Abstract:
A method of hydraulically prestraining the tubes of a once-through steam generator by welding the tubes to their respective tubesheets prior to the application of hydraulic expansion to produce tensile stresses which shorten the tubes an equal and predetermined amount to increase the margin to buckling and increase the natural frequency of the tubes reducing flow induced vibration.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates generally to heat exchangers and more particularly to the pre-treatment of tubes for such heat exchangers. 
     DESCRIPTION OF THE PRIOR ART 
     The once-through steam generators or heat exchangers, associated with nuclear power stations and which transfer the reactor-produced heat from the primary coolant to the secondary coolant that drives the plant turbines may be as long as 75 feet and have an outside diameter of about 12 feet. Within one of these heat exchangers, tubes through which the primary coolant flows may be no more than ⅝ inch in outside diameter, but have an effective length of as long as 52 feet between the tube-end mountings and the imposing faces of the tubesheets. Typically, there may be a bundle of more than 15,000 tubes in one of these heat exchangers. 
     In the construction of a once-through steam generator, a plurality of these small diameter long length tubes are configured in a square array where they are welded at their top and bottom ends to a tubesheet to maintain this array in the once-through steam generator. 
     The original once-through steam generators were fabricated using a sequence where tubes, prior to welding to both tubesheets, were individually electrically heated such that cooling of the hot tubes after welding to the tubesheet resulted in tensile strains. This fabrication method is not recommended since in the thermal method of tube prestraining used on the original once-through steam generator the tubes were heated individually until the desired thermal strain was achieved and then seal-welded in place. Thus, for the first seal welded tubes, the desired prestrain was achieved exactly. As the procedure progressed, the previously welded tubes cooled and started to load the secondary shell and the tubesheets. In response, these components deflected in the direction of the load and effectively decreased the length of the subsequently welded tubes. This mechanism introduced an unwanted, uncontrolled and undefined tensile strain in these tubes. Excessive tensile stress was detrimental to the tube life. In addition, thermal prestraining is an expensive and time consuming process. 
     Since both the tubes and the shell of the once-through steam generator are restrained by the tubesheets at both ends, interaction stresses develop during operation due to the relative deformation of the steam generator shell and the tubes. These interaction stresses come from several sources. (1) Both the primary and secondary pressures elongate the secondary shell of the vessel between the two tubesheets; (2) the combined action of the primary and secondary pressures changes the tube radius which, in turn, causes a length change of the tube (“Poisson effect”), or a stress from resisting that change; (3) the tube temperature varies along its length and is different from the lengthwise temperature distribution of the secondary shell. This causes differential expansion of the two; (4) the tubes have a higher coefficient of thermal expansion than the secondary shell which causes a differential expansion; (5) tubesheet bowing, created by primary and secondary pressures combined with induced shell and head deflection loads; and (6) the tube preload introduced during manufacturing. 
     In the case of the once-through steam generator, tube buckling is caused by deformation controlled loads and thus is not a catastrophic primary stress failure mode. However, analysis of a tube shows that tube touching would occur very soon after the tube assumed a bowed shape. Therefore, the load which causes tubes to touch is considered as the limit load on the tube in compression. 
     A slight manufacturing tube prestrain of about ⅛ inch over the length of the tube is considered beneficial to reduce compressive loads on the tubes under all operating conditions. This has the added benefit of preventing stress softening and the resultant reduction in tube natural frequency for flow induced vibration considerations. 
     In view of the foregoing it is seen that an improved method of prestraining the tubes of a once-through steam generator was needed which would not subject the tubes to interaction stresses during welding. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention solves the prior art once-through steam generator or heat exchanger assembly problems and other problems by providing a method of prestressing the once-through steam generator tubes in which the tubes of the once-through steam generator are prestrained to the desired level using the hydraulic expansion of the tubes in the tubesheet. Prior to performing the tube joint hydraulic expansion, both ends of the tubes are welded to their respective tubesheet. The subsequent tube radial expansion within the hydraulic expansion zone creates the desired axial preload. It has been demonstrated both analytically and experimentally that tensile stresses are developed during the hydraulic expansion of tubes which are restrained at both ends. The obtained stresses are of the desired magnitude to increase margin to buckling and increase tube natural frequency to thus increase the margin to detrimental flow induced vibration. 
