Choose the best heat exchanger cleaning .doc

Choose the best heat exchanger cleaning methodHeat-transfer equipment can be cleaned chemically or mechanically. This article offers guidance on selecting the method and the final procedure. he majority of fouling deposits that form in chemical process equipment originate in the proces fluids themselves. Waterbased fluids can transport and deposit a wide variety of minerals, and corrosion products form due to the reaction of the aqueous fluids with the metals of construction. Hydrocarbon and petrochemical fluids transport and deposit a variety of organic scales (most of which include polymeric materials). Hydrocarbons can also transport sulfides and mercaptans that can react with the construction materials to form iron or copper sulfides. Common inorganic-scale-forming compounds include various iron oxides (red rust and magnetite), hardness deposits (carbonates), and silicates. The organic deposits are much more complex than inorganic scale, because they almost always contain mixtures of chemicals, including low- to high-molecular-weight polymers and a wide variety of functional groups. Surface deposits must be removed for many reasons: to eliminate scale that contributes to increased corrosion (such as iron oxides and copper found on the water side of many types of process equipment); to increase heat transfer; to increase fluid-flow rates and decrease pressure drops (since small changes in pipe diameter have large effects on flow rate and pressure drops); to reduce damage to downstream components; to make vessels safe by eliminating toxic (e.g., benzene) or flammable (hydrocarbon) vapors prior to vessel entry; to prepare a surface for inspection or maintenance; and to make disassembly of equipment easier (especially for heavily scaled shell-and-tube exchangers that have close assembly tolerances). This article describes general methods for cleaning heat exchange equipment, including both mechanical and chemical procedures, and gives guidelines for selecting between chemical and mechanical cleaning, and among the various types of chemical cleaning processes. Basic considerations Shell-and-tube heat exchangers (including air coolers and condensers) can be cleaned using chemical or mechanical methods or a combination of the two. The ultimate choice will depend on a number of factors: 1. The goal of the cleaning, such as a temporary increase in heat exchange capacity or a need to have perfectly cleaned bare metal prior to a mechanical inspection. Chemical methods usually produce cleaner surfaces than mechanical methods. 2. Physical accessibility of equipment and possibility (or desirability) of physical dismantling. If there is time and space, it may be economical to pull a single tube bundle and clean it with high-pressure water, but time constraints may dictate cleaning a series of exchangers with chemicals without removing the bundles. Chemical methods will be preferred if the plant wishes to clean a series of exchangers as well as the connecting piping. 3. Short-term vs. long-term costs. In the long run, it may be more effective (and ultimately cheaper) to clean an entire system with chemicals, but immediate budget restraints may require that only the most critical bundles be cleaned mechanically. 4. Metallurgical, heating, and disposal factors. These must be considered both when choosing between chemical and mechanical cleaning and when choosing a particular chemical solvent. For example, hydrochloric acid may not be suitable for use on 300 Series stainless steel surfaces. 5. Deposit characteristics. Deposits that are very hard, highly polymerized, or converted to coke may not be soluble in any liquid. If a tube is completely plugged, chemical cleaning will not be effective. In these cases, mechanical cleaning must be employed. The entire cleaning situation must be considered when choosing between mechanical and chemical cleaning, as well as the specific technique within the general category. Weigh all of the above factors and make a decision based on current and long-term budget considerations. Consult with a reliable cleaning contractor. Additional guidance can be found in (1). Mechanical cleaning The general categories of mechanical cleaning are abrasive, abrasive/hydraulic, hydraulic, and thermal. (Refer to (1) for further details.) Here we concentrate on the hydraulic and abrasive/hydraulic methods, because they account for the majority of procedures that require the expertise of an outside contractor. Table I lists the major techniques used for hydraulic cleaning of heat exchange equipment. Figure 1 is a water lance for cleaning the tube side of a heat exchanger bundle. Hydroblasting, or jetting, seems simple, but it is definitely not lowtech. The pressure developed by the jetting pump may exceed 40,000 psig, and the jetting system must deliver that energy to the surface to be cleaned. The nozzle design is critical to delivering the desired pattern of energy to the surface at the desired distance. The design and length of hose are important to delivering the hydraulic horsepower acting on the surface HHP = (flow rate x pump pressure)/1,714. Spin nozzles, flow splitters, and automated devices improve the efficiency and safety of jetting operations. Two major productivity and safety improvements are automated tube lances (Figure 2) and automated bundle blasters. Automation greatly improves safety as well as efficiency by removing the operator from direct contact with the lance. Power-assisted lances use mechanical forces to push a lance through a plugged tube. Multitube lances can clean several tubes simultaneously. Abrasives such as sodium bicarbonate can be added to the water stream to increase the efficiency of deposit removal (compared to water alone). Figure 3 is a shell-side hydraulic cleaning system. The high-pressure water (in this case, from two jets) is directed at a bundle and the