Maintaining and Repairing Heat Exchanger Tubes L e a r n h o w t o l o c a t e t u b e f ai ai l u r e s a n d r e p a i r t h em em w i t h t h e p r o p e r p l u g g i n g , f e r r u l i n g o r s l e ev ev i n g m e t h o d Stanley Yokell, MGT Inc.
Tube plugging is probably the most frequently used maintenance and repair technique for the tube side of an exchanger. This article provides information on how to locate the positions of tube failures and discusses why it is important to know such information. It describes how to find out where the tubes failed and explains how to analyze what caused failures, seen from the tube side. Descriptions of several kinds of plugs and their uses and plugging techniques are included. Preparation of a plug map, a key to tubeside maintenance, is also discussed. Although tube ferruling and sleeving ar e less common, some of the r equirements for these maintenance techniques are so similar to those for plugging that this article includes them.
Locating Tube Failures Tube failures may occur any time in the life of an exchanger. To get the longest, most-effective exchanger service life you must find out where and why the tubes failed. Two techniques that contribute to the failure analysis are 1) establishing the pattern of the failures and their locations along the length of the tube, and 2) extracting the failed tube for chemical analysis and metallurgical examination. Maintenance workers are usually under pressure to return a shutdown unit to service as quickly as possible. They are constantly aware of the cost of the lost production of power or product. Therefore, they often take the quickest route to restore production, rather than spending the time required to get to the root of the problem. This poor practice provides no insight into changes that would prevent similar problems in a replacement exchanger and may lead to an exchanger’s early demise and replacement. Here are some simple techniques for determining the axial position of tube failures. Preliminary steps Some preparation is required before you probe the tubes. The first step is to isolate both sides of the exchanger from the process and utility streams. The preferred way is to insert line blinds to and from the unit. Closing leak-tight valves may be acceptable for some services where exposure to the fluids is not harmful. Where the hazards of contact with process fluids is severe, use double block valves. Do not enter an exchanger channel or bonnet unless the process lines are fitted with double block valves or the unit is off the line, and follow plant safety practices for entering closed spaces. You must do some initial cleaning of the tube interiors before you attempt to locate leak positions. If the tubeside stream is not hazardous or noxious, you may be able to collect corrosion products by inserting an air nozzle into the individual tubes, blowing through and collecting the material in a strainer at the other end. Chemical analysis of the collected material in the laboratory may provide clues to the failure mechanism. The bare minimum is to flush out the process fluids and water wash the tube side. Blow-dry the tubes with clean, filtered dry air before examining the tube interiors. Commercial devices At present there are no com mercial devices designed specifically to f ind out where tubes are leaking. Products are available for testing individual tube hydraulically or with air pressure. One manufacturer markets a system for hydrostatically testing individual tube-to-tubesheet joints.
Pressurizing the shell and traversing the tube with a plug The classical way to find the position of the leak in a tube is to pressurize the shell with air. You then push a tight-fitting rubber plug into the tube to seal it. When a seal is achieved you traverse the length of the tube until the plug passes a perforation. Air leaks out, indicating that the break position has been found. The disadvantage of this method is that it locates only one leak position. Axial position tube leak finder The author has developed an axial position tube leak finder as a device that plant machine shops can readily make. It consists of a metal plug just smaller than the tube I.D. with a blind hole down its length and a pipe tap at the open end. On the barrel of the plug are two grooves fitted with O-rings. Between the O-rings is a small drilled hole that intersects with the blind hole. The way it works is almost self-evident. An armored air hose or section of pipe is connected to the threaded end. An air pressure gage is installed in the air line. The O-rings are lubricated and the tube is traversed with the position finder for its full straight length. If pipe sections are used, additional lengths are attached as the position finder is run along the tube. When a perforation is encountered, the air pressure drops. Mark the position of the leak and continue to traverse until the whole tube has been scanned. If the tube is a U-tube, and no leak is encountered in either leg, you can assume that the perforation is in the U-bend section. The condition of the tube interior must be such that the probe can traverse the tube without encountering excessive scale or other foulants. Borescopy Fiber-optic examination with video recording capability is an excellent way to examine tube interiors for their general appearance. Flexible probes are available in various lengths. Both the axial and radial leak position can be found and recorded on video tape for offsite review and study. Depending upon the configuration of the shell side, it may be capable of being examined. Unless the tube interior is so fouled that it is too clogged to permit access, no special cleaning is required.
