Guidelines for Nonchemical Methods of Heat Exchange Equipment Cleaning from Deposits

DEVELOPED BY All-Russian Thermal Engineering Institute, JSC (JSCVTI)
PREPARED BY R.P. Sazonov 
APPROVED BY the Department of Development Strategies and Scientific and Technological Policy of RAO UES of Russia on July 18, 2000. First Deputy Head A.P. Bersenev

FIRST INTRODUCTION
The present guiding document is applicable to water-heating heat exchange equipment (excluding water heating boilers) installed at power plants (TPPs) and in heat supply systems. It contains requirements to various nonchemical methods of heat exchanger tubes cleaning, ensuring reliable and high-quality operation of the main (at TTPs and boiler houses) and auxiliary (at heat point of centralized heat supply systems) equipment.
This document shall be guided by enterprises located in the territory of the Russian Federation, including unions, associations, corporate groups, joint-stock companies, as well as inter-branch, regional and other associations that incorporate thermal power plants, boiler houses and heat points regardless of the form of property and subordination.
1. GENERAL PROVISIONS
1.1 Tubular water-heating equipment connected with centralized heat supply systems is widely used at thermal power plants, in boiler houses, and heat points of heat consumers: delivery and make-up water heaters at TPPs and in boiler houses, heaters of hot water supply and heat point heating. While in operation, heat exchange equipment tubes are contaminated from the inside with various kinds of deposits, including ferruginous and carbonate ones.
1.2 Make-up water heaters at TTPs and hot water supply heaters at heat points are intensively contaminated due to the source water quality. Make-up water heater tubes are often clogged by mechanical inclusions.
Contamination of line heaters is less intensive and depends significantly on maintaining water-chemistry conditions at TTPs and in boiler houses. The rate of deposition in line heater tubes increases in case of violation of operational rules with respect to make-up and delivery water indicators, associated with increased hardness, lower pH, and excess of dissolved oxygen content. This process is also aggravated by the carry-over of ferruginous deposits generated during the heat pipeline corrosion from a heat network and heating systems.
1.3 The heat exchange equipment contamination results in:
- decrease of heating rating due to the drop of actual heat transfer coefficients due to increasing heat transfer resistance in tubes;
- increase of temperature difference in delivery water heaters, resulting in the energy performance degradation and possible excessive fuel consumption;
- increase of hydraulic resistance in tubes due to the reduction of their flow cross section and growth of roughness.
A significant decrease of heat transfer coefficients in hot water supply heaters operating with tap untreated water leads to a decrease of water temperature below 60-65 °C, required as per regulatory data.
Based on the above, a regular cleaning of the water heating equipment heat exchange tubes is required.
1.4 The chemical method has been the basic method of pipe cleaning from deposits for a long period of time. This method allows washing the entire internal heat exchange surface with water solutions of various chemicals without opening water heating equipment.
However, this method has significant disadvantages:
- significant consumption of expensive chemicals;
- requirement to neutralize and recycle waste water after treatment.
1.5 The hydraulic high-pressure method of cleaning from deposits of line heater and make-up water heater tubes is widely used at power plants along with different chemical pipe cleaning methods.
In the past few years, other physical, i.e. nonchemical pipe cleaning methods have been applied:
- hydro-cavitation pipe cleaning;
- thermo-abrasive pipe cleaning;
- hydromechanical pipe cleaning;
- electric hydro-pulse pipe cleaning;
- ultrasonic pipe cleaning.
2. GENERAL REQUIREMENTS TO SELECTING THE OPTIMAL NONCHEMICAL METHOD OF CLEANING HEAT EXCHANGE EQUIPMENT FROM DEPOSITS:
- complete absence of deposit residues that foster the formation of new deposits;
- tube cleaning from deposits of any composition, including mechanical inclusions clogging the tubes;
- low power consumption (compressed air <2.5 kW/h; water <3.5 kW/h).
