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Anti-Vibration Technologies for Heat Exchangers
NPRA Maintenance and Reliability Conference May 26, 2011
Amar S. Wanni Jerard T. Smith Zdenka F. Ruzek
2
Outline
Flow-Induced Vibration Problems in Heat Exchangers Including Examples of Failures
Overview of Vibration Analysis
Example Anti-Vibration Solutions for Heat Exchangers
Analysis of Vibration with HTRI Xvib
Questions / Discussion
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3
The Flow-Induced Vibration Problem In Heat Exchangers: Background
Tube damage from flow-induced vibrations has increased over the past 5 to 10 years, primarily from two main sources: Advances in catalyst and control technologies allow
operators to increase plant capacity by simply increasing flow through existing process equipment
New exchanger designs are smaller than in the past, which entails greater shell side velocities
Tube failures are always very expensive and lead to lost production, contaminated products, additional energy usage, and/or high repair costs
4
Most Likely Cause for Tube Vibration Damage: Fluidelastic Instability When Local Velocity Exceeds Critical Velocity, Tubes Experience
Orbital Motion that Could Lead to Necking at Baffle Holes HTRI Programs (Xist and Xvib) Predict Fluidelastic Instability Ratio
(FIR = Local Velocity/Critical Velocity) FIR Must be Kept Below 0.8 for high-quality Design Rate of Tube Failure Proportional to FIR4 if FIR > 1
Vortex Shedding Could Lead to Tube-to-Tube Collisions, and Failure at Baffles, Near Tubesheet, and U-Bend Regions Vortex Shedding Frequency Ratio Preferably Kept Below 0.8 Limit Cross-Flow Amplitudes to Less Than 10% of Tube Spacing
The Flow-Induced Vibration Problem In Heat Exchangers: Background (continued)
5
Acoustic Vibration Not Known to Have Caused Tube Failures
Often, Flow-Induced Vibration (FIV) Conditions are Misdiagnosed
Often, Plant Operations Do Not Facilitate Proper Root Cause Failure Analyses
Leaky Tubes are Plugged and Exchanger Quickly Put Back in Operation to Avoid Throughput Losses
Photographs of Failed Tubes are often Unavailable
The Flow-Induced Vibration Problem In Heat Exchangers: Background (continued)
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Examples of Tube Failures in Field
Feed/Effluent Exchanger Tube Damage Shellside : Reactor Effluent Failure Mode: Vortex Shedding
7
Examples of Tube Failures in Field (Continued)
Liquid on Shellside Failure Mode: Fluidelastic Instability
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Examples of Tube Failures in Field (Continued)
Gas on Shellside Tube Fretting at Baffle Hole
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Anti-Vibration Solutions for Heat Exchangers
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Needs for Anti-Vibration Technologies
Improve reliability of existing equipment that may have already suffered vibration damage
Provide tube vibration mitigation to an existing bundle predicted to have vibration problems at a future/planned increased throughput
Modify baffle design to decrease shellside pressure drop while also providing vibration mitigation
Design new exchangers with axial shell side flow to substantially decrease pressure drop (e.g., compressor circuits)
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Anti-Vibration Technologies Open Substantial Opportunities
In Existing Bundles Allow greater flow rates while minimizing risk of vibration
damage Allow re-use of bundles that failed due to flow-induced vibration
damage of tubes
In New Bundles... Allow use of fewer baffles (e.g., 50% Less) decreasing shellside
p by as much as 75% Allow design of exchangers with axial shellside flow providing
smaller footprint, lower cost, and superior performance than other technologies available in todays market
Allow design of heat exchangers at their optimum configuration based on heat transfer and pressure drop considerations.
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ExxonMobil has Developed a Suite of New Anti-Vibration Technologies:
Dimpled Tube Support (DTS)
Saddled Tube Support (STS)
Slotted Baffle Exchangers (SBX)
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Dimpled Tube Support (DTS)
Each strip is fabricated from a thin strip of metal
Outer end consists of a dimpled region Dimples lock into tubes avoiding
accidental dislodging Remainder of strip has corrugations
somewhat similar to dimpled region Both dimpled and corrugated regions
deflect tubes slightly thereby stiffening tubes and avoiding tube chatter
Suitable for all tube layouts (30, 45, 60, and 90)
Suitable for vertical tube bundles Modified DTS strip used as a U-Bend
stiffener Applicable even when some of the
tubes are warped 13
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Dimpled Tube Supports - Installation
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Saddled Tube Supports (STS)
Each support is formed by welding of two strips that are fabricated to provide Saddles
A locking device prevents accidental dislodging of tube support
Similar to DTS strips, each tube is slightly deflected to stiffen tubes and provide vibration mitigation
Suitable for 45 and 90 tube layouts only
Suitable for low-finned tubes made of softer metals such as carbon steel and brass
Suitable for exchangers with axial shell side flow
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Saddled Tube Supports - Installation
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DTS and STS Strip Concept
DTS and STS strips are used to mitigate vibrations perpendicular to the tube axis
Inserting strips into every other tube lane diminishes
tube chatter as the tubes are slightly deformed and pushed against the baffle holes - Tube-to-baffle impacts reduced - Natural frequency of the tubes is substantially
increased with a negligible rise in shellside pressure drop
The tube bundle as a whole tightens up and will act as one rigid entity, significantly lessening relative motion of components
