresses and mapped thermal stress response.

RemLife – High Temperature Remaining Life Economic Viability Module – is a simulator that allows the effects of unit cycling on both the damage sustained by the equipment and the economic benefits or not of the chosen cycling regime to be studied.

The principal factors to be considered during cyclic operation are:

• Increased capital spend for component replacement;
• Increased routine 0&M cost from higher wear and tear;
• Lower availability due to increase in failure rate and increased outage time;
• Increased fuel cost from reduced efficiency and non-optimum heat rate.

This gives the ability to rapidly run different load profiles and shifting strategies to gauge the impact on individual components and the unit as a whole.

R5-R6, API579 and RCC-MR:

• Time Fraction Approach
• Ductility Exhaustion
• Strain Energy Density Exhaustion

• Elastic and physical constants
• Monotonic tensile data
• Criteria for insignificant creep
• Distinction between creep brittle and creep ductile material
• Creep rupture data
• Shakedown factors
• Continuous cycling (fatigue) data
• Stress relaxation data
• Creep ductility data

• Determine the loading history
• Calculate metastable thermal stresses
• Determine the cyclic stress-strain deformation loops
• Obtain creep-fatigue endurances
• Calculate the total creep-fatigue damage
• Assess whether crack initiation will occur
• Calculate economic consequences of cyclic damage