Redundant Ply Design Bellows Expansion Joints
- If one ply fails, the other ply will take its place until a scheduled replacement.
- 2 – Ply Construction.
- Each ply designed to withstand the Design / Operating conditions.
2-Ply Testable Bellows
If a hole or stress crack develops in the inner ply during service, the outer ply takes over without exposing operators to increased risk or creating the need for an unscheduled shutdown.
FCCU expansion joints generally incorporate 2-ply testable bellows where each ply is designed for the full operating conditions.
Monitored Bellows
The annular space between plies can be monitored for leakage to detect a ply failure. This will serve as a warning of an imminent problem. A pressure device in the outer ply alerts about the inner ply failure.
The 2 ply testable bellows from simple pressure gauges to electronic devices and can be categorized as
• Active and
• Passive Monitors.
(a) Passive Monitor
When the inner ply fails the monitor is activated by the pressure between the plies.
(b) Active Monitor
The active monitor can detect inner and outer ply failures. A vacuum is created between the inner and outer ply before installing the monitoring device. If the inner ply fails, the pressure between the plies will activate the monitoring device and if the outer ply fails the vacuum will be lost and the monitoring device will be activated.