     Optimized selection of the final total tube stress is controlled by controlling the length of hydraulic expansion in the upper tubesheet. The main advantage is that the developed prestrain is independent of the tube load prior to the expansion. Therefore, the achieved pre-set of a given tube will be independent of the state of the other tubes resulting in the desired uniform foreshortening of each tube. 
     A tensile prestrain of ⅛ inch over the tube length will assure that all tubing stress limits will be met and the tubes will be at a very low tensile stress of approximately 3 ksi during full power operation. This tensile prestrain is achieved by controlling the hydraulic expansion process. 
     In view of the foregoing it will be seen that one aspect of the present invention is to provide a prestraining method for once-through steam generator tube assemblies which will be constant for all the individual tubes. 
     Another aspect is to provide a prestraining method for once-through steam generator tubes once they are assembled into a once-through steam generator tube array. 
     These and other aspects of the present invention will be more fully understood from the following description of the invention when considered along with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical elevation view in full section of a once-through steam generator embodying the principles of the invention; 
     FIG. 2 is a perspective view of a test stand used to develop once-through steam generator tubing hydraulic stress verification data. 
     FIG. 3 is an end view of the FIG. 2 test stand. 
     FIG. 4 is an expanded view of strain gauge location on two tubes of the FIG. 2 test stand. 
     FIG. 5 depicts test results of a full tube length expansion strain. 
     FIG. 6 depicts test results of half tube length expansion strain. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is described in connection with a once-through steam generator for a nuclear power plant, although these principles are generally applicable to shell and tube heat exchangers in any number of diverse fields of activities. Thus, as shown in FIG. 1 for the purpose of illustration, a once-through steam generator unit  10  comprising a vertically elongated cylindrical pressure vessel or shell  11  closed at its opposite ends by an upper head member  12  and a lower head member  13 . 
     The upper head includes an upper tubesheet  14 , a primary coolant inlet  15 , a manway  16  and a handhole  17 . The manway  16  and the handhole  17  are used for inspection and repair during times when the once-through steam generator unit  10  is not in operation. The lower head  13  includes a drain  18 , a coolant outlet  20 , a handhole  21 , a manway  22  and a lower tubesheet  23 . 
     The once-through steam generator  10  is supported on a conical or cylindrical skirt  24  which engages the outer surface of the lower head  13  in order to support the generator unit  10  above structural flooring  25 . 
     As hereinbefore mentioned, the overall length of a typical once-through generator unit of the sort under consideration is about 75 feet between the flooring  25  and the upper extreme end of the primary coolant inlet  15 . The overall diameter of the unit  10  moreover, is in excess of  12  feet. 
     Within the pressure vessel  11 , a lower cylindrical tube shroud wrapper or baffle  26  encloses a bundle of heat exchanger tubes  27 , a portion of which is shown illustratively in FIG.  1 . In a once-through steam generator unit of the type under consideration moreover, the number of tubes  27  enclosed within the baffle  26  is in excess of 15,000, each of the tubes  27  having an outside diameter (OD) of ⅝ inch. It has been found that Alloy  690  is a preferred tube material for use in once-through steam generators of the type described. The individual tubes  27  in the bundle each are anchored in respective holes formed in the upper and lower tubesheets  14  and  23  through seal welding the tube ends at the tubesheets. To support the tubes  27  in their proper positions, and array of drilled and broached substantially flat support plates  45  is positioned transverse to the longitudinal axes of the tubes  27  and the axes of the pressure vessel  11 . 
