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Lateral Expansion Joint
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Angular (Hinged) Expansion Joint
Topic:
Is it necessary to install an expansion joint at pump discharge piping?
Is it necessary to install an expansion joint at pump discharge piping?
Expansion joints can be of great benefit in a pumping system when used properly. They are, however, only needed when pipe strain is present. In addition, the correct installation of an expansion joint is also necessary.
It is not simply a matter of bolting them into the pipeline. This merely transfers the strain through the joint and onto the pump casing, which will ultimately cause seal and bearing failure. In order to protect the pump from such problems, it is necessary to independently secure the end of the expansion joint closest to the pump. This allows the expansion joint to absorb the entire strain coming from the pipeline and thus protects the pump.
It is not simply a matter of bolting them into the pipeline. This merely transfers the strain through the joint and onto the pump casing, which will ultimately cause seal and bearing failure. In order to protect the pump from such problems, it is necessary to independently secure the end of the expansion joint closest to the pump. This allows the expansion joint to absorb the entire strain coming from the pipeline and thus protects the pump.
2 - The use of expansion (or flexible) pipe joints in a pump system that has no excessive temperature differential problems is only necessary when the pipe fitters installing the piping are unable or unwilling to install the piping without imposing any strain on the pump nozzles.
When they are used, it should be identified why theyre being used. The most common reason is to protect the pump against pipe strain that may be imposed by the movement of the piping during system operation. In such case, the side of the flexible pipe joint closest to the pump needs to be independently supported and secured against movement. This allows all the movement of the piping to be absorbed by the flexible joint, and none of it transferred to the pump.
Another thing to note is that an expansion joint will not necessary accommodate radial movement of the piping. Some of them are designed to accommodate only axial movement where the pipe and pump flanges are trying to get closer together or further apart. These are of no value against poor piping misalignment or thermal movement of the piping.
When they are used, it should be identified why theyre being used. The most common reason is to protect the pump against pipe strain that may be imposed by the movement of the piping during system operation. In such case, the side of the flexible pipe joint closest to the pump needs to be independently supported and secured against movement. This allows all the movement of the piping to be absorbed by the flexible joint, and none of it transferred to the pump.
Another thing to note is that an expansion joint will not necessary accommodate radial movement of the piping. Some of them are designed to accommodate only axial movement where the pipe and pump flanges are trying to get closer together or further apart. These are of no value against poor piping misalignment or thermal movement of the piping.
What is PTB-3
ASME problem sample manuals PTB-3 and PTB-4 are well kept secrets. The samples that used to be in the back of ASME VIII-1 in Appendix L have been changed, expanded and published as PTB-4. The ASME VIII-2 rewritten in 2007 got its own new PTB-3 problem sample manual in 2010. PTB-3 contains worked examples with numerical results. Although meant more as an educational guide than a verification set, here we compare our own results in both ABAQUS and SolidWorks against published PTB-3 results.
ASME problem sample manuals PTB-3 and PTB-4 are well kept secrets. The samples that used to be in the back of ASME VIII-1 in Appendix L have been changed, expanded and published as PTB-4. The ASME VIII-2 rewritten in 2007 got its own new PTB-3 problem sample manual in 2010. PTB-3 contains worked examples with numerical results. Although meant more as an educational guide than a verification set, here we compare our own results in both ABAQUS and SolidWorks against published PTB-3 results.
PTB-3 Example E5.2.1 and E5.3.2
PTB-3 example E5.2.1 “Elastic Stress Analysis” covers the correct use of stress linearization and provides numerical results. The same model is used for sample E5.3.2 “Elastic Analysis”. Here both are run.
PTB-3 example E5.2.1 “Elastic Stress Analysis” covers the correct use of stress linearization and provides numerical results. The same model is used for sample E5.3.2 “Elastic Analysis”. Here both are run.
[From E5.2.1] Evaluate the vessel top head and shell region for compliance with respect to the elastic stress analysis criteria for plastic collapse provided in [VIII-2] paragraph 5.2.2. Do not include the standard flanges or NPS 6 piping in the assessment for compliance to allowable stresses. Internal pressure is the only load that is to be considered. Relevant design data and geometry are provided below and in Figures E5.2.1-1 and E5.2.1-2.
In other words analyse the head and a nozzle in the top of a pressure vessel to determine its acceptability against ASME code rules for FEA. The instructions for E5.3.2 are:
Evaluate the vessel top head and shell region given in Example Problem E5.2.1 for compliance with respect to the elastic and elastic-plastic local failure criteria provided in [VIII-2] paragraphs 5.3.2 and 5.3.3. The same model and material conditions were used as in Example Problem E5.2.1.
Methods
This example provides enough dimensional and material information to attempt to duplicate the results. Exactly matching the published results is not possible because not all not all model geometry is given and some linearization locations are not exactly provided. The 2D 8 node ABAQUS element type CAX8R was provided, however mesh sizes were missing. Where information exists, we replicated the PTB-3 exactly. Where information is missing, we tried to get a model that looked similar to the one in the publication. Given these limitations, we hoped for results that match PTB-3 with less than 5% error.
