Avoid Costly Failures Of Shell-And-Tube Heat Exchangers
Many processes use heat exchangers to adjust or maintain the temperature of streams and unit operations. An efficient heat-transfer system plays a key role in cost-effective manufacturing — while a heat exchanger failure can lead to costly downtime.
Plants often rely on shell-and-tube heat exchangers. Properly selected, installed and maintained, these units can operate reliably and efficiently for long periods. Failures, when they occur, commonly stem from three causes:
1. Chemically induced corrosion resulting from a chemical interaction with circulating fluids;
2. Mechanical damage due to, e.g., metal erosion, steam or water hammer, or thermal expansion; and
3. Scale, mud or algae fouling that creates an insulating effect and, ultimately, corrosion.
You can avoid each of these types of failure.
Here, we’ll review the operational problems that can develop in a shell-and-tube heat exchanger and describe corrective actions you can take to prevent them.
Chemically Induced Corrosion
These failures result from the complex chemical interaction between the materials of the heat exchanger and the fluids circulated through it and numerous other system controls. Common types of chemically induced corrosion failures include:
• pitting corrosion;
• stress corrosion;
• dezincification;
• galvanic corrosion;
• crevice corrosion; and
• condensate grooving.
General corrosion. A relatively uniform attack over the tube, tube sheet, inlet bonnet/channel or shell characterizes this type of corrosion. You may see no evidence that corrosion is occurring.
Fairly stable aggressive conditions generate this type of attack. Low (< 7) pH combined with either carbon dioxide or oxygen can produce this attack on copper. A blue or bluish-green color can appear on the tubes as a result of a carbon dioxide attack on the surface of a copper tube. Various chemicals such as acid also create this type of metal loss.
The solution: Choose a material with adequate corrosion resistance for the environment and use proper treatment chemicals to maximize heat exchanger life. It’s important to keep in mind the various factors working in combination. Most material/chemical compatibility charts don’t account for this, so you many need to consult a metallurgist.
Pitting corrosion. Localized pitting frequently occurs in both ferrous and nonferrous metals. It results from the electrochemical potential set up by differences in oxygen concentration within and outside of the pit; this often is referred to as a concentration cell. The oxygen-starved pit acts as an anode while the unprotected metal surface serves as a cathode. You only may see a small number of pits — however, any one can cause a heat exchanger failure.
Pitting corrosion is most likely to occur during shutdown periods when there is no flow and the environment is most suitable for the buildup of concentration cells. Scratches, dirt or scale deposits, surface defects, fractures in protective scale layers, breaks in metal surface films and grain boundary conditions can further enhance the susceptibility to pitting corrosion.
The solution: Select suitable materials of construction. In addition, properly clean and prepare the heat exchanger for shutdown periods. Failure to do so can result in pitting corrosion beginning within a matter of days; it eventually will lead to failure of the surface and cross contamination of the two fluids.
Stress corrosion. This form of corrosion attacks the grain boundaries in stressed areas. Heat exchanger tubes usually have both avoidable and unavoidable residual stresses. These stresses result from drawing or forming the tube during manufacture, forming U-bends or expanding the tubes into tube sheets. Failures from this corrosion take the form of fine cracks that follow lines of stress and material grain boundaries.
Chloride ions can cause stress corrosion on stainless steel tubes while ammonia can prompt stress corrosion cracking on copper or copper alloy tubes.
The solution: Keep tube wall temperatures below 115°F (calculated with maximum, not average, fluid temperatures) to prevent stress corrosion cracking problems with a chloride ion concentration up to 50 ppm. Where you expect low concentrations of ammonia, use copper-nickel alloys because they have good resistance to stress corrosion cracking.
Dezincification. This creates a porous surface due to chemical removal of zinc from the alloy. Dezincification occurs in copper-zinc alloys (containing less than 85% copper) when they come in contact with either stagnant solutions or water with a high oxygen and carbon dioxide content. The effect tends to accelerate as temperature increases or pH decreases below 7.
The solution: Use a brass with lower zinc content or one containing tin or arsenic to inhibit the chemical action. You also can forestall the problem by controlling the environment, i.e., avoiding contact with stagnant solutions or water with a high oxygen/carbon dioxide content.
Galvanic corrosion. This occurs when dissimilar metals are joined in the presence of an electrolyte such as acidic water. It usually produces a higher rate of reaction on the less noble metal, causing it to corrode quickly.
The solution: First, check the galvanic chart, which shows the relative potential of materials to support this type of corrosion; metals grouped together have lower tendencies to produce galvanic corrosion. Avoid coupling two metals from substantially different groups in an electrolyte, otherwise substantial corrosion of the less noble metal will result. Typically, a voltage difference greater than 0.2 V suggests a galvanic risk; the further apart the metals are, the greater the risk of corrosion. Consider not only the different materials for the heat exchanger components but also for piping and fittings connected to the heat exchanger.
Crevice corrosion. Such corrosion originates in and around hidden and secluded areas, such as between baffles and tubes, or under loose scale or dirt. It requires oxygen to begin. A localized cell develops, with the resulting corrosion appearing as a metal loss with local pits. This often gives the impression that erosion is taking place. Relatively stagnant conditions must exist for crevice corrosion to occur.
The solution: You often can control the attack by ensuring that velocities suffice to prevent stagnation or the accumulation of solids. Also, bear in mind that process fluids believed to be oxygen free may not be; achieving oxygen-free process fluids is difficult.
Condensate grooving. This occurs on the outside of steam-to-water heat exchanger tubes, particularly in the U-bend area. It shows up as an irregular groove or channel cut in the tube as the condensate, in the form of rivulets, drains from the tubing. A corrosion cell usually develops in the wetted area because of the electrical potential difference between the dry and wet areas. The condensate, which must be aggressive for grooving to take place, wears away the protective oxide film as it drains from the tubing.
