Below is a list of the most commonly asked hydraulic expansion questions. Click a question to expand it and view the answer.
- 1.I've already invested in Tube Rollers. Why do i need HydroPro's technology?
You may not. Our technology is only justified when:
- We replace step rolling with a single step expansion on thick tubesheets, thereby increasing productivity.
- You are having difficulty expanding some tube materials such as duplex SS or titanium.
- You wish to extend exchanger life by reducing stress corrosion cracking of the tubes.
- 2.How does my tooling cost compare with rollers?
- Normally the system and mandrel assembles can be used over and over. These costs are normally capitalized. The consumable parts used to do any given job are minimal as compared to roller parts used.
- 3.What is the difference between hydroexpanding and "hydroswaging"?
- Hydro-expanding is the generic term used for hydraulic tube expansion. HydroSwage ® is the registered trade mark of Haskel International, another manufacturer of hydro-expanding equipment. Other manufacturers (brands) include Maus and Sugino.
- 4.How does hydroexpanding compare with explosive expansion
- Both hydroexpanding and explosive expanding are recognized as "uniform pressure expansion". However, hydroexpanding requires no permits, no transportation difficulties, no neighborhood notification and no separate shop area dedicated just for tube expansion.
- 5.My tube rolling spec is based on tube wall reduction. If I go to hydraulic expansion, can I use the existing specs?
- No. the amount of wall thinning when rolling is much more severe than with hydro-expanding in order to achieve a satisfactory pull out and joint tightness. This reduced wall thinning can contribute to extended exchanger life.
- 6.How is the ideal pressure for production use determined?
By first knowing the tensile strengths and yields of the tube and tubesheet material, preferably as determined by the tube mill report, the ideal pressure is based on enough pressure to cause the tube material to become plastic while making sure the ligaments stay in their elastic range. While we use formulas and tables to arrive at a recommended pressure, we always recommend mockup testing to verify results.
The ideal expanding pressure is theoretically determined first from equations and tables based upon the curve published in "Hydro expanding; the Current State of Art" by Stanley Yokell, P.E. presented at the Joint power General Conference in October 1982. The curve was adopted 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 at its inside; and (2) will not cause the pressure of the tube on the hole to exceed the elastic 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 the tube and tubesheet yield shown on mill test reports. Specifically, it should be pointed out that there may be a wide variation in tube yields from heat to heat, and the minimum values shown in the ASME code cannot be used for setting expanding pressures. They suggest that a tube map be maintained showing where tubes from each heat are located.
Empirical determination through tests and the use of coupons covers items not considered in the theoretical approach:
- Tube spring back varies with different tube material. Titanium and similar alloys are some of the worst, showing about .004" spring back or relaxation 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 are best determined by coupon tests.
- Effect of variations in surface conditions of tube O.D. and tubesheet hole are very difficult to calculate and one needs to resort to empirical findings.
- 7.What is the permissible variation from expansion pressure (+or-)?
Expansion pressure should be held as close as possible. With the HydroPro, a +/- of 1000 psi is easily held. The controller set points and indicating lights give the operator good control of the swaging process. This is in stark contrast to the lack of control with roller expansion which is dependent upon operator "feel" and skill, or torque control. Furthermore, the HydroPro performance can be accurately measured and recorded.
It should be noted, that the variation from set pressure on the HydroPro system is well within the limits of precision of data on the tube and tubesheet properties.
- 8.For 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. Typical time would be 1 to 2 seconds for tubes 1 inch and below. Excessively long expansion zones and/or larger diameter tubes may require a slight increase in this time.
- Expansion Time: This is the expansion pressure holding time and will vary depending on tube material. 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. Setting of the actual time is a function of the specific material characteristics and it's resulting ability to take a "set". Typical numbers for a material such as carbon or stainless steel are no more than 1 to 2 seconds. A material such as titanium with a high springback rate may require a slight increase in this value to the 3 to 4 second range.
- 9.What is the interfacial pressure between tube and tubesheet with expansion pressure applied.
