Seismic Qualification of Equipment by Analysis

Steve J. EderFacility Risk Consultants, Inc.

Seismic Qualification of Equipment by Analysis

• General Approach– Electrical Equipment– Mechanical Equipment

• Alternative – Earthquake Experience Data– Seismic Qualification Utility Group (SQUG)– Technical Requirements

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General Approach • Review the equipment to assess the dynamic

characteristics• Determine the response by analytical methods• Determine the stresses and displacements that result

from the response• Compare the calculated responses with those that

ensure compliance with the design requirements– Function after design earthquake

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Review Equipment to Assess Dynamic Characteristics

• The review stage must take into account the complexity of the equipment and the adequacy of analytical techniques to properly predict the equipment’s safety functions after a design earthquake.

• The review should determine which analytical method will most accurately represent the equipment’s performance under seismic conditions.– Dynamic analysis methods– Equivalent static analysis methods

• Choosing between the dynamic analysis method and the static coefficient method:– In general, the choice is based on the perceived margin of

strength of the equipment since the static coefficient method, while easier and more economical to perform, is generally more conservative.

– Dynamic analysis or tests may indicate that the equipment is either rigid or flexible. Rigid equipment may be analyzed using static analysis and the seismic acceleration associated with the mounting location. Flexible equipment, on the other hand, is analyzed using its dynamic response computed from a response spectrum time history, or other analysis methods.

• The mathematical models used for analysis can be based on calculated structural parameters, or on those established by test, or by a combination of these.

• Where complex mathematical models are based solely on calculated structural parameters, the use of verification testing is highly recommended for validation of structural dynamic parameters.

• The equipment and any secondary structural supports must be modeled to adequately represent their mass distribution and stiffness characteristics.

Stresses and Displacements• In seismic qualification by analysis, stress levels in general

must be well within the elastic range.

• Displacement checks are required for verification of critical clearances. This can be a simple validation of rigidity for certain sub-assemblies, or a detailed review of geometry of bearings, shafts, or conductors.

• Sub-assembly force checks may be necessary for clips, brackets, bushings, etc.

Electrical Equipment• Seismic qualification by analysis is generally limited to

verification of structural integrity (modeling of enclosures and framework).– Functional qualification is generally not possible solely by analysis.

Electrical issues such as those resulting from electro-mechanical device trips, short circuits, etc. during the shaking cannot in general be captured by analysis.

– Dynamic analysis can be used to ascertain in-equipment response levels for subcomponent shake table testing (response amplification due to panel flexibility).

Mechanical Equipment• More complex finite element models are used for pressure

boundary components with moving parts such as pumps and valves.– Simplified models can be used for air handling equipment with

rigid sub-assemblies such as motors.– Close interface with manufacturers is necessary for verification of

shaft clearances, strength of bearings, nozzles, and mechanical joints.

– The analysis must address the interface with piping and has to include thermal effects.

Alternative Method – Earthquake Experience Data

• In lieu of qualification by analysis, testing, or combined analysis and testing, earthquake experience data can be used for seismic qualification of certain types of equipment within aset of strictly defined limitations for the method.

• The Seismic Qualification Utility Group (SQUG) provides the technical requirements.

What is SQUG?• In 1980, the Nuclear Regulatory Commission (NRC) issued

Unresolved Safety Issue (USI) A-46 to address concerns with seismic equipment qualification in older plants

• Utilities banded and formed the Seismic Qualification Utility Group to address USI A-46.

• The use of earthquake experience data was developed and detailed, including procedures for application in nuclear power plants andDOE facilities.

SQUG Accomplishments• The USI A-46 program was implemented and resolved by

most nuclear utilities from 1993 to 2000.

• The SQUG Generic Implementation Procedure (GIP) methods remained proprietary to SQUG.

• Experience data is currently used on a regular basis for seismicqualification of new and replacement equipment in nuclear power plants.

• Collaboration with MCEER is now underway.

• Electric Power Research Institute (EPRI) maintains the electronic earthquake experience data base (eSQUG).

Earthquake Experience Data Base• Industrial facilities and power plants subjected to strong

motion earthquakes were reviewed to determine performance of equipment during and after earthquakes

• Detailed documentation was collected and studied for 20 classes of equipment plus other features.

• In general, lessons learned follow a clear trend: Well anchored equipment, free of certain caveats and seismic interaction concerns, performs well in major earthquakes.

The 20 Classes of Equipment• SQUG Equipment Classes (functionality) Include:

I&C Panels and Cabinets

Temperature sensors

Instruments on racks

Engine generators

Battery chargers & inverters

Batteries on Racks

Distribution panelsAir compressors

Air HandlersChillersFansSolenoid-operated valves

Motor-operated valves

Fluid-operated valvesVertical pumpsHorizontal

pumps

TransformersMedium voltage switchgear

Low voltage switchgear

Motor control centers

I&C Panels and Cabinets

Temperature sensors

Instruments on racks

Engine generators

Battery chargers & inverters

Batteries on Racks

Distribution panelsAir compressors

Air HandlersChillersFansSolenoid-operated valves

Motor-operated valves

Fluid-operated valvesVertical pumpsHorizontal

pumps

TransformersMedium voltage switchgear

Low voltage switchgear

Motor control centers

eSQUG Provides Archive of Raw Data

Generic Implementation Procedure• Four criteria are used to verify seismic capacity:

– Seismic Capacity vs. Demand (Comparison with SQUG Bounding Spectrum)

– Earthquake Experience Database Caveats and Inclusion Rules

– Anchorage Evaluation

– Seismic Interaction Evaluation

• Bounding Spectra are used to define the seismic capacity, based on the experienced level of ground shaking, for each equipment class whose seismic adequacy is verified by the earthquake experience success data.

Seismic Capacity vs. Seismic Demand

SED ≤ 1.2

SED = [ 1 + 2 (z/h) ] × SDS ≥ 1.5 x SDS

Using Earthquake Experience Data• Inclusion rules are used to define the bounds of each

equipment class whose seismic adequacy is verified by the earthquake experience success data. These address:– Configuration, size limitations, weight limitations, operating

capacity limits, etc.– Materials, subcomponents, manufacturer classifications,

design & construction standards, etc.– Other issues derived from investigation of shake table fragility tests,

analyses, expert panels, etc.

Caveats and Inclusion Rules

Other Features Also Covered by SQUG• Tanks• Heat Exchangers• Conduit and supports• Cable trays and supports• Piping and pipe supports; tubing• HVAC ducting and supports• Dampers• Cranes

Summary – Qualification by Analysis• Analysis methods can be used for seismic

qualification for only certain kinds of equipment, with physical parameters and functional/performance requirements that can be addressed by stress and deflection limitations.

• Analysis methods can be used in conjunction with testing methods and can significantly reduce the required scope for the testing.

• Earthquake experience methods provide an attractive alternative for certain types of equipment within certain seismic motion bounds.