HOW IS THE IDEAL PRESSURE FOR PRODUCTION USE DETERMINED?
The ideal expansion pressure is theoretically determined first from equations and tables based upon the curve published in "Hydroexpanding: The Current State of the Art" presented at the Joint Power Generation Conference in October 1982. The curve was adapted from, Goodier, J.N. & Schoessow, G.J., "The Holding Power and Hydraulic Tightness of Expanded Tube Joints; Analysis of the Stress and Deformation", Trans. ASME July 1943.
It provides the theoretically highest expanding pressure that will: (1) not exceed the plastic limit of the tube that it is inside of; and (2) will not cause the pressure of the tube on hole to exceed the plastic limit of the tubesheet hole. In the Goodier & Schoessow work, it was demonstrated that the highest expanding pressure that met these conditions would produce the maximum interfacial fit pressure. It was also demonstrated that this was the pressure that would produce the highest residual interfacial pressure when expanding pressure was released.
It is extremely important to note that the expanding pressure chosen from the chart must be related to the actual mill test values of tube and tubesheet yield values detailed on the mill test reports. Specifically, it should be pointed out that there may be a wide variation in tube yields from one heat to another, and the general minimum values shown in the ASME Code cannot be used for setting expansion pressures. It is strongly suggested that they maintain a tubesheet map indicating where tubes from each heat are located.
Empirical determination through tests and die use of coupons covers items not considered in the theoretical approach:
Tube spring back, or relaxation, varies with different tube material. Titanium is the worst, showing about .004” spring back when pressure is released. This degree of spring back is not available in the published data on material properties such as tensile, yield, elongation, modulus, etc. It must be considered that hydraulic expanding, unlike roller expanding, DOES NOT mechanically work harden the tube; therefore, the tube should be deflected sufficiently to overcome spring back. Work hardening does tend to overcome spring back. The various deleterious effects of this are well known in the industry.
Effect of grooves upon sealing and pull strength is best determined by coupon tests.
Effect of variations in surface conditions of tube O.D. and tubesheet hole cannot be calculated and one needs to resort to empirical findings.
Report HAR114, Appendix, shows a typical test program on a particular tube and tubesheet combination. Through a test such as this, the following can be determined:
Tube O.D. change vs. pressure, including spring back characteristics.
O.D. change of tube and coupon or portion of tubesheet, which determines pressure, needed to overcome tube spring back characteristics.
Groove width and depth effect on penetration of tube and ability to cut through tube O.D. surface discontinuities. It also shows amount of shear shoulder which produces pull strength. This data is evaluated against a) and b) and the optimum groove configuration determined.
WHAT IS THE PERMISSIBLE VARIATION (+/-) OF SWAGE PRESSURE?
Swage pressure should be held as close as possible. With the HydroSwage® system, a tolerance of +/- 1000 psi is easily held. The controller set points and indicating lights give the operator clear knowledge of the swaging process status. This is in contrast to the lack of control with typical roller expansion, which is dependent upon operator "feel" and "intuition". Furthermore, the HydroSwage® system performance can be accurately measured and recorded.
It should be noted that the set pressure tolerance of the HydroSwage® system is calculated well within the limits of the tube and tubesheet properties.
WHAT ARE THE EFFECTS OF EXCEEDING THE SWAGE PRESSURE RANGE?
The effects of exceeding the maximum calculated swage pressure is an extrusion of the tube or tubesheet . The actual degree can be determined by calculation or review of test data. It can also reduce leakage and increase joint strength. This is in contrast to roller expansion, where over rolling will break the bond between tube and tubesheet, Increase leakage, and reduce joint strength.
An under pressure condition will reduce joint strength and increase the tendency to leak. This can normally be corrected by re-swaging at a higher pressure.
WHAT TIME PERIOD IS THE PRESSURE HELD?
There are two adjustable time periods:
Prefill Time. This is the time to fill the tube at low pressure and is determined by the diameter and length of tube being pressurized. Time can be determined by observing the time it takes for the pressure to stabilize.