jet head moves back and forth as the bundle is rotated under it. The articulated arm can also be rotated to blast the tube side of a heat exchanger bundle. Chemical cleaning If chemical cleaning is to be used, it is very beneficial to obtain a sample of the deposit so that its composition can be determined. Based on the chemical analysis of the deposit, an optimal treatment plan can be developed and the best solvent selected. If it is not possible to analyze a desposit sample, the composition must be deduced from past cleanings or based on the composition of the process fluid. However, this can decrease the probability of performing a completely successful cleaning. Deposits are classified generically as organic (process side) or inorganic (water-side). Solvents for process-side deposits Process-side deposits may range from light hydrocarbons to polymers, and they generally are similar to the fluids from which they originate. Figures 4 and 5 show the locations (F1-F10) where deposits are formed (in a petroleum refinery as an example) and the processes that generate them. General removal techniques for the various types of deposits are listed in Table 2. Deposits found in other types of chemical process equipment tend to be similar in that low-molecularweight fluids frequently form lowmolecular-weight deposits, and the chemical composition parallels the composition of the fluid. Because of the very large range of chemical processes in use, deposit identification by chemical means takes on added importance. The general categories of solvents for process-side scales include aqueous detergent solutions, true organic solvents, and emulsions. Aqueous detergent formulations always contain a surfactant-type component. In addition, they can contain alkaline agents, such as sodium hydroxide, sodium silicate, or sodium phosphate. Builder molecules such as ethylenediaminetetraacetic acid (EDTA) suppress the effects of hard water, and coupling agents such as glycol ethers improve the dissolution of some organic deposits. Detergent formulations are effective only for removing the lighter (F1-F4) deposits and for degassing towers and tanks. Before environmental concerns about chlorinated hydrocarbons were recognized, these chemicals were used extensively to remove heavy organic deposits. They have been replaced by refinery fluids, aromatics, and terpenes to dissolve the F4, F5, and F9 deposits. Nmethyl-2-pyrrolidinone also is a very effective polar solvent with low toxicity characteristics. The effectiveness of the application depends greatly on proper application conditions, such as flow rate and temperature. The last category of fluids for removing organic deposits combines surfactants, organic solvents, and water to form emulsions. These formulations can extend the efficiency of the organic phase by dispersing it into a large amount of water. Emulsions with an organic outer phase are particularly useful for cleaning large vessels, such as tanks and towers, that have very large volumes but small surface areas. Acidic emulsions combining an acid and an organic solvent can dissolve deposits that have both organic and inorganic compositions, such as an oily rust deposit. Solvents for water-side deposits Water-side deposits usually contain minerals such as iron oxides (corrosion deposits), hardness (Ca and Mg carbonates), and silica; in individual cases other minerals can also be found. The solvents for removing inorganic deposits usually contain mineral acids, organic acids, or chelating agents. Mineral acids used in chemical cleaning include hydrochloric acid (HCl), hydrofluoric acid (HF), sulfuric acid (Hsub 2SOsub 4), phosphoric acid (Hsub 3POsub 4), nitric acid (HNOsub 3), and sulfamic acid (Hsub 2NSOsub 3H). Most of these acids have very low pKa values and are completely ionized at the use strength. The exceptions are HF, which has a pKa value similar to formic acid, and Hsub 3POsub 4, which is about ten times as ionized as HF. Hydrocloric acid is the simplest, most common, and most versatile mineral acid. It is used on virtually all types of industrial process equipment, at strengths from 5% to 28% (5-10% is the most usual range). It can be inhibited at temperatures up to about 180 deg F (oilfield acids are used up to about 350 deg F, but different standards of protection apply). HCl usually is not used to clean Series 300 stainless steel, free machining alloys, magnesium, zinc, aluminum, cadmium, or galvanized steel because of the potential for generalized or localized attack. HCl will dissolve carbonates, phosphates, most sulfate scales, ferrous sulfide, iron oxides, and copper oxides. With appropriate additives, fluoride deposits, copper, and silica can also be removed from surfaces with inhibited HCI. The equations in Figure 6 represent the basic reactions between common deposit materials and HCI. Sulfate and phosphate scales are converted to the more-soluble chloride salts bv the action of HC. Often, it is not desirable to contact the fouled metal with a strong mineral acid because of the danger of damage to the equipment during or after cleaning. An alternative solvent family consists of aqueous solutions of chelating agents and organic acids with pH values of about 2 to 12. A chelating agent forms an equilibrium complex between the metal ion and the complexing agent, characterized by the formation of more than one bond between the metal and the molecule of the complexing agent, and resulting in the formation of a ring structure incorporating the metal ion. This is different from the reaction of a complexing agent, such as ammonia or thiourea, with a metal ion, such as Cusup 2+ or Cusup +, in which the complexing agent coordinates with only one reactive site on the metal ion. The formation of rings by the multidentate (many reactive sites) ligand gives added stability to the complex. The equilibrium for the reaction: M + nL - MLsub n does not go