Vibration Vulnerability of Rod Baffle Type Heat Exchanger

VIBRATION VULNERABILITY OF ROD BAFFLE TYPE HEAT EXCHANGER: CASE STUDY BADAK LNG MCR AFTERCOOLER

Frilo Fitrasali Hutagalung

Badak LNG Technical Department

Badak LNG Bontang 75324 East Kalimantan, Indonesia

[email protected]

ABSTRACT Badak LNG plant utilizes Multi Component Refrigerant

(MCR) as cooling medium for natural gas liquefaction. In the process system, a heat exchanger is serving as compressor aftercooler that cools down this MCR (gas with mostly Methane and Ethane) by means of sea cooling water. This particular heat exchanger is constructed with Rod-Baffle design with Titanium tubes. The design also incorporates a vapour belt at shell inlet/outlet that is functioned as flow distributor.

Badak LNG has 8 plants called process trains; with each process train has one MCR aftercooler. The aftercooler at each train has difference in terms of vapour belt design. During plant operations, vibration problem has repetitively occurred in all MCR aftercooler and caused tube leaks. However, a computational fluid dynamics study revealed that the fluid velocity was low enough to generate tube vibrations. However, Rod-Baffle design uses single support direction per baffle. With 4 axis of support (X+, X-, Y+, Y-), one direction support will repeat at the 5th support, 10th support, and so on. Calculation was then made to confirm the effect if one of the baffle failed to support. This condition will increase the length between baffle by 2 times. Consequently, the fluid critical velocity will decrease by 4 times.

Fig.1 Rod-Baffle Support Arrangement

This approach was then compared with the fluid velocity pattern inside the exchanger shell. It concludes that the fluid velocity is above its critical velocity. Thus, the exchanger is vulnerable to vibrations. Each MCR aftercooler has different vulnerability to vibrations due to vapour belt different designs. The study has concluded the comparison of vulnerability of the exchangers. Exchanger A has 25.9 % tubes having tubes vulnerable to vibrations, while Exchanger F (with different vapour belt design) has only 1.92 % tubes vulnerable to vibrations. With 2000 tubes quantity, the effect to process conditions is significantly different between exchangers. This paper concludes that design of vapour belt can significantly improve the reliability of Rod-Baffle type heat exchanger.

INTRODUCTION

Experience of vibration in shell and tube heat exchanger has been extensively covered in several technical papers. TEMA (Tubular Exchanger Manufacturers Association) has also elaborated the vibration analysis on its standards. In Badak LNG, heat exchanger is designed in accordance with TEMA Standard for the process sizing (including vibration evaluation) and ASME Boiler code for mechanical strength sizing. the MCR aftercooler heat exchanger is designed as shell and tube rod-baffle type. The intent of selecting the type is maximizing heat exchange in smaller exchanger size [7]. Of all eight typical LNG process trains, there is difference in the aftercooler design which exist on the annular vapor distribution belt design. Vapor belt is incorporated the exchanger entrance and exit to allow low cross flow velocity hitting the tubes [4].

Vibration, however, is consistently experienced in all aftercoolers [2]. It makes sense that they are equipped with

Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition

IMECE2014 November 14-20, 2014, Montreal, Quebec, Canada

IMECE2014-39673

1 Copyright © 2014 by ASME

different vapor belt design. Therefore, the vibration problem is recommended to be solved in same approach. ANALYSIS BASIS

It is clear that all aftercoolers are vulnerable to vibration with different level of significance. As an LNG plant, the analysis shall be ended to an implementable solutions. Vapor belt design will not be the focus of analysis due to no practicable solution can be made to this item. In our experience, since the vibration problem has been repetitively causing tube leakage, the most practical solution is to plug the leaking tubes. Therefore, the analysis is based on how to determine which tubes to plug.

But before jumping to the determination of plugged tubes, it will be described how vapor belt designs can create significant effect to tube vibration.

