Metal Bellow

This expansion joint is simply a bellows element with end connections. Regardless of accessories, such as liners and covers, it will deflect in any direction or plane that the bellows will. It is the least expensive type, but requires that the piping be controlled as to the direction of the movements required of the unit. The expansion joint should not be expected to control the movement of the pipe. If the piping analysis shows that the expansion joint must accept axial compression, then the piping must be guided and constrained so that only that movement will occur. 

 

This expansion joint will not resist any deflections with any force other than the resistance of the bellows, which is a function of the spring rate times the deflection amount. It is incapable of resisting the pressure thrust along its axis, which is the product of the pressure times the effective, or cross sectional, area of the bellows. Large diameter units, even with low pressures, can generate very large axial pressure thrust forces, which must be reacted by main and directional anchors. Otherwise the expansion joint will extend with disastrous results.

The gimbal expansion joint is basically the same as the hinge type, except that instead of being limited to deflection in only one plane, it can accept bending or angulation in any plane. It contains two sets of hinge pins or pivots, the axis of each set perpendicular to the other. Each set of pins is connected to each other with a central gimbal ring, in much the same way that a universal joint on an automobile works. This unit provides the same type of restraint and resistance to axial forces, such as the pressure thrust, and to shear forces as the hinge type.

Hinged Expansion Joints contain hinges or pivots which allow the unit to bend in a single plane. These units are designed to restrict axial deflection, either in extension or compression. The hinge mechanism is typically designed to accept full pressure thrust. Also, because of the hinge mechanism's design, shear loads, such as from the weight of adjacent piping, can be accepted by this expansion joint, relieving the piping designer of having to provide additional supports and anchors required by the Single type.

Some hinge types can be provided with hinge pin holes which are slotted to permit limited axial travel. These "slotted hinge" types will not resist pressure thrust forces, and anchoring must be provided. This catalog only depicts the fully restrained hinged type. If axial travel is desired, the piping designer should clearly state that the slotted hinge type is wanted in the design requirements, and he should provide for pressure thrust anchoring. On the other hand, if the full axial restraint of the hinged type is desired, the piping designer should understand that there is no allowance in the expansion joint for any axial travel, including none for any installation misalignment.

When axial deflections exist, and anchoring is impractical for structural or economic reasons, such as high in the air or short straight pipe runs between two large vessels, the in-line pressure balanced expansion joint is a powerful solution to a difficult design problem. The principle of this type of unit is essentially the same as the pressure balanced elbow type, in that the axial pressure thrust is reacted by the pressure acting on a cross-sectional area equal to the area of the working or primary bellows. 

 

Since this unit is entirely axial, and there are no directional changes in the pipe, such as with the elbow in the previous discussion, the cross-sectional area needed to balance the pressure is placed around the outside of the unit. Since the pressure forces are generated by the pressure acting on the annular surface between the primary and outer, or balancing bellows, the arrangement of the tie rods transfers and balances the pressure thrust created in the pipe on each end. Now the forces needed to compress or extend the unit are only the result of the spring resistance of the bellows, and main anchoring of the pipe or vessels is not required.

 

This expansion joint is obviously more expensive than the simpler types; however, they may result in a lower overall system cost when the elimination of main anchoring is considered. This expansion joint can also be used to replace pipe loops, and its cost advantage may be seen in reduced pumping energy by the elimination of the loop's elbow-generated pressure losses. This may also allow reducing the pipe size for the entire system.

This type of expansion joint is really a combination of several types. Its purpose is to retain and balance the pressure thrust so that main anchoring of the pipe or adjacent equipment is not required, and forces and movements on attachment flanges of delicate equipment, such as turbines, are kept to acceptably low levels. The deflections to be accepted are handled by the proper type of expansion joint, which normally, and as shown in the above sketch, is a tied universal type to accept lateral movements. However, the pressure balanced elbow is usually required because axial deflections are also present. In order to accept these movements, a bellows is added beyond the elbow with the same cross-sectional area as the ones in the universal section.

 

This balancing bellows is connected by the tie rods to the pipe beyond the universal section; in this way the pressure thrust is contained as tension in the tie rods. The section of the expansion joint between the tie rods, which includes the elbow, is now free to move axially, with the only resistance being a function of the spring rates of the bellows. Because of their arrangement, however, the spring rate of the entire expansion joint is the sum of the spring rates of the balancing and the universal bellows. This is a constant volume system, in that when the universal end compresses, the balancing end extends the same amount. All of the lateral deflection is absorbed by the universal end, and there is no lateral deflection imposed on the balancing end. Therefore, the balancing bellows is almost always a single bellows type.

The universal expansion joint consists of two bellows separated by a pipe section or spool. The primary purpose of this arrangement is to have a unit which will accept large amounts of lateral deflection. The amount of lateral deflection they can accept is a function of the amount of angulations each bellows can absorb and the distance between the bellows. For a given bellows element, the amount of lateral deflection capability can be increased or decreased by simply changing the length of the center spool. In this catalog three standard overall lengths are given with their lateral movement capability. If the piping problem requires greater capability, then the overall length can be increased to suit.

 

Since deflections are usually the result of piping thermal expansion, and universal expansion joints are usually long, our units are designed so that the thermal expansion of the entire unit's length is accepted as compression by the two bellows elements. In this way, the overall length of the unit does not change when the piping is heated. The standard units in this catalog have all been designed to accept the thermal expansion of their length when the temperature is up to the design temperature shown.

Rectangular metal expansion joints have a variety of applications in the power, petrochemical, refining, chemical, and steel industries. Since there are no standard duct sizes, and due to the wide range of pressure and temperature combinations, each rectangular metal expansion joint is custom-engineered to provide the most economical design that will absorb the thermal movements of the system in which it is installed. Like circular expansion joints, rectangular expansion joints absorb three types of movement: axial, lateral and angular. For the purpose of designing rectangular bellows, it is critical to know in which direction the lateral and angular movements will occur, i.e. parallel to the long and/or short side of the bellows.

These may be cost-effective for large-diameter piping systems which operate at low pressure.  Metals can be selected to satisfy different temperature conditions.  The distribution of axial, angular, and lateral  forces will be different when thick-wall expansion joints are used.  We can provide your design engineers with the potential forces and movements for proper design of the structural members supporting the system.  These joints have a long life which justifies the initial investment.

Any standard-size bellows can be fabricated with multiply construction up to four plies of equal thickness. The maximum pressure, spring rate, and stability pressure are increased in direct proportion to the number of plies, but with the same axial deflection as a single ply. Multiply design permits a lower spring rate and a greater cycle life than a single ply configuration for an equivalent pressure. This type of construction is recommended for applications with vibration or rapid cyclic motion because of the inherent damping provided by the relative movement of the plies. Multiply construction in not recommended for vacuum application requiring a mass spectrometer helium leak test because of possible out-gassing from an undetectable leak in an inner ply.