An Elbow Pressure Balanced Expansion Joint is designed to absorb externally imposed axial movement without imposing pressure loading on the system. This is accomplished by using two bellows both at line pressure tied together and acting in opposite directions. It is used where pressure loading on piping or equipment is not acceptable.
To move the joint itself is not balanced. Both the flow bellows and balancing bellows must be compressed or elongated in operation and the combined axial spring force acts on the piping or equipment. Since the forces needed to move the bellows of these joints are of low magnitude these are usually not objectionable.
The in-line pressure balanced bellows is a unique design which absorbs thermal motion while eliminating the pressure thrust loads on a piping system without a change in the direction of the piping consists of a constant volume device which is created by the addition of a balancing bellows whose cross sectional area is exactly twice the cross sectional area of the line or flow bellows.
By proper cross linking the change in the volume of the line or flow bellows due to change in the length (i.e. due to compression or extension) can be made to cause an equal but opposite change in the volume of the balancing bellows, Thus since the volume changes are of equal value the pressure thrust forces that are normally present in the piping system containing bellows expansion joints are eliminated. In-line pressure balanced expansion joint is used where the location of the expansion joint prohibits or makes it very costly to install main anchors.