Following is the tubes physical characteristic: Material : ASTM B-338 Grade 2 Tube OD : 19.05 mm Tube Type : Finned Tube Tube Thickness : 1.24 mm Tube Number : 2396 Baffle Number : 47 Tube Pitch : 25.4 mm Tube Lay Out : 90 degrees Tube unsupported span : 12 inch (304.8 mm)

HYPHOTESIS Rod-Baffle type heat exchanger is constructed by four

baffle segments. The segments represent support direction in X+, X-, Y+, and Y+. This construction results that tube is supported between two same direction in four segment span.

Based on guidelines in TEMA [1], the critical velocity of

fluid hitting the tubes is calculated using the following formula;

�� = ��.��

��

��

Critical velocity (VC) depends on two aspects. First is the

material properties of the tubes. Second is that it depends also to the design of heat exchanger. In that formula, the critical velocity of tubes is square of the unsupported tube length (l).

If one of the tube support is not effectively supporting, the unsupported tube length becomes 2l. This conditions will significantly lower the critical velocity by 4 times.

Baffle design in shell and tube heat exchanger mostly comprises of plate baffle and rod baffle. There is a significant difference between the two types. Plate Baffle design incorporates all directions of support in each segment. Rod Baffle designs do not include all direction of support in each segment. In exchange, it incorporates four segment to support each direction.

This difference gives significantly different effect to tube critical velocity. If one of the plate baffle fails to support, the critical velocity will become four times of its normal value. In rod baffle design, the unsupported tube length for one direction is two times the length of the plate baffle type (4l). Therefore, if one segment fail to support, the unsupported tube length becomes 2l. Consequently, the critical velocity becomes 1/4 times of its normal value.

Failures of support baffle shall be interpreted as looseness of the rod that makes space between rod and the tubes in rod-baffle heat exchanger. Probability of rod to baffle clearance is deemed plausible to occur [6]. The aftercoolers which have length of 7300 mm is constructed by 43 baffles with 6 inch distance between baffle. Setting up many baffles in a 7 m heat exchanger can create misalignment in the baffle direction. This idea is taken as hypothetical basis of the vibration analysis.

COMPARISON: TEMA AND PETTIGREW

Vibration in heat exchanger has been researched so extensively that it is essential to compare which research approach shall be utilized. Since author has already known the pattern of leaking tube, it is valid to justify that the most suitable approach is the one that is closer to the actual leaking condition.

There are 2 (two) approaches used in comparison. First is from Standard of TEMA [1], second is from Pettigrew [2].

TEMA defines damping in heat exchanger vibration through experimental observations and idealized models. Damping depends on shell media. Equations in TEMA describes damping with shell side liquids, shell side vapors, and two phase shell side media. Damping is defined as logarithmic decrement of the vibration.

Since the aftercooler utilized MCR gas at shell side, the damping equation is as follows:

� = 0.314 ��

� ��

The equation defines damping as the function of number of

spans (N), baffle thickness (tb), and tube unsupported spans (l). Pettigrew [2] defines damping with the following equation:

!" = 4 �� #

�$�

��

This Pettigrew equation is derived for finned tubes which

accounts the effect of baffle thickness (L), tube unsupported span (lm), and number of spans (N).

Damping ratio is significantly different between the TEMA

equations and Pettigrew equation.

Eq.1

Eq.2

Eq.3


Critical velocity is also compared in accordance with Pettigrew equation:

&'()*

= + � ,-./� �

��

This equation incorporates effect of tube natural frequency

(f), tube diameter (D), damping (ξ), tube mass per unit length (m), fluid density (ρ), and fluidelastic instability constant (K).

Pettigrew [2] defines K = 3 as recommended design

guidelines for finned tube bundles. However, since Pitch to Diameter ratio is below 1.47, then K is calculated as 2.75.