Swage Time. This is the swage pressure holding time. This time can be determined by observing the pressure read-out. It can also be observed more scientifically by attaching a strip chart recorder and observing the profile of the pressure trace.
WHAT IS THE INTERFACIAL PRESSURE BETWEEN TUBE AND TUBESHEET WITH SWAGE PRESSURE APPLIED?
No attempt is made to idealize interfacial pressure during application of expanding pressure. Based upon the Goodier & Schoessow work, the maximum calculated expansion pressure to be applied will not cause extrusion of tube or tubesheet. This will provide the highest residual interfacial fit pressure when the expansion pressure is withdrawn.
The original Goodier & Schoessow paper can be used to determine the stress state of the tube- tubesheet structure under various pressure load conditions. Podhorsky & Krips papers provide a theoretical consideration which gives results' somewhat different from the Goodier & Schoessow investigation, Uragami, K. Sugino, M. Urushibatat, S. Kodama, and Fujiwara's. "Experimental Residual Stress Analysis of Tube to Tube Sheet Joints During Expansion", ASME paper 82PV-61, gives other results.
Since the interfacial pressure during application of pressure is not idealized, permissible variations have not been established. The variation in interfacial fit pressure during application of expanding pressure will essentially vary linearly with the limits of variation of the HydroSwage® set pressure, which is + or - 500 psi. There is no way to directly measure the variations in interfacial pressure during or after expanding.
If the expanding pressure is exceeded, it may cause either the tube or the tube hole to extrude.
If the tubesheet extrudes, it will distort. When this happens, joint strength declines.
WHAT IS THE REMAINING RESIDUAL INTERFACIAL FIT PRESSURE AFTER SWAGE PRESSURE IS REMOVED?
Theoretical calculations for residual pressure can be made by using the Soler-Xu Hong work, embodied in chapter 7 of Soler, A.I. and Singh; K.P., "Mechanical Design of Heat Exchangers and Pressure Vessels", Arcturus Publishers, Inc., Cherry Hill, NJ, 1984. This work provides, in Appendix 7.D, the computer code for calculating residual fit pressure, among other things.
However, the theoretical calculations notwithstanding, relying on theoretical joint strength calculations using any expanding method is not recommended. Instead, actual test models should be made and pullout results should be correlated with expanding pressure.
Consider this: The interfacial pressure is one of three major factors in determining joint strength and tightness. The other factors are total surface area in contact and the effective coefficient of friction. You can make an approximate calculation of area in contact, but unless you can control both tube surface and hole surface within narrow limits, you can not truly predict the coefficient of friction that exists in any tube-tubesheet assembly. In addition, the coefficient of friction will vary from hole to hole depending on variations in surface machining.
When the idealized residual pressure is exceeded, the joint strength declines. This relationship is illustrated in the work by Goodicr and Schocssow.
WHAT IS THE EFFECT OF A DIAMETRICAL GAP BETWEEN THE TUBE AND TUBESHEET?
Ideally, there would be no gap between the tube and tubesheet hole when expanding pressure is applied. As a general rule from an expansion point of view, no matter what expanding method is used, the smaller the gap the better. What establishes the tube-to-tubesheet gap used by manufacturers is their ability to install the tubes through the tubesheets and baffles. This varies with the size of the structure, its configuration, and the tube diameter.
From a practical standpoint, the best quality will be obtained by using the TEMA Special Close Fit drilling tolerances and adhering to tubing manufactured in complete conformity with Section II of the ASME code.
WHAT ARE THE EFFECTS OF TUBESHEET SURFACE PROFILES AND GROOVES?
Circumferential markings are beneficial and will indent themselves into the tube O.D. surface. This will Increase joint strength and reduce leakage. Longitudinal marking form leak paths and increase leakage. It is therefore recommended that the tubesheet holes are drilled only, no reaming afterward.
Grooves are recommended and serve to interrupt longitudinal markings, thus providing a more reliable seal. Various engineers have examined the effects of grooves on joint strength and tightness, both for roller expanded and hydraulically expanded tube configurations. Specifically, joint strength increases linearly with groove depth. The minimum effective groove depth for hydraulic expansion was shown to be about 1/64" by Yoshitomi, and others, in "Tube- Hole Structure for Expanded Tube-to-Tubesheet Joints", U.S. Patent No. 4,142,581.