- long service life of cleaning unit components (minimum 400 hours prior to the first routine repair);
- small unit dimensions and weight (<660x600x305 mm; <60 kg);
- low consumption of water or other medium (water <2.0 t/h; air <1.5 m3/min);
- high reliability with respect to safety in accordance with the "Safety Regulations for Operation of Thermal Mechanical Equipment of Power Stations and Thermal Networks", 1997;
- no mechanical and invisible damage of heat exchanger tubes while cleaning;
- environmental safety;
- affordable cost of cleaning (<12 RUB/rm).
Considering the above listed requirements, all the main currently used physical methods of cleaning heat exchange equipment for heat supply are given below.
None of them fully meets the listed requirements. The applicability of each method should be estimated with account of the above requirements and local conditions.
The optimal method of tube cleaning from deposits shall be selected based on a preliminary condition analysis for heaters to be cleaned and cleaning unit specifications.
3. CHARACTERISTICS OF METHODS OF HEAT EXCHANGER TUBE CLEANING FROM DEPOSITS AND RECOMMENDATIONS FOR THEIR SELECTION
3.1 The following methods for cleaning heat exchanger tubes are recommended:
- high-pressure hydraulic method;
- hydro-cavitation method;
- thermo-abrasive method;
- hydromechanical method;
- electric hydro-pulse method;
- ultrasonic method.
3.2 A number of additional factors shall be considered when selecting the cleaning method:
- heat exchanger design (straight or U-shaped tubes);
- location (vertical or horizontal);
- tube material (brass, stainless steel, carbon steel);
- deposits composition (carbonate, ferruginous, mixed, silt, sandy and stony);
- presence of completely clogged or plugged tubes and their quantity;
- type of equipment (delivery water heaters, make-up water heaters, hot water supply heaters, heating heaters);
- acceptable cleaning time periods;
- corrosion condition of heat exchanger tubes;
- cleaning on one's own account or under contracts with specialized organisations;
- cost of cleaning in comparable prices.
3.3 Tables 1, 2 and 3 show the technical characteristics of the applied cleaning methods (Table 1), specifications of equipment to be cleaned (Table 2) and recommendations on selecting the optimal cleaning method (Table 3). The following organisations are listed in Table 2: cleaning equipment designers, manufacturers and actual performers of works.
Each of these methods is reviewed in details below.
Table 1 - Technical Characteristics of Nonchemical Methods of Tubular Heat Exchangers Cleaning from Various Deposits 
Table 3 - Summary Table of Correspondence of Various Cleaning Methods to the Specified Requirements of Section 2 and Recommendations on Selecting the Optimal Cleaning Method
The data given in Tables 1 and 2 have been obtained directly from the equipment designers and manufacturers, as well as the organizations performing heat exchanger cleaning.
3.4 High-pressure hydraulic method
3.4.1 This method is used to clean 16 to 25 mm diameter brass tubes of line heaters from deposits up to 3 mm thick.
3.4.2 The method is based on converting the energy of high pressure of water supplied into the tube through a special nozzle into the kinetic energy of a high-speed flow at the outlet of the nozzle gradually being moved in the tube under cleaning. As a result, the flow withdraws deposits from the tube internal surface.
3.4.3 Units of this type shall be operated at a working pressure of water upstream of the nozzle of 63 MPa, which is generated by a plunger pump. Moreover, the units of some foreign companies are applied: Atümat, Woma, Hannemann (Germany) with the pressure of up to 98 MPa.
3.4.4 Pressure decrease below 63 MPa impairs the quality of cleaning the tubes with hard carbonate deposits, particularly stainless steel tubes. Deposits shall be preliminary drilled to a depth of 80-100 mm to insert the nozzle in order to clean the tubes fully clogged with deposits or local plugs.
3.4.5 The disadvantages of the method are as follows:
- requirement for highly qualified service personnel in accordance with the "Safety Regulations for Operation of Thermal Mechanical Equipment of Power Stations and Thermal Networks" (1997);
- complexity of transportation due to significant unit dimensions and weight;
- high labor intensity;
- rapid wear of plunger pump seals and high pressure hoses;
- requirement for two to three passes through the tubes for high-quality cleaning to the base metal.
