The two-ply testable bellows (also referred to as a redundant ply design) can, in majority of cases, provide early warning about bellows failure. Although a representative sample performs in a statistically predictable way, any single expansion joint’s exact fatigue life is unpredictable.
Installation of a two-ply testable bellows can, in a majority of cases, provide early warning about bellows failure.
Many refineries use two-ply testable bellows especially in critical process equipment such as the FCC unit, but they are also used in the spent catalyst standpipe, regenerated catalyst standpipe, recirculation cooled catalyst flue gas piping and the turbo expander. Gas and steam turbine applications also find these bellows ideal in the exhaust, crossover and flue gas piping.
Two-Ply Testable Bellows
A two-ply testable bellows consists of two plies of identical thickness, with a small cavity between the plies.
The two-ply design is used when it is necessary to monitor the integrity of the bellows inner or outer ply. Both plies are designed for the full pressure and temperature cycles required. If one ply fails, the second will ensure pressure integrity and take over until a scheduled repair or replacement can be facilitated. This allows maintenance personnel to take action and plan solutions accordingly to minimize downtime.
Active and Passive Monitors
There are various methods to monitor a two-ply testable bellows, from simple pressure gauges to electronic sensing devices. Overall, they are categorized as active and passive monitors. Passive monitors utilize the line pressure to indicate an inner ply failure. When the inner ply fails, the internal pressure between the plies activate the monitoring device. Passive monitors will only sense an inner ply failure.
Active monitors will detect both inner and outer ply failures. A vacuum is created between the plies before the monitoring device is installed. In case the inner ply fails, the pressure between the plies will increase to the line pressure. If the outer ply fails, the vacuum will be lost. In both cases, the monitoring device will be activated.
Monitoring devices used are most commonly pressure gauges, pressure transducers and pop up detectors. Direct monitoring to the control room is also an option and can be attached directly to the test ports.
In case the monitoring device indicates pressure between the plies, our recommendation is (1) during operation, use the bleeder valve to evacuate the pressure and close valve immediately. Then check for pressure build up. (2) If pressure builds up again, the inner ply may have cracks that allow the system pressure to fill the space between the two plies. A replacement bellows should then be considered.
Two cylindrical tubes are formed with thin wire mesh between to generate a cavity between the plies in the two-ply testable bellows. The purpose of the mesh is to ensure that there is a flow path between the plies.
The test ports are placed at each end of the bellows, 180° apart; a test port tube is welded to the outer bellows ply. Typically, one test port will be blinded off with a pipe cap and the other connected to the desired monitoring device.
Note: When installing an expansion joint with two-ply testable bellows, test ports/valves must be kept open the first 24 hours to release any moisture or condensate that may have collected between the plies during storage or manufacturing.
Benefits of two-ply testable bellows
- Bellows monitoring (deterioration & leakage)
- Early warning of leak detection
- Two (2) plies designed for the full system design conditions
- Minimizing risk of unplanned shutdowns
- System can maintain operation while a suitable repair or replacement can be arranged
- Reduced downtime in services where the bellows failure could cause forced outage
These are welded to the outer bellows ply, placed at each end 180° apart as shown below. Typically, one test port will be closed off with a pipe cap and the other connected to the desired monitoring device.
To prevent measuring equipment from being affected by heat radiation, pressure gauges are placed at a safe distance and connected to the two-ply testable bellows’ test port with a braided hose.
Testing of the two-ply testable bellows can be performed with limited means. Often maintenance staff is limited to readily available portable testing equipment as shown below.
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