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Piping Expansion Joints

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EXPANSION JOINTS
T. N. GOPINATH

1. INTRODUCTION

When piping lacks inherent flexibility due to routing and/or develops large reactions or detrimental overstrain on the strain sensitive equipments, the Piping Engineer considers provision of expansion joints to overcome the same. Expansion joints are also provided to isolate the vibrating equipment from piping and also to facilitate free movement of the equipment mounted on load cells.

2. TYPES OF EXPANSION JOINTS

The expansion joints can be slip type or the bellows type.

2.1 Slip Type of Expansion Joint In slip type of expansion joint one pipe slides into another and the assembly is sealed by means of packing between the sliding pipes. This device has the limitation that it permits only axial movement in the direction of pipe axis. Small amount of lateral and/ or angular movement will cause binding and eventually leakage. It is extremely difficult to seal it off completely. The limitations on packing makes it suitable only for very low temperature and low pressure services. Fig 2.1 indicates the general arrangement of a slip type expansion joint.
[pic]
Fig 2.1
2.2 Bellow Type Expansion Joint The bellow type expansion joint is extensively used as the most efficient and functionally reliable elongation compensator and/ or vibration isolator. These are capable of compensating for large amounts of axial and/ or lateral and/ or angular movements as a single unit. It lends itself to piping configurations that are much more compact than those using bends or loops to provide flexibility.

3. USAGE AND RESTRICTIONS

Based on the above, the point of usage could be identified as below.

• At the suction and discharge nozzles of vibrating equipments such as pumps, blowers etc.,
• On large diameter pipes and ducts operating at high temperatures but at lower pressures.
• In piping where the space is inadequate for conventional arrangement for providing flexibilities.

It is not advisable to use the expansion joint in all piping systems.

The major areas of applications where its use is not advisable are following piping systems.
• where hazardous chemicals are handled.
• where the service is high pressure.
• in which slurry or suspended solids are handled.

4. MATERIALS OF CONSTRUCTION

Based on the service for which the expansion joint is selected/used, the material of construction of the same is selected. Expansion joints are available in the following materials of construction.

° Rubber ° PTFE ° Canvas ° Metal

Except for the metallic expansions joints, all others are used to isolate the vibrating equipment from the ducting/piping. The selection is limited by pressure, temperature and the compatibility with the service fluid. The rubber expansion joints are available with single convolution or multiple convolutions. Metallic split flanges are provided as retaining lugs to ensure pressure tight seal. These are provided with tie rods which restrict the lateral movements. To ensure that no damage is done to the expansion joint, pipes should be anchored at change in pipe direction, branching of pipe, change in pipe size and end of pipe run. The PTFE expansion joints are mainly used in glass piping to ensure that no strain is transmitted to the pipeline. Such joints are provided at all equipment connections and change in direction on piping. Expansion joints made from canvas are not suitable for liquid service. These are used in very low pressure systems and can be used in services where operating temperatures are moderate. These are mainly used in ducting to isolate the vibrating equipment. Rubber, PTFE, Canvas expansion joints are designed and manufactured as per manufacturers’ standard. The metallic expansion joint is manufactured from austenitic stainless steel of required grade depending upon the service conditions. These are designed and manufactured as per E J M A (Expansion Joint Manufacturers Association) standard.

5. TYPES OF EXPANSION JOINT MOVEMENTS

It has been indicated that the expansion joint is used to absorb the movement in the pipeline. Let us consider the various possible movements to be absorbed by an expansion joint.

1. Axial Movement

The dimensional lengthening or shortening of a bellow parallel to its longitudinal axis is termed as the axial movement. (Refer Fig 5.1)

[pic]

Fig. 5.1

5.2 Lateral Deflection

The displacement of one end of the bellow expansion joint relative to the other end, perpendicular to the longitudinal axis is termed as lateral deflection. (Refer Fig. 5.2)

[pic]

Fig. 5.2

The deflection could be multidirectional as well. (Refer Fig. 5.3)

[pic]

Fig. 5.3

3. Angular Rotation

The displacement of the longitudinal axis of the expansion joint from its initial straight line position to a circular arc is termed as angular rotation. (Refer Fig 5.4)
[pic]

Fig. 5.4

4. Torsional Movement

The twisting of one end of the expansion joint with respect to the other end is called the torsional movement (Refer Fig. 5.5). Torsional movement imposes severe stresses in the expansion joint and special care has to be taken for the use of expansion joint for such application.

