ML20128C315

Responds to NRC Concerns Re Design of HVAC Sys Utilizing Circular cross-section Duct & Reg Guide 1.61 Structural Damping Levels for Bolted Structures,Per Sser 18 (NUREG-0797)

ML20128C315
Person / Time
Site:	Comanche Peak
Issue date:	01/28/1993
From:	William Cahill, Walker R TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
RTR-NUREG-0797, RTR-NUREG-797, RTR-REGGD-01.061, RTR-REGGD-1.061 TXX-93068, NUDOCS 9302030384
Download: ML20128C315 (4)

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N Log # TXX-93068

"~ M File # 10010 TUELECTRIC January 28. 1993 William J. Cub!H, Jr.

a,mna rmanu U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555

SUBJECT:

COMANCHE PEAK STEAM ELECTRIC STATION (CPSES) - UNIT 2 00CLET NO. 50 446 DESIGN OF HVAC SYSTEM UTillZING CIRCULAR CROSS SECTION DUCT REF: 1) 0B0 C5-086. Rev. 2, dated January 7, 1993 CPSES Design Basis Document HVAC Duct And Duct Supports

2) Supplement No. 18 to NUREG-0797 Related to Operation of the Comanche Peak Steam Electric Station, Unit 2-(November 1988)

3) Project Technical Report No. CECO-PTR-008, Rev, 0, dated Hay 1989 Damping Values for y Seismic Analysis of CPSES HVAC Duct Systems

4) TU Electric Calculation CS-CA-0000-3293, Rev. 0, dated January 1993, Evaluation of Round HVAC.DVct and Duct Supports for 2% OBE and 4% SSE Damping Gentlemen:

V The purpose of this letter is to respond to NRC steff concerns relative to the design of HVAC systems utilizing circular cross-section duct and Regulatory Guide 1.61 structural damping levels for bolted structures, The CPSES Unit 2 HVAC systems are air distribution systems consisting of cold formed sheet steel ducting and supports constructed from structural steel shapes. The majority of the ducting is rectangular in cross-section

{ rectangular duct) but, circular cross-section<[ circular duct] has also been m used. Rectangular and circular duct runs are constructed from a. number of.

h chort duct sections which are bolted together using gasket 9d, _ flanged joints.

The flanges are fabricated at the duct section ends by attaching?a

^ rolled or f abricated angle to the duct (companion angle flange) or by .i bending the_ duct (hemmed or bent flange), The duct rests'on-the supports  !

and are' restrained in the transverse direction by the. support members with the use of shims as required and in the longitudinal direction by bolted or-  !

-welded connections to approximately every fourth or fifth: support. The duct supports:are spaced approximately every eight feet and are constructed-of' dtructural steel 1 shapes which are welded together. The supports are y

typically attached to the_ building structures by clip angles utilizing'one '

or more concrete expansion anchors.

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'TXX-93068 ~i Page 2 of 5 CPSES Unit 2 HVAC duct _ systems located within Seismic Cat ,ory l buildings.

are designed to meet Seismic Category I or Seismic Category 11 design requirements as provided in the CPSES FSAR and DBD CS-086 (Reference 1).

DBD CS-086 used the same acceptance criteria for Seismic-Category II' ducts and duct supports as reviewed and approved in Section 4'.1'.1.4 of SSER;18 (Reference 2). The Unit 2 HVAC duct-and supports which are within the-boundaries of HVAC systems, or portions of HVAC systems, which are vital-support systems for equipment (and personnel) required to remain functional during and after the SSE, have been designated Seismic Category I and '

designed to Seismic Category I criteria. As discussed later, the-Unit 2 design does not include Seismic Category I systems utilizing circular duct.