Assessing Failure In order to establish a basis for repairs and to prevent failure recurrences, you have to determine what caused the damage. Shell side conditions are very often the causes of tube failures. Therefore, you have to look at the modes of operation on both sides and both stream flows. The analysis has several facets: Look for a pattern. Locate the failed tube positions in the tube field and along the lengths of the tubes Obtain specimens of the failed tubes in the failure region and perform metallurgical and chemical analyses See if the operating mode, or more likely a change to the operating mode, is a primary cause Search out the failure mechanisms
Patterns of failure Some typical failure patterns follow: Multiple failures in tubes near the inlet connection. When several failures occur in the region of the inlet connection, the likely cause is inadequate or broken impingement protection. Repair requires cutting into the shell.
In new exchangers, this pattern of failures probably results from inadequately designed impingement protection. The perforation and crack locations may be some distance from the impingement plate edges. This kind of failure in new units is usually self-limiting because the failed tubes protect the ones behind them. Here are some alternatives for fixing this problem: 1. If the bundle is removable, extract it. Replace the impingement plate on the bundle with a wider one, and perhaps one that wraps around the bundle. Extract and replace the damaged tubes. 2. Install welded-in round-bar plugs that extend well past the inlet nozzle. Plug off the failed tubes and those in one or two succeeding rows near the inlet. Insert close-fitting round bars into the plugged tubes. The plugged tubes then become impingement protection for the bundle. These two alternatives assume that some loss of heat-transfer surface is tolerable. 3. Plug off the failed tubes. Cut out a circular section of the shell centered on the inlet nozzle. Make the hole large enough to accept the large end of a reducer, to the small end of which you weld the inlet nozzle. You can combine this with the fix described in 1. If you can get access to the impingement plate, replace it with one that is 25-mm (about 1 in.) larger in diameter than the largeend diameter. If the plate is rectangular, replace it with one that extends 25 mm beyond the circular opening. To do this, there must be enough room between the inlet opening and inlet-end tubesheet or shell flange. 4. Cut out the inlet nozzle. Patch the hole. Cut four equally spaced holes, which are equal in size to the inlet nozzle, into the shell. Place the first hole 45-deg. away from the patched hole in the shell. Construct and weld a shell distribution belt to the shell that overlaps the region of the holes cut into the shell. Install the cut-out inlet nozzle in the distribution belt. This procedure also permits replacement of the damaged tubes in straight tube units of all types and in removable U-Tube bundles. Failures in the outer tubes near baffles or tube supports. Failures in outer tubes near the baffles or supports are generally attributable to shell-flow induced vibration. However, they may also stem from crevice corrosion or corrosion due to differences in the electromotive force between the baffles/supports and the tubes. Fixing vibration failures may be difficult. The principal way is to stake the tubes in one way or another. Tube stabilizers and stakes are commercially available for this purpose. Effects of operating mode Properly designed exchangers — made from materials that are corrosion resistant to the process streams — in steady-state operation at design-point conditions require the least maintenance and have the longest life. Ordinarily, moderate continuous deviations from the design point do not seriously affect exchanger life or require extra maintenance. Wide continuous deviations above or below design points can, however, increase the rate of deterioration. Stream flows greater than designed. When stream flows are greater than designed, exchangers behave as if they are underdesigned, increasing the flow velocities of both streams. Pressure loss increases as the square of the flow velocity. Inlet turbulence effects, such as erosion and corrosion on the channels, tubesheets, tube ends, pass partitions and turnaround compartments of multipass units also increase exponentially. On both sides of the tubes, the higher velocity reduces the tendency to foul and increases the film coefficient of heat transfer. Because of the higher heat-transfer rates, the average tube-wall temperature varies from that calculated for the design. In fixed-tubesheet exchangers without shell expansion joints, the tube-metal temperature change can cause overstress in the tubes, shell, tubesheets and tube-to-
tubesheet joints. In units fitted with expansion joints, deflections greater than planned, which result from the different tube-wall metal temperature, can shorten the expansion-joint life. If the tube metal is hotter than designed and the material flowing is temperature sensitive, it may degrade or char, leading to tube corrosion. If it is lower than designed, fluids close to their freezing point may deposit on the tubes. Tubes may buckle or bow as a result. Austenitic stainless steel and titan ium tubes can tolerate linear velocit ies considerably higher than those recommended by the manufacturers’ standards without damage to the protective oxide coating on which they depend for corrosion resistance. However, carbon-steel tubes and most nonferrous varieties suffer loss of corrosion resistance when the flow velocity is much above that recommended. The acceptable exit velocity of fluid between impingement plates and shell-and-tube bundles that the Tubular Exchanger Manufacturers Association, Inc. (Tarrytown, N.Y.; www.tema.org; TEMA) standards permit is an average. Velocities in some regions of the escaping stream may far exceed the average even at design-point conditions. The greater-than-design-point flow may damage tubes that would not be harmed at design-point flows. The higher flow also increases the likelihood of shell-flow-induced vibration