With tube natural frequency of 108 Hz, following is the

result of comparison between TEMA method and Pettigrew method (for Aftercooler G):

Table 1 Comparison TEMA & Pettigrew

Parameter TEMA Pettigrew Damping Ratio 0.0393 0.5007 Critical Velocity (m/s) 5.94 64.56

It appears that Pettigrew approach is far higher than the

TEMA approach. Comparing the result with the fluid velocity inside the aftercooler, no velocity is found at that high while the tube keeps leaking. That concludes the Pettigrew approach is not suitable to apply in this aftercooler leak case.

LEAKING TUBE PATTERN

Our records show that the leaking tubes were located at the opening area of the vapor belt. It makes sense that it is the relatively higher velocity area.

Fig.2 Tube Leaking Pattern in Aftercooler G

The other aftercoolers also provide leaking tube patterns as

above. Tubes were also spotted leaking at only in the inlet side of the exchanger. That concludes that the leaking tubes were caused by vibration that induced by high velocity of fluid that exceeds tube critical velocity.

TUBE CRITICAL VELOCITY Refer to critical velocity formula in Equation 1, it was

revealed that the critical velocity of the tubes ranges 15.87 m/s to 19.88 m/s. Based on Computational Fluid Dynamics (CFD) analysis, the fluid velocity at the tubes ranges between 5 m/s to 7 m/s. If the fluid velocity is lower than the tube critical velocity, no tube vibration will occur [1]. However, in fact, tube vibration still occurs on the exchanger tubes.

Previous experience [6] shows tube fretting was spotted during exchanger inspection.

Fig.3 Tube Fretting in MCR Aftercooler (6)

The size of fretting matched with the rod size that concludes that tube has been repetitively hit the rod. Implementing the hypothesis resulted that the tube critical velocity drops to 4.7 m/s. This value is found matched to the range of fluid velocity around the tubes.

The calculation was made to all other aftercooler. The phenomenon is consistent and convergent. All other aftercooler, even with different vapour belt design, has tube critical velocity that fell in the range of fluid velocity around its tubes.

Calculation of critical velocity (m/s) and fluid velocity (m/s) for five different aftercoolers was made and resulted as follows;

Table 2 Fluid Velocity Threshold

Aftercooler Critical Velocity Fluid Velocity Normal Hypothetical

D 18.28 5.15 4.12 E 17.21 4.71 3.77 F 16.70 4.66 3.73 G 19.88 5.39 4.31 H 15.87 4.28 3.42

Wanni et al [8] describes for reliable operation the

Fluidelastic Instability Ratio (FIR) must be kept below 0.8. FIR is defined as ratio between local fluid velocities to critical velocity.

012 = 334

Eq.5

Eq.4


Taken the FIR as 0.8, the fluid velocity that is taken as the threshold of vibration prone velocity is provided in Table 1. Tubes with fluid velocity above the value in the table are considered vulnerable to vibrate [5]. The amount of tubes that fall into the range depends on the velocity profile around the tube banks.

This velocity profile differs among aftercoolers due to different vapour belt design. The following chapter describes the difference and how many tubes are vulnerable to vibration.

VAPOR BELT DESIGN Design of vapour belt in the five aftercoolers is different.

Different design created difference fluid velocity patterns inside the tube banks. There was no evidence why the vapour belt designs are different among aftercoolers.

Vapor belt of aftercooler D was full circular annulus that flows fluid to the direction of tubesheet. Vapor belt of Aftercooler E have large side opening and end opening that distribute flows to tubesheet direction and side direction. Vapor belt of aftercooler G have large side slot that flow the fluid in angular direction. Meanwhile vapour belt of Aftercooler F and H consist of multiple side channel.

All aftercoolers have inlet nozzle of 30 inch with shell diameter of 1575 mm (62 inch). The vapour belt designs are as follows:

Fig. 4 Full Circular Vapor Belt of Aftercooler D

Fig. 5 Semi-Circular Vapor Belt of Aftercooler E

Fig. 6 Multiple-Channel Vapor Belt of Aftercooler F

Fig. 7 Double-Channel Vapor Belt of Aftercooler G

Fig. 8 Multiple-Channel Vapor Belt of Aftercooler H

The above figures only show the shape of vapour belt

without the covering inlet shell (except for Aftercooler D).

