CHAPTER 1 INTRODUCTION
1.1. GENERAL INTRODUCTION
Shell and tubes heat exchangers are used in a wide variety of applications in many industries for heat transfer purposes. The integrity of tube-tubesheet joints plays a critical rule in heat exchanger durability and life cycle cost. A failure in the tube-tubesheet joints can occur due to poorly formed joints, or later failure may develop due to corrosion cracking of the tube material resulting from the tensile residual stresses in the transition zone between the expanded and unexpanded parts of the tubes. The tube-tubesheet joint must be tight enough to withstand the operating conditions without leakage, and so the residual contact pressure should be as highest as possible to transfer loads from tubes to the tubesheet. There are many parameters that affect the joint strength that include joint configurations, expansion process and materials properties. To guarantee reliable strength of the tube-tubesheet joints Tubular Exchanger Manufacturers Association (TEMA) [2] calls for some specifications which should be maintained during the expansion process. Heat exchangers tubes can be attached to the tubesheet either by welding or expansion. The expansion methods include: roller expanding, explosive expanding, uniform pressure expanding and hybrid expanding. The objective of tube expanding is to create a residual interfacial pressure between the tube and surrounding tubesheet. Among these methods
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2 roller expanding is the most commonly used method in industry to form the tubetubesheet joints. This chapter provides an overall introduction and outlines the thesis organization. Section 1.2 highlights the roller expanding process and section 1.3 describes the parameters that affect the joint strength. Section 1.4 explains the phenomena of the residual stresses in the tube transition zone since it is one of the main issues addressed in the thesis. The problem is defined in section 1.5 followed by the thesis objectives and approach to the problem in section 1.6. Thesis organization is provided in section 1.7.
1.2. ROLLER EXPANDING:
Roller expanding depicted in Figure 1.1 is the most common procedure for expanding heat exchangers tubes into the tubesheet holes. Tube rollers consist of a cylindrical cage with three to five (and for special cases seven) equally spaced longitudinal slots. Hardened steel rolls larger in diameter than slot width are nested in the slots and held in place by a hardened steel taper mandrel that fits inside the nest. Figure 1.2 shows a cross section of three rollers expander. Current practice is to manufacture pins with a taper angle approximately opposite that of the mandrel in order to have the whole active length of the pin bear against the tube wall. The slots into which the pins fit in the cage may be parallel with the centerline or set at an angle. When the slots are parallel axial force is required to insert and withdraw the mandrel. Slots set at an angle cause rotation of the roller in one direction to screw the mandrel in or self-feed and reverse rotation to back of it [3].
3 During rolling tubes in tubesheet holes, the outer end of the mandrel is chucked into the rolling tool driver. A ball bearing thrust collar and locking nut screwed onto the cage’s drive end permits the cage to rotate opposite the direction in which the rolling machine turns the mandrel. The axial position of the rolls in the tube is set by adjusting the position of the thrust collar [3].
Groove Tube projection Tube Mandrel
Roller Tubesheet
Clearance
Figure 1-1 Rolling expansion process To perform the roller expanding process the roller expander is lubricated and inserted into the tube and a torque is applied to the mandrill. As the rolls travel up the mandrel, they cause an increasing radial force exerted at the contact point between the rolls and the tube. This increasing force moves the tube material outwards until it contacts the internal diameter of the tubesheet hole and continues until supposedly the tubesheet material is just below its yield point.
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Figure 1-2 Cross section of three rollers expander
1.2.1. Factors that affect the quality of roller expanded joints:
The roller expanding process is sensitive to some parameters. Some of these parameters can be controlled during drilling, boring or reaming of the tubesheet holes, whereas other parameters can be controlled during the rolling process itself. The factors that affect the quality and tightness of the rolled tube-tubesheet joints are:
Cleanliness of the tube, tubesheet and roller
Condition of the cage, rolls and mandrel
Lubrication and cooling
Tubes and tubesheet holes dimensions
Rolling technique
Roller rotation speed
Number of rolls
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Angle of rolls relative to the tube axis
Shape of the rolls
1.2.2. Advantages and disadvantages of roller expansion process:
Rolling expansion has been used by industry to create heat exchangers tube-tubesheet joint for many decades because it gives reliable joint with relatively low cost. Some advantages which attract people to use rolling expansion for creating tube-tubesheet joint are:
Simple and offer time and cost saving
Availability for commercial applications.
Provides good structural integrity and reliable quality over other methods.
Has been used for a long time and many operators were trained to handle it.
Very few parameters included in the process.
Can be performed by one operator.
The load that can be applied during the process is much higher than the hydroforming approach.
Despite all the above mentioned advantages rolling expansion has some limitations and disadvantages which need to be considered when rolling tubes into tubesheet to form the tube-tubesheet joint. Some of these disadvantages are:
Process is quite difficult to control and somehow subjective to the operator experience
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Lead to joint leak if the proper amount of radial force to establish the contact between the tube and tubesheet is not applied to the joint
Like other expansion methods it provides tensile residual stresses at the transition zone of the tube which under certain environmental condition may provide a source for stress corrosion cracking.
1.3. TUBE-TUBESHEET JOINT STRENGTH:
The residual contact pressure between the tube and tubesheet after rolling is a measure of the tube-tubesheet joint strength and it is equal to the difference between the loading and unloading radial stresses. The residual contact pressure is usually dependent on many parameters these parameters can be grouped into three main groups: 1. Joint configuration which includes:
Initial radial clearance
Presence and geometry of tubesheet grooves
Projection of the tubes
Tubesheet hole surface finish
Tube and tubesheet surface cleanness.
2. Tube and tubesheet materials properties which can be either:
Tube material has greater yield stress than tubesheet material
Tube material has less yield stress than tubesheet material
Both the tube and tubesheet materials has the same yield stress
3. Expansion process parameters which include:
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The expansion pressure or the rolling torque during loading and unloading Using lubrication during the expansion process Operator skills
Because the residual contact stress, between the tube and tube sheet is difficult to measure experimentally, researchers usually measure the push or pull out force and relate it to the residual contact stress.
1.3.1. Grooved Joint:
To enhance joint tightness the industry uses grooved tubesheet. Grooves are recommended and serve to interrupt longitudinal marking, thus providing a seal that will increase joint strength and reduce leakage. Various investigators have examined the effects of grooves on strength and tightness, both for roller expanded and hydraulically expanded tube configurations. Various groove shapes can be used, TEMA [2] specifies that for a design pressure over 2 MPa and/or a temperature exceeding 177 C, all tubesheet holes must be machined with at least two grooves each approximately 3.175 mm wide and 0.397 mm in depth. The effect of the grooves is controlled by the following parameters:
Groove depth
Groove width
Number of the grooves
Spacing between the grooves
Location of the grooves
8 1.3.2. Tube Projection:
Tube projection is the length of heat exchanger tubes extended beyond the tubesheet. TEMA [2] specifies that, tubes shall be flush with or extended by no more than one half of the tube diameter beyond the face of each tubesheet. Expander manufacturer usually specifies a range of projection lengths to be used with the expander. Failure to comply with the recommended tube projection will cause extreme side load pressure and reduce the tube expander’s life. Not like other parameters, the effect of tube projection on the tube-tubesheet joint has received less attention and it is not well addressed in the literature.
1.4. RESIDUAL STRESSES IN THE TRANSITION ZONE
Residual stresses are stresses that remain upon releasing the load. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment and can be sufficient to cause a metal part to suddenly split into two or more pieces after it has been resting on a table or floor without external load being applied. In the case of heat exchanger, the resulting differential displacement of the inside diameter surface of the tubes in the transition from the fully expanded to unexpanded zones obviously creates both tensile and compressive residual stresses. The tensile residual stresses together with operating conditions of the heat exchanger and a corrosive environment can cause stress corrosion cracking of the tube which will lead to failure of the heat exchanger and stoppage of the production which cost a lot of money. On the other hand compressive
9 stresses are beneficial especially in the radial direction because they cause the contact between the tube and tubesheet and contribute to closing any cracks.