and erosion of the shell, and cage. Continuous flow less than designed. Continuous flows less than designed make an exchanger act as if it were overdesigned. The fouling rate varies inversely with flow velocity. Therefore, you will probably have to clean the unit more frequently than anticipated. The tube-metal temperature will also deviate from that used in design with similar results to those described above. In process condensers, condensing on the shell side, a low flowrate in the shell and design rate in the tubes causes low tube-wall temperatures and reduces the shellside pressure. The low-pressure gas and vapor flow may induce tubes to vibrate that would not do so at design conditions. Continuously deviant stream-inlet temperatures. The effects of higher or lower stream-inlet temperatures are like those described above. Continuously deviant operating pressures. Safety considerations set upper operating pressure limits. Modest elevations within the limits rarely affect exchanger life. It may be necessary to adjust level-control devices to protect tubes that must remain submerged in condensate when the operating pressure deviates from design. Changes in operating mode Many an exchanger that has operated satisfactorily over long periods at moderate rates of deterioration has rapidly degenerated after the mode of operation was changed. A common example is when a baseloaded power station with carbon-steel tubed feedwater heaters is changed over to peaking service. In this situation, even when the shells of feedwater heaters are inerted as the heaters come off the line, acid formed by noncondensibles and wet steam in the regions of the tubesheets rapidly corrodes the tubes, thereby shortening heater life. More subtle is the effect of many unanticipated startups and shutdowns. This could be caused by converting an exchanger designed for continuous production to batch operation, or starting up and shutting down a plant in which there is overcapacity relative to the market for its products. In these circumstances, fixed tubesheets, welded to shells or channels are apt to suffer fatigue cracks in the shell or barrel near the tubesheets or in the welds of the tubesheets to the channel or shell. Weld failures may spread into the tubesheets. Tube-to-tubesheet joint failure
Failures at or near the rear face of a tubesheet. Many failures occur in the region of the rear face of the tubesheet where most tubes are anchored to the tubesheet and receive bending and torsional loads. Tubes subjected to shell-flow induced vibration may also fail in this area. When you expand tubes into the tube holes, there is a transition from the expanded diameter to the original diameter near the back face. Stress in the tube may vary from compressive to tensile in and beyond the transition zone, sensitizing the tubes to stress-corrosion cracking (SCC) behind the tubesheet. SCC is a phenomenon related to the material, the fluid environment and tensile stress in the tubes. The tensile stress level in the transition zone may be as high as 90% of the tube yield stress. If the tubes have been bulged by rolling behind the tubesheet rear face, the residual tensile stress may be close to 100% of the tube yield. In thin-walled tubes, an error in the expanding technique, in which the tubes are inadvertently expanded beyond the rear tubesheet face, may lead to cracking. Excessive columnar loads on the tubes created by forward step rolling may contribute to tube breakage in the region of the transition zone. Failures at tube entrance. When fluid enters the tubes, entrance friction may — in addition to creating pressure-drop — cause washing away of welds and erosion of tube ends (feathering). Because the fluid regime does not stabilize for a distance of 10 to 20 tube diameters, there may also be enhancement of tube-end erosion and corrosion. Expanding may compound this effect because of the reduction of flow area as the stream emerges from the expanded to the unexpanded region. When austenitic tubes are fusion-welded to tubesheets, there may be precipitation of complex carbides in the heat-affected zone behind the weld. An atmosphere that the austenitic stainless steel ordinarily resists may corrode this zone. Bubble testing welded tube-to-tubesheet joints with a soap solution or other homemade bubble former may be the cause of stress SCC in sensitive tube and weld materials because most such solutions are high in chlorides. Failures inside the tube end. Inside the tube near its end, failures may result from work hardening, SCC and fatigue. SCC can occur both inside and outside the tube end, and may occur for some distance beyond the inner face of the tubesheet. Although drilling the tube holes to TEMA special close-fit tolerances helps to keep the tensile stress down by limiting the amount of expansion, tubes sensitive to SCC that are expanded into tubesheets will eventually crack in the vicinity of the transition from expanded to unexpanded tube. Soap solutions or other high-chlorides bubble formers that run down the tubes when testing welded tubeto-tubesheet joints also promote SCC in austenitic tube ends. Failures on tube exteriors. The exteriors of tubes joined only by welding may undergo crevice corrosion. (Similarly, the interior metal in the tube holes would suffer crevice corrosion.) Tubes sensitive to SCC in the fluid environment that are expanded into the holes will eventually crack. Failures in tube-to-tubesheet welds. Some of the many cause of failures in tube-to-tubesheet welds are as follows: Improper thermocouple placement during welding: When a tube-to-tubesheet welding procedure requires preheat or post-weld heat treatment, the emplacement of thermocouples to control temperature is critical to avoid distortion and undesired metallurgical changes. Improper placement may be the hidden cause of failures due to the distortion and altered metal structure Localized stress: Front-face fusion welds probably fail more from localized stress than from axial loading. Here is a general description of the mechanism: The tubesheets flex under load, intensifying
