Vessel Nozzle Reinforcement

1. Purpose

This procedure describes the Inflection Point Engineering practice for reinforcement of nozzle or attachment connections to pressure vessels built according to the ASME Boiler and Pressure Vessel Code, Section VIII Division 1. Inflection Point Engineering Design guidelines and criteria are provided to ensure that the requirements of the Codes are met.

2. References

[1] FE/Pipe Analysis Software Manual (COADE Inc.)

[2] ASME Boiler and Pressure Vessel Code Section VIII, Divisions 1 & 2 and Section II Part D

[3] ASME Process Piping Code B31.3

[4] W. J. Koves, “Evaluation of Pressure Design Criteria for Nozzles (II),” PVP – Vol 383-1999

[5] Welding Research Council Bulletin Number 107

[6] Welding Research Council Bulletin Number 297

[7] Welding Research Council Bulletin Number 368

3. Definitions

3.1 Nozzle Reinforcement

The additional plate material that is provided in the vicinity of the vessel nozzle opening to limit stresses to Code allowable. Stresses result from internal pressure, external pressure, and nozzle loadings.

3.2 Reinforcing Pad

An annular plate, with ID equal to the OD of the nozzle, curved in the shape of the vessel shell, and welded onto the shell to provide additional local strength.

3.3 Insert Plate

An annular plate similar to a reinforcing pad, but of a thickness equal to the sum of the vessel and reinforcing pad thicknesses, used to provide additional local strength. Inflection Point Engineering recommends that this type of reinforcement be used on large diameter nozzles subjected to external loads (e.g., FCC standpipes, and vessel inlets and outlets).

4. Introduction

Before the availability of finite element analysis, simplified methods were used to design nozzle reinforcement of vessel openings. Today, the finite element method and advancements in desktop computing allow one to economically analyze configurations of complex geometry.

The ASME Code, Section VIII, Division 1, gives formulas and rules for the design of nozzle reinforcement for pressure loads only, but does require (Section UG-22) that external loads such as the weight of attached equipment and other vessels and piping loads be included in the analysis (refer to Section U-2(g)). Section VIII, Division 2 (refer to Section 4.1 and Section 5) offers guidance on both stress classification and allowable stress limits, for operating metal temperatures below the creep range. Inflection Point Engineering has developed its own design philosophy for nozzles in the creep range.

The Welding Research Council Bulletins provide guidelines for determining stresses at nozzle-to-shell connections. WRC 107, issued in 1979, is used for the design of pressure vessel nozzles under external loading. In 1989, WRC 297 was published as an improvement to WRC 107 for cylinder-to-cylinder connections. In 1991, WRC 368 added local stresses due to pressure, and work is currently in progress by the Pressure Vessel Research Council to extend these relations to connections with large diameter ratios.

Work done at Inflection Point Engineering in recent years suggests that integration of stresses over a certain length results in some weight savings. This averaging procedure can be used on an individual basis when the computed stresses are being evaluated, to show that some connections, which would otherwise not have met the local membrane stress criteria, are adequate.

Section VIII, Division 2 was extensively re-written in 2007 and this version will supersede the 2004 edition after July of 2009. Changes include new load combination methods, replacement of the Stress Intensity criteria with the Von Mises criteria, and nozzle design based on membrane stress determination, not area replacement. The user can select an option in FE/Pipe to use the rules published in 2007 or later for load cases and stress calculations.

5. Analysis Procedure

The following procedure outlines the steps for determining nozzle reinforcement requirements, by utilizing Finite Element (FE) methods, or publications. Nozzle reinforcement analysis and design is best-done using the finite element method. Commercially available programs like FE/Pipe are particularly well suited for this purpose. The analyst’s judgement is still needed in choosing the model and model parameters, as well as in the interpretation of results. It should be noted that analysis techniques cannot be used to supersede formulas in ASME Code, Section VIII, Division 1.

