2 ply testable bellows can also be referred to using slightly different names such as two-ply testable bellows, two ply bellows and redundant ply or double ply bellows. No matter what the designation, the type is the same. In many situations, 2 ply testable bellows can provide early warning if a bellow fails. In this way it can monitor for leakages and provide a warning if a leakage occurs.


What are 2-ply testable bellows?
2-ply testable bellows consist of two plies of material with the same thickness, the plies can be made from either the same or from different materials. A small space exists between the plies. The bellow is designed such, that each ply will be able to take up the full load independently. So if one ply fails/is leaking the other ply will take up the load, operate and ensure pressure integrity alone until the next scheduled shutdown. This feature improves the on-site crew/maintenance team’s ability to efficiently plan the replacement, solution to the problem and/or repair.


How to monitor if the bellow is leaking? (Active and Passive monitors)
A 2-ply testable bellow design improves both the safety and monitoring options, typically the bellow is monitored through a leak indicator (pressure gauge monitor).

Additionally, the Expansion Joints are fitted with a pressure gauge monitor which can indicate the failure of the inner ply.
Generally speaking, there are two types of monitors:

  • Passive monitor
  • Active monitor


Passive monitor
If the inner ply fails the monitor is activated by the increasing pressure between the plies. This monitor will only indicate the failure in an inner ply.


Active monitor
This monitor can detect failures on both the inner- and the outer ply. A vacuum is created between the inner and the outer ply before installing this monitor. If the inner ply fails, the pressure between the plies will activate the monitor. If the outer ply fails, the monitor is activated by the loss of vacuum.


Advantages of 2 ply testable bellows?

  • Early warning if a bellow fails
  • Minimized risk of unplanned and unexpected shutdown
  • Option of monitoring the bellows; leakage, failure
  • The bellow is designed in such a way that one ply is able to take up the full load if the other ply fails (irrespective of which ply fails)
  • Even though a leakage occurs the system can run until the next scheduled shutdown


Applications of 2 ply testable bellows
2 ply testable bellows are widely used in critical process equipment such as:

  • Oil refineries (FCC unit, spent catalyst standpipe, regenerated catalyst standpipe, recirculation cooled catalyst flue gas piping, turbo expander among others).
  • Power plants
  • At Gas turbines (crossover piping, flue gas piping, exhaust piping)
  • At Steam turbines (crossover piping, flue gas piping, exhaust piping)
  • Other applications where it is crucial to avoid unplanned shutdowns


Belman and two-ply testable bellows
Belman have designed and manufactured 2 ply testable bellows for many years and for various projects, and can therefore provide expert advice concerning this type of bellows and its benefits for you. Further Belman can engineer and design the bellows for your project with the 2 ply testable bellows incorporated.