the stress in the ligaments. Residual stress due to weld cooling can approach the yield stress of the weld metal. When added to the operational stress, the residual stress can provide the energy level for crack propagation. If the tube is not expanded into the tubesheet after welding, there is free relative deformation between tubes and tubesheets due to expansion and contraction Undetected flaws. The analysis of the causes of tube-to-tubesheet failures should include the possibility of undetected flaws in the joints. Here are some common ones. Hidden gas pockets: Failure may occur because of large gas pockets that are either open to the surface or covered by a thin membrane. This porosity may be caused by lubricants used in prerolling, or the tubes may not have been sufficiently cleansed of drawing lubricants, oxides or protective coatings. In Gas Tungsten Arc Welding (GTAW), also called TIG, nearly surgical cleanliness is required to prevent porosity Blowholes: Incompletely deoxidized steels may cause blowholes. It is a good practice to machine the oxide coating off the tube ends and abrasively clean the tube exteriors and hole interiors just before welding. Inadequate welding techniques that permit introduction of nonmetallic inclusions or that allows burning through at the tube wall may also cause blowholes Weld cracking: Buried porosity, weld-shrinkage, fatigue and corrosion-fatigue cause cracks in the weld that propagate into the tube and tubesheet. Weld-shrinkage stresses can initiate cracking in fissure or crack-sensitive materials. Filet weld designs have exposed roots in which cracks can be caused by improper weld composition, heat-affected-zone hardening, hot cracking in all metal combinations, and after-cooling-cracking in carbon steel
Filet welds may fail by propagation of a leak path from the root notch. The failures are attributable to fatigue or brittle fracture. Crack-failure paths are at the fusion line of weld-metal to base-metal junctions, and along the weld grain boundaries. The boundary between weld metal and parent metal may be the weakest link in the joint, especially with different tube and tubesheet thermal-expansion coefficients. Tube-wall cracks propagate normal to the wall; cracks in fissure-sensitive materials propagate along the weld grain boundaries. Leakage between the shell-and-tube sides occurs when tube walls are penetrated and when tube-totubesheet joints are not hydraulically tight. When it is not possible, convenient or desirable to replace leaking tubes or to repair tube-to-tubesheet joints, the common strategy is to plug the tube ends. Plugging may be temporary until other maintenance is performed, or it may be permanent. More on the symptoms and causes of heat exchanger trouble can be found in Reference [ 4].
Comparing Tube Plugs The following paragraphs discuss various types of plugs and their uses, advantages and disadvantages. The types of tube plugs and plugging methods discussed include the following:
One-piece taper plugs Two-piece plain plugs Explosive plugs Breakaway plugs Elastomeric front and rear seals Far end or remote position sealing with steel wool and lead Torque plugs Thimble plugs
Plugs for fusion welding Removable plugs
One-piece tapered plugs One-piece tapered plugs are tapered to an included angle of about 5-1/2 deg. They are commercially available in fibrous materials and most common metals. Typical plug length is 2-1/2 in. (63.5 mm). However, for small diameter tubes, shorter lengths are used. When a supply is not on hand, most maintenance shops can produce them. The mechanic drives them into the tube ends to create an interference fit between the plug and inner surface of the tube, and between the outer surface of the tube and the tube hole. The tightness of the plug in the tube depends on the interfacial fit pressure (equilibrium unit force) between the tube and plug, coefficient of friction and area of tube and plug in contact with each other. The tightness between the tube and hole similarly depends on interfacial fit pressure between tube and hole, coefficient of friction and surface in contact. To install one-piece taper plugs, descale the tube end before plugging. Use only enough force to contain the leak. Excessive driving may deform ligaments surrounding the tube to be plugged and start leaks in adjacent tube-to-tubesheet joints. It is also possible to crack tubesheet ligaments by over-driving the plugs. Fibrous taper plugs are useful for low-pressure water service. Absorbed water causes the plugs to swell, thereby enhancing tightness. Advantages of one-piece taper plugs. Here are some of the advantages of one-piece taper plugs: They are readily available They are inexpensive They can be made in the maintenance shops They can be used to provide a backup for welding the tube end shut when friction plugging does not suffice, but with disadvantages described below
Disadvantages of one-piece taper plugs. These are some of the disadvantages of one-piece taper plugs:
Improper installation can result in damage to ligaments and spring nearby tubes loose A crevice is created behind the contact surface of the plug and tube They may further sensitize the tube and hole to SCC
Two-piece plain plugs In two-piece plain plugs (Figure 1) the tapered inner element (pin) is driven into a machined taper on the outer element (ring) to expand it into the tube. To be able to insert the assembly into the tube end, some initial clearance is required. The theory that applies to expansion of the plug into the tube and the secondary expansion of the tube into the tube hole is the same theory that applies to uniform pressure expanding during application of pressure. However, there is no pressure release or concern with residual pressure after spring-back. Residual unit forces may be less in two-piece taper plugs than that in one-piece versions, but the greater contact area compensates and may even provide more frictional resistance and a longer leak path than what is available with one-piece plugs. The tube end must be free of scale and axial scores, or tightness will not be achieved. A disadvantage is that the plug may leak at the matched tapers if there is insufficient interfacial pressure.
