VVD Code Assistant

VVD Code Assistant is an integrated database of technical articles to help and aid the user in the design process. The screen window below shows the startup screen for VVD Code Assistant.

The content of VVD Code Assistant for the section 4.1 Flanges is shown below:

4.1 Flanges

4.1.1 Terminology

Welding Neck:
A flange with hub welded to a pipe/shell.

Long Welding Neck:
A flange with neck/pipe as an integral part.

Slip On:
A flange that is slipped over the shell/nozzle and welded to it at both ends.

Lap Joint:
A flange assembly in which the bolt load is transmitted through a loose backing flange onto a stub flange. The stub flange incorporates the gasket contact face.

Narrow Face Flange:
A flange in which the gasket is entirely inside the circle enclosed by the bolts and there is no contact outside the bolt circle.

Full Face Flange:
A flange in which the face contact area, either direct or through a gasket or spacer, extends outside the circle enclosing the bolts.

Reverse Flange:
A flange attached at its outside diameter to the shell.

Split Flange:
A split loose flange in a Lap Joint to make it readily removable from the nozzle neck or vessel.

Smooth Bore:
The bore of the flange coincides with the bore of the shell.

Stepped Bore:
The bore of the flange is different than the bore of the shell.

Bolting-up Condition:
This condition applies when the gasket or joint contact surface is seated during assembly of the joint at ambient temperature and the only loading comes from the bolts.

Operating Condition:
A condition when the hydrostatic end force due to the design pressure acts on the flange.



This module includes design of flanges with full face and narrow face flanges, subject to both internal and/or external pressure. Also included are reverse flanges, split flanges and seal welded flanges. The design method is based on the well-established Taylor Forge method.

The first required input is selection of standard ANSI or DIN flanges, or User Specified Flange. A dimensional database is included for standard ANSI and DIN flanges that also includes facing data. This reduces the number of input required significantly.

In case of non-standard flanges, six different flange types have been defined as listed below:

Description Bore Method Facing Options
RT Ring Type sm/st a or b Narrow/Full Reverse
WN Welding Neck smooth a or b Narrow/Full Reverse
LWN Long Welding Neck smooth a or b Narrow/Full Reverse
SO Slip On stepped b or c Narrow/Full Reverse
SF Stub Flange smooth a or b Narrow
LJ Lap Joint NA b or c NA Split


4.1.3 Design Methods

According to the available methods for solving the stresses in flange connections under pressure, flanges can be classified under three main categories as described below.

1) Narrow Face Flanges.
Flanges with a ring-type gasket inside the bolt circle. One of the three following design methods are applied to circular narrow face flanges with gaskets or joints under internal pressure, taking account of the exceptions given.

a) Integral method. The integral method shall not be applied to the slip-on hubbed flange or to the loose flange in a lap joint. The integral design method allows for a taper hub, which may be a weld; the hub assumed for purposes of calculation shall not have a slope of more than 1:1, i.e. g1 = h + g0.

b) Loose method. The loose method shall only be applied, except for loose flanges in lap joints, if all of the following requirements are met
g0 £ 16 mm and P £ 2 N/mm2 and B /go £ 300 and operating temperature £ 370 °C.

c) Loose hubbed flange method. This shall be applied to the slip-on hubbed flange and the loose hubbed flange in a lap joint.

2) Full Face Flanges with Soft Ring Type Gaskets
This type of flange is only used for lower operating pressures. Select facing alternative 11 to use this design method.

3) Full Face Flanges with Metal to Metal Contact
This type of flange can be fitted with a self-energizing o-ring gasket. Select facing alternative 12 to use this design method.

4.1.4 Flange Bore

The flange bore can either be smooth or stepped. For a Slip On flange the bore is always stepped, as the outside diameter of the shell/nozzle becomes the inside diameter of the flange. For a Ring Type flange the user can select either bores, but for all other flange types the bore is assumed to be smooth. The bore selection has an impact on flange loads and moment arms.

4.1.5 Bolting Data

There shall be at least four bolts and the total number of bolts should be a multiple of four. The bolts are designed to produce enough load for gasket seating in the Bolting Up Condition and sustaining both the hydrostatic end force and maintaining sufficient gasket pressure to ensure a leak-free joint in Operating Conditions. ASME VIII also requires the consideration of the minimum required bolt load for operating conditions, which is a requirement not contained in BS5500 or EN13445.

The VVD database contains root area for the bolts as a function of bolt diameters for both imperial and metric screw threads. The bolting database also includes suggested values for the edge clearance, hub clearance and radial clearance for the bolts. The recommended minimum clearance is based on TEMA (Standard of the Tubular Manufacturer Association) TABLE D-5.

Bolt loads Wop and Wamb are calculated for the operating and bolting-up conditions respectively.

To facilitate the design of pairs of flanges the applied bolt loads from the mating flange for operating and bolting-up conditions may be specified if required. These applied loads are checked against the calculated loads and the higher values are used to evaluate the required bolt cross-section areas Am2 and Am2. The total required bolt cross-section area Am is taken as the greater of Am1 and Am2 and checked against the actual cross-section area of the bolts Ab.

An equal pretightening of all the flange bolts on pressure vessel gasketed connections is important. For pretightening usually the torque wrenches are satisfactory, but for critical applications with high pressure and large size bolting hydraulic bolt tightening tools should be used to ensure correct preload of the bolts. Special means are required to ensure that an adequate preload is obtained when tightening large diameter bolts. With larger flange sizes it is often necessary to generate a bolting-up procedure with clearly defined preload increments and bolt a bolt tightening sequence. In the case of small diameter bolts consideration should be given to prevent the application of excessive load on the bolt.

The program can calculate the required bolt torque for operating and hydrotest conditions. For operating conditions the torque is based on the largest load values of Wamb and Wop. In order to ensure a seal at the hydrotest pressure, which is at least 25% and can be as much as 50% higher than the design pressure, the initial bolt loads Wamb and Wop are recalculated using the hydrotest pressure instead of the design pressure.

4.1.6 Gaskets Data

The gasket manufacturer should normally provide the values of the gasket factors m and the gasket seating stress y, but suggested values are included in the gasket material database.

Not all combinations of gasket material and facing type are permitted. Only valid combinations based on those permitted in ASME VIII Div.1, Table 2-5.1 is permitted by the program.

NOTE: Asbestos containing gaskets are forbidden in many European countries.

4.1.7 Lap Joints

The design of a Lap Joint flange requires two separate calculations:

1) Calculate the Stub Flange

2) Calculate the Lap Joint Flange and attach it to the Stub Flange

The Stub Flange needs always to be calculated prior to adding the Lap Joint flange. A Lap Joint flange has the option of being split, to make it readily removable from the nozzle neck or vessel. The lap or stub flange may be specified with or without a hub.

When calculating the Lap Joint the bearing stresses in the lap joint are checked against the allowable stresses for the lap and stub.