1.5. PROBLEM DEFINITION:
Failure of heat exchangers may have both economic and catastrophic effects on plants operation and even human lives. The most likely reasons for failure are due to lack of tube-tubesheet joint integrity. A failure in the joints in the first instant may occur due to poorly formed joints, but later develop due to corrosion cracking of the tube material resulting from the tensile residual stresses in the transition zone between the expanded and unexpanded tubes. The joint integrity is affected by many parameters that include the radial clearance between the tube and tubesheet, tubesheet grooves and tube projection. The radial initial clearance or overtolerance between heat exchanger tubesheet and tubes increases over time because of repeated maintenance processes. This clearance is considered as an important factor in retiring the tubesheet from service when it exceeds prescribed industrial limits such as that of TEMA [2]. A recent work by Shuaib et al [1] indicates that overtolerance greater than that stated by TEMA standards will still give acceptable joint strength. However Shuaib et al [1] work focused on joint strength and did not address residual stresses which could be a source of failures due to fatigue or stress corrosion cracking. The literature review in chapter 2 revealed that the effect of clearance on residual stresses in the tube transition zone of tube-tubesheet joint with overenlarged holes has not been investigated using finite element analysis.
Tubesheet grooves
geometry has a direct effect on the joint strength [4]. Any increase in the clearance will be
10 combined with a decrease in the grooves depth. The effect of the projection of tubes beyond the tubesheet on the joint strength according to the best knowledge of the researcher was not experimentally addressed in the literature. A need exist to fill the above mentioned gaps in the literature. The problem under consideration deals with the design and maintenance of heat exchangers. The knowledge about the effect of clearances, grooves and projection on the strength and residual stresses at the tube-tube sheet joints will assist in designing heat exchangers and developing adequate maintenance polices for them. Therefore our problem in this thesis is to study the effect of clearance, grooves and projection on tube tube-sheet joints having overenlarged holes with focus on residual stresses in the transition zone between expanded and unexpanded tube lengths.
1.6. THESIS OBJECTIVES:
The general objective of this work is to study the effect of tubesheet grooves and tube projection on the integrity of rolled tube-tubesheet joints having overenlarged holes using finite element analysis and an experimental setting. The specific objectives are: 1. Evaluate the effect of grooves on the residual contact pressure and residual stresses in the transition zone of tube-tubesheet joints with overenlarged holes using axisymmetric finite element model. 2. Study the effect of grooves on the axial strength of overenlarged tube-tubesheet joints experimentally by measuring the pullout force and joint hydraulic tightness.
11 3. Evaluate the effect of friction coefficient on tube-tubesheet joints contact pressure using axisymmetric finite element model 4. Evaluate the effect of tube projection on the residual contact pressure and residual stresses in the transition zone of overenlarged tube-tubesheet joints using axisymmetric finite element model. 5. Study the effect of projection on the strength of overenlarged tube-tubesheet joints experimentally by measuring the pullout force and joint hydraulic tightness. 6. Verify the finite element model results by comparing the joint strengths and the deformation obtained from the model and the experiment. The above objectives will be accomplished via two approaches. The first approach is to employ finite element analysis and the second one is to use well designed experiments. Objectives 1, 3 and 4 will achieved using nonlinear axisymmetric finite element analysis utilizing ANSYS finite element software [5]. Objectives 2 and 5 are accomplished using the experimental approach. Objective 6 will be achieved by comparing the results of the two approaches.
1.7. THESIS ORGNIZATION:
This thesis is divided into five chapters. The first chapter defines the problem and states the objectives and approaches to the problem. Chapter two presents the literature review. The finite element model and the experimental procedure are described in the third chapter together with the results and discussions for the effect of tubesheet grooves and radial initial clearance on the joint strength and residual stresses on the transition zone of
12 the tube. The fourth chapter presents the results and analysis for the effect of tube projection and radial clearance on the tube-tubesheet joints integrity. Chapter five provides the conclusions and recommendations of the thesis.
