Inspectioneering
Inspectioneering Journal

The Challenge of Inspection and Assessment of Critical Piping Systems in Chemical Plants

By James Widrig at Quest Integrity Group. This article appears in the July/August 2013 issue of Inspectioneering Journal

Introduction

Inspection and fitness-for-service assessments of critical in-plant piping systems are a concern for the chemical industry. This presents a potentially insurmountable task and discovery of a number of areas where the condition is at risk. Recent failures in chemical facilities indicate that there is a present and severe risk in piping systems. Many in-plant piping systems have historically been unpiggable due to their small diameters and the obstacles inherent in valves and bends, as well as diameter changes. However, many of these piping systems can be inspected with advanced smart pigging technology that provides 100% coverage of the internal and external pipe surfaces. The high resolution inspection data captured by these advanced in-line inspection tools is analyzed to identify the location and degree of potential corrosion, as well as deformities in the piping. Assessment of the corrosion and deformation damage using API 579 Fitness-for-Service methodology dra-matically improves the ability of a plant to understand and improve the mechanical integrity of these systems and better manage environmental, safety, and operational risks.

Plant Piping Design and Damage

Process piping within a chemical plant is typically designed to ASME B31.3 - Process Piping1 standard. Pipelines and interconnecting piping operated outside the fence is typically designed to ASME B31.4 - Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids2 or ASME B31.8 - Gas Transmission and Distribution Piping Systems3 codes. These product distribution, raw material supply, or intermediate transfer lines may also come under the jurisdiction of the Pipeline and Hazardous Materials Safety Administration (PHMSA), which is the primary federal agency responsible for pipeline safety and enforcement.

The causes of serious incidents in pipelines are summarized in Figure 1. The proportion of incidents resulting from excavation damage has decreased and is no longer the most significant factor in pipeline incidents. Additional causes such as mechanical damage not associated with excavation (vehicle incidents, vandalism, etc.), corrosion, and material failure are also of potential concern.

Figure 1. Causes of Significant Pipeline Incidents on Hazardous Liquid Transmission Pipelines from 1988-2008 [4]  Figure 1. Causes of Significant Pipeline Incidents on Hazardous Liquid Transmission Pipelines from 1988-2008 [4]

Pipelines and piping most often fail because of the development of critical flaws such as cracks, corrosion (including wall thinning, erosion, erosion-corrosion, and pitting), mechanical fatigue, and mechanical damage. Once initiated, defects in the pipe may continue to grow while the equipment is in service to the point at which a line can either leak or rupture. With a leak, there is typically no extension of the length of the defect. A rupture will bulge outward prior to failure and then the defect will extend into the surrounding material before either propagating or arresting, depending upon the material, the pressure, and the physical state (liquid or gas).

Some examples include:

  • A partial wall defect (corrosion, cracking, etc.) eventually fails and becomes a through-wall defect.
  • A pipeline may leak when the corrosion or defect reaches the point where the stress carried by the remaining ligament or section of pipe becomes too great and the material cannot sustain the load.
  • A pipeline may leak when the through-wall defect is small and the pressure is low. The through-wall defect may not propagate to the surrounding material and the line may leak.
  • A pipeline may rupture when the through-wall defect is large or long and the pressure is high.

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