Two-piece serrated plugs Serrations are not threads, but complete machined rings with roots and crowns. Serrating the outer surface of the rings of two-piece taper plugs confers these advantages: The serrations may penetrate residual scale in areas of the tube that cannot be adequately cleaned High contact forces are created under the crowns of the serrations, causing them to dig into the tube and forcing the tube to dig into the tube hole, thereby increasing tightness
Explosive plugs Explosive plugs may be plain or serrated. They are used with thick tubesheets to seal as much of the tubesheet depth as is desired [ 3]. Advantages of explosive plugs. The advantages of explosive plugs are as follows: The shock of the detonation attenuates so rapidly that adjacent ligaments are not damaged, and adjacent tube are not loosened Explosive plugs can be inserted and detonated where tube ends are inaccessible to plugging by other means Serrated explosive plugs have been tested at blowout pressures of 10,000 psi (68,950 kPa) without leakage A current version requires very little expertise to install and ordinarily is not subject to regulations concerning transport of explosives
Disadvantages of explosive plugs. These are the disadvantages of explosive plugs: Unless the maintenance force has received special training, explosive plugs must be installed by qualified technicians employed by a plugging or maintenance contractor Explosive plugs cannot be used where there is a hazard of igniting volatile solvent vapors
Breakaway plugs Breakaway plugs consist of two-piece serrated taper plugs in which the small end of the pin is attached to a rod somewhat longer than the tube (or tubesheet if a thick tubesheet is being plugged at its rear face). The rod is notched or scored at its attachment to the pin so that under sufficient tension it will break before the rod yields (Figure 2). Installation kits for breakaway plugs typically include a brush, used for cleaning and rounding the tube before inserting the plug, and a go/no-go gage used for measuring the tube I.D. and selecting the appropriate plug size. The sequence of installing the plugs is as follows. 1. 2. 3. 4. 5. 6.
Clean the tubes thoroughly with the brush. Measure the tube I.D. with a micrometer to set the plug gage range. Plug gage with the go/no-go gage to determine the plug O.D. to use. Measure the actual tube length. Set the standoff ring to fix the plug position. Set the plug position and compression tube.
7. Retract the hydraulic ram until the rod breaks away. Elastomeric seals Elastomeric front and rear seals, separated by a compression tube and compressed hydraulically may be used to seal tubes in relatively low-pressure service. The whole assembly remains in place. A variety of elastomeric plugs with washers on each end of the elastomeric cylinder, captured by washers fastened by through bolts is commercially available. Compressed cylinder plugs Elastomeric compressed cylinder plugs consist of two cylinders, three washers and a through bolt. Temporary homemade plugs Homemade plugs can be made for insertion into the end of a failed tube to seal it off during bubble testing by machining a rubber laboratory stopper to fit the tube I.D. Drill a hole through the length of the stopper and insert a small cap screw and nut. The author has made one that held tight under 100 psi shellside air pressure. Far-end or remote-position sealing with steel wool and lead Inner tubesheets and tube regions behind the front tubesheet may be sealed off by inserting a long rod into the tube as far as it can go. The rod may butt against the rear cover or against the bend of a U-tube. A sleeve, of a length selected to posit ion the plug, is slid down the r od as far as it can go. Steel wool and lead are wrapped into a roughly cylindrical plug that fits snugly over the rod. The plug O.D. is somewhat smaller than the tube I.D. A second sleeve is slid over the rod to compress the plug. This kind of plugging is a stopgap. Torque plugs This type of plug expands a serrated ring by means of a torque drive. It is installed by means of a driver and torque wrench provided by the supplier. Some variations on this plug consist of a thick walled, flanged elastomeric thimble into which is fitted an adjusting screw mechanism with screwdriver slot, capped by a screwdriver-slotted cover. The left-hand threaded cover fits snugly into the opening in the flanged end of the elastomeric thimble and protects the threaded adjusting screw. To insert the plug, remove the cover by turning it clockwise. Turn the adjusting screw counter-clockwise, which stretches the thimble, thereby reducing its diameter. Insert the plug in the tube and turn it clockwise. The thimble returns to its original length and diameter, with its exterior bearing firmly against the tube interior. Replace the protective cap by turning it counter-clockwise. Thimble plugs Thimble plugs (Figure 3) consist of closed-ended, thin-walled cylinders with flanged-over lips on the open end. The O.D. of the lip is no greater than the tube O.D. plus 3/4 of a ligament thickness. The O.D. of the cylinder is a maximum of 0.040-in. (1.02-mm) smaller than the I.D. of the tube. The depth of the thimble is great enough to allow insertion of a hydraulic mandrel or tube-rolling tool. Plugs for welding High-pressure equipment should be plugged with welded in plugs. A variety of configurations has been recommended by heat exchanger manufacturers. Figure 4 shows three types that have been used successfully. The left sketch requires cutting out a section of tubing and cleaning the tube hole. For this