5.1 Meets ASME Code Requirements

Listed here are the relevant items to review in order to meet the Code requirements.

a. The ASME Code requires area replacement based upon the required thickness (tr in Section UG-37) for internal pressure and 50% area replacement for external pressure.

b. The Code requires that external loadings (e.g., superimposed static reactions from the weight of attached equipment, such as motors, machinery, other vessels, piping and insulation) be taken into account in the design of the nozzle reinforcement (Section UG-22). Other external loads include wind, earthquake, pipe hangers, piping displacement loads, etc.

c. In general, the Code requires that the minimum nozzle neck thickness be at least the greater of the following:

(1) The minimum thickness required for the nozzle cylinder by utilizing the Code design equations for pressure plus external loads.

(2) The minimum thickness of standard wall pipe plus corrosion allowance. Exceptions are access openings and openings for inspection only, and the nozzle thickness need not exceed that required for the vessel shell or head (assuming E=1) at the point of attachment. Refer to paragraph UG-45 of the Code for complete requirements.

d. Small nozzles with the required neck thickness do not require any additional reinforcement, providing the vessel is not subject to rapid pressure fluctuations and the Code’s size limitations are met. Refer to paragraph UG-36(c)(3) of the Code for complete requirements.

e. Reinforce openings in flat heads with a diameter less than one half the shell diameter, by replacing half of the area removed. Refer to paragraph UG-39 of the Code for complete requirements.

f. In addition to the Code rules given in UG-36 through UG-43, supplemental rules are required for openings exceeding the following limits:

(1) For vessels not exceeding 60 inch (1520 mm) ID, openings greater than 20 inches (508mm) or one half of the vessel diameter.

(2) For vessels greater than 60 inch (1520 mm) ID, openings greater than 40 inches (1000 mm) or one third of the vessel diameter.

The supplemental rules are in Appendix 1 paragraph 1-7 and they require two thirds of the reinforcement to be within three fourths of the appropriate limit. Openings exceeding these limits, and with nozzle-to-vessel diameter ratio of d/D<0.7 must meet the requirements in paragraphs 1-7(b) 2, 3 and 4. If d/D<0.3, the WRC Bulletins may be used, but in the range of 0.3<d/D<0.7, use Finite Element Analysis (FEA).

For openings with d/D>0.7, the Code does not provide design equations and the design shall be in accordance with Code section U-2(g), which states that it is the responsibility of the manufacturer to provide the details for a design as safe as those provided by the Code. The requirements of U-2(g) are met by performing an FEA.

g. The ASME Code has additional rules for the case when multiple openings are spaced close enough together that their limits of reinforcement overlap. A given piece of the cross section cannot be considered to apply to more than one opening. The combined reinforcement has to provide an area that is greater than or equal to the sum of the areas required for each opening. Refer to paragraph UG-42 of the Code for complete requirements.

5.2 Determine Local Stresses

The computed elastic stresses from FE analysis methods or WRC 297 is in general conservative as the results include the determination of local elastic discontinuity stress. These stresses would be important if fatigue due to cycling is an issue. In most other cases, these are conservative in that local deformation will work to relieve these stresses, refer to WJKoves paper [4] for additional details on methods to average this stress over a distance.

The Design Engineer is responsible for selecting the appropriate method to determine the local stresses in the nozzle-to-shell connection. The available methods include:

a. WRC Bulletins 107 for Spheres and 297 for Cylinders

Add pressure stresses using WRC 368. Further, when d/D>0.3, or the nozzle is not radial, use FEA procedures. For connections with d/D>0.7, use FEA procedures.

Use FEA procedures for non-cylindrical nozzles or for vessels that are not cylindrical or spherical, such as nozzles attached to a vessel cone.

The vessel analysis software Compress contains WRC 107, while the piping software program Caesar II contains both WRC 107 and WRC 297.

b. Commercially Available Nozzle Analysis Software

Analyze nozzles by using the finite element method with the commercially available computer program NozzlePro. Use this program to easily model most shell and nozzle configurations, and to separate the stresses into the various categories described in ASME Section VIII, Division 2 and to compare them to the proper allowable stress.

For geometries and configurations that NozzlePro does not handle, use the more general purpose program FE/Pipe (from which NozzlePro is derived). Most geometries can be modeled using FE/Pipe and the Template of the General Nozzle Plate and Shell Model. This template is best suited for designs where stresses due to temperature differentials within the connection are known not to be a significant contribution to the overall stress.

c. Other FEA Models

If for any reason a more detailed and case-specific analysis with unusual conditions is needed, the general-purpose finite element computer program ABAQUS may be better suited for that purpose.

