Brittle Scenarios Process Hazards Analysis (PHA) Using Method Based on API 579 (Recommended Practice for Fitness-for-Service)

In a Process Safety Management system, one of the core elements it is the process hazard analysis that spreads into many others elements, once the results of the risk analysis address recommendations to mitigate risk scenarios, feed mechanical integrity program through the prioritization of critical equipments, procedures revisions, emergency response plans and training programs for contractors and employees

The process hazards analysis are the first step to process safety management. A strong and robust risk scenarios identification is extremely important to identified the most possible scenarios and then to proceed with risk management.

In process hazards analysis all possible risk scenarios about process deviations are identified. One of the deviations observed in HAZOP analysis, for instance, is low temperature. One kind of scenario of low temperature is brittle fracture scenarios. Brittle fracture scenarios are caused by exposure to low temperatures where an unintentional or uncontrolled phase change for a hydrocarbon from a liquid state to a vapor occurs, resulting in a very rapid chilling (refrigeration) of the liquid containing local equipment. For brittle fracture occurs, three elements must be present simultaneously: a) a susceptible steel to low temperature b) a stress riser, as a crack or a notch c) applied stress above a minimum stress level.

The olefin and hydrocarbon processing industries in general have already experienced a number of brittle fracture failures caused by exposure to low temperature due auto-refrigeration.

When it is identified, a brittle fracture in a qualitative risk analysis as HAZOP the first step it is to analyze the pair pressure-temperature allowed in material specification. However this approach is very conservative.

This work was developed in the Braskem Basic Petrochemical Unit of South Brazil (BPU-RS). During the five year process hazards analysis review plan they were found scenarios related to brittle fracture. Due the scenarios complexity, it was necessary to develop a more detailed methodology to evaluate these scenarios. It was developed an innovative method according fitness for service for low temperature conditions using the concepts of API 579, considering the installed process versus process conditions risk scenarios. In the literature, there are papers about API 579 use in brittle scenarios evaluation. The use of this technique it is to evaluate existing carbon steel equipments. The method reduces considerably the necessity of physical substitutions and investments keeping risks acceptable in Braskem matrix.

This work aims to present a brittle fracture scenario during start-up procedure, identified in ethylene compressor discharge pipeline in Olefins unit

The methodology approach involves the analysis methods:

1. PHA- Process hazard analysis- It is applied HAZOP method to identify the risk scenario.
2. Mechanical evaluation analysis using API 579 methodology. The equipment is analyzed for brittle fracture potential using API 579. The result of this evaluation is to generate a curve that reflects a equipment’s MAT (material) for all pressure conditions.
3. The purpose of this analysis is to generate a CET (Critical Exposure Temperature) for all equipment subject to exposure to brittle fracture. The CET is represented by a curve of process temperature conditions for pressures below the MAWP (Maximum Allowable Working Pressure) of the equipment.

The scenario was identified in the five year process hazard analysis first cycle. If it would be applied the pair pressure temperature material specification it would be necessary to change the pipeline material. In discharge pipeline the temperature would be around -60°C during start-ups. The pair pressure-temperature specification material is -29°C for the pressure observed.

It was plotted in a diagram the pipeline and its accessories mechanical evaluation according API 579 and the process conditions as the method described above.

The result of this study identified only the necessity to change the cases pipeline. The investment was lower compared to change all discharge pipeline if a simple pair pressure temperature material specification would be considered.

Although the scenario object of this study was ethylene discharge compressor pipeline, the developed method is applied for scenarios involving hydrocarbons from 1 to 4 carbons with brittle scenarios potential and carbon steel existing equipments and or pipelines. This approach is applied for existing equipments or pipelines that would be exposure to low temperature due auto-refrigeration. The scenarios would be identified in process hazards analysis.