purpose, the end of a suitably sized twist drill may be turned down to provide a pilot and cutting edge. Either of these is more satisfactory than the sketch that shows the plug inserted into the tube, since the weld of the latter bridges the connection of the tube to the tubesheet. Removable plugs Various patents have been issued for removable plugs. Be aware that removable plugs are intended for temporary service. The period of successful operation depends on the rate at which the given polymer deteriorates in the service to which it is exposed. One variety shown in Figure 5 consists of a reverse, two-piece, serrated plug, in which the outer piece is extended and closed off with a section thick enough to drill and tap. The inner piece terminates at its small end in a threaded section that is threaded into the front end of the outer piece so as to protrude far enough for a nut to fit on the end. Tightening the nut pulls the taper part forward, thereby providing expanding force to both pieces. The plug is removed by backing off on the nut and gently tapping the threaded end to release the expansion. The polymer used with this plug has withstood years of operation in high-pressure steam service. Figure 6 shows a variety of high confidence plugs, which are also removable.
Effects of Plugging Plugging tubes results in loss of effective surface and increase of pressure drop on the tube side. The pressure-drop increase will be approximately equal to the 1.8 power of the velocity increase. You can readily estimate the increase in pressure drop by raising the ratio of the number of new tubes to the number of unplugged ones to the 1.8 power and multiplying the pressure drop that existed before any plugging took place. Common practice is to retube the unit or replace the tubes or bundle when 5% of the tubes are plugged. However, if there is sufficient pressure drop and surface available for the operation, a better indicator is found by plotting the cumulative number of plugs against months of operation. This presents a clear picture of the trend of failures requiring plugging. You can plot the information using a standard spreadsheet program to plot cumulative numbers of plugs versus months of operation and convert it to percent plugged versus months of operation. (Figure 7). Typically, there is an initial period when there is a surge of plugging until either the causes of the failures are found and remedied or the failure mechanism is self-limiting. Then there is a long period when failures occur because of anticipated deterioration. At the point where the plot bends sharply, prepare to retube, rebundle or replace the unit.
Venting and Stabilizing Following are two requirements that should be considered when plugging. Venting process exchangers Before plugging a tubular heat transfer device, it is necessary to make sure that materials in the streams will not reseal the perforations in the tube. Consequently, it is advisable to vent the tube before plugging. In oil coolers and other exchangers that handle oils or other organic materials, make sure there is a large enough hole or cut through the tube wall in each plugged straight length to permit complete venting. Process fluids from the shell may be entrapped in the tube if the hole becomes sealed by polymerized material or deposition of foulants. This can present a hazard to workers during maintenance and retubing because the entrapped material may develop enough pressure to eject a disturbed plug with considerable force, or it may be explosive or hazardous. Therefore, make sure that before plugging such equipment a hole or slot large enough to vent the tube is cut into the tube behind the tubesheet. When plugging tubes,
make sure to consider safety both during the remaining life of the exchanger and after it is consigned to the scrap heap. Venting feedwater heaters In feedwater heaters, current practice is to remove plugs prior to sleeving repairs because of the potential for pressure to remain in plugged tubes. Schafer and Wahlmeir [ 5 ] describe a protective shield used to permit safe plug removal prior to making repairs to feedwater heaters in the Wolf Creek nuclear station. However, it would be far less expensive to use the venting tool described above before plugging than to employ such a device at a subsequent time. Stabilizing tubes to be plugged Stabilize plugged tubes to prevent them from whipping around and damaging other ones in the event they become completely severed. There are various commercially available and homemade devices for stabilizing tubes. Where access for insertion of stabilizer rods is limited by the space in front of the tubesheet, jointed rods may be produced in the maintenance shop or purchased commercially (Figure 8). Power stations may use stainless steel cables for such stabilization. Cables have the advantage of being capable of traversing large diameter U-bends, such as the outer bends of nuclear steam generators and closed feedwater heaters. Plugged tubes in vertical nuclear steam generators have been successfully stabilized with stainless-steel cables. They can also be considered for closed feedwater heaters where accessibility is limited. Remove plugs before scrapping When scrapping an old exchanger or bundle and before cutting the tube nest into pieces by burning or sawing, be sure to remove the plugs from both ends of the tube. Do this before any cleaning process used to remove harmful substances to prevent explosions, fires and pressure-caused ejection of the plugs.
