Metal Expansion Joints - ThomasNet

Multi-ply Bellows
has two or more plies of material that are independently
designed for the full system design
conditions. This is achieved by nesting thin tubes
inside each other before the initial punch stage of
manufacturing.
Multi-ply bellows have the distinct advantage of
containing the same pressure as an equivalently
thick single ply design, but with much lighter spring
rates and much higher cycle life. For example, high
pressure might require a 0.060” thick bellows to
contain hoop stress. A single ply of 0.060” thick
or three plies of 0.020” thick material will both
handle the hoop stress in the bellows, but each ply
in the multi-ply bellows will act individually where
sidewall bending is a concerned. This reduces the
spring rate and significantly increases cycle life.
Two-ply testable bellows with pressure gauges
Multi-ply
Bellows
Redundant Ply Bellows
or sometimes referred to as a two-ply testable bellows,
are used when it is necessary to monitor the
integrity of the bellows inner or outer ply. Not to be
confused with the multi-ply design, both bellows
plies are designed for the full pressure and temperature
cycles. If one ply fails, the second one will
take its place until a scheduled repair can be made.
The first style, referred to as “passive”, only monitors
the pressure between the bellows plies. If the
inner ply is breached, then the incident is detected
as an increase in pressure on the measuring device.
Lighter Spring Rates
Higher Cycle Life
Two-Ply Testable
Pressure gauges and pressure transducers
are the most common types of measuring
devices. The second style, referred to as “active”,
requires a vacuum between the plies. Depending
on the pressure reading, an inner or outer ply failure
can be detected.
7