The unit operating time prior to the first routine repair shall be minimum 400 hours. Three operators work with the unit.
3.5 Hydro-cavitation method
3.5.1 The method is used to clean 16 to 25 mm diameter brass, stainless steel tubes with deposits of any composition and thickness, including plugs, regardless of their operating life.
3.5.2 This method represents the advanced high-pressure hydraulic method of cleaning heat exchanger tubes from deposits.
High-pressure water (up to 60 MPa) enters the tube not as a solid jet, but as a cavitating jet, generated by navigation nozzles with the help of a special profile.
3.5.3 The method is based on the cavitation effect associated with the discontinuity inside the liquid flow and formation of dissolved gas bubbles therein. As the liquid flow velocity increases, the pressure in it decreases and drops to zero at a certain critical velocity. As a result, saturated vapors increase their volume and turn into large cavitation bubbles. The bubbles collapse at a very high speed, resulting in multiple microexplosions that clean the surface. The repeated explosions destruct the deposits, withdraw them from the surface and remove out of the tubes with the help of flowing water.
3.5.4 This method has the following advantages, if compared with the high-pressure hydraulic method:
- faster cleaning of tubes partially clogged with deposits;
- cleaning of tubes completely clogged with deposits or containing solid inclusions;
- cleaning of tubes to the base metal.
When the tubes are completely clogged, the through channel is preliminary cleaned with special nozzles on both sides of the tubes. Then the final cleaning follows.
3.5.5 The hydro-cavitation cleaning unit is installed on a common frame. Its weight is 1500 kg including component parts. The unit incorporates the following assemblies and components:
- three-plunger water pump designed for 4-6 m3/h water flow rate;
- 90 kW electric motor with solenoid starter;
- high-pressure hose designed for 60 MPa;
- drain hose;
- special extension along the length of tubes;
- set of cavitation nozzles;
- hydraulic control pedal;
- intercom.
3.5.6 The unit is used to clean horizontal and vertical heat exchangers. If the heat exchanger can't be cleaned on-site, it shall be located on a mounting platform in the horizontal position prior to cleaning.
Three operators work with the unit.
3.5.7 The disadvantages of the unit in view of this cleaning method are as follows:
- requirement for highly qualified service personnel in accordance with the "Safety Regulations for Operation of Thermal Mechanical Equipment of Power Stations and Thermal Networks" (1997);
- complexity of transportation due to significant unit dimensions and weight;
- insufficient time to failure due to the damage of the high-pressure hose and failure of the pump sealing.
3.6 Thermo-abrasive method
3.6.1 The method is used to clean any diameter brass and stainless steel tubes of delivery and make-up water heaters from deposits of any composition and thickness, including solid deposits and plugs.
3.6.2 The method is based on creating a supersonic heated gas-fuel jet that moves slag waste abrasive powder in the tube. The jet is created in a special device (thermo-abrasive gun) . Figure 1 shows a schematic diagram for cleaning tubes 5 by means of the thermo-abrasive method, including a gun 1 for abrasive-jet cleaning of tubes, a container 2 with abrasive material, a fuel cylinder 3, and a starter 4.
Figure 1 - Schematic Diagram of Tube Cleaning Using Thermo-Abrasive Method
3.6.3 The device (gun) for abrasive-jet cleaning (Figure 2) includes a distribution head 1 with a fuel channel 2 in it to supply low-grade petrol in the amount of 5-7 l/h, a fuel swirler 3, a swirler 4 of compressed air, that is supplied from an external source in the amount of 5-6 m3/min. An abrasive supply nozzle 5 and a fuel supply nozzle 6 connected to the fuel channel 2 are installed in the head. There is an opening along the axis of the distribution head 1 and the abrasive supply nozzle 5 with an abrasive supply nipple 7 inserted in it. Air is supplied from an airline or a compressor into a nozzle 9. A flow combustion chamber 10 with holes is installed in the body of the gun 8 coaxially with it. A channel 16 of an abrasive acceleration nozzle 12 is a wear-proof removable insert.
