Seismic and Weak - Link Analysis

HWTS can provide complete weak link/seismic analysis of the entire valve. It can address all major areas of concern. The first step in this process will be a review of all existing qualification documentation supplied by the OEM. In addition to this, details pertaining to the stem-disc interface is necessary to do a complete analysis. The extended structure dimensional information can usually be obtained in the OEM's original analysis and if this is not the case the information can be obtained from the OEM or through a physical measurement of the existing valve. Once the dimensional information is confirmed and the acceptance criteria outlined, a detailed analysis can be performed.

Areas Addressed

Stem

A stem analysis is comprised of three major items, buckling, threaded end and stem head. The major modes of buckling are addressed and the lowest value used with respect to buckling. The threaded portion of the stem is treated as a power screw and the treads in both the stem and stem nut are looked at. In addition, the minor diameter of the stem and the major diameter is used to determine the weakest section. The stem head is analyzed with respect to bending forces. The weakest portion of the stem is then used to determine the overall thrust capacity of the stem.

Disc

The disc analysis addresses two main areas; the disc lugs and the pressure force on the disc. As the disc lugs are usually the most sensitive area when opening a valve, the lugs are modeled a number of different ways in order to most closely predict the stresses. The disc is usually treated as a flat plate model which represents the typical installed configuration.

Yoke and Supporting Structure

The yoke is treated as a frame with loads typically applied at both the top and bottom. These load induce stresses related to bending/torsion and are added to the torsional load from the operator. In addition to the yoke, the body/bonnet flange interface can also be addressed.

Bolting

The criteria for analyzing a bolted flange connection is well defined in the ASME Boiler and Pressure Vessel Code Section NF. Based on the symmetry of the bolt patterns this calculation provides a very accurate analysis.

FEA Capabilities

Finite Element Analysis of specific components is highly dependent on having detailed design drawings. The analysis can be performed on the major components and the results that are given are usually in the same order as those obtain by simple models. This is useful in the area of stress concentrations and how significant they are in respect to the over all stresses.

	Fatigue Analysis Fatigue analysis can be used to justify higher levels of stress when applied to key areas. When the number of operating cycles is known for a valve, or can be estimated, a fatigue curve can be plotted and the failure of the component can be predicted.
	Design Support Complete design analysis can be provided with regards to working with the OEM to support valve modifications. This can include all analysis to support replacement of operators and yoke to changing of design material to provide for more thrust capacity.

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