No attempt is made to idealize interfacial pressure during application of expanding pressure. Based upon the Goodier & Schoessow work, maximum expanding pressure is applied that will not cause extrusion of tube or tubesheet. This will provide the highest residual interfacial fit pressure when the expanding 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 Jujiwara's, "Experimental Residual Stress Analysis Of Tube To Tube Sheet Joints During Expansion", ASME paper 82-PV-61, give 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 HydroPro set pressure which is + or – 1000 psi. There is no way directly to measure the variations in interfacial pressure during or after expanding.
If the expanding pressure, that will produce the maximum residual interfacial pressure after expanding pressure is released, 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.
- 10.What is the residual interfacial fit pressure remaining after expansion pressure is removed.
Theoretical calculations for residual pressure can be made by using 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, other than as a starting point. Instead, test models should be made and pullout results should be correlated with expanding pressure.
Consider this—the interfacial pressure is but one of three major factors in determining joint strength and tightness. The other factors are surface area in contact and 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 cannot 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 upon the variations in machining.
When the idealized residual pressure is exceeded, the joint strength declines. This relationship is illustrated in the work by Goodier and Schoessow.
- 11.What is the recommended diametrical clearance between tube and tubesheet?
- 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. However this is not always practical (or warranted) and in these case TEMA standard fit should be adhered to.
- 12.What is effect of tubesheet hole surface/profile 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 markings form leak paths and increase leakage. It is therefore recommended, that the tubesheet holes be drilled only with no reaming afterward.
Grooves are recommended and serve to interrupt longitudinal markings, thus providing a seal. Various investigators have examined the effects grooves on 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 by Yoshitomi, and others, "Tube-Hole Structure For Expanded Tube-To-Tubesheet Joints", U.S. Patent No. 4,142,581, to be about 1/64".
While the TEMA specified groove configuration for hydraulically expanded tubes is sufficient for the vast majority of all applications generally optimum groove dimensions are approximately 2-1/2 times wall thickness in width with a depth of approximately 20% times wall thickness. Final dimensional determination can be further optimized by test.
Out of round and tapered holes (within reason) are easily handled as pressure is equal in all directions and the tube will form itself to the I.D. of the hole.
- 13.TEMA recommends a 1/4 inch wide groove if expanding tubes hydraulically. Why?
- Roller expanding depends on very high contact forces to expand tubes. These high stresses mash the tube material into the groove. The wider groove used with hydroexpanding allows the tube material to flow around the groove edges and bottom out in the bottom of the groove.
- 14.What are the effects of the respective physical properties of tube and tubesheet?
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 Goodier & 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, 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).
- 15.What are limitations of ligament and tube wall thickness?
The relationship between tube wall and ligament thickness is a design consideration. There is 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 expandability. The relationship between wall thickness and tube O.D., along with its tensile strength, determines how much pressure a tube will withstand. Therefore, since hydro expanding does not apply force cyclically but only by uniformly applied pressure, any tube-ligament configuration that can be successfully rolled is capable of being successfully hydro expanded.
Variations in wall thickness do not limit the effectiveness of hydraulic expansion.
The position of the tube hole relative to the center edge or corner of the tubesheet doesn't limit the effectiveness of hydraulic expansion. It may, however, affect the deflection characteristics of the ligaments.
- 16.What is the corrective action for joint leak and limitations on expansion pressure?
Any joint can be re-expanded at the same or a higher pressure or with a longer dwell time. In some cases, a light short tack roll (Hybrid) can be added if kept well inside the tubesheet. This would utilize the tighter fit produced by the roller work hardening without it extending into the transition zone.
The HydroPro system is set at the factory for a maximum swage pressure of 60,000 psi. This can be increased, but it is not generally recommended
- 17.Can HydroPro meet military spec requirement of a pull out strength equal to tube yield strength?
- This can be accomplished with the use of properly designed grooves. The joint strength is determined by the shearing strength of the expanded portion of the tube within the groove.
- 18.What is function and speed of tubepro tube set tool?
The TubePro tube setting tool serves to position the tube relative to tubesheet face. It also opens the diameter of the tube and makes it easier to insert the mandrel o-rings into the tube. The normal time to position the tube and set it in is 3 to 5 seconds.