as far to the right for monodentate ligands as it does when a chelating agent is present. This is known as the chelate effect. The equilibrium constants for these reactions, K = MLsub n/MLsup n, define the strength of the chelate complex (Table 3). For example, the equilibrium constant for Cusukp 2+ reacting with four molecules of ammonia (NHsub 3) is 10sup 13 (log K = 13), while for the complex of Cusup 2+ with the chelating agent EDTA, log K = 19. Citric acid. This material was one of the first organic acids used in industrial cleaning operations (2). One formulation for removing iron oxide from steel surfaces consists of a mixture of citric acid and formic acid (3). It was claimed that the mixture of the two acids would hold more iron in solution than either of the acids alone (citric acid alone precipitated iron after several hours of contact). A process for cleaning iron oxide and copper from boilers uses monoammonium citrate at pH values of 3-5 to remove the iron oxide (4). The ammonium citrate solved the precipitation problems encountered with the use of citric acid alone. The pH is then raised to 9 with ammonia or an amine, and an oxidizer is introduced, which dissolves the copper. The iron oxide removal step is conducted at 150 deg F to 212 deg F, while copper removal is conducted at 150 deg F. Ammonium citrate and sodium citrate solvents are currently used to clean a wide variety of heat-transfer equipment, including boilers and various types of service water systems. A major advantage of citric acid formulations is their low toxicity and ready biodegradability in most waste treatment plants. EDTA. EDTA is a very versatile chemical that forms metal ion complexes with higher equilibrium constants than citric acid (Table 3). As a result, chemical cleaning solvents with pH values from 4.5 to about 9.2 have been formulated that can remove Fe and Cu (as well as Ca, Ni, and Cr). A high-pH iron oxide removal and passivating solvent is described in (5). Ammonium salts of alkylenepolyamine polyacetic acids, especially EDTA, were shown to dissolve iron oxides from steel at a pH of 9.0 and at temperatures up to 300 deg F, and would leave the steel in a passive condition (i.e., resists rusting). The copper in the scale is not removed by this process, but can be dissolved by contacting the surfaces being cleaned with ferric EDTA produced by oxidizing the ferrous EDTA generated during the iron oxide dissolution process (6). This copper removal stage is usually conducted at temperatures of 150 deg F to 180 deg F (65 deg C to 82 deg C). Introducing an oxidizing agent, such as air, sodium nitrite, hydrogen peroxide (7), or gaseous oxygen (8) generates the ferric ion. EDTA formulations buffered to pH 9 or 5 with ammonia or to pH 5 with sodium hydroxide are used to clean the same types of equipment as the citrate salts described above. The major advantage of the EDTA solvents is that they are much more aggressive than citrate salts for removing very heavy iron oxide deposits especially if they contain copper. Two major disadvantages are the high cost per pound of metal removed and low biodegradability. HEDTA. Hydroxyethylethylenediaminetriacetic acid was tested as a solvent for iron oxides at a high pH (9), and is about as effective as EDTA. However, HEDTA has much higher solubility in water than EDTA (at room temperature, 6% for HEDTA vs. 0.1% for EDTA). Therefore, low-pH chelating solvents with exceptional iron oxide dissolution behaviors are possible (10). Tests have shown that formulations of HEDTA, HEDTA with formic acid, or HEDTA with Hsub 2SOsub 4 will clean iron oxides from heat exchanger tubing much faster than other acidic solvents, such as hydroxyacetic/ formic or diammonium EDTA (11). The increase in cleaning rate is attributed to the low pH (1.2 to 2.3) and the strong chelate bonds formed with Fe(III) even at a low pH. Figure 7 compares the iron removal rate from a pipe scaled with red rust for two different solvents - the low-pH solvent, consisting of HEDTA/Hsub 2SOsub 4, removed the scale much faster than the pH-5 diammonium EDTA formulation. If the scale is mostly ferric oxide (FeOOH, or red rust), a safe solvent using HEDTA and the reducing agent erythorbic acid can be used to clean cooling water systems at temperatures at or below 150 deg F (12). Organic acids. All of the chelating agents described above are also organic acids. Eberhard and Rosene taught the use of solvents consisting of formic acid or citric acid for cleaning nondrainable tubes in superheaters and reheaters (13). Reich developed a solvent that contained both acids, at (weight) ratios of 3:1 to 1:6 of formic acid to citric acid, as a solvent for removing iron oxide deposits (3). The advantage of these mixtures was that they avoided the precipitation of solids that formed in pure formic or citric acid solutions. Other mixtures of organic acids include formulations of formic acid with hydroxyacetic acid (HA), and citric acid with hydroxyacetic acid (14). A mixture of HA with formic acid at a 2:1 ratio held more iron in solution than other mixtures. Table 4 summarizes some of the benefits and drawbacks for common solvents used to remove water-side deposits. Corrosion inhibition Most of the chemical solvents for removing water-side deposits and some of the solvents for cleaning process-side deposits contain corrosion inhibitors to protect the equipments metal from attack by the solvent. The subject of corrosion inhibition is beyond the scope of this article; see (15, 16) for more information about inhibitor types and mechanisms of action. For this discussion, it is important to know that there are two major types of corrosion inhibitors used in cleaning formulations - inhibitors used in HCl-containing solvents and inhibitors used in organic acids, chelants, and sulfur acids. Essentially all of the materials used by contractors are proprietary blends. Examples of HCI inhi