Many materials will foul heat exchangers over time. This fouling leads to less efficient plant operations due to reduced heat-transfer capacity of exchangers and lower fluid flow through fouled equipment. Increased corrosion of base metal resulting from metal oxide deposition and overheating will also contribute to lower plant efficiency.
Removing the fouling deposits is crucial to restore equipment efficiency, which is most readily done during plant turnaround. Turnarounds offer other reasons for cleaning, such as to ease disassembly of equipment, prepare metal surfaces for inspection or maintenance, minimize vapor releases when opening vessels, and to ensure the atmosphere inside the vessel is safe for entry.
Heat exchange equipment can be cleaned mechanically or chemically. The choice depends upon several factors:
1. How clean do you want the metal surface? Chemical cleaning usually produces a cleaner metal surface than mechanical cleaning can achieve. Some chemical cleaning procedures are designed to condition a vessel for safe entry by only removing volatile fluids or toxic vapors. These applications are not designed to remove all of the deposits on the heat exchange equipment.
2. Is the equipment going to be disassembled? Most mechanical methods require disassembly, whereas chemical cleaning techniques require little or no disassembly. So, if disassembly isn’;t otherwise required, chemical cleaning may be preferred.
3. Are the deposits in the heat exchange equipment economically soluble in a chemical cleaning solution? Most metal-oxide and hard-water deposits are easy to remove with chemicals. Deposits resulting from hydrocarbons may be removed by chemical methods, but sampling is highly recommended in these cases. This will allow the cleaning vendor to perform solubility tests on the deposits in various solvent formulations.
If a deposit is not readily soluble (less than 80% solubility) or if the deposit does not slough off the metal surface, then chemical cleaning applications will be less successful. The cost of chemical cleaning may become too high if deposit removal requires more than two stages of solution application to obtain the desired level of cleanliness. Deposits that consist mostly of very hard or highly polymerized organic deposits usually cannot be economically removed via chemical cleaning. However, some newer solvents can remove a number of deposits that occur in plastic elastomer manufacturing processes, and are proving to be more economical than mechanical cleaning.
4. What is the physical condition of the heat exchanger equipment? The equipment metallurgy must be compatible with the cleaning solvent. Also, the heat exchanger cannot be chemically cleaned if it is so plugged with deposits that liquid will not flow through it.
If you choose chemical cleaning, then consider several factors to ensure that you obtain maximum benefit from the cleaning.
Picking a product
There are many chemical cleaning products available on the market that will dissolve most deposits found in process equipment. The success of each formulation depends on the following:
The deposit’;s contamination matrix. Deposits usually arise from several sources of contamination: metal corrosion products, water hardness, product impurities and overheated product contamination subproducts. The deposit composition matrix can be both organic and inorganic in nature. Organic deposits can be made up of light-ends on one end of the spectrum, to heavily coked and carbonized debris on the other end. In general, the more homogeneous a deposit is, the easier it will be to remove with a single chemical cleaning product.
Submit deposit samples for testing. If you have no history of cleaning a particular operating system, sampling deposits for laboratory analysis and solubility testing can greatly enhance the chances of success for a planned cleaning. If a system has been subject to operational upsets (treatment chemicals, tube leaks, etc.) or a major change in feedstock composition, the resulting deposits can be quite different from those encountered during previous cleanings, and you should consider sampling. If you were not able to sample the deposits prior to the cleaning and the chemical is not working as well as expected, consider sampling the remaining deposits so that future adjustments can be made.
Finding a chemical cleaning product that best dissolves or loosens the adhering deposits from the equipment is just the beginning of the job design process. The next question is how to best apply the chemistry.
The job design process takes into consideration the engineering requirements of the cleaning procedure. This process addresses several needs, such as isolation of equipment being cleaned, appropriate solvent flow, venting and draining, temperature, and post-cleaning neutralizing and preservation of system base metal.
Isolate equipment for cleaning
One of the leading causes of a less-than-satisfactory chemical cleaning is when the chemicals that are pumped into the exchanger leak through valves into other process equipment, or fluid from other process equipment leaks into the chemical cleaning solvent stream.
Existing isolation valves do not always close completely, which might be due to worn valve seats. This occurs because the valve cannot fully seal if there are deposits on the valve seats. In this case, valves may stop cross-contamination of fluids at the beginning of the cleaning, but they will start to leak as the deposits are removed from the valve seat.
It is best practice to place a blind flange or pancake blind (a.k.a. slip blind) in these valve connections to isolate the heat exchanger. If you don’;t do this, be prepared to tighten down on isolation valves during cleaning and address any leaks through the valves. You can minimize leak-through by keeping equal pressure on both sides of the isolation valves.