[pic]

Fig. 5.5

6. COMPONENTS AND

ACCESSORIES

Expansion joints are used in the industry for multiple applications. To perform such functions and to meet complex requirements of industrial applications, there are various additions to the basic components and accessories of an expansion bellow. (Refer Fig. 6.1 and 6.2)

[pic]

Fig. 6.1

[pic]

Fig. 6.2

6.1 Bellow Bellow is the corrugated portion of the expansion joint responsible for absorbing movement. Bellow consists of a number of convolutions, which is directly proportional to the total movement to be absorbed by the bellows. The top most position of the convolution is called the crest and the bottom most portion is called the root. The depth of the convolution is the total height of the same and is the distance between the crest and the root. The average of the diameters of the crest and the root is termed as the mean diameter of the bellow. When the internal pressure demands a higher thickness of bellow, the flexibility gets reduced. To overcome this, bellows are made with multi ply thin wall sections. This will permit larger movements withstanding high pressure and will provide same service life.

6.2 Tangent The straight portion at the two ends of the bellows on which the end connections are made is termed as tangent. The end connections could be provided with beveled joint to connect the expansion joint to the piping by welding. It could be provided with either welded or loose flanged connections depending on the piping specification.

6.3 Collar This is a ring of suitable thickness by which the bellow is secured to the tangent. This prevents the bellow from bulging due to pressure.

6.4 Reinforcing Rings When bellows are meant to be used for high pressure service, reinforcing rings made out of either tubing or solid bar will be fitted strongly to the roof of the convolutions. This is considered as a safety measure to ensure that the convolutions do not open out due to extra pressure that gets applied occasionally.

6.5 Lagging Shroud This is an external cover provided over the bellows. In addition to providing protection to the bellows from mechanical damage, this also prevents the insulation material from entering the root of the convolutions which may prevent the bellow from functioning. This is also termed as cover or external shroud.

6.6 Internal Sleeves This is a thin pipe section placed inside the bellows to prevent contact between the inner surface of the bellows and the fluid flowing through it. This device is provided to protect the bellows convolutions from damage due to resonant vibration when the fluid velocity is high. This will also prevent erosion when the service involves abrasive media. While installing a bellow with internal sleeve, care should be taken to mount the same in proper direction with respect to the direction of flow. The internal sleeve is sometimes referred to as liner as well.

6.7 Limit Rods To restrict the movement of the bellow in axial, angular or lateral direction during the normal operation, solid rods are provided to space the bellow assembly. These are designed to prevent the bellows from over extension or over compression by dynamic loading generated due to the pressure loading. These are used in pressure balanced type of joints.

6.8 Tie Rods These are solid rods or bars spacing the bellow assembly provided to restrict the axial movement and permitting the lateral deflection during the normal operation. These are suitably designed to absorb the pressure thrust due to internal pressure.

6.9 Shipping Devices This device is provided as a protection against damages which can occur to the assembly during transportation. This will maintain the installation tight by keeping configuration of the convolution. Care should be taken to keep this device till the lines are hydro tested and removed prior to start up of the system.

6.10 Pantographic Linkages This arrangement is done in hinged or gimbal type expansion joints. This is a scissor like device, which will allow rotational movement while restricting the axial and lateral movement of the bellow.

7. TYPES OF EXPANSION JOINTS

There are different types of expansion joints manufactured to take care of various requirements of the industry. The various types available are as follows:-

• Axial - Single/ Double
• Universal
• Swing
• Hinged
• Gimbal
• Pressure Balanced
• Tied

A brief description of these is given below.

1. Axial Expansion Joint

7.1a AXIAL- SINGLE EXPANSION JOINT

[pic]

Fig. 7.1a

This is the simplest form of expansion joint of single bellows construction. It absorbs all of the movement of the pipe section into which it is installed. These bellows are capable of absorbing only small amounts of lateral or angular movement.

7.1b AXIAL- DOUBLE EXPANSION JOINT

[pic]

Fig. 7.1b

A double expansion joint consists of two bellows jointed by a common connector which is anchored to some rigid part of the installation by means of an anchor base. The anchor base may be attached to the common connector either at installation or at the time of manufacturing. Each bellow of a double expansion joint functions independently as a single unit. Double bellow expansion joints should not be confused with universal expansion joints.