     The lower baffle or wrapper  26  is aligned within the pressure vessel  11  by means of pins (not shown). The lower baffle  26  is secured by bolts (not shown) to the lower tubesheet  23  or by welding to lugs (not shown) projecting from the lower end of the pressure vessel  11 . The lower edge of the baffle  26  has a group of rectangular water ports  30  or, alternatively, a single full circumferential opening (not shown) to accommodate the inlet feedwater flow to the riser chamber  19 . The upper end of the baffle  26  also establishes fluid communication between the riser chamber  19  within the baffle  26  and annular downcomer space  31  that is formed between the outer surface of the lower baffle  26  and the inner surface of the cylindrical pressure vessel  11  through a gap or steam bleed port  32 . 
     A support rod system  28  is secured at the uppermost support plate  45 B, and consists of threaded segments spanning between the lower tubesheet  23  and the lowest support plate  45 A and thereafter between all support plates  45  up to the uppermost support plate  45 B. 
     A hollow toroid shaped secondary coolant feedwater inlet header  34  circumscribes the outer surface of the pressure vessel  11 . The header  34  is in fluid communication with the annular downcomer space  31  through an array of radially disposed feedwater inlet nozzles  35 . As shown by the direction of the FIG. 1 arrows, feedwater flows from the header  34  into the once-through steam generating unit  10  by way of the nozzles  35  and  36 . The feedwater is discharged from the nozzles downwardly through the annular downcomer  31  and through the water ports  30  into the riser chamber  19 . Within the riser chamber  19 , the secondary coolant feedwater flows upwardly within the baffle  26  in a direction that is counter to the downward flow of the primary coolant within the tubes  27 . An annular plate  37 , welded between the inner surface of the pressure vessel  11  and the outer surface of the bottom edge of an upper cylindrical baffle or wrapper  33  insures that feedwater entering the downcomer  31  will flow downwardly toward the water ports  30  in the direction indicated by the arrows. The secondary fluid absorbs heat from the primary fluid through the tubes  27  in the bundle and rises to steam within the chamber  19  that is defined by the baffles  26  and  33 . 
     The upper baffle  33 , also aligned with the pressure vessel  11  by means of alignment pins (not shown), is fixed in an appropriate position because it is welded to the pressure vessel  11  through the plate  37 , immediately below steam outlet nozzles  40 . The upper baffle  33 , furthermore, enshrouds about one third of the tube bundle. 
     An auxiliary feedwater header  41  is in fluid communication with the upper portion of the tube bundle through one or more nozzles  42  that penetrate the pressure vessel  11  and the upper baffle  33 . This auxiliary feedwater system is used, for example, to fill the once-through steam generator  10  in the unlikely event that there is an interruption in the feedwater flow from the header  34 . As hereinbefore mentioned, the feedwater, or secondary coolant that flows upwardly through the tube bank  27  in the direction shown by the arrows rises into steam. In the illustrative embodiment, moreover, this steam is superheated before it reaches the top edge of the upper baffle  33 . This superheated steam flows in the direction shown by the arrow, over the top of the baffle  33  and downwardly through an annular outlet passageway  43  that is formed between the outer surface of the upper cylindrical baffle  33  and the inner surface of the pressure vessel  11 . 
     The steam in the passageway  43  leaves the generating unit  10  through steam outlet nozzles  40  which are in communication with the passageway  43 . In this foregoing manner, the secondary coolant is raised from the feed water inlet temperature through to a superheated steam temperature at the outlet nozzles  40 . The annular plate  37  prevents the steam from mixing with the incoming feedwater in the downcomer  31 . The primary coolant, in giving up this heat to the secondary coolant, flows from a nuclear reactor (not shown) to the primary coolant inlet  15  in the upper head  12 , through individual tubes  27  in the heat exchanger tube bundle, into the lower head  13  and is discharged through the outlet  20  to complete a loop back to the nuclear reactor which generates the heat from which useful work is ultimately extracted. 