Other HVAC duct and supports within Seismic Category I structures have been U designed to Seismic Category II criteria to assure that their structural integrity is maintained during and after the SSE and thus preclude.the potential for their failure and falling which could reduce the functioning-of equipment (or personnel) required to remain functional during and after the SSE 08D-05-086 provides the design basis and technical descriptive information for-the HVAC duct and duct supports et the Comanche Peak Steam Electric Station, Units 1 and 2. This document addresses:the design of both Seismic Category I and Seismic Category II HVAC duct and supports. The DBD addresses CPSES FSAR ' commitments. required system interfaces. ;and the requirements of applicable codes and regulatory authorities. The DES along

- with the applicable installation specifications assures that installation is in accordance with the design requirements.

Included in DBD-CS-086 are the requirements for the input motion to the seismic design of'the HVAC duct and supports. ' tut specification for _ seismic input motion was developed in accordance' with Regulatory- Cuide 1.61 which is the licensing basis for CPSES Unit 2. The levels of structural damping specified are in accordance with RG 1.61-for the seismic analysis of bolted ,

structures and were selected based on the general structural connection details used in the design and fabrication of the HVAC duct and supports.

It.was determined that these damping levels were further substantiated by full scale testing of similar structural systems (Referance_3),

TU Electric made the decision to utilize the seismic damping levels for bolted structures in accordance with RG 1.61. Since the connection details =

- for the rectangular and circular ducts are similar,;no distinction was made in the seismic camping levels. The technical basis' for this is that the; dynamic behavior of the HVAC structural systems when subje :ted to seismic inertia icads .is dominated by the behavior. of the structural connection-between the H','AC duct sections, the ducts and its supports, and the supports

( and the building structure.

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'TXX 93068 I Page 3 of 5 Recently, the NRC staff _has raised concerns with regard to the1 appropriateness of the use of 4% OBE and 71 SSE damped-spectra (RG 1.61 for  ;

bolted structures) for the ' seismic de.ign of HVAC_ systems utilizing circularL l

duct with bolted flange connections. -The concern has been based on the lack-of representative test data' for circular duct whsreas recent conservative tests do exist for rectangular duct with bolted flange connections which -

confirm industry design practice.

L To address the NRC staff concerns. TV Electric agreeu to review Unit 2 HVAC i' duct and supports to identify the use of circular (or non-rectangular) duct'.-

This review inriuded a detailed assessment to demonstrate that increased ,

seismic inertia loads resulting from reduced structural damping (i.e., RG 1.61 damping for welded _ structures) would not result in-reduction in the safety of the design of CPSES Unit 2. A discussion of this review and its conclusions follows.

Discussion A review of CPSES Unit 2 HVAC system design identified HVAC' systems which utilized circular duct. The Unit 2 design does not include Seismic Category I HVAC systems 'lizing the circular duct. The Unit 2-design does _ include:

a number of Se' .c Category II HVAC systems which utilize circular duct.

The remainder of this discussion then, focuses only on the design of Unit _-2 Seismic Category II HVAC circular duct and its. supports.

The design qualification of CPSES Unit 2 Seismic Category !! HVAC duct and support was accomplished by two separate engineering methods within the Unit 2 Civil / Structural Engineering organization. The methods used depended upon wnether the duct and supports were installed prior to the restart _of Unit 2 Engineering in June of 1990 or were designed and-installed as part of the completion of Unit 2 (post-June 1990h The former was qualified by the Seismic II/I Program; the latter by' detailed _HVAC structural design. ,

The Unit 2 Seismic 11/1 Program utilized a " source. oriented" approach:'that is, an approach focused on. demonstrating sufficient seismic capacity sof non-seismic commodities to withstand the CPSES SSE without failure and falling.

The_ Unit: 2- program was reported to the NRC in'TU Electric = letter TXX-92022-dated January 8.-1992. During the- NRC staff's ' review of. the mechanical; attributes associated with the Post Construction Hardware Validation Program, it was noted by NRC Inspection Report Nos. 50'445/92 13:

50 446/92 13 that "...the nonsafety HVAC supports were' reinspected _to.

criteria consistent with the seismic adequacy evaluations.and-Category-11/I walkdown_ criteria....' The program was implemented-by:the following primary steps:

1) Established _ guidelines / checklists for the purpose of identifying Unit-2 HVAC duct and; support instellations or configurations which.

cannot be qualified on the basis of performance of. similar -

p installations during'past strong motion earthquakes or the

qualification of similar configurations by test or analysis:

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2) Performed area walkdowns to select bounding configurations and suspect details for further evaluation: and

3) Evaluated, through analysis, the structural integrity of the bounding configurations and suspect details when subjected to the SSE.