2. Universal Expansion Joint

[pic]

Fig. 7.2

A universal expansion joint is one containing two bellows joined by a common connector for the purpose of absorbing any combination of three (3) basic movements. A universal expansion joint is used in cases where it is necessary to accommodate greater amounts of lateral movement than can be absorbed by a single expansion joint.

7.3 Swing Expansion Joint

[pic]

Fig. 7.3

A swing expansion joint is designed to absorb lateral deflection and/or angular rotation in one plane only by the use of swing bars, each of which is pinned at or near the ends of the unit.

7.4 Hinged Expansion Joint

A hinged expansion joint contains one bellow and is designed to permit angular rotation in one plane only by the use of a pair of pins running through plates attached to the expansion joint’s ends. Hinged expansion joints should be used in sets of 2 or 3 to function properly.

[pic]

Fig. 7.4

5. Gimbal Expansion Joint
[pic]
Fig 7.5

A gimbal expansion joint is designed to permit angular rotation in any plane by the use of two pairs of hinges affixed to a common floating gimbal ring.

7.6 Pressure Balanced Joint
A pressure balanced expansion joint is designed to absorb axial movement and/or lateral deflection while restraining the bellows pressure thrust force by means of the devices interconnecting the flow bellow with an opposed bellow also subjected to line pressure. This type of joint is installed where a change of direction occurs in a run of pipe, where it is not possible to provide suitable anchors.
[pic]

Fig. 7.6

7.7 Tied Expansion Joint These are bellows provided with tie rods to restrict axial movement, while the bellow is subjected to high pressure services. Tie rods can be provided on single, universal or pressure balanced type of expansion joints.

Selection Chart
|Sr. |Type of |Axial |Lateral |Angular |Elimination |
|No. |Expansion Joint |Movement |Movement |Rotation |of Pr. Thrust |
|1 |Axial |Yes |No |No |No |
|2 |Universal |Yes |Yes |Yes |No |
|3 |Swing |No |Yes |Yes |Yes |
|4 |Hinged |No |No |Yes |Yes |
|5 |Gimbal |No |No |Yes |Yes |
|6 |Pressure Balanced |Yes |Yes |No |Yes |
|7 |Tied |No |Yes |No |Yes |

The selection of a proper expansion joint involves a number of variables, including piping configuration, the operating conditions, derived cycle life, load limitations on the equipment etc. The major factor is the unique character available with the type of design, which makes it suitable for a particular application. The selection chart will facilitate a selection. Before discussing the applications of the various types of expansion joints, it is required to derive formulas, which contain terms which are of common use and which are required for the proper selection and application of the various types of expansion joints.

8. GLOSSARY OF TERMS
8.1 Pipe Anchor The purpose of anchor is to divide a pipeline into individual expanding/ contracting sections. The function of pipe anchor is to limit and control the movement with expansion joints located between the anchors absorbing the same.

8.2 Main Anchor Main anchor is located at any of the following points in a pipe section.
• Between two bellow units installed on the same pipeline.
• Change in direction as at elbows when the advantage of elbow is not considered in flexibilities.
• At the entrance of a side stream into main pipeline i.e. in “T” section.
• At bend ends of pipe containing bellows. Main anchor should be so designed that it is capable of withstanding forces and moments imposed by the pipe section between which bellows are located. When main anchor is installed at the pipe bend, the centrifugal thrust as a result of change in direction of flow should also be considered.

8.3 Intermediate Anchor These are anchors provided in between the main anchor dividing the pipeline into individual expanding pipe sections.

8.4 Pipe Guides Pipe guides are those, which permit axial movement while preventing angular or lateral movement. These are of significant importance for proper functioning of the expansion joint.

8.5 Directional Stop/Anchor Directional stop/anchor is a device, which is designed to absorb loading in one direction while allowing the movement in another.

8.6 Spring Rate This is a measure of bellows flexibility. It is the force required to extend or compress the bellow per unit length in the axial direction parallel to its longitudinal axis. It is expressed in kg/mm or lbs/in.

8.7 Spring Force While absorbing the movements, the bellow imparts forces and moments to the piping system, which should be absorbed by proper provision of support and structures. This force is the product of the deflection absorbed and the spring rate of the bellow.

8.8 Pressure Thrust This is the force due to internal pressure acting to open out the bellows. The magnitude of the pressure thrust is the product of the system pressure and the area at mean diameter of the bellow. In case of positive pressure, the convolutions are pushed apart causing the bellows to elongate while the case is reverse in the case of an external pressure.