Generally, groove dimensions are approximately 2.5 times the wall thickness in width, with a depth of approximately 20% of the wall thickness. Final dimensional determination can be further optimized through testing.
Out-of-round and tapered holes are not a problem due to the hydraulic pressure being applied equally in all directions so that the tube will form itself to the deformed I.D. of the hole.
WHAT ARE THE EFFECTS OF THE PHYSICAL PROPERTIES OF TUBES AND TUBESHEETS?
Generally speaking, the tubesheet should be stronger than the tube. With hydraulic expansion, the tube is deformed until it contacts the tubesheet. The tubesheet is then deflected within its elastic range sufficiently so that, when expanding pressure is released, the tubesheet relaxes and creates an interference fit with the tube O.D.
Specifically: It is always preferable to have the tubesheet yield strength higher than the tube yield strength.
It is always preferable to have the tubesheet modulus of elasticity lower than the tube modulus so that the tube hole will spring back more than the tube.
It is always preferable to have a harder hole than tube. According to the Goodicr & Schoessow analysis, "no joint can be made if the tube is much harder than the plate, unless the tube is sufficiently thick".
If the tubesheet yield is less than 60% of the tube yield, it is problematical whether you can get a satisfactory expanded joint, no matter how you expand it. If the tube modulus of elasticity is very low and the tube yield stress very high compared with relatively high tubesheet elastic modulus and low tubesheet yield stress, satisfactorily tight, a strong expanded joint made by any process will be difficult to achieve and may not be achieved. This is especially true when the ratio of tube diameter to wall is greater than 20 (thin walled tubes).
WHAT ARE THE LIMITATIONS OF LIGAMENT AND TUBE WALL THICKNESSES?
The relationship between tube wall and ligament thickness is a design consideration. There are an infinite number of combinations. Each combination requires its own evaluation and no general answer is available.
Wall thickness by itself doesn't determine its swageability. The relationship between wall thickness and tube O.D., along with its tensile strength, determines how much pressure a tube will withstand. Therefore, since hydroexpanding does not apply force cyclically but only by uniformly applied pressure, any tube-ligament configuration that can be successfully rolled should be more capable of being successfully hydroexpanded. Generally, it has been found that HydroSwage® pressure is approximately 2 1/2 times the calculated tube burst pressure.
Variations in wall thickness do not limit the effectiveness of HydroSwage®.
The position of the tube hole relative to the center edge or corner of the tubesheet doesn’t limit the effectiveness of HydroSwage. It may, however, affect the deflection characteristics of the ligaments.
WHAT IS CORRECTIVE ACTION FOR A JOINT LEAK?
Any Joint can be re-swaged at the same, or possibly higher, pressure. In some cases, a short tack roll can be added if it is kept well within the tubesheet. This would benefit from the tighter fit produced by the roller work hardening without it extending into the transition zone.
WHAT ARE THE LIMITATIONS ON SWAGE PRESSURE?
The HydroSwage® system is factory set for a maximum swage pressure of 60,000 psi. This can be increased, but it is generally not recommended.
CAN HYDROSWAGE® MEET MILITARY SPEC REQUIREMENT OF PULL OUT STRENGTH EQUAL TO TUBE YIELD STRENGTH?
This can be accomplished with use of properly designed grooves. The joint strength is determined by the shearing strength of the expanded portion of the tube within the groove.
WHAT IS THE FUNCTION AND SPEED OF THE TUBE-LOC™ TOOL?
The Tube-Loc™ tool serves to position the tube within the tubesheet face. It also opens the diameter of the tube and makes it easier to insert the HydroSwage® mandrel O-rings into the tube. The normal time to position the tube and lock it is around 3 to 5 seconds.
WHAT ARE THE SPECIAL CONSIDERATIONS WHEN USING HYDROSWAGE® MANDRELS?
