10 Tips: Troubleshooting Metal Coils
Heat exchangers and heating or cooling coils are designed for the demands of corrosive chemical heating. A variety of standard and custom configurations are available to suit specific process applications. Materials of construction include fluoropolymer, titanium, 316 stainless steel, plain steel and exotic metals such as zirconium.
Should you encounter problems with your metal coil heat exchangers, consult these 10 troubleshooting tips.
TIP 1: Consider Chemical IncompatibilityChemical corrosion usually is characterized by general etching along the entire heated length, the weld seam or the solution-interface areas of the heat exchanger. It may be seen as small, irregular pits, patchy discoloration or cracks along the coil sheath that are usually most severe in the heated inlet tube areas. Weld failures at cross tubes or splices and coil-sheath splits also can be caused by severe chemical attack and thinning of the tubing. In addition, chemical incompatibility can cause inlet tube/header corrosion on hottest portion of coil.
To remedy chemical incompatibility issues, check the chemistry or MSDS sheets and replace the coil with one constructed of the appropriate coil material.
TIP 2: Contemplate Galvanic CorrosionGalvanic corrosion usually is caused by dissimilar metals between the tank and coil sheath. For example, a 304 stainless coil used in a 316 stainless tank may cause corrosion on the 304 stainless material, as the less noble metal on the electromotive chart (304 stainless) will become anodic and corrode to the cathodic (316 stainless) metal.
This type of corrosion may be seen as a general etching or pitting along the side of the coil facing the more noble metal, and may sometimes be seen in oval patterns. In the case of a coil made with the same metal combinations, corrosion typically will start on the less noble metal, about 0.25" back from the junction of the two metals. Usually the relationship of more surface area of the noble metal will cause increased corrosion on the lesser surface area of the less noble metal. Weld failures at cross tubes or splices also can be caused by galvanic corrosion.
To remedy galvanic corrosion issues, replace the coil or the tank to match materials, or isolate components constructed of different materials.
TIP 3: Investigate Oxygen-Deprivation CorrosionOxygen-deprivation corrosion also can cause holes and pitting in heat exchanger tubes. It can be seen as pits or cracks under areas of buildup, or in areas where metal parts overlap and prevent contact with the oxygenated solution. One of the hallmarks of this type of corrosion is that oxygen is necessary to form the corrosion-resistant oxide layer on metals (figure 1).
To prevent this type of corrosion, coils should either be cleaned frequently or replaced with a material less prone to this type of attack.
TIP 4: Think About the Effect of Cathodic ChargeSometimes with titanium coils, the tube is etched or has a porous appearance on the outside surface. This usually is caused by operation with a cathodic charge applied to the coil in a normally compatible application. To remedy the problem, check with the chemical supplier to verify proper sheath material selection. Also, install isolators to the inlet/outlet, and protect the coil from contact with charged parts.
TIP 5: Avoid Excess AirIf the inside of the tubes in your heat exchanger are etched or have a porous appearance, consider whether this could be caused by excess air. Tube etch and a porous appearance on the inside of the tubes usually are caused by carbonic acid formation in condensate water due to excessive air. To remedy this problem, you have three options: install a steam trap that is capable of venting all air from the system; install a bypass vent; or replace the exchanger with a type compatible with the level of evolved carbonic acid.
TIP 6: Avoid Contact with Electrically Charged WorkContact of any metal coil with DC power within the plating tank will cause a discharge to the sheath, possibly resulting in any of the following conditions:
- Arc-type hole through the sheath.
- Increased corrosion in the area of contact.
- Excessive buildup or plate-out, depending on the polarity of the DC power in contact with the coil.
To avoid problems with electrically charged work, you have three options: locate the coil in the tank to avoid contact with charged work; protect the coil from contact with a nonconductive guard material; or install the coil with isolator couplings to isolate it from the piping.
TIP 7: Don't Let the Gunk Pile UpIf the problem is metal plate-out on the coil surface, inspect the tank for contact between the coil sheath and the plating tank cathode, the work, or parts that have accumulated on tank bottom. Metal plate-out also may be caused by a difference in potential between the coil supply piping and the rectifier ground. To solve these problems, either isolate the coil or connect the coil and rectifier ground wire to the same verified ground source.
TIP 8: Don't Pick Up StraysHoles in metal coils also can be caused by stray voltage/current from other sources entering the tank through the coil sheath. To determine whether stray voltage might be a factor, ground the coil and test the coil-to-ground connection for voltage/current flow to verify presence of flow. If stray voltage is affecting your heat exchanger, locate the source of voltage/current flow and correct the condition. Dielectric insulators can be added to the plumbing connections to disrupt flow.
TIP 9: Make Your Solution Part of the SolutionSometimes, the inlet/outlet tubing can corrode at the solution interface. To resolve this, replace the affected area with heavier gauge material and cover or coat the interface area with a chemically compatible heat-shrink tubing or coating.
TIP 10: Give SupportPhysical damage to coils such as collapsed or twisted inlet/outlet tubes can occur for a number of reasons, but proper installation and operational practices can help avoid the damage.
Collapsed or twisted inlet/outlet tubes that are caused by insufficient support of the inlet/outlet pipe nipples during installation can be fixed by repairing or replacing the damaged components and supporting the inlet/outlet nipples during reinstallation.
Collapsed or twisted inlet/outlet tubes also can occur if the coil is not supported on the adjacent end, allowing the weight or buoyancy of the coil to distort and damage the tubing. To resolve this, repair the coil and reinstall it with the appropriate hangers or anti-floatation weights.
Thermal expansion of the header and/or piping can lead to tubing damage as well. To fix this problem, repair or replace the damaged components and install pipe elbows at the coil connections to allow for expansion.
Finally, the exchanger's end caps can be broken off or cracked from water hammer. To prevent this damage, install a condensate drip trap ahead of the steam valve to reduce the accumulation of condensate, and a water hammer arrester in the plumbing to reduce water/steam hammer. PH