A glow plug 18 is screwed into a sleeve 17 for fuel ignition.
Figure 2 - Jet-Abrasive Cleaning Device
3.6.4 The air-pressurized abrasive from the storage container and the air mixture from the compressor and burning fuel are supplied to the gun and then to the tube to be cleaned. The starter is switched off and the high voltage cable is disconnected from the gun following the gun start-up. The deposits are withdrawn and removed from the cleaned tube together with the abrasive as a result of the combined effect of hot air and heated abrasive on the deposits of any composition and thickness.
3.6.5 The cleaning unit is not mass-produced, it is unconventional, and is manufactured by the developer in single quantities.
3.6.6. The unit is intended to clean straight tubes of horizontal and vertical heaters. Three operators work with the unit.
3.6.7 The advantage of the method is the high tube cleaning rate (up to 60 rm/min for brass tubes). Brass tubes shall be cleaned upon the deactivated impulse burner, with two-sided purging, and using a fine-particle abrasive powder with fraction size of up to 1 mm. Stainless steel tubes shall be cleaned with the activated impulse burner, using a coarse-particle powder with fraction size above 1 mm.
3.6.8 The method disadvantages include:
- high abrasive consumption (5-10 kg per m2 of tubes to be cleaned), that increases the cleaning cost;
- air pollution in premises where the cleaning takes place, unless a special suction device is used, or the area of deposits discharge together with abrasive is covered with canvas;
- stringent fire safety requirements when working with the impulse burner;
- possibility to damage an oxide film inside the brass tubes.
3.7 Hydromechanical method
3.7.1 Straight and bent pipes, spiral tubes (coils), sewage system, and 7-200 mm diameter pipelines up to 50 m long are cleaned with the help of KROT fixed pneumatic units, that transfer rotation with the particular torque to a cleaning tool via a flexible shaft.
They are used to clean any kind of metal tubes from deposits of any composition and thickness, including solid deposits and plugs (completely clogged pipes).
3.7.2 The method involves the mechanical destruction of hard deposits (usually carbonate) on a heat exchanger tube internal surface by chipping them with a rotating cleaning tool (roller head, roller cutters, or special hard-alloy drills; with rollers or hard-alloy segments with a specialized profile, respectively) with subsequent removal of deposits with flowing water. The water supply to the tool working area also significantly improves pipe cleaning efficiency and tool service life. The cleaning tool design along with the accurate selection of its dimensions relative to the diameter of the pipe to be cleaned eliminates the possibility of the pipe internal surface damage.
3.7.3 Krot unit comprises the following basic assemblies, schematically shown in Figure 3:
Figure 3 - Krot Unit
3.7.4 Figure 4 shows the heat exchanger tube cleaning diagram
Figure 4 - Heat Exchanger Tube Cleaning Diagram
The supply of compressed air (with 0.4-0.7 MPa pressure and 1.5 m3/min flow rate, with no air handling possible) and technical water (with 0.2-0.5 MPa pressure) to the valve assembly nozzles shall be ensured to provide for the operation of KROT fixed pneumatic unit. 
A foot pedal controls the valve opening/closing to supply compressed air to a pneumatic engine that transfers rotation with the particular torque and speed to the unit output shaft through a reducing gear. The pedal also controls the valve opening/closing to supply water to the cleaning tool working area. Water is supplied along the channel in the unit body through the flexible shaft braid directly to the cleaning tool.
The braided flexible shaft (the diameter and length to be selected relative to the pipe to be cleaned) is attached to the fixed pneumatic unit output shaft. The cleaning tool is attached to the flexible shaft opposite end.
3.7.5 An axial force shall be applied to the cleaning tool to clean the pipes from hard deposits. The cleaning tool is self-screwed in case of soft deposits.
The pipe cleaning from soft or insignificant deposits can be done in one pass using a roller head (for straight pipes), a disk roller cutter (for U-shaped pipes), or metal brushes.
