Head/Conical Reducer - Internal Pressure

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Last updated: Feb 28, 2017 by Mike ClarkVersion comment
6 min read

(Includes Jacket and Heat Exchanger Components)

Use Diameter: This option is only available for some components. The user may choose to input dimensions as the inside dimensions or the outside dimensions where this option is available. For some components (such as shell), the use of outside dimensions for internal pressure calculations may result in a slightly higher required thickness.

Solve For: This option is only available for some components. The user may choose to solve for thickness or for pressure where this option is available. The “Solve for Thickness” option is more favorable when doing nozzle calculations for nozzles that use the current component as host.

Head/Reducer Design

Pressure: The internal design pressure (pressure on the concave side). This value is gauge pressure. When “Solve for Thickness” is selected, this value is an input and should not include static head. When “Solve for Pressure” is selected, this value is a result. In the latter case, it represents the total internal pressure (design pressure plus head) that the component can handle and meet code in the absence of any other loadings.

Static Head: The internal pressure (pressure on the concave side) resulting from the static head of the fluid. The user must determine this value and input it accordingly. It will be added to the Pressure input and the sum will be used in the internal pressure calculations for the component. This field will not be present when “Solve for Pressure” is selected.

Temperature: The maximum mean metal design temperature for the internal pressure case as defined in UG-20(a).

The following fields are not used by this software; they are listed here for the convenience of filling out the data forms in DesignDocs and FormPro.

Radiography: This field represents the degree of radiography performed on the joints. See UW-11 for more information.

Joint type: This field represents the joint type for the joints. See Table UW-12 for more information. 

Is an inner head: Select the check box to indicate the entire head (both the concave and convex sides) is in the interior of the vessel.

Head/Reducer Dimensions

Diameter: The component diameter in the new condition. The selection in the “Use Diameter” area determines whether this is the inside or outside diameter of the component. In the context of curved heads, this value is specifically the skirt diameter. For circular flat heads, this is the diameter as defined in per the configuration in UG-34.

Long Span: This field is only available for the Non-circular Flat head type. These head types are more specifically considered rectangular and this represents the long side of that head.

Inside CA: Corrosion allowance on the inside of the component (concave side).

External CA: Corrosion allowance on the outside of the component (convex side). This field is available for internal heads and for the inner components of a jacketed vessel.

Efficiency: The joint efficiency of the component, which is determined from Table UW-12 for welded joints and may also represent ligament efficiency per UG-53. When both ligaments and welded joints exist, the lowest efficiency is used. See Appendix L for further help in determining the efficiency.  UG-32 defines E to include the head to shell seam for hemi-spherical heads.

Crown Radius: The component crown radius in the new condition. The selection in the “Use Diameter” area determines whether this is the inside or outside crown radius. This field is only available for heads that have a crown or spherical portion – ASME F & D, Torispherical, Hemispherical, and Dish Cover. If the head type is ASME F & D, the value will be locked to the “Diameter” field to meet the dimension requirements of UG-32. If the head type is Torispherical, the value will default based on the settings under Tools > Defaults.

Dimension h/ho: The dished head depth in the new condition of an elliptical head. The selection in the “Use Diameter” area determines whether this is the inside or outside head depth. This field is only available for elliptical heads. The head depth will default based on the “Diameter” field so that the inside dimensions have a ratio of D/2h = 2.0. This ratio is based on the new inside dimensions.

Cone Height: The axial length of a cone. This field is algebraically connected to the “Diameter”, “Small End Diameter”, and “Cone Angle” fields. Entering the “Cone Angle” will solve for the “Cone Height”; entering the “Cone Height” will solve for the “Cone Angle.”

Cone Angle: The half-apex angle (half of the included angle) of the cone. This field is algebraically connected to the “Diameter”, “Small End Diameter”, and “Cone Height” fields. Entering the “Cone Angle” will solve for the “Cone Height”; entering the “Cone Height” will solve for the “Cone Angle.”

Knuckle Radius: The component inside knuckle radius in the new condition. This field is only available for heads that have a knuckle region: ASME F & D, Torispherical, and Toriconical.

Large End Diameter: The large end inside diameter of the conical section of a toriconical head in the new condition. See dimension Di in Figure 1-4 (sketch e) for more information.

Small End Diameter: The diameter of the cone’s small end in the new condition. The selection in the “Use Diameter” area determines whether this is the inside or outside diameter at the small end.

Dimension L: This value is only for toriconical heads and is a displayed value that is not available for input. This value is what the inside crown radius (new condition) would be if a crown existed in place of a conical section at the end of the knuckle.

Short Span: This field is only available for the Non-circular Flat head type. These head types are more specifically considered rectangular and this represents the short side of that head.

Maximum Pitch: This field is only available for the Braced, Stayed head type. The pitch is the distance between any two adjacent stays. The maximum pitch is the largest value from the collection of pitches.

Factor C: This field is only available for Flat; Non-circular Flat; and Braced, Stayed head types. The default value is based on the head type and configuration selected, however the user may be able to change it to better advantage. Review the sketches in UG-34 for more information.

Factor M: This is a displayed value that is not available for input. This value is used in the head design calculations for heads that have a knuckle (ASME F & D, Torispherical, and Toriconical). See Appendix 1-4 for more information.

Factor K: This is a displayed value that is not available for input. This value is used in the head design calculations for elliptical heads. See Appendix 1-4 for more information.

Thin Out: The material thickness lost to the forming process. To determine the minimum thickness after forming, the un-corroded nominal thickness is reduced by this amount. When an ellipsoidal head is used to make a pipe cap, this field is replaced with a 12-1/2% undertolerance field.

12 ½% Pipe: The thickness that represents the 12-1/2% undertolerance of a pipe cap. For this field to be visible, the user must create a pipe cap by selecting an Elliptical head type and using the Pipe Search. This is a calculated value and is not available for input. When a pipe cap is not being made, this field is replaced with the "Thin Out" field.

Straight Flange: The length of the cylindrical straight flange of the head. Straight flange calculations are not performed automatically; a shell must be created to run calculations on the straight flange.

Include thinout in 1-4(e): This option is only available for toriconical heads. Toriconical heads have two design equations for thickness (for internal pressure): 1-4(d) for the knuckle and 1-4(e) for the conical section. Thin Out is automatically considered in the former equation; this box must be selected for it to be considered in the latter equation as well.


Nominal t: This value is in the new condition. For the component to pass, this value must be at least the sum of the thickness necessary for pressure and temperature, corrosion allowances, and forming allowances or under-tolerances. If the thickness necessary for pressure and temperature is less than the thickness required by UG-16, the nominal thickness must be at least the sum of the UG-16 thickness and the tolerances, etc. In some cases, under-tolerance is not considered (e.g., for nozzle reinforcement, the under-tolerance of the nozzle neck is ignored). When “Solve for Thickness” is selected, the software will determine the smallest standard size that passes. The user is able to manually edit this value.