Anti-Vibration Technologies for Heat Exchangers
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EXAMPLES DTD / STS
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Repair of Failed Heat Exchangers (Example 1) An LNG Plant experienced tube failures in a
number of kettle reboilers Vibration analyses showed a very high
probability for tube damage due to fluidelastic instability as well as excessive cross-flow amplitudes due to vortex shedding
A field inspection showed heavy fretting at tube support locations in top 10 tube rows
Unsupported tube span was around 4.5 ft for tubes having a diameter of 0.75 in and a wall thickness of 0.065 in
DTS strips inserted at 1/3 and 2/3 locations within each unsupported span
Free movement of tubes non-existent following DTS installation; no further vibration failures reported
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Kettle Reboiler
Original Conditions
Conditions with DTS
Duty (MBtu/hr) 244 244
P (psi) 0.29 0.29 Fluidelastic Instability Ratio (FIR)
4.1 0.2
Vibration Problems YES NO
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Application of DTS Retrofit into Treat Gas Heater (Example 2)
Current Case has no vibration problems
Future operating case with 30% increase in capacity predicted to have vibration problems
Use of DTS allowed re-use of existing equipment while eliminating vibration potential
A new design would have required replacement of both shell and bundle and potentially piping modifications
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Feed / Effluent Heat
Exchanger
TEMA BEU 33 x 126, 1 tubes 1,180 ft2 Segmented baffles
Existing Design
Conditions
30% Capacity
Creep without DTS
30% Capacity
Creep with DTS
Duty (MBtu/hr) 49.4 64.2 64.2
p (psi) 8.4 10.0 10.2 Vibration Problems NO YES NO
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An Optimum Design (Example 3)
Optimum conventional design even with double-segmental baffles has vibration problems
Alternate conventional design to avoid vibration requires a larger shell diameter (44 in vs. 36 in)
Alternate exchanger also requires No-Tube-In-Window (NTIW) design
Optimum Design with DTS: No vibration problem
Smaller shell diameter
Lower shell side pressure drop 1.4 psi vs. 1.8 psi
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Condensing Service
Exchanger
Conventional Design
(with no vibration)
Optimum Design with DTS
TEMA: AES
44 x 192
Plain Tubes NTIW/Seg
2p / 1s
36 x 192
Plain Tubes Double seg
2p / 1s
Duty (MBtu/hr) 17.7 17.7
p (psi)
1.8 1.4
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October 20, 2011
Application of SBX to Debottleneck Gas Loop (Example 4)
Original Bundle with Conventional Baffles Experienced a Shellside Pressure Drop of 110 kPa
SBX Bundle Designed for 24 kPa Axial Shellside Flow Decreases
Shellside Heat-Transfer Coefficient
To Maintain Heat Duty, Low-Finned Tubes Were Used
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Exchanger: TEMA AEU
Feed Preheat w/ Steam
Original bundle (at debottleneck
conditions) SBX Bundle
25"x126" Shell
1200 ft2 0.75 plain
tubes Segmental
baffles
2140 ft2 0.75 finned
tubes No baffles (axial
flow)
Duty (MBtu/hr) 97.6 103
p(psi) 16 3.5 Vibration Problem? Yes No
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Example of DTS Installation
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Example of DTS Installation in U-Bend
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Example of DTS Installation
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HTRI Xvib DTS/STS Modeling Options
Capability to simulate shell and tube heat exchanger with specific locations of DTS or STS and their effect on flow induced vibration probability
Capability to model DTS and STS as tube supports to reduce unsupported tube span to affect flow induced vibration however without impacting pressure drop or flow velocity
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Anti-Vibration Technologies for Heat Exchangers
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HTRI Xvib
Anti-Vibration Technologies for Heat Exchangers
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SAMPLE CASE DETAILS
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Anti-Vibration Technologies for Heat Exchangers
29
QUESTIONS AND DISCUSSION
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In Conclusion
AVTs technology will eliminate tube chatter Suitable for U-bend and vertical bundles Applicable even when some of the tubes are
warped HTRI software (Xvib) now includes DTS
and STS modeling options Licensed to a select number of qualified
heat exchanger manufacturers for applications worldwide
Successfully used at ExxonMobil and third party sites
ExxonMobil Research and Engineering Company proprietary technology
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Anti-Vibration Technologiesfor Heat ExchangersOutlineThe Flow-Induced Vibration Problem In Heat Exchangers: BackgroundThe Flow-Induced Vibration Problem In Heat Exchangers: Background (continued)Slide Number 5Examples of Tube Failures in FieldExamples of Tube Failures in Field (Continued)Examples of Tube Failures in Field (Continued)Anti-Vibration Solutions for Heat ExchangersNeeds for Anti-Vibration TechnologiesAnti-Vibration Technologies Open Substantial OpportunitiesExxonMobil has Developed a Suite of New Anti-Vibration Technologies:Dimpled Tube Support (DTS)Dimpled Tube Supports - InstallationSaddled Tube Supports (STS)Saddled Tube Supports - InstallationDTS and STS Strip ConceptEXAMPLES DTD / STSRepair of Failed Heat Exchangers(Example 1)Application of DTS Retrofit into Treat Gas Heater (Example 2)An Optimum Design(Example 3)Application of SBX to Debottleneck Gas Loop (Example 4)Example of DTS InstallationExample of DTS Installation in U-BendExample of DTS InstallationHTRI Xvib DTS/STS Modeling OptionsHTRI XvibSAMPLE CASE DETAILSQUESTIONS AND DISCUSSIONIn Conclusion