The solution: You usually can reduce condensate grooving by controlling condensate pH and dissolved gases, and through cleaning the outside surface of the tube bundle to remove oils that prevent uniform wetting.
Mechanical Failures
These can take many different forms including, but not limited to:
• metal erosion;
• steam or water hammer; and
• thermal expansion and cycling.
Metal erosion. Excessive fluid velocity on either the shell or tube side of the heat exchanger can cause damaging erosion of metal from the tubing. This can accelerate any corrosion already present because erosion potentially can remove the tube material’s protective film, exposing fresh metal to further attack.
The areas most prone to erosion are the U-bend of U-type heat exchangers and the tube entrances. Tube entrance areas can experience material loss when excessively high velocity fluid from a nozzle splits into much smaller streams as it enters the heat exchanger. Excessive velocity occurring at the entrance area of tubes typically produces a horseshoe-shaped erosion pattern.
The solution: Keep flow to the maximum recommended velocity in the tubes and entrance nozzle. This value depends on many variables including tube material, the fluid handled and temperature. Materials such as carbon steel, copper-nickel and stainless steel withstand higher tube velocities than copper. Typical tube velocity limits are: copper — 8 ft/s; carbon steel — 9 ft/s; 90/10 copper-nickel — 11 ft/s; and stainless steel — 11 ft/s.
Erosion problems on the outside of tubes can occur with impingement of wet high-velocity gases such as steam. You can control wet gas impingement by oversizing inlet nozzles or placing impingement baffles in the inlet nozzle.
You can determine typical shell-side nozzle velocity limits to prevent impingement erosion on the outside of tubes via an equation involving density, ρ, and shell nozzle velocity, V2:
ρ × V2 = 1,500
where density is in lb-m/ft3 and velocity is in ft/s.
Steam or water hammer. Pressure surges or shock waves caused by the sudden and rapid acceleration or deceleration of a liquid can create steam or water hammer. The resulting pressure surges can reach levels up to 20,000 psi, which is high enough to rupture or collapse the tubing in a heat exchanger.
In a water/steam heating application, damaging pressure surges can lead to an interruption to the flow of cooling water. The stagnant cooling water is heated beyond its boiling point to generate steam; the resumption of the flow causes a sudden condensing of the steam that produces a damaging pressure surge or water hammer.
The solution: Always start cooling water flow before applying heat to the exchanger. Also, use modulating control valves rather than fast-acting shut-off valves, which open or close suddenly and cause water hammer. If you handle condensable fluids in either the shell or tubes, vacuum breaker vents can help prevent steam hammer damage resulting from condensate accumulation.
The installation of properly sized steam traps with return lines can help forestall steam hammer by stopping condensation from accumulating in the shell. Also, ensure the lines are pitched to a condensate receiver or condensate return pump.
Thermal expansion and cycling. Accumulated stresses associated with repeated thermal cycling or expansion can result in tube failure. Exchangers with U-tube-type construction best handle thermal expansion and cycling because the bundle can expand and contract within the shell. With a straight-tube fixed-tube-sheet design, the tubing can’t expand or contract.
The problem becomes much worse as the temperature difference across the length of the tube increases. The temperature difference causes tube flexing, which produces a stress that acts additively until it exceeds the tensile strength of the material, which then cracks. The crack usually runs radially around the tube and often results in a total break. In other cases, the crack occurs halfway through the tube and runs longitudinally along it. Failures due to thermal expansion of fluids most commonly afflict steam-heated exchangers.
The solution: Put relief valves in the heated fluid system to avoid this kind of failure. Also, it’s advisable to provide some means to absorb fluid expansion. For example, installing a tank in the heated fluid system prevents periodic discharge of relief valves, which results in a loss of system fluid and places an undue burden on the valve. Position these devices between the heat exchanger and any shut-off or control valves.
Scale, Mud And Algae Fouling
Various marine organisms or deposits can leave a film or coating on the surfaces of heat transfer tubes. The film acts as an insulator, restricting heat flow and protecting the corrosive components. As a result of this insulating effect, tube wall temperatures rise and corrosion increases.
Scale results from dissolved minerals precipitating out of heat transfer fluids. Forces within the heat exchanger, such as changes in temperature or chemical reactions, alter the solubility of these minerals. For example, calcium bicarbonate, a common constituent of many waters, releases carbon dioxide when heated. This reduces the material to calcium carbonate, which is a relatively insoluble compound that precipitates and coats heat transfer surfaces.
The Solution: Experience shows that increasing the fluid velocity reduces the rate of precipitation. Of course, you must match fluid velocity to the tube material’s ability to withstand the erosive effects of velocity.
Suspended solids usually are found in the form of sand, iron, silt or other visible particles in one or both of the heat transfer fluids. If velocities aren’t high enough to keep them in suspension, particles settle out, causing the same kinds of problems associated with scale from dissolved solids. In addition, many suspended solids are very abrasive to tubing and other heat exchanger parts. When handling abrasive suspended solids in a heat exchanger, you must keep fluid velocity low enough to prevent erosion.
Algae and other marine organisms are a serious problem if they get in the heat exchanger. In many cases, the environment in the heat exchanger spurs their rapid proliferation, which restricts flow and impedes heat transfer.
The solution: Chemical algaecides such as chlorine can effectively control algae and other marine organisms; always check to ensure any chemical treatment is compatible with the materials of construction. High fluid velocities also discourage their attachment and expansion.
JOHN BOYER is heat transfer commercial team manager for Xylem, Cheektowaga, NY. JIM KLIMEK is heat transfer product/business development manager for Xylem, Cheektowaga, NY. Email them at [email protected] and [email protected].