The scope of study in Examples E5.2.1 and E5.3.2 is the shell, head and nozzle. These are symmetric about the centerline allowing a 2D axisymmetric analysis to be chosen by the authors. This reduced the complexity of the analysis and allows a refined mesh to be used. Most model dimensions were provided in drawings E5.2.1-1 and -2. We re-created the 2D model geometry in SolidWorks. Where model dimensions were not available, we made our model visually match the published drawing. A link to a drawing of our model is provided in the resources section below.
We used the same model in both ABAQUS – the software used by the authors and SolidWorks Simulation (SWS). We inferred the mesh size used by counting the number of elements in areas of known dimensions. We used this size of 0.015″ in both programs. The materials were modeled using the two different material moduli as outlined in PTB-3. The exact location of the change in modulus was not given, so we chose SCL #4 as the transition.
This example provides enough dimensional and material information to attempt to duplicate the results. Exactly matching the published results is not possible because not all not all model geometry is given and some linearization locations are not exactly provided. The 2D 8 node ABAQUS element type CAX8R was provided, however mesh sizes were missing. Where information exists, we replicated the PTB-3 exactly. Where information is missing, we tried to get a model that looked similar to the one in the publication. Given these limitations, we hoped for results that match PTB-3 with less than 5% error.
The scope of study in Examples E5.2.1 and E5.3.2 is the shell, head and nozzle. These are symmetric about the centerline allowing a 2D axisymmetric analysis to be chosen by the authors. This reduced the complexity of the analysis and allows a refined mesh to be used. Most model dimensions were provided in drawings E5.2.1-1 and -2. We re-created the 2D model geometry in SolidWorks. Where model dimensions were not available, we made our model visually match the published drawing. A link to a drawing of our model is provided in the resources section below.
We used the same model in both ABAQUS – the software used by the authors and SolidWorks Simulation (SWS). We inferred the mesh size used by counting the number of elements in areas of known dimensions. We used this size of 0.015″ in both programs. The materials were modeled using the two different material moduli as outlined in PTB-3. The exact location of the change in modulus was not given, so we chose SCL #4 as the transition.
We split the model at Stress Classification Line (SCL) locations 1-9 as shown in the PTB-3 figures E5.2.1-10 and E5.2.1-11. The exact location was not provided for SCL 5 and SCL 9. We attempted to visually match the publication. We used exactly same location in both SWS and ABAQUS even if we could not exactly match PTB-3.
SCL Methods
Two issues stand in the way of getting good SCL data. 1) taking a SCL at a bad location, and 2) setting up the tool poorly. Getting good SCL locations is not always possible. Our article “ASME VIII-2 Permissible Cycle Life” discusses what to do when a good SCL is not possible. PTB-3 does not discuss the reason for the 9 SCL locations chosen. VIII-2 Annex 5-A.3 discusses the selection of SCLs Because we often encounter results from improperly configured SCL tools some detail is provided here.
The SCL starts with stress data taken from the model. The data set is taken on a straight line from the inside to the outside of the model. The data is rotated from global (or model) coordinates to local. When the SCL is on the X axis (like SCL #1 above) no rotation is required. The local direction 1-1 is the direction of the SCL. Stress in this direction is S11. Likewise S22 is perpendicular to the line on the plane of the SCL. S33 is perpendicular to the line out of plane. S12 is the shear stress in the plane of study. For 2D axisymmetric studies S13 and S23 are zero.
Two issues stand in the way of getting good SCL data. 1) taking a SCL at a bad location, and 2) setting up the tool poorly. Getting good SCL locations is not always possible. Our article “ASME VIII-2 Permissible Cycle Life” discusses what to do when a good SCL is not possible. PTB-3 does not discuss the reason for the 9 SCL locations chosen. VIII-2 Annex 5-A.3 discusses the selection of SCLs Because we often encounter results from improperly configured SCL tools some detail is provided here.
The SCL starts with stress data taken from the model. The data set is taken on a straight line from the inside to the outside of the model. The data is rotated from global (or model) coordinates to local. When the SCL is on the X axis (like SCL #1 above) no rotation is required. The local direction 1-1 is the direction of the SCL. Stress in this direction is S11. Likewise S22 is perpendicular to the line on the plane of the SCL. S33 is perpendicular to the line out of plane. S12 is the shear stress in the plane of study. For 2D axisymmetric studies S13 and S23 are zero.
The correct SCL components must be included to get the correct membrane and membrane + bending results in the SCL. The default settings in most SCL tools will not work for pressure vessel studies. The ABAQUS tool must be configured to include S11, S22, S33 and S12 (all the available data) in the membrane stress calculation. Here we have also included the same S11, S22, S33 and S12 components in the bending calculation – however the bending result has no defined meaning in pressure vessel studies and is ignored.
The “Bending Components for Computing Invariants” is the calculation of the averaged difference in stress from one end of the line to the other. Only bending components are included in the invariant calculation. For this 2D study stresses S11 in the direction of the SCL and shear stress S12 are not perpendicular to the SCL and can not create a stress bending the SCL. Stresses S11 and S12 are removed from the invariants.
The “Bending Components for Computing Invariants” is the calculation of the averaged difference in stress from one end of the line to the other. Only bending components are included in the invariant calculation. For this 2D study stresses S11 in the direction of the SCL and shear stress S12 are not perpendicular to the SCL and can not create a stress bending the SCL. Stresses S11 and S12 are removed from the invariants.