HydroSwage® mandrels come in 0.5mm increments. O-rings, to be effective, must have squeeze contact within the tube I.D. which is carefully calculated within the 0.5mm increment sizing.
Tube ends should be chamfered on the I.D. to permit easy O-ring insertion.
O-ring seal life is usually determined by the condition of the tube entrance. It is not unusual to get 200 to 500 or more swages from each O-ring.
The O-ring does not function alone. It is supported by a polyurethane backup ring which is, in turn, supported by a six-piece steel expansion segment. These components are designed to support pressures up to 60,000 psi so there is little likelihood of a tube rupture, given the expansion zone is within the tubesheet.
Normal production rate is determined by factors such as dwell times as well as the speed of the operator inserting and removing tooling.
WHAT ARE THE PREPARATIONS FOR HYDROSWAGE® PROCESS?
Measure and mic tube I.D.'s for proper mandrel assembly selection. (Refer to the HydroSwage® mandrel sizing chart in the tooling handbook).
Make sure tubes are clean and that the I.D. of the tube ends are properly chamfered for easy insertion of the Tube-Loc™ assembly and the HydroSwage® mandrel assembly.
Measure and mic the I.D. of the tubesheet hole to get an accurate measure of the expansion zone. Also, measure the exact depth of the chamfer in the tubesheet holes, if applicable.
Example: T/S hole thickness is 7/8". Expansion zone is normally 3/4" (1/8" & 1/8”). If there is a chamfer of 1/32" in the tubesheet hole, that would necessitate an additional depth of 1/32” to the placement of the HydroSwage® mandrel inside the tubesheet. If this is not done, the danger would be blown segments and O-rings due to the chamfer being within the expansion zone.
Always check the swage pressure for the specific application. Refer to the pressure setting chart and feel free to contact the HydroSwage® International, LLC Sales & Support Team for recommended pressure settings as well as time settings.
If you have sized the HydroSwage® mandrel assembly for your specific tube to tubesheet job, and, after mandrel insertion, you find that you can't achieve a seal, go to the next largest O-ring size.
Always use a liberal amount of lubricant with both the Tube-Loc™ tool as well as the HydroSwage® mandrel. We recommended using HydroLube™ (P/N CN002082) lubricant for most applications.
Always refer to the HydroSwage® Operation Manual before starting a job, and always feel free to contact HydroSwage® International, LLC, or your local Representative, for advice.
TUBE EXPANSION BEFORE WELDING?
Welding after HydroSwage® may leave difficulties with the out flow of weld gases. There are numerous reports and studies that indicate the negative effects of welding gases that were not allowed to escape. For this reason, we recommend the use of a Tube-Loc™ tool to position and lock the tube prior to welding. This process allows the welding gases to properly escape.
TUBE EXPANSION AFTER WELDING?
Weld roll-over can affect the entry of the mandrel into the tube. The shop personnel should be instructed to run a cleaning reamer into the tube.
After welding, the welded surfaces should be fluid penetrant examined. When all welds are shown to be joint tested by an air-bubble test; depending upon how critical the welded joint is, a halogen or a helium leak test may be considered. Only after the weld is shown to be tight should you expand the tube. After expanding, the standard hydrostatic test is used to verify that the structure can withstand the stress imposed by applying pressure of 1.5x the maximum allowable pressure shown on the nameplate, corrected for temperature.
If the expanded zone is started about 3/8" into the tube interior, behind the weld, the expansion would not affect the weld at all, except to protect it from the effects of tube vibration. If expansion was to start right at the welded region, the weld would be stressed uniformly and have a slight tendency to work the weld. However, it is doubtful that enough force would be applied to achieve the benefit of changing its character from a cast structure to a forged grain structure.
WHAT IS THE EFFECT OF A WELD BEAD?
The weld bead effects the size of the HydroSwage® mandrel that can be inserted into the tube. A limited size weld bead can be sealed by the O-ring. Usually the tube is drawn after welding, in which case, there is no weld bead, and thus no effect from it.
Expanded joints can be tested by any means previously used by the manufacturer. Leak test fixtures are available for leak testing individual tube stubs.