The bounding HVAC duct and support configurations selected by the walkdown teams included three (3) configurations containing circular duct. These configurations had been analyzed and qualified to the Seismic Category 11 design reautrements of DBD-Ct 086.

The Unit 2 HVAC structural design group maintained structural design responsibility for Unit 2 Seismic Category I HVAC duct and supports and any new (i.e.. installed after June 1990) Unit 2 Seismic Category 11 HVAC duct and supports. Design was developed in compliance with the requirements of DBD CS-086. As previously stated the Unit 2 design does not include any Seismit Category I HVAC systems utilizing circular duct. The new Unit 2 Seismic Category Il design does include seven (7) HVAC configurations utilizing circular duct.

To address higher seismic inertia loads resultant from higher accelerations at structurcl damping levels below RG 1.61 values for bolted structures, a review of Unit 2 HVAC system configurations using circular duct was performed. This review, documented in calculation CS-CA 0000-3293, Rev. 0 (Reference 4), provides a conservative estimate of increased seismic inertia loads and load / stress it.teractions which result f rom accelerations at damping levels expected in welded structures (RG 1.61). Increased inertia loads and load / stress interactions were estimated utilizing the following met hodol ogy :

1) Generate a list of Unit 2 HVAC supports which support circular duct:

2) Identify the calculations which are the design basis for each of the supports listed above. If the Response Spectrum Method (RSM) of analysis had been used in the calculation, all supports in the analysis were reviewed. If the Equivalent Static Method (EbH) of analysis had been used then each support with a tributary span utilizing circular duct was reviewed:

3) Determine the seismic input used in the calculation:

4) Select the appropriate 4% damped SSE seismic response spectra curves (3% is used if the 4% curves are not available):

5) Develop a ratio (increase factor) of the 4% (or 3%) damped spectral accelerations to the 7% damped spectral accelerations:

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6) Apply-this' ratio to the load / stress interactions to-conservatively R

estimate . interaction ratios (ratio of actual over allowable) at.

reduced damning levels: and, ,

7) If interaction ratios are greater than 1.0. provide a more detailed-review to identify conservatisms in the existing calculation or factoring techniques.  ;

The results of this calculation demonstrate that increased inertial loads 1 resulting from reduced structural damping (RG 1.61 damping for welded- .

structures) would not result.in increases in Unit 2 HVAC duct and support Li load / stress interactions above the CPSES Unit 2 design requirements os documented in DBD CS 086.

Conclusion n

The NRC staff has recently expressed a concern with' regard to the use of RG 1.61 damping _ levels for bolted structures (4% OBE and 7% SSE)'for-the seismic design of circular ducts and- their supports at CPSES Unit 2. To address this concern TV Electric has performed a review of CPSES Unit 2:HVAC design utilizing circular (non-rectangular) cross-section duct to assess the; impact of higher seismic inertial loading resulting from lower structural damping (RG 1.61 for welded structures). This review, documented'in calculation CS-CA 0000-3293, Rev. O, has demonstrated'that the higher _ .

a accelerations resultant from potentially-lower structural damping:would not result in load / stress interactions above the design basis-for CPSES: Unit.2  :

[ .as documented in DBD CS 086; Therefore,:the original CPSES Unit 2 design -

calculations along with the review documented in calculation CS-CA-0000-3293, Rev. 0,~provida assurance that=this-concern d es-not impact plant safety.

- Sincerely, William.J Cahill .; Jr.

i a By: d~b D.' R. Woodlan L Generic Licensing Hanager CBC/grp i

c Mr._-J. L. Hilhoan. Region.IV Resident inspectors, CPSES (2)

Mr. T. A. Bergman, NRR Hr. B. E..Hol.ian.-NRR

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