The TubePro is also used to set tubes to tubesheet prior to welding as it can be adjusted to firmly lock tubes to a predetermined stick-out from the tubesheet (or flush/recessed) while still allowing gases to pass through the joint. This results in much improved quality of the welded joint.
- 19.What are special considerations when using mandrels?
Mandrels come in 1/2 mm increments. O-Rings, to be effective, must have squeeze within the tube I.D. The amount of squeeze is carefully worked out within the 1/2 mm increment sizing.
Tube ends should be chamfered on the I.D. to permit easy O-ring insertion.
O-Ring seal life is in part determined by condition of tube entrance. It is not unusual to get 200 to 500 or more expansions per O-ring.
The O-ring doesn't function alone. It is supported by a polyurethane backup ring which is in turn supported by a six piece steel segment. These are designed to support pressures up to 60,000 psi.
There is scant likelihood of a tube rupture as long as the pressure zone is within the tubesheet.
Normal production rate is determines by dwell times and speed of operator inserting and removing tooling, 15 to 20 seconds is very common on most tube to tubesheet jobs.
- 20.What are the "pre" preparations to hydraulic expansion?
Measure and mike tube I.D.'s for proper mandrel selection. (Refer to mandrel sizing chart in tooling handbook.)
Make sure tubes are clean and I.D. of tubes are properly chamfered for easy insertion of mandrel and tube lock.
Measure and mike I.D. of tubesheet hole for accurate expansion zone. Also be alert for exact depth of chamfer in tubesheet hole, if applicable. Example: T/S hole thickness is 7/8". Expansion zone is normally ¾" (1/8" & 1/8"). If there is a chamfer of 1/32", that makes the placement of the mandrel inside the tubesheet necessary to be 1/32" more inside. The consequence, if not done, is loss of seal, and possible parts damage.
Always check expansion pressure for desired job. Please refer to pressure setting chart and please feel free to contact HydroPro, Inc. for confirmation of recommended pressure settings as well as time settings.
If you have sized the mandrel for said tube and 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 set tool as well as the expanding mandrel. Our own recommended lubricant is HydroLube (containing no halides / chlorides) and is available from us in 1 and 5 gallon sizes .
When sizing the tube set tool, drawbar and anvil, always down size by 1/2 mm (or size), i.e.: if proper HydroPro mandrel size should be 1550, go down to 1500 on TubePro mandrel size.
Always refer to operating manual on HydroPro before starting a job and always feel to contact HydroPro, Inc. or your local Representative for advice.
- 21.Exactly what is hybrid expansion and when is it used?
- Hybrid expansion is used in situations when the tube sheet has significantly lower yields and tensile strength than the tube or when the tube holes are damaged to point where getting a seal is difficult. The most common hybrid procedure is to first hydraulically expand the tubes to contact with the holes at a pressure that will not distort the ligaments. After all the tubes have been so expanded, a light tack roll is applied to the tube. This second step is also being done with explosives.
- 22.What are recommendations when expanding tube prior to welding>
Welding after Hydraulic Expansion (except in the case of titanium tubes) 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 set tool to position and set the tube prior to welding. This process allows the welding gases to escape.
Distilled or purified water is used, as this is preferred by most manufacturers. Any other fluid must be suitable for use in the pumping system. Acetone is not a suitable fluid.
- 23.What are recommendations when expanding tube after welding?
There ought not be any weld roll-over welding. The shop personnel should be instructed to run a cleaning reamer into the tube.
After welding, the welded surfaces should be fluid penetrant examined. Only after the weld is shown to be tight (following an air test for example) should you expand. After expanding, the standard hydrostatic test is used to verify that the structure can withstand the stresses imposed by applying pressure of 1.5 X the maximum allowable pressure shown on the nameplate, corrected for temperature.
If the expanded, zone started about 3/8" into the tube interior behind the weld, the expansion would not affect the weld at all, except to protect if from the effects of tube vibration. If expansion were to start right at the welded region, the weld would be stressed uniformly and have a slight tendency to work the weld.
- 24.What is effect of weld bead seam inside the tube diameter?
When using welded (instead of seamless) tubing an excessive weld bead seam may affect the size of 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.