FLUID VELOCITY PATTERNS Different vapour belt designs generate different result in the

CFD analysis. Following are the CFD simulation that is done using Solidworks Flow Simulation 2013.

The simulation was intended to map the fluid velocity

profile. The profile is used to determine which tubes are prone to vibrate.

The focus of the patterns shall be the fluid velocity

surrounding each tube.

Vapor belt


Fig.9 Fluid Velocity Pattern of Aftercooler D

Fig.10 Fluid Velocity Pattern of Aftercooler E

Fig.11 Fluid Velocity Pattern of Aftercooler F

Fig.12 Fluid Velocity Pattern of Aftercooler G

Fig.13 Fluid Velocity Pattern of Aftercooler H

By probing each individual tube fluid velocity during CFD

simulation, the pattern was made that a number of tubes shall be plugged to prevent leaking due to vibrations [3].

The result is shown in the following table:

Table 3 Vibration Prone Tubes

Aftercooler Vibration Prone Tubes D 552 tubes (25.9%) E 276 tubes (11.4%) F 46 tubes (1.92%) G 492 tubes (20.5%) H 248 tubes (10.1%)

The result shows that Aftercooler F has the least vulnerable

to vibration tubes compared to other Aftercoolers. The patterns also support the statement from Pettigrew et al that tube vibrations are mainly in the entrance area [2].


ANALYSIS

Vapor distribution belt effectiveness is dependant to its design. Aftercooler F vapour belt design is quite similar to Aftercooler H vapour belt design. However the result turns significantly different. This phenomenon was affected by the design of Aftercooler H vapour belt that is located in the mid span of the vapour belt. That position is right below the inlet nozzle that makes fluid velocity is relatively higher.

Aftercooler D has the most tubes that are vulnerable to vibration. This is because it has the smallest entrance area compared to other Aftercoolers.

Aftercooler D has also the highest ρV2 parameter among the Aftercoolers.

The width of vapour belt channel in Aftercooler G is deemed not adequate to uniformly distribute the flow velocity. Thus the flow tends to ingress further inside the banks.

Comparing the CFD result for Aftercooler G (Fig.12) and the record of leaking tubes for Aftercooler G (Fig.2), it shows that the TEMA approach is very close to the leaking pattern.

This aftercooler is in service from idle (plant shut down) to

start up. Based on the compressor operating point from idle to steady operation, the vulnerability of tubes to vibrates is different. The fluid is not running at one fluid velocity. By the time the compressor is loaded, the fluid velocity entering the MCR Aftercooler increases.

The following table shows the record of compressor operating from idle (Point 1) to steady operating condition (Point 10).

Table 4 Vibration Prone Tubes

Point Pressure

(kg/cm2g)

Critical Velocity

(m/s)

Fluid Velocity

(m/s)

Ratio

1 5.93 12.37 0.47 26.328 2 10.13 10.51 1.1 9.553 3 18.54 8.29 2.2 3.769 4 25.20 7.31 3.35 2.182 5 31.42 6.67 4.3 1.550 6 34.60 6.39 4.89 1.306 7 37.36 6.21 5.26 1.180 8 39.89 6.16 5.7 1.081 9 43.69 5.97 6.25 0.954 10 44.27 5.94 6.32 0.939

The above data is calculated for MCR Aftercooler G during

LNG Plant Start Up. The point where the fluid velocity exceeds its critical fluid velocity is more apparent from the following figure.

Fig.14 Fluid Velocity Changes in Operation

The figure above shows that the moment where the fluid

exceeds its critical velocity (Ratio < 1) occurs when the compressor reaches approximately 90% of its load. The figure shows also that it will extend farther beyond its critical velocity as its load is increased to steady operating conditions.

Therefore, taking the basis of calculating the vulnerability at the steady operating condition is valid and a more conservative approach.

CONCLUSION Badak LNG utilizes different design of MCR Aftercoolers

in its process trains. It consists of five different designs, which mainly focused on the vapour belt design.