     Referring now to the drawings generally and FIGS. 2 and 3 in particular, it will be noted that a test stand or rig  50  has been designed to investigate two areas of special interest. (1) Explore the effects of tube insertion into a once-through steam generator configuration and (2) quantify residual strain during axially constrained hydraulic expansion of the tubes  27  alone. 
     The capability to accurately and analytically predict the hydraulic expansion mechanics was confirmed using finite element modeling as will be discussed later. 
     Generally, for once-through steam generator tube applications, the expansion takes place at each end of the straight tube which has both ends fastened by seal welds at the tubesheet. An expansion in a U-bend or free ended straight tube results in contraction of the free end of the tube. This contraction is in proportion to the length of the expansion in accordance with the Poisson effect. For a 26 inch expansion length, this axial movement has been observed to be approximately ⅛ inch. Expansion in a fixed ended tube induces strain to the tube instead. What is not known is the influence of the expansion zone plastic deformation in the distribution of the strain i.e. the strain in the tube could be evenly distributed throughout the expanded and unexpanded tube or could accumulate in the plastically flowing region. An even distribution is expected based on theoretical material mechanics; however, the magnitude must be verified to be analytically predictable so that it may be considered in the residual stress, flow induced vibration, and tube/shell interaction analyses of the once-through steam generator tube design. 
     The once-through steam generator proposed design consists of ⅝ inch OD, 0.038 inch wall Sumitomo Alloy 690TT on a ⅞ inch pitch. Tubesheets are sized at 22 inches thick each. Fifteen tube support probes exist over the bundle length. 
     The test rig  50  shown in FIGS. 2 and 3 has tube/hole and pitch geometries selected based on availability of equipment. The stand  50  includes two tubesheet blocks  51  and  52  gun drilled to a 0.93 inch triangular tube pitch, and two broached plates  53  and  54 . The tubes  55 ,  56  and  57  are {fraction (11/16)} inch OD, 0.040 inch wall Sumitomo Alloy 690 TT. The broached plates  53  and  54  are of similar material and tube-to-hole clearance as the once-through steam generator broached plate  45 , and holes  58  drilled to a 0.95 inch triangular tube pitch. The edge condition of the hole  58  is much rougher than the broached plate  45  hole to provide a conservative condition for tube abrasion assessment. The pitch of the drilled broached plates  53  and  54  for the test arrangement was larger than the pitch of the holes  59  of the tube sheet blocks  12  and  14 . As such one central hole  59  was used to align the tube passage by typical production techniques with the surrounding holes  59  being progressively further out of alignment. The test holes were the aligned hole and an adjacent hole that represents a 0.020 inch offset of the tube passage. A third hole in the periphery of the pattern was used to assess tubeability and entry abrasion for a conservative out-of-tolerance offset, i.e., ˜0.050 inch misalignment. 
     The assembly was mounted between heavy structural beams  60  and  61  to approximate the rigidity of the tubesheet/pressure boundary assembly, and offer stiffness in excess of the tubes  55 ,  56  and  57  being investigated. 
     The tubes  55 ,  56  and  57  were eddy current inspected for manufacturing burnish mark in full accordance with accepted testing procedures before insertion into the test rig  50 . 
     The tube  55  was then inserted by normal practice into corresponding ideally aligned holes  58  and  59  while tube  56  was inserted into the holes  58  and  59  having a 0.020 inch displacement between the corresponding broached plate holes  58  and the tubesheet block holes  59 . A third tube  57  was inserted into a peripheral hole  59  with ˜0.050 inch offset relative to the corresponding broached plate holes  58 . The ends of the inserted tubes  55 ,  56  and  57  were tack expanded at each end in preparation for welding. 