5.3 Determine Stresses and Allowables

• Stress Classification

After the analysis is run, combine the stresses from the WRC Bulletins or FEA tools into a stress intensity value before comparison to the appropriate Code allowables. Note that FE/Pipe has built-in post-processing that performs ASME Code allowable comparisons. Ultimately the responsibility still lies with the user to assure that allowables are being compared correctly in his/her situation.

Inflection Point Engineering utilizes the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, for determining the stress classification. The Code provides that stresses in each piping or vessel component be classified in the three categories of primary, secondary and peak stresses. Examples and typical cases of stress classification are given in Table 5.6 of Division 2.

Primary stresses are stresses due to weight and pressure and are broken down into three separate categories:

• Pm, General Primary Membrane Stresses
• Pl, Local Primary Membrane Stresses, and
• Pb, Primary Bending Stress

Primary membrane stress is the average stress through the thickness of the component that exists over the entire cross section of the component. For example, the stress caused by pressure (pr/t) is a primary membrane stress.

Local primary membrane stress is a stress due to a primary load such as pressure or weight, but that is localized around a discontinuity.

Primary bending stress varies through the thickness in proportion to the distance from the centroid. It is a bending stress due to a primary load, such as the bending stress in a flat head.

Secondary stresses, Q, are self-equilibrating stresses necessary to satisfy continuity of structure. They occur at structural discontinuities and can be caused by thermal or mechanical loads.

Peak stresses, F, include increments added to primary or secondary stresses due to load concentration (notch) effects.

b. Stress Allowables

At temperatures below the range where creep and stress rupture strength govern the selection of allowable stress, the basic allowable stress, Sm, is defined by the Code as the lesser of:

• Two sevenths of the specified minimum tensile strength at room temperature (more recently, a factor of two sevenths is used)

• Two sevenths of the minimum tensile strength at temperature

• Two thirds of the specified minimum yield strength at room temperature

• Two thirds of the minimum yield strength at temperature

Prior to 1999, the ASME Code, Section VIII, Division 1, limited the allowable stress to one fourth rather than two sevenths of the specified minimum tensile strength.

At temperatures in the range where the creep and stress rupture strength govern the selection, the maximum allowable stress value for all materials is established by the Code not to exceed:

• 100% of the average stress to produce a creep rate of 0.01% in 1000 hours

• 67% of the average stress to cause rupture at the end of 100,000 hours

• 80% of the minimum stress to cause rupture at the end of 100,000 hours

For design in the creep range, additional considerations may apply.

Once the value of Sm is determined, the following limits apply to the listed stress classifications:

• The general primary membrane stress is limited to the value Sm.

• The primary local membrane and bending stress is limited to 1.5 times the value of Sm.

• Secondary membrane plus bending stresses are limited to 3 Sm.

Peak stresses are limited by the allowable stress amplitude of the material, Sa, found in the appropriate fatigue curve, and the desired number of cycles.

The Codes provide that in the case of occasional loads, the allowable stresses may be increased by a factor of 1.2 (ASME Boiler and Pressure Vessel Code) or 1.33 (ASME B31.3 Piping Code). Inflection Point Engineering adheres to the 1.2 factor for vessels, but uses 1.33 for piping.

c. Average the Local Membrane Stress

The designs tend to be governed by local membrane stress criteria on the longitudinal plane. If the local membrane stress is averaged over a median distance of 0.78 RT from the nozzle junction, it produces a better, and less conservative, correlation with burst test results. A conservative equation for computing the average local membrane stress from the maximum local membrane stress is

/ S Max = 0.4+a/L[0.4-0.3(1+L/a)-0.1/(1+L/a)3]

Where

and S Max are the average and maximum local membrane stresses,

L is equal to 0.78 RT

a is the mean nozzle radius.

This equation may be applied to the WRC 368, WRC 297 or Finite element analysis results. Justification is based on Reference [4] and Code precedent is in ASME Section VIII, Division 1, Appendix 1, paragraphs 1-5 and 1-8.