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The figure on the right shows PBEXJ used when the direction of the pipe does not change. In this example two smaller bellows have opposite movement to the single larger bellows. When they are compressed, it extends. When they extend, the larger bellows is compressed. The sizes are chosen so the ratio of the mean diameter of the larger bellows to the diameter of the smaller bellows is equal to the square root of 2 and this PBEXJ maintains a constant volume when the bellows change length.The piping designer sizes the line bellows to accommodate changes in length expected in the piping system at the point where the PBEXJ is installed. The spring rates of both PBEXJs in Figures 1 and 2 are defined by the spring rates of the mechanically linked bellows.

Pressure is a used to describe the force transmitted by a fluid to the solid boundaries it contacts. It is expressed as a force per unit area and this force is at right angles to the surface at each point. Pascal’s Principle states that pressure (a force) applied to an enclosed fluid is transmitted to every portion of the fluid and to the walls of the containing vessel. Piping designers must consider all forces operating on the system. When the fluid is incompressible, forces can be transmitted rapidly. With a PBEXJ properly placed, forces which change the length of the bellows will not be transferred to sensitive connections of the piping to the system.

Mechanical changes caused by various factors are a major problem in piping design. During operation of an industrial process the following steps occur:

1. Process changes are made including changes in temperature, pressure and flow rate. Certain mechanical changes such as opening or closing a valve may be part of the control system used to make process changes. This often changes the INTERNAL WORKING PRESSURE inside the piping system.
2. Other mechanical changes, such as thermal expansion of the pipe, result from the process changes.

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