The cleaning is recommended to be done in two passes in case of cleaning completely clogged pipes (to reduce the force transferred by the flexible shaft, in order to increase its service life). First, make a hole in the fully clogged tube using a special hard-alloy drill, then clean the remaining deposits with the roller head (for straight pipes) or the disk roller cutter (for U-shaped pipes).
3.7.6 It is better to use Krot unit at power plants and in boiler houses at the available supply of compressed air with 0.63 MPa pressure, significant contamination (including a completely blocked section) of tubes of any material at different operating time periods. The method may be used directly where the heat exchangers are installed without dismantling them, at relatively 
insignificant costs and a high cleaning rate. The method advantages include high quality of cleaning, small unit dimensions (460x240x200 mm) and weight (12 kg excluding the flexible shaft and supply hoses), its transportability and ease of installation. The disadvantage involves the relatively short service life of the roller heads and roller cutters, but only when cleaning the tubes from very hard deposits, which requires their periodic replacement.
3.7.6 Tubecleaner-2000 unit is designed and produced by Rädler (Austria). It is used to clean tubes made of steel, copper, brass and other materials with deposits of any degree of hardness.
3.7.7 The unit consists of the following basic assemblies, as shown in Figure 8: a frame 1 to suspend a hydraulic cylinder 2; a rod with a bit 3; an air turbine 4; a water pump 5; a foot pedal 6; an oil pump 7; a control unit 8; a support shaft 9; a clamp 10. Compressed air rotates the air turbine 4 which, in its turn, rotates the rod with the bit 3 that destructs deposits. The movement in horizontal (or vertical) position is done by the hydraulic cylinder 2 using the control unit 8 handle.
Figure 8 - Tubecleaner-2000 Unit General View
3.8 Electric hydro-pulse method
3.8.1. The method is used to clean straight and U-shaped 10 to 25 inside diameter brass and steel tubes with deposits up to 3 mm thick. Cleaning is carried out with the help of ZEVS-16 unit.
3.8.2 The working principle is based on converting the electrical energy into mechanical one using the energy of a high-voltage electric discharge in water. The shock wave and hydrodynamic flows generated by the electric discharge in water destruct scale and 
other deposits on a heat exchange equipment tube internal surface. When cleaning the tubes of an exposed heat exchanger, water is supplied from one end of the tube. The unit working tool is entered from the other end and moves gradually in the course of cleaning.
The water flowing through the full cross-section tube is drained on the entry side of the working tool being a flexible coaxial cable. The source of electromagnetic pulses is a converter of the network supplied electrical energy (220 V, 50 Hz) into high-voltage electromagnetic pulses transmitted to the cleaning area via the coaxial cable. The electric discharge occurs at the end of the cable put in water. The pulse frequency is set at a level of 1-10 Hz. The coaxial cable burns out when cleaning for 3-4 to 10-15 mm per 10 rm of a tube depending on the deposits hardness.
3.8.3 This method may be used to clean both large-sized heaters in power plants and boiler houses, and heaters at heat points. However, it is preferred to be applied to clean hot water supply and heating steam-water and water-water heat exchangers at central and individual heat points. This is due to the relatively low tube cleaning rate (up to 5 rm/min), small unit dimensions (660x600x305 mm), unit weight (57 kg), and possibility to locate it at heat points at insignificant power (2.5 kW) and water consumption (0.36 t/h). The number of tubes to be cleaned in hot water supply heaters is significantly less than that in the line and make-up heaters at power plants. Therefore, the lower tube cleaning rate compared to other methods does not remarkably affect the overall cleaning duration.
The method disadvantage consists in the possibility to damage the heat exchanger brass tubes that have been in operation for a long time and have mechanical or corrosion damage. The tubes may unseal during cleaning and leak water under pressure via through openings or openings formed in the course of cleaning due to tube wall thinning or for other reasons.
Therefore, prior to the full-scale tube cleaning, several control tubes shall be removed from the heater, cleaned with ZEVS-16, pressurized and cut along the generator for visual inspection of their condition after cleaning. If defects are found in at least one removed tube, the tubes SHