Although the valve body and bonnet are the major pressure-containing components, other components such as the body-to-bonnet bolting can influence the pressure and temperature limits of a control valve assembly.
Characteristics Of Common Bolting Materials – The graph in Figure 6 shows the relative strength vs. temperature of various bolting materials. The table lists key material characteristics and typical applications. Section 10.2.1 of SAES-L-009, Metallic Flanges, Gaskets And Bolts, lists B7 studs (with 2H nuts) as a standard bolting material.
Bolting Materials And Pressure Deratings – Some body-to-bonnet bolting materials that are specified for ANSI Class 600 and above valve bodies have pressure limits that are lower than the normal ANSI Class pressure limits. The reduced pressure limits are referred to as deratings. PS Sheets in Fisher Catalog 71 include tables that list the pressure deratings for different bolting materials at various temperatures. These tables show that the pressure limits are unique for each valve type. Figure 7 is an excerpt from PS Sheet 59.1:031(A)(S1).
For reference, the standard ANSI Class pressure limits are included in the top portion of the table. Under the heading ‘Bolting Pressure Limits’, the maximum pressure for each bolting material, valve size, and temperature is listed. Note that, in some instances, the bolting pressure limit is greater the standard ANSI Class pressure limit, and, that in some instances, the bolting pressure limit is less than the standard ANSI Class pressure limit.