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Why You Should Qualify Mechanical Plants and Equipment by Seismic Walkdown

Extreme Loads and structural risk

Why Qualify Mechanical Plant and Equipment by Seismic Walkdown

The seismic qualification of mechanical plants and equipment, which help form shutdown systems, is a crucial part of secure operating procedures for environmental, and in the case of nuclear facilities, radiological safety.

What is a Seismic Walkdown?

The seismic walkdown methodology was first developed for chemical and nuclear facilities in the early 1980s to create a practical, reliable and cost-effective alternative to the more rigorous seismic qualification of equipment achieved via testing or analysis. This new experience-based approach involved an in-depth review of the performance of conventional power plant equipment during strong-motion earthquakes at over 100 facilities around the U.S. This review helped form the experience database, which is the key reference in the assessment approach for qualification by seismic walkdown. Now, the methodology has been widely adapted for other sectors, making it a universally accepted approach for seismic qualification.

What are the Benefits of a Seismic Walkdown?

Reduced Expenses 

A seismic walkdown review allows organizations to identify specific instances where detailed design calculations or assessments are not necessarily required to substantiate a particular plant item or structure. The known seismic performance of similar plant items or structures can then be used to demonstrate the inherent seismic withstand and capacity of the plant item or structure. This negates the requirement to assess every component in a safe shutdown system via calculation or testing and is a significant cost-benefit.

Simultaneous Assessments 

The seismic walkdown methodology can be implemented in both the assessment of existing plants and equipment and in the design and assessment of new plants and equipment. It can be utilized for the qualification of systems across all industries including nuclear, chemical, industrial, commercial and offshore.

A critical part of the seismic walkdown review is that it considers the plant and equipment in their current condition and location. This is important as it takes account of any variations from concept, design or installation drawings or references and assesses the true condition, including material degradation and damage. Additionally, the seismic walkdown review enables potential seismic interaction concerns from adjacent plants and equipment to be identified, which cannot be considered satisfied by a desktop-based assessment.

How is a Seismic Walkdown Performed?

Seismic walkdowns must be completed by experienced engineers who have a broad range of knowledge and analytical insight into the performance of various structural forms. To perform a seismic walkdown, engineers visit a site and follow the means and guidance of their earthquake experience (as previously mentioned), to demonstrate the seismic adequacy of new or existing plants and equipment. There are 20 generic classes of conventional plant equipment, the most common of which include control and instrument panels, batteries, pumps, valves, fans, chiller units and instrument racks. In addition to this equipment, new seismic guidelines have been developed for other items such as relays, tanks and heat exchangers, pipework, cranes and cable and conduit raceway systems. 

Ensuring Compliance 

To confirm that the plant and equipment are in compliance with the guidelines and are similar to the equipment previously reviewed, a number of stipulations, or caveats, must be met. Different caveats apply for each equipment class, taking cognizance of the known seismic vulnerabilities. Compliance with the intent of the caveats indicates that the structural strength of the item or plant being assessed is adequate based on the experience database.

Items of plants that do not satisfy certain screening guidelines (or the intent of all of the caveats) for a particular class of equipment are considered to be outliers. Further investigation is required to mitigate outliers, which involves increased assessment work or consideration of additional caveats.

What are the Typical Findings of a Seismic Walkdown?

The review of the available information, such as drawings and plant manuals prior to the seismic walkdown, enables the team to (1) confirm that the installed equipment arrangements are representative of the design intent and (2) to identify any configurational differences which may exist. Typical deficiencies identified during these reviews include:

  • Incorrectly or Inadequately Installed Anchors
  • Corroded Support Steelwork
  • Missing Supports
  • Inadequately Braced or Restrained Items

ABS Group: Your Seismic Walkdown Experts

For the past 25 years, ABS Group has assisted several clients and sites across a wide range of industries in both the U.S. and UK in completing successful reviews of safe shutdown systems and components utilizing the seismic walkdown methodology. Our engineers have significant experience and are widely recognized as subject matter experts. Our organization has been instrumental in the compilation of earthquake experience databases, with access to all original records. This has allowed us to provide our clients with unique, cost-effective and practical solutions to resolving their pressing needs. 

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