Expansion joints must be very flexible in order to absorb axial movement. This inherent flexibility prevents the expansion joint from restraining longitudinal pressure loads and results in a force being exerted on the piping system known as pressure thrust.
Pressure thrust is calculated by multiplying the effective area of the bellows by the system pressure. With larger diameter bellows and/or higher pressure applications, the pressure thrust exerted by the bellows can be quite significant — hundreds of thousands of pounds.

If the ends of an expansion joint were capped and it was then pressurized, the bellows would extend freely until it formed a tube (its original shape). This type of unrestrained extension would destroy the expansion joint. To maintain the structural integrity of the piping system, the pressure thrust must be balanced or restrained. Often, pipe anchors can be designed to withstand the pressure thrust. If no axial movement is required, tie rods can be installed on the expansion joint to contain the pressure thrust. In some situations, where axial movement is required and pipe anchors are not practical, a pressure-balanced expansion joint is the solution.

Pressure-balanced expansion joints completely balance the pressure thrust within the expansion joint assembly through the addition of an opposing (or balancing) bellows.  This greatly simplifies piping anchor and support requirements. Turbines and other reciprocating equipment frequently require the implementation of pressure-balanced expansion joints to eliminate nozzle loads due to pressure thrust.