Tube Plug Maps Whenever you plug a tube, be sure to note the location, axial position of the failure, date and suspected cause of the failure on a layout of the tubesheet. If possible, the layout should be to full scale. Figure 9 illustrates a typical plug map for a U-tube bundle.
Plugging Techniques Following are some techniques to use when plugging tubes. Testing before plugging It is necessary to know whether a tube, a tube-to-tubesheet joint, or both are leaking before deciding to plug. In the operation of feedwater heaters it is customary to monitor the tube-wall thickness. In the circumstance where a tube has not failed, but is considered to be so thin that its failure is likely between scheduled outages, the practice is either to sleeve the length of the tube or to plug the tube with the thin wall. Manufacturers market hydraulic pressure systems for this purpose. Supporting ligaments
Plugging tubes with taper plugs is generally done in a great rush in order to get the unit back on stream as soon as possible. Maintenance people are apt to drive the plugs too vigorously on the assumption that more force will make for a tighter plug. However, the ligaments that surround the tube are very easily moved, and it is common to find that the plugged tube is tight, but the expanded tube-to-tubesheet joints in the surrounding tubes have begun to leak. If the tubes have first been welded into the tubesheets and then expanded, it is even possible to crack the welds in the surrounding tubes. In any event, in the surrounding holes, the seals of the expanded region to the holes may be broken. This can be prevented by using ligament supports. Ligament supports consist of a snug fitting rod with a lip at the outer end. The outer end is tapped to enable removing the ligament support. Insurance plugging Insurance plugging is plugging tubes that have not failed in service, but for which there is reason to suspect may fail at an inconvenient time. Insurance plugging without first examining the surrounding tubes is a poor way to proceed. A better way, if time permits, is to eddy current or ultrasonically scan the surrounding tubes. Only plug those that show evidence of damage. The author strongly recommend against insurance plugging tubes in the upper tube rows of subcooler section of a horizontal heater built with a thin end plate (less than 3-in. thick), unless the cause of the failures in the tubes that failed has been determined. Otherwise, it may simply lead to more damage. Testing after plugging It is advisable to bubble test at the tubesheet face after plugging to verify tightness of the tube to the tubesheet and of the plug to the tube. If you have plugged with welded plugs — using the technique of cutting back the tube and welding the plug to the hole — the purpose is to test the integrity of the weld of the plug to the tubesheet. The pressure applied to the shell side need not be much more than 350 kPa (about 50 psig). Sometimes a higher pressure may be desirable. However, under no circumstances should the shell-side air-test pressure be taken higher than the maximum allowable working pressure (MAWP) stamped on the nameplate.
Sleeves and Ferrules Sleeves and ferrules are installed in tube ends to protect against entrance erosion caused by entrance turbulence. Ferrules may also be used as heat shields when the tubeside inlet temperature is too high to permit direct contact with the tube metal. Sleeves are also used to extend the life of tubes by bridging across a region where corrosion or erosion has caused leaks but the rest of the tube is undamaged. Ferrules are snug fitting and may be flanged over to a diameter slightly greater than the tube O.D. When installing ferrules and sleeves, it is necessary to bear in mind that the fluid regime does not stabilize after entrance into the tube for a distance of 10 to 20 tube diameters, and that further restricting the flow can cause additional downstream turbulence. Sleeves for bridging damaged regions of tubes are therefore about 0.25 mm (about 0.01 in.) thick in order to cause minimal downstream turbulence. Ferrules, which are usually thicker, should have their I.D.s tapered out to a thickness at the inner end of about 0.25 mm (about 0.01 in.). Most sleeving contractors expand the sleeves into position. However, the sleeve may also be gastungsten arc welded to the tube I.D. A recent innovation is applying hydraulic pressure inside the sleeve, in much the same fashion as in hydroexpanding tubes into tubesheets. However, instead of controlling the expanding pressure, the volume of water fed into the mandrel is controlled. This limits the deflection of the sleeve or ferrule, thereby preventing overexpanding and consequent tube bulging. When crevice corrosion is not likely to occur, ferrules and sleeves are simply pressed into place. However, if crevice corrosion is likely, they are also expanded to make an interference fit with the tube. A
technique sometimes practiced to avoid crevice corrosion is to gas-tungsten arc weld the inner end of the ferrule to the tube before expanding.
