Selecting Flanges for High Pressure and High Temperature

High pressure and high temperature are demanding on their own; together they force every part of a flanged joint to be reconsidered.

Class falls with temperature

Class is a nominal designation, not a pressure value. A 300# A105 flange carries roughly 50 bar at ambient, and markedly less at 400 °C. Selection therefore weighs design temperature and pressure together, and the table changes with the material group.

Material

Heat treatment records and PMI reports belong in the shipping file for alloy items.

Gasket and face

RTJ faces are common from class 600 upwards. Where an RF face is retained, graphite filled spiral wound or kammprofile gaskets are selected; PTFE filler is unsuitable at these temperatures.

Bolting and creep

Metal creeps at high temperature: it elongates over time under constant load, which means loss of preload and loosening. Countermeasures:

• Use B16 studs instead of B7 in hot service
• Re-check torque after the first heat up
• Apply hydraulic tensioning on critical joints
• Plan periodic checks on frequently cycled lines

On the design side

Thermal expansion loads the line, and flexibility analysis should limit the moment reaching the flanged joint. Otherwise the gasket opens on one side and leakage begins. See our power plants page for our approach on plant piping.

Send your material and class list through the enquiry form.

FAQ

How many bar does Class 300 carry?
There is no single value; it depends on material group and temperature and is read from the pressure temperature table.

Which gasket suits high temperature?
Graphite filled spiral wound or kammprofile gaskets, with metal ring (RTJ) common from class 600 upwards.

Why do bolts loosen?
Creep at high temperature causes loss of preload, so suitable bolt grades and periodic torque checks are needed.