During operations, Badak LNG has experienced a number of tube-leaking events which caused by tube vibration. Calculation shows that the existing fluid velocity is still lower than the tube critical velocity. However, vibration still occurred.

A prevention attempt shall be made since leaking tube can cause loss of primary containment and loss of production. A method was raised by hypothetically assume that one support of that rod-baffle heat exchanger failed to support. This leads to smaller tube critical velocity that eventually falls under fluid velocity. TEMA has concluded that conditions will certainly causing tube vibration.

By implementing that basis, the fluid velocity is compared. Fluid velocity is determined using CFD simulation and concludes that a number of tubes in the Aftercoolers are prone to vibration.

The result shows that Aftercooler F with its vapour belt design has the least vibration vulnerability compared to other Aftercooler.

In this opportunity, Author would like to raise this issue for further and deeper research in fluid induced vibration specifically in the design of vapour distribution belt and rod-baffle heat exchanger.

0

5

10

15

20

25

30

0 10 20 30 40 50

Ve

loci

ty (

m/s

), R

ati

o

Compressor Load in Pressure (kg/cm2g)

Critical Velocity

Fluid Velocity

Ratio

Ratio = 1


NOMENCLATURE

A = Tube axial stress multiplier C = Constant depending on edge condition

geometry l = Tube unsupported span E = Elastic modulus of tube material at the tube

metal temperature I = Moment of inertia of the tube cross section ω0 = Effective weight of the tube per unit length VC = Critical flow velocity fn = Fundamental natural frequency d0 = Outside diameter of tube D = Dimensionless Critical Flow Velocity Factor, N = Number of spans lm = Tube unsupported span (Pettigrew) K = Fluid Instability Constant Upc = Critical Fluid Velocity (Pettigrew) Dx = Outside diameter of tube (Pettigrew) ξ = Damping ratio (Pettigrew) m = Tube mass per unit length tb = Baffle thickness L = Baffle thickness (Pettigrew) ρ = Fluid density f = Tube natural frequency (Pettigrew)

ACKNOWLEDGMENTS Author greatly appreciates the support of Badak LNG in

this paper development and presentation in ASME forum.

REFERENCES

[1] TEMA “Standards of Heat Exchanger Manufacturer Association”, 1999, Tubular Exchanger Manufacturers Association.

[2] Pettigrew, M. J., and Taylor, C. E.N, 2003, “Vibration Analysis of Shell-And-Tube Heat Exchangers: An Overview, Part I: Flow, Damping, FluidelasticInstability,” J. Fluids Struct., 18, pp. 469–483.

[3] Pettigrew, M. J., and Taylor, C. E., 2003, “Vibration Analysis of Shell-And-Tube Heat Exchangers: An Overview, Part II: Vibration Response, Fretting-Wear, Guidelines,” J. Fluids Struct.,18, pp. 485–500.

[4] Khulief, Y.A., Al-Kaabi, S.A., Said, S.A., and Anis, M., 2009, Prediction of Flow-InducedVibrations in Tubular HeatExchangers—Part I: Numerical Modeling, Journal of Pressure Vessel Technology, vol.131.

[5] Patil, R.V., Bhutada, S.S., Katruwar, N.R., Rai, R.R., Dhumke, K.N., 2014, Vibrational Analysis of a Shell and Tube Type of Heat Exchanger In Accordance With Tubular Exchanger Manufacturer’s Association (TEMA) Norms.

[6] Riza, M.F., 2000, Metallurgy Improvement of MCR Coolers in Badak LNG Plant, an APCI Paper.

[7] Kartoyo, C.P., 2003, Evolution Of “Asset Management” at Badak LNG Plant, Bontang, Indonesia, World Gas Conference.

[8] Wanni, A.S., Smith, J.T., Ruzek Z.F., 2011, Anti-Vibration Technologies for Heat Exchangers, NPRA Maintenance and Reliability Conference.


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Vibration Vulnerability of Rod Baffle Type Heat Exchanger