Heat & Erosion Shields Various means are used to protect tubes and tubesheets from heat and erosion. The following paragraphs discuss some of these. Some confusion exists in describing heat shields in closed feedwater heaters because the same terminology is used for the cylindrical sleeve installed in the steam inlet nozzle that conducts the hot, superheated steam into the desuperheating zone, and the perforated plate attached to the shellside face of the tubesheet in the desuperheating zone to shield it from the high temperature superheated steam. Heat shields in feedwater nozzles Feedwater-heater inlet nozzles for high-pressure, superheated steam are usually fitted with a tubing sleeve or liner, slightly smaller in diameter than the steam nozzle. Near the steam inlet, the sleeve is flared to make contact with the steam nozzle and welded to it. At the other end, the sleeve is welded to the desuperheating-zone shroud to conduct the steam into the desuperheater. When liners are made of carbon or austenitic steel, they may deteriorate and crack. In this event, we recommend replacement with an Inconel shield. Tubesheet heat shields in desuperheaters Tubesheet heat shields in desuperheaters are preferably fastened directly to the rear face of the tubesheet in order to make intimate contact. In some constructions where the heat shield does not lie directly on the tubesheet face, insufficient venting of the space between the shield and tubesheet, combined with peaking service conditions that promote introduction of moisture-saturated noncondensibles, can cause corrosion of carbon steel tubes. Tubeside shields Tubeside shields are an effective method for protecting against high tubeside inlet-fluid temperatures and erosive inlet fluids. Sleeve shields. To protect tubes against channel-side high-temperature inlet fluids and erosion, weld sleeves into a thin plate drilled on the tubesheet template. Slide all of the sleeves into the tubes simultaneously. Expand the sleeve ends to intimate contact with the tube, or gas-tungsten arc weld and expand. Perforated-plate shields. When the placement of channel inlet nozzles is such that entering fluid erodes tube ends and the tubesheet, a strategy for protecting against further damage is to drill a plate segment that will fit into the inlet compartment on the same drilling template as the tubesheet. The holes are drilled to the I.D. of the tubes. The back sides of the holes in the plate segment are relieved to clear any tube or weld protrusion. The plate is then fastened to the tubesheet and serves as a replaceable wasting plate. Edited by Rebekkah Marshall
References 1. Materials Evaluation, published monthly by the American Society for Nondestructive Testing (ASNT), Columbus, Ohio
2. Lohmeir, A. and Reynolds, S.D. Jr., "Carbon-Steel, Feedwater-Heater Tube-to-Tube-Sheet Joints," ASME Paper 65-WA/PWE-10 August, 1965, ASME New York. 3. Editors, Explosive Welding does Bang-Up Job in Heater Repair, Welding Journal , February 1980. 4. Yokell, S., Troubleshooting Shell-and-Tube Heat Exchangers, Chem. Eng., July 25, 1983. 5. Schafer, Bruce W., and Wahlmeir, "Wolf Creek Feedwater Heater Repair", Proceedings EPRI Feedwater Heater Technology Seminar and Symposium, October 2001, Birmingham, Ala.
Author Stanley Yokell , P.E., is president of MGT Inc. (4390 Caddo Parkway, Boulder, CO 80303-3607; Phone: 303 494 9608, Mobile: 303 817 1721; Fax: 303 499 1849; Email:
[email protected], Website: www.mgt-inc.com), a consulting engineering firm. Previously, he was president of PEMCO, a subsidiary of Ecolaire Heat Transfer and, before that, founder and head of Process Engineering and Machine Co. He is author of "A Working Guide to Shell-and-Tube Heat Exchangers" (McGraw-Hill, 1990), coauthor of "Tubular Heat Exchanger Inspection, Maintenance & Repair" (McGraw-Hill, 1997), and author or coauthor of numerous journal articles. He has presented over 100 offerings of a course, "Shell-and-Tube Heat Exchangers – Mechanical Aspects," in Canada, Denmark, the Netherlands and the U.S., plus many offerings of other courses on tubular exchangers and closed feedwater heaters. A Fellow of the American Soc. of Mechanical Engineers (ASME), he has been a member of the subgroup on Heat Transfer Equipment, formerly the Special Working Group on Heat Transfer Equipment of Subcommittee VIII of the ASME Boiler and Pressure Vessel Code Committee, and is a member of AIChE, the National Soc. of Professional Engineers and the Ameri can Soc. for Nondestructi ve Testing. He holds a B.Ch.E. from New York University and has done graduate work at Newark College of Engineering.
