Tubesheet Troubleshooting

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Last updated: Oct 06, 2016 by Mike Hartnett
5 min read

Tubesheets per Part UHX require many more inputs and calculations than most components in ASME Section VIII-I. Because of this, it is very important that all of the dimensions, temperatures, materials, thermal values, etc., are entered correctly. Once this has been done, it is still possible that the design will be failing, incomplete, or passing with an unreasonable thickness. Due to the complex nature of the tubesheet design, this is very likely to occur. In order to finalize the tubesheet design, you can follow the suggestions below to aid in optimizing the tubesheet.

Incomplete Tubesheet

If the tubesheet is Incomplete, click the Check Status icon. This will give you a message telling you why the design is incomplete. If the message indicates that one or more values is unacceptable, check the tabs to make sure the appropriate values were entered for every field. A second possibility is that the MDMT calculations are checked on the MDMT tab but an MDMT loading condition row is not present on the Conditions tab.

Failed Tubesheet

If the tubesheet is Failed, click the Check Status icon. If the message indicates that the value of "mu*" exceeds 0.6 or that it is less than 0.1, a more fundamental design change must occur to meet UHX. Review the section in UHX on tubesheet effective properties for triangular or square tube patterns (whichever pattern you selected) if this is the case.

If the value of "mu*" is not the issue, review the different failure modes present and make sure that a value NAN does not appear in the message. This is a case that may occur in fixed and floating tubesheet configuration "a" designs where certain thermal values are identical. To remedy this, make sure that the thermal values entered are accurate for the operating conditions for each thermal loading case. Another reason this may occur is if cumulative corrosion allowances are greater than the plate thickness. This type of failure is typically very easy to resolve by altering the inputs.

If neither the "mu*" or NAN issue is present in the information window, view the different failure modes to decide the best approach to resolve the failing status.

Design failing due to sigmatmax or sigmatmin

If the design if failing due to sigmatmax, the design is failing due to the tensile stress in the tubes. This case is the most difficult to adjust.

  1. Make sure that all of the tube information is entered correctly (especially the values for the thermal loading cases).
  2. Ensure that the corrosion values are accurate for the tubes if a corrosion allowance was selected.
  3. Check that the vacuum design pressure for the tube side entered on the Tube tab is the vacuum pressure in the channel and not the external pressure on the tubes.

If all of these items are correct and the tubes are still failing, a more aggressive change is needed, such as adding an expansion joint, increasing the tube gauge, or changing the tube material. Before performing these changes, address the other failure modes to limit the need to make further changes to the tube bundle and to help optimize the expansion joint design if one is required.

If the design is failing due to sigmatmin, the design is failing due to the critical buckling stress in the tubes.

  1. Follow the steps listed above for tubes failing due to tensile stress.
  2. If the design is still failing for any loading case, look at the length of the unsupported spans and see if they can be reduced by adding tube supports. If tube supports are present, make sure the longest unsupported span is between a tubesheet and a tube support, not between two tube supports.
  3. If the tubes are failing in the thermal loading cases, you also have the option to use the material properties for loading cases at operating conditions. These values can be adjusted by changing the temperatures on the conditions grid or manually altering the properties on the grid.

If these items have all been addressed and the tubes are still failing, a more aggressive approach, such as that described above for tubes failing due to tensile stress, will need to be taken.

Design failing due to sigma S or sigma C

If the design is failing due to sigma S, the design is failing due to the stress in the shell integral with the tubesheet. If it is failing in any of the loading cases, increasing the shell thickness is the most effective way of reducing this stress. However, this solution may not be acceptable because of geometric constraints with the tube bundle, the necessity for an unavailable plate size, or cost. If the shell is failing in the design stress cases, the option to implement elastic plastic calculations (U-Tube and Fixed only) is available. See the requirements for elastic plastic analysis in UHX to determine if this is acceptable in your engineering judgment. It may be necessary for the shell to satisfy the requirements of UG-23(e) in order to allow two times Yield for the SPS,svalue to qualify for the elastic plastic calculations.

If the shell is failing in the thermal cases, several options exist. If the requirements of UG-23(e) are met, it may be beneficial to change the SPS,s value to two times Yield to increase the allowable primary plus secondary stress beyond that of three times Stress. You also have the option to use the material properties for loading cases at operating conditions. These values can be adjusted by changing the temperatures on the conditions grid or manually altering the properties on the grid.

If the design is failing due to sigma C, the design is failing due to the stress in the channel integral with the tubesheet. If it is failing in any of the loading cases, increasing the channel thickness is the most effective way of reducing this stress. However, this solution may not be acceptable because of geometric constraints with the tube hold pattern on the tubesheet bundle, the necessity for an unavailable plate size, or cost. If the channel is failing in the design stress cases, the option to implement elastic plastic calculations (U-Tube and Fixed only) is available. See the requirements for elastic plastic analysis in UHX to determine if this is acceptable in your engineering judgment. It may be necessary for the channel to satisfy the requirements of UG-23(e) in order to allow two times Yield for the SPS,c value to qualify for the elastic plastic calculations.

If the channel is failing in the thermal cases, several options exist. If the requirements of UG-23(e) are met, it may be beneficial to change the SPS,c value to two times Yield to increase the allowable primary plus secondary stress beyond that of three times Stress. You also have the option to use the material properties for loading cases at operating conditions. These values can be adjusted by changing the temperatures on the conditions grid or manually altering the properties on the grid.

Design failing due to tube-to-tubesheet welds

There are several ways to make these welds pass, but most are not practical. Typically, the best way to make the tube-to-tubesheet welds pass is to redesign them. If this is not an option or if the weld seizes become unrealistic, the next step is to carefully review the thermal cases for the tubesheet design. You also have the option to use the material properties for loading cases at operating conditions. . These values can be adjusted by changing the temperatures on the conditions grid or manually altering the properties on the grid.