Determine flow requirements
Any cleaning solution must contact the deposits in order to work. Hence, a tube that is completely plugged with deposits will not allow fluid flow, and cannot be chemically cleaned.
Flow requirements vary with the number of tubes in the heat exchanger that is being cleaned. Flow is even more critical if deposit components slough off surfaces rather than dissolving in the solution, and subsequently need to be removed by filtering the cleaning solvent.
If a particularly aggressive solution is used, such as hydrochloric acid, too much flow can be detrimental to metal surfaces.
Recommended guidelines for flow requirements are:
1. A minimum flow rate of 1 ft./sec. through each tube that is being cleaned.
- Heat exchanger with 100
1/2-in. tubes = 100 gpm flow requirement
- Heat exchanger with 500
1/2-in. tubes = 500 gpm flow requirement
2. A minimum flow rate of 1 ft./sec. through the exchanger shell being cleaned.
3. If you’;re using an aggressive acid, do not exceed flows of 2-3 ft./sec. Alkaline solutions, surfactants and chelant-type solvents can effectively and safely be circulated at flows greater than 10 ft./sec. without experiencing corrosion problems.
Flow requirements exist for several reasons. You must have enough flow to maintain the solution concentration at the deposit sites to achieve sufficient cleaning. An adequate flow also will flush any insoluble material that sloughs off out of the system. Once the cleaning process is completed, you must be able to displace the cleaning solution from the system. Excessive flow, however, can cause erosion or corrosion when using certain chemicals.
Adequately sized connections on heat exchangers are generally not present. For example,
If the heat-exchange equipment is so large that flow requirements cannot practically be achieved, there are alternative methods for applying the chemicals, such as foam or vapor phases. Foam cleaning has been effectively used on 8,000-tube surface condensers in the power industry with liquid-chemical injection rates as low as 100 gpm. The liquid-to-gas ratios of chemical foams are in the range of 10:1 to 15:1. This results in a foam flow rate in the range of 1,000 gpm to 1,500 gpm at a liquid solvent flow rate of 100 gpm.
The minimum required flow rate is often the determining factor when deciding on an application method. But consider other factors when looking at alternative application methods. What is the weight of the liquid? Can the vessel support the liquid volume weight? What temperature is required for the cleaning solution to effectively dissolve the deposits? Will the piping experience thermal expansion during alternative application methods? Can the system be vented and drained during cleaning?
Alternatives to the traditional fill-and-circulate cleaning techniques exist, but it is not the intent of this article to address them in detail. Alternative methods include on-line cleaning, foam cleaning, vapor phase with steam or hot inert gases, and vacuum vapor cleaning.
During most chemical cleaning applications, various gases are produced as a result of the reaction between the cleaning solution and the deposits. These gases can be flammable, explosive or toxic. Testing deposit samples will normally identify whether they will generate toxic gases. However, some gases almost always evolve from a chemical cleaning process. Certain precautions must be made in the job design for safely venting the heat exchanger during cleaning.
Vent connections should be made at all high-point locations on the heat exchanger or system piping. Do not allow the heat exchanger to become gas-locked or to accumulate pressure. This will impede flow through the exchanger.
The evolving gas is usually composed mostly of hydrogen. Because of its flammability, no open flames or smoking should be allowed near a chemical cleaning project.
If toxic gases (chlorine gas, hydrogen sulfide, etc.) are expected to evolve, be prepared to treat them. This may require scrubbing, adding chemicals to prevent formation of the gases, or flaring to contain and dispose of the gases.
Drain connections on exchanger equipment are usually located to induce proper flow during a cleaning. The inlet connection is generally located on the bottom of the heat exchanger and the outlet or return connection off the top of the exchanger, which allows the evolving gases to rise to the outlet. However, to drain the cleaning solution from the heat exchanger, connections must be installed at low points and dead legs. Strategically located low-point drains also provide a means of flushing out insoluble deposit components that may not have been carried out of the equipment with the solvent.
Heat up and clean up
Some chemical cleaning solvents work at ambient temperatures. Those that do are usually aggressive, highly concentrated acids that dissolve hard-water salts or soft, iron-oxide type deposits.
Chemical cleaning of exchanger equipment is generally completed at temperatures below 200 F. There are specialty chemical cleaning systems available to clean polymeric plastic resin from heat exchange equipment that require temperatures in excess of 400 F.
Since it is likely you will need to heat a chemical cleaning solution, consider a few general rules:
- Elevated temperatures will likely be required for the cleaning application if the equipment normally operates at high temperature and the resulting scale is formed as a highly dehydrated deposit. Solubility testing of the deposits will determine the required temperature.
- Most cleaning solutions that remove organic materials almost always require temperatures higher than ambient temperature to be effective.
- Almost any chemical cleaning solution will work faster at elevated temperatures.
- When determining the solvent temperature, consider the temperature limitations of the equipment being cleaned and that of any corrosion inhibitors that may be used (inhibitors are sometimes part of the cleaning solution formulation).
Passivate clean metal surfaces
Many types of chemicals can be applied during cleaning applications. These solvents are effective in removing the fouling deposits; however, chemicals left in the system can be detrimental to the heat exchange equipment at operating conditions. Since the metal surfaces are chemically activated by acidic solutions, these surfaces should be deactivated by neutralization and/or passivation.
Neutralizing is imperative after cleaning equipment with highly alkaline or acidic chemical solutions and is usually required before opening equipment for inspection. Both neutralizing and flushing the equipment is critical if you’;re using solutions that are high in chlorides, fluorides or sodium salts. This is less of an issue when you’;re cleaning with low-chloride or low-halogen solutions, such as chelants or surfactant-type chemical solutions.
You must also decide whether to passivate the metal surfaces after deposit removal is complete. If the metal surfaces are chemically activated after applying an acidic solution, rusting or re-oxidation can quickly occur. The resulting rust is not only unsightly, but will encourage and speed up the redeposition of fouling products. Chemical passivation is done to protect the metal surfaces if they are to be exposed to atmospheric conditions where re-oxidation can occur.
If the heat-exchange equipment is cleaned early in a shutdown and remains out of service for a period longer than a couple of days, passivation should be considered. This is especially true for heat exchangers with copper and iron metallurgy.
If the equipment is immediately going back into service, or is blanketed with an inert gas, such as nitrogen, passivation of the metal surfaces is not necessary.
For years, chemical cleaning has proved to be a cost-effective way to remove deposits from heat exchanger equipment. Equipment that has been thoroughly cleaned will operate longer before requiring another cleaning. Cost recovery from chemical cleaning is realized through:
- Increased production or decreased cooling/heating requirements;
- Extended equipment life due to reduced on-line corrosion;
- Minimized disassembly labor and material cost to complete the cleaning; and
- Faster turnarounds since chemical cleaning is usually completed in less time than that required to disassemble, mechanically clean and reassemble equipment.
To ensure that a safe, successful and efficient chemical cleaning project is completed requires effective job design, planning and execution.
Steven Barber is business development manager for BJ Services Co. E-mail him at SBarber@bjservices.com.