     Ten strain gauges  62  were mounted and equally spaced across the free span of tube  55  and tube  56  as shown at FIG.  4 . The tubes  55  and  56  were welded at their respective ends to tubesheet blocks  51  and  52 . Each of the gauges  62  was used to measure tube expansion in an axial or transverse direction at its respective location, and strain data was then recorded to assess any imparted strains from the welding operation. Thereafter, tube  55  was full depth hydraulically expanded, i.e., 26{fraction (9/16)} inch length, while the digital data acquisition system recorded the resulting strain development in the tube  55 . After a cursory data review, the partial depth hydraulic expansion, i.e., 13.25 inch length at the second tubesheet block  52  was performed with strain data recorded. The process was then repeated for tube  56  at the full and partial depth hydraulic expansion as shown at FIGS. 5 and 6, respectively. 
     Eddy current evaluation was performed on the two completely assembled/instrumented tubes  55  and  56  and the tube  57  which had been inserted into the 0.050 inch offset tube holes  58  and  59 . 
     After two weeks, tube  55  was cut at a point ˜8 inch from the first, i.e., full depth hydraulically expanded, tubesheet block  51  and the strain relaxation was measured with a dial gauge. 
     A linear elastic plastic finite element model of the experimental test stand  50  was developed to provide comparative analytical predictions of the strain development with hydraulic expansion. The model was an adaptation of the 3-D axisymmetric hydraulic expansion model developed for in-house Tube to Tubesheet Joint Qualification Programs. 
     Tubes  55  and  56  were inserted through exactly aligned and 0.020 inch offset corresponding holes  58  and  59  and passed through both tubesheet holes  59  and broached plate holes  58  with no substantial resistance relative to U-tube steam generator tubing experience. Mild resistant ‘stop and starts’ were encountered by the tubes  55  and  56  as they were passed through the first tubesheet hole  59  which is a typical response to ‘eyeball’ estimation of a perpendicular entry alignment of the tube at the beginning of its insertion. Experience has shown that once the tube is sufficiently inserted, it guides itself and resistance is virtually eliminated. 
     Slight resistance was sensed on insertion of the tube  57  through the corresponding 0.050 inch offset holes  58  and  59 . However, this was well within the range of experience with pressurized once-through steam generator tube insertion. Manual thumb pressure on the tube end was sufficient to smoothly move the tube  57  into position. 
     The eddy current evaluation showed no manufacturing burnish mark in any of the tubes before or after insertion and expansion, when subjected to eddy current test processes and criterion suitable to typical Baseline evaluations. 
     Visual evaluation indicated superficial discontinuity in the tube surface finish on tubes  55  and  56 . A mild burnish was visible on tube  57  which had no raised metal or discernible depth relative to calibrated scratch standards i.e., &lt;0.0005 inch. 
     No strains in the tube free span were detectable from the welding operation. In response to hydraulic expansion, the strain gauges showed strain development in the free span of both tubes  55  and  56  that was uniform and consistent throughout. Finite element prediction of the expansion strain was shown as very close to that experimentally measured on tube  56  as shown in FIG.  5 . The strain levels as experimentally measured on tube  56  increased again by an expected 50% in response to the second tubesheet expansion of half the length of the original as seen in FIG.  6 . 
     After two weeks, tube  55  was cut and the strain relaxation was measured with a dial gauge. This was intended to investigate any unknown relaxation effects. The tube relaxed over 0.130 inch which is comparable to prediction by the finite element model. 
     The development of the strains to predicted levels shows that the plastic expansion regions have not absorbed a disproportional share of the strain due to unexpected non-linearities. The strain is uniformly distributed throughout the expanded and free span regions. It is repeatable, analytically predictable, controllable (by setting expansion length) and permanent under the conditions tested in this experiment. 
     No manufacturing burnish marks were detected by eddy current testing in spite of attempts to create a worst case tube passage. 
     From the foregoing test results it is seen that hydraulic prestressing of once-through steam generator tubes is possible when done according to the developed empirical data. 
     It will be understood that certain modifications and improvements obvious to people of ordinary skill in this art area were deleted herein for the sake of conciseness and readability. It is intended, however that all such be included in the scope of the following claims.