[pic]

Fig 8.1

8.9 Cycle Life This is defined as the number of movements an expansion joint is able to perform from the initial position to the

operating position and then return to initial position before it fails.

Fig. 9.1 Pipe Guide Location

9. APPLICATION The knowledge of the application of the various types of expansion joints is important, as it is required for the selection. The location of anchors as well as guides are also very important for the proper functioning of the same. The general guideline used in the location of guides is that the first pipe guide must be located within a distance of four pipe diameters from the end of the bellow and the second guide must be located within a distance of fourteen pipe diameter from the first guide. The subsequent support could be placed at the maximum span allowed as per the pipe size and the service for which it is meant for.

1. Axial Expansion Joints Tied/Untied
[pic]
Fig. 9.2
The axial expansion joint could be single or double with an intermediate anchor. Because it offers the lowest expansion joint cost, the single expansion joint is usually considered first for any application. Fig 9.2 shows a typical application of a single expansion joint absorbing combined axial movement and lateral deflection. The expansion joint is located at one end of the long piping leg with main anchors at each end and guides properly placed for both movement control and protection of the piping against buckling. In this case, however, the anchor at the left end of the line is a directional main anchor, which while absorbing the main anchor loading in the direction of the expansion joint axis, permits the thermal expansion of the short piping leg to act upon the expansion joint as lateral deflection. Because the main anchor loading exists only in the piping segment containing the expansion joint, the anchor at the end of the shorter piping leg is an intermediate anchor.

[pic]

Fig 9.3

Fig 9.3 shows an alternate arrangement in which the expansion joint is installed in the short piping leg and the principal expansion is absorbed as lateral deflection. Note that in this case, the longer piping leg is free of compressive pressure loading and requires only an intermediate anchor and directional guiding. The functions of the directional anchor and the pipe guide may be combined in a single device.

[pic]

Fig. 9.4

Fig. 9.4 and 9.5 represent modifications over Fig. 9.3 in which the main anchors at either end of the expansion joint are replaced by tie rods. Where the piping configuration
[pic]

Fig. 9.5

Permits, the use of tie rods frequently simplifies and reduces the cost of the installation. Because of these tie rods, the expansion joint is not capable of absorbing any axial movement other than its own thermal expansion. The thermal expansion of the piping in the shorter leg is, as a result, imposed as deflection on the longer piping leg. In some cases, where the longer piping leg is not sufficiently flexible and where the dimension of the shorter leg is suitable, the rods may be installed spanning the entire short leg so that no deflection is imposed on the longer run from this source. Where appreciable amounts of lateral deflections are imposed upon the expansion joint, some shortening of the expansion joint results from the displacement of the tie rods as shown in Fig. 9.4 Care should be taken to insure that sufficient piping flexibility exists to absorb this deflection and that adequate clearances are provided in the guide to permit deflection of the piping. The amount of this deflection can be minimized by cold springing the expansion joint in the lateral direction as shown in Fig.9.5 The principal restriction upon the use of single expansion joint for lateral deflection or combined axial movement and lateral deflection is the limited amount of lateral deflection, which such an expansion joint can absorb. The allowable lateral deflection is directly proportional to the ratio of corrugated length to diameter, which, in turn, is restricted by considerations of stability and manufacturing limitations. Thus, while eminently suitable for applications such as Fig 9.2 where the principle movement is axial, the relatively small available lateral movement severely limits the type of application illustrated in Fig 9.3, 9.4 and 9.5. Where operating pressures and temperatures are high, or where availability of suitable structures precludes the use of main anchors and multiple guides, the application shown in Fig 9.2 may not be feasible and another type of expansion joint may result in a far more economical installation.

9.2 Universal Expansion Joints The universal expansion joint is particularly well adapted to the absorption of lateral deflection. In addition, this design may be used to absorb axial movement, angular rotation or any combination of the three. The most common application of the universal expansion joint is its use as a tied expansion joint in a 90 degree piping offset., with the tie rods adjusted to prevent external axial movement. Two such applications are shown in Fig. 9.6 and 9.7
[pic]

Fig. 9.6

[pic]

Fig. 9.7

Fig 9.6 shows a tied universal expansion joint used to absorb lateral deflection in a single plane “Z” bend. Where dimensionally feasible, the expansion joint should be designed to fill the entire offset leg so that its expansion is absorbed within the tie rods as axial movement. The thermal movement of the horizontal lines is absorbed as lateral deflection by the expansion joint. Both anchors are intermediate anchors since the pressure loading is absorbed by the tie rods. Only directional guiding is required since the compressive load on the pipe consists only of the force necessary to deflect the expansion joint. Any thermal expansion of the offset leg external to the tie rods, such as that of the elbows at either end, must be absorbed by bending of the horizontal pipe legs. Provision should be made in the design of the guides to allow for both this deflection and the reduced length of the expansion joint in its deflected position. In addition, particularly in the case of long universal expansion joints under high pressure, additional allowances may be necessary to compensate for stretching of the tie rods under load. The expansion joint manufacturer should be consulted for recommended minimum guide clearances. Fig 9.7 shows a typical application of a tied universal expansion joint in a three-plane “Z” bend. Since the universal expansion joint can absorb lateral deflection in any direction, the two horizontal piping legs may lie at any angle in the horizontal plane.
[pic]

Fig 9.8

In cases where a universal expansion joint must absorb axial movement other than its own thermal growth, it cannot function as a tied expansion joint and must be used in combination with main anchors to absorb pressure loading. One such case is shown in Fig 9.8 The relative expansion between the two vessels results in both axial movement and lateral deflection on the expansion joint. Both vessels must be designed to absorb main anchor loading. Limit rods may be used to distribute the movement between the bellows and to control their movements. As a direct result of increasingly high operating pressures and temperatures, and lighter building construction methods, universal expansion joints are finding increasing use in steam and hot water distribution systems where, due to their ability to absorb large amounts of movement with minimum guiding and anchoring, they offer impressive savings in overall cost. Numerous variations are possible in the design of universal expansion joints. In a horizontal installation, for example, where it is desirable to support the center pipe section of the expansion joint independently of the bellows, tie rods or external structural members may be used. In a single plane system, the tie rods may be placed by two bars with pinned connections at either end of the expansion joint. This construction is so commonly used that it has been given the standard nomenclature of “swing expansion joint”. In some cases, two sets of short control rods, one spanning each of the two bellows in the universal expansion joint, are used instead of the overall tie rods shown in most of the illustrations. This arrangement is frequently used where the expansion joint must absorb axial movement and where the control rods are used primarily for control and stability rather than for absorption of pressure loading. Where the universal expansion joint is very long in relation to its diameter, where a large number of corrugations are used at each end of the expansion joint or where the expansion joint is subject to external forces such as wind loading, vibrations, etc. it may be desirable to incorporate control devices in the expansion joint to prevent excessive displacement of the bellows and the relatively free pipe section between them.

Fig. 9.9 and 9.10 show two forms of controls which may be used for this purpose. In Fig 9.9, short rods are used spanning each of the bellows in the expansion joint. Stops are provided on the rods so that, once the expansion joint has reached its rated lateral deflection, the stops will be engaged by members rigidly fastened to the pipe portions of the expansions joint and no further displacement will be possible.

[pic] [pic] Fig. 9.9 Fig.9.10

Fig 9.10 shows a similar device adapted to an expansion joint with overall tie rods. In this case, the rods’ tops are engaged by a plate or lug attached to the center pipe portion and movement of this part beyond its design deflection is prevented. In order to obtain maximum control from these devices, the stops are usually oriented to lie in the plane of resultant movement of the expansion joint, affording maximum leverage as well as greater sensitivity to small movement. Devices of this nature are usually stipulated by the manufacturer when the design characteristics of the expansion joint warrant. Despite the versatility of the universal expansion joint, its use is sometimes precluded by the configurations of the piping, the operating conditions or even by manufacturing and transportation limitations. Where, for example, the length of the offset leg in a “Z” bend is extremely long, it may be undesirable or impossible to fabricate, ship to the job site and install a universal expansion joint which would span the full length of the offsets. Further, where the expansion joint is very long in relation to its diameter, the flexibility of overall rods may reduce the effectiveness of the control so that the center pipe section becomes unstable. Where such limits are encountered, other types of expansion joints may offer a more desirable solution.

9.3 Pressure Balanced Expansion Joints The pressure balanced expansion joint is used most frequently in applications similar to those shown for the single expansion joint, but where pressure loading upon piping or equipment is considered excessive or objectionable. The major advantage of the pressure balanced design is its ability to absorb externally imposed axial movement without imposing pressure loading on the system. It should be noted, however, that the force required to move the expansion joint is not balanced. In fact, it is increased over that of a single expansion joint. Since both the flow bellows and the balancing bellows must be compressed or elongated, the combined axial force acts upon the piping or equipment. Since the forces to move the bellows are generally of a low order of magnitude, these are usually not objectionable, except in cases involving extremely light equipment with close clearance moving parts which might be affected by small forces.

[pic]

Fig 9.11

Fig 9.11 shows a typical application of a pressure-balanced expansion joint for combined axial movement and lateral deflection. Both the anchor at the end of the piping run and that on the turbine are intermediate anchors and only directional guiding is required. By proper design, the guide directly above the turbine can be made to absorb the axial movement forces of the expansion joint without imposing these on the turbine. The only force imposed on the turbine is that which is required to deflect the expansion joint laterally.
[pic]

Fig 9.12

Fig 9.12 shows another turbine application, but, in this case, the anchor point of the turbine is located some distance from the expansion joint and the expansion of the turbine between its anchor and the expansion joint is absorbed as lateral deflection. An intermediate anchor is used at the center fitting of the expansion joint. Since the expansion joint is located close to the turbine, guiding between the turbine and expansion joint is not required.

[pic]
Fig 9.13

Figure 9.13 shows that a pressure balanced expansion joint can be used at changes in direction other that 90 degrees. In this case, the growth of the longer piping run is absorbed as axial movement on the expansion joint, while the thermal expansion of the offset piping run introduces both axial and lateral components of deflection on the expansion joint. Again, only intermediate anchors are required at the ends of the lines and directional guiding is used. The guide on the offset run may be used to absorb the axial movement forces of the expansion joint, if the piping is not sufficiently stiff to transmit this directly to the intermediate anchor.
[pic]

Fig 9.14

Fig 9.14 shows a common application for which a pressure balanced expansion joint is well suited. Under various process conditions, the vessel and the vertical pipe may expand at different rates. By installing a pressure balanced expansion joint, as shown, the differential vertical movement is absorbed as axial movement in the expansion joint and the thermal expansion from the center line of the process vessel to the piping is absorbed as lateral deflection. The piping may then be secured by an intermediate anchor at the bottom and furnished with a directional guide adjacent to the expansion joint. as shown. In many cases no external structure is available at the upper elevation of the process vessel and the guide must be connected to the vessel itself. Using this arrangement may especially help where the vessel is tall and is subject to wind loading deflection, or similar effects where the guide is attached to a rigid external structure. The expansion joint must be designed to absorb wind loading deflection, etc., as lateral deflection are involved, Pressure balanced universal expansion joints are used in the flow end of the expansion joint and a single bellow in the balancing end. Normally, as shown in Fig. 9.15, the balancing bellows will be subjected to axial movement only if the tie rods are properly designed to rotate or pivot at their attachment points.
[pic]

Fig 9.15

In order for a pressure balanced expansion joint to function properly, the pressure thrust restrained by the tie rods must exceed the axial movement forces of the expansion joint. In a large diameter, low pressure application, it may be impossible to utilize the pressure balanced expansion joint to eliminate the pressure loading or at best, the effect may be uncertain. In such cases, some other expansion joint design must be considered.
Pressure balanced expansion joints are not recommended for use in services where the pressure equalizing connections between the flow bellows and the balancing bellows may become plugged or blocked by the flowing medium or contaminants. Where flow considerations permit, this problem may be overcome by the use of a tee as a center fitting of the expansion joint rather that an elbow. In some cases, the pressure for the balancing end of the expansion joint has been introduced from a separate pressure source. A control failure or even a slow control response might result in partial or full pressure loading being imposed upon the piping or equipment, thus defeating the initial reason for using the pressure balanced expansion joint. From the view point of cost, it must be considered that the pressure balanced expansion joint requires the use of an extra bellow which does not add to its ability to absorb movement. In addition, the expansion joint is usually furnished with a center fitting, either elbow or tee, which would otherwise constitute a portion of the piping cost. Further, the necessary structure i.e blind flanges, tie rods and attachment structures add appreciably to the cost of the expansion joint. The use of a pressure balanced expansion joint can be justified economically only where the problems created by the pressure loading represent an even greater cost. The pressure balanced expansion joint is finding increasing use for the sole function of relieving loads upon equipment such as pumps, compressors and turbines. In many cases, the cost of the pressure balanced expansion joint will be negligible when compared to the cost of additional equipment, piping and building space which would be necessary for safe functioning of the equipment without the expansion joint.

9.4 Hinged Expansion Joints Hinged expansion joints are usually used in sets of two or three, to absorb lateral deflection in one or more directions in a single plane piping system. Each individual expansion joint in such a system is restricted to pure angular rotation by its hinges. However, each pair of hinged expansion joints, separated by a segment of piping will act in unison to absorb lateral deflection in much the same manner as a swing or universal expansion joint in a single plane application. For a given angular rotation of the individual expansion joint, the amount of lateral deflection which a pair of hinged expansion joints can absorb is directly proportional to the distance between hinge pins. Therefore, in order to utilize the expansion joints most efficiently, this distance should be made as large as possible.
Expansion joint hinges are normally designed to absorb the full pressure thrust of the expansion joint and, in addition, may be designed to support the weight of piping and equipment, wind loads or similar externally applied forces. Where such external forces are anticipated, their direction and magnitude must be indicated to the expansion joint manufacturer so that the hinges can be adequately designed to withstand these forces.
[pic]

Fig 9.16

Fig 9.16 illustrates the use of a two hinge system to absorb the major thermal expansion in a single-plane “Z” bend. Since the pressure thrust is absorbed by the hinges on the expansion joints, only intermediate anchors are required at each end of the piping system. The thermal expansion if the offset section containing the expansion joints must be absorbed by bending of the piping legs perpendicular to that segment, since the expansion joints are restricted to pure angular rotation by their hinges and cannot extend or compress. The amount of bending deflection imposed on each of the two long piping legs may be controlled by proper design of guides and supports. Where one long leg is sufficiently flexible to absorb the full thermal growth of the offset leg, the other long leg may be controlled to permit longitudinal movement only. The planar guides shown at the ends of the long piping runs near the elbow are intended to maintain the planarity of the piping system only and must of course allow for the bending deflections of the long piping legs. In calculating guide clearances, consideration should be given to the fact that the thermal expansion of the offset piping leg containing the expansion joints will be partially offset by the reduction in length resulting from the displacement of the center pipe section. The latter effect may be neglected only where the distance between hinge pins is very large and the lateral displacement small. This effect can be minimized by cold springing the expansion joints 50% of the full rated deflection. Because of the ability of the hinges to transmit loads, support of a hinged piping system can frequently be simplified. Assuming that Fig. 9.16 is an elevation view, for example and that the upper piping leg is sufficiently flexible to absorb the total expansion of the vertical leg, it would be possible to use sliding supports on the lower horizontal run to support its weight and restrict it to longitudinal movement only. By utilizing the rigidity of the hinges, a substantial portion of the weight of the upper horizontal leg may also be carried on these lower supports. It should be noted, however, that the sliding support nearest to the vertical leg must be designed to resist the force required to deflect the piping. Spring supports must be used throughout the length of the upper horizontal leg where bending occurs. Beyond that point, sliding supports may be used. In locating hinged expansion joints for more efficient use, it should be noted that the hinges need not be collinear in order to function properly.

[pic]

Fig 9.17

Fig 9.17 illustrates a two-hinge expansion joint system similar to the pressure balance expansion joint application of Fig 9.14. In this case, the expansion joints will absorb only the differential vertical growth between the vessel and the pipe riser. Any horizontal movement due to piping expansion, vibration, wind loads etc., will be absorbed by the bending of the vertical pipe leg. A planar guide may be installed near the top of the vessel to protect the hinged expansion joints from wind loads at right angles to the plane of the piping. The anchor shown at the bottom of the riser is an intermediate anchor only. The pressure load is absorbed by the expansion joint hinges. However, this anchor must be capable of withstanding the forces created by bending of the riser. Depending upon the dimensions and weight of the piping system, complete support may be obtained from the process vessel and from the intermediate anchor. If additional supports are required , spring type supports should be used. If desired, the vertical piping may be cold sprung to reduce bending stresses, utilizing the hinges to withstand the cold spring force. Where the piping in a single plane system is not sufficiently flexible to absorb the bending deflections involved in a two hinge system, or where the loads resulting from such bending exceed the allowable limits for connected equipment, a system of three hinged expansion joints may be used.

[pic]

Fig 9.18

Fig 9.18 illustrates a system of three hinged expansion joints in a single plane “Z” bend. The thermal expansion of the offset piping section is absorbed by the action of expansion joints B and C. It is therefore evident that expansion joint B must be capable of absorbing the total of the rotations of expansion joints A and C. Hence, it is frequently necessary that the r expansion joint at the center contain a greater number of corrugations than those at either end. As in the previous cases, the anchors at the ends of the piping system are intermediate anchors only. In this case, all deflection is absorbed by the expansion joint and no pipe bending loads will be imposed upon these anchors. Where the distance between the anchor at the left and the first hinged expansion joint C is large, a pipe guide should be installed adjacent to the expansion joint, as shown in Fig.9.18. This pipe guide will minimize bending of the pipe section between expansion joint C and the left hand anchor, which might otherwise result from the moment required to rotate the expansion joint. One or more additional guides may be used to maintain the planarity of the piping system and relieve the hinges of bending forces, which may be created by external loads. Support of the piping system may be effected in various ways, utilizing available supporting structures with greatest efficiency. Here again, however, it is essential that spring supports be used to permit free movement of the piping between the expansion joints. [pic]

Fig.9.19

Fig 9.19 illustrates the principle that systems of hinged expansion joints may be used in other than 90o bends. In such applications, a three hinge system is usually most suitable, since the components of movement may be quite large and excessive bending stresses would result from the use of a two hinge system. Except for this point, the system is similar in every respect to the previous ones containing 90o bends. Only intermediate anchors and planar guides are required.

[pic]

Fig. 9.20

A hinged expansion joint system may be used effectively in applications involving movement other that the pure thermal growth of piping. Fig. 9.20 illustrates an application combining the thermal expansion of piping system with the single plane movements of a piece of connected equipment. So long as all movements are restricted to a single plane, the behavior of the expansion joint system is quite similar to that of the system shown in Fig. 9.18. In this case, an intermediate anchor is required at one end of the piping. The equipment serves as an intermediate anchor at the opposite end. The displacements of the equipment are totaled with those of the piping in order to evaluate the movements of the expansion joints. Planar guide clearances in the plane of the piping must be adequate to allow for the equipment movement as well as for the piping rotations.
Among the major advantages of hinged expansion joints are their compact size, which facilitates installation, and the great rigidity and strength which can be incorporated into the hinge structures. By the use of these individual units, it is frequently possible to compensate for the thermal expansion of irregular and complex piping configurations which might preclude the use of other types of expansion joints. Because of the ability of the hinge structure to transmit loads, piping system containing hinged expansion joints impose minimum force on the pipe anchors. Furthermore, such systems may be supported at virtually any point which does not interfere with the free movement.

9.5 Gimbal Expansion Joints Just as hinged expansion joints may offer great advantages in single plane applications, gimbal expansion joints are designed to offer similar advantages in multiplane systems. The ability of the gimbal expansion joint to absorb angular rotation in any plane is most frequently applied by utilizing two such units to absorb lateral deflection. An application of this type is shown in figure 9.22. Since the pressure loading is absorbed by the gimbal structure, intermediate anchors only are required. Planar guides are provided to restrict the movement of each piping leg. As in the case of hinged expansion joints, the location of pipe supports is simplified by the load carrying ability of the gimbal structure. Since, in a two gimbal system, the growth of the vertical pipe leg will be absorbed by bending of the long legs, spring supports may be required on either or both of these. Guides must be designed to allow for the thermal expansion of the leg containing the expansion joints and for the shortening of this leg due to deflection. Where it is impossible or undesirable for the piping to absorb the growth of the offset leg, a system consisting of two gimbal and one hinged expansion joints may be used as shown in Fig. 9.21.

[pic]

Fig. 9.21

[pic]

Fig. 9.22

The gimbal expansion joints function in unison to absorb the combined movements of the upper and lower legs, while the hinged expansion joints and the upper gimbal expansion joint act in combination to absorb deflection of the offset leg. Since the expansion of the offset leg takes place in one plane only, the use of the simpler hinged expansion joint is justified. The advantages of using gimbal expansion joint system are simpler to those previously mentioned for systems containing hinged expansion joints. Greater flexibility of usage is, however possible since gimbal expansion joints are not restricted to single plane systems.

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