Page 36 - Design Criteria.qxd

37 ® Welcome to the Next Generation of Sealing Technology
SECTION II
Joint Integrity Calculations
This section is designed to enable a flange designer or gasket user to:
1. Calculate a bolt stress required for a particular gasket in a known flange.
2. Modify both gasket and bolting parameters in the relevant calculations to
arrive at a suitable gasket type and dimension, and bolt pattern to suit
a given application.
A Torque Guide is included to enable the user to obtain a torque figure once the bolt stress has been calculated.
See the installation section for a controlled bolting procedure in which to apply these torque values.
Gasket Type
The engineer must always be aware of the abilities and limitations of the gasket types and materials. Factors such as blow out resist-
ance, creep resistance, stress retention, recovery characteristics and cost must be considered.
Application
When determining the type of gasket to be used, design pressures and temperatures must always be considered. Media will further dic-
tate gasket selection and what materials may or may not be utilized, ensuring chemical compatibility. Always consider special condi-
tions such as thermal cycling, thermal shock, vibration, and erosion.
Flange Design
Attention to the flange design is critical when designing a gasket. Flange configuration, available bolt
load and materials all have obvious effects on gasket selection. Flange configuration determines the
style and basic dimensions of the gasket. Compatibility between flange and gasket material must be
ensured, thus minimizing the possibility of galvanic corrosion.
When a joint assembly is placed in service, three basic forces become active and affect its seal-
ing qualities.
1
2
3
1) END FORCE - which originates with the pressure of confined gases or liquids
that tends to separate the flange faces.
2) GASKET LOAD - the function of the bolting or other means which applies force
upon the flange faces to compress the gasket and withstand
internal pressure
3) INTERNAL PRESSURE - force which tends to move, permeate or bypass the gasket.
Taking the above factors into consideration, attention must be paid to the initial force applied to a joint. Firstly, the applied preload
must be sufficient to seat the gasket upon the flange faces, compensating for any surface imperfections which may be present. Secondly,
the force must be sufficient to compensate for the internal pressures acting against the flange assembly. i.e. the hydrostatic end force
and internal pressure. Finally, the applied force must be sufficient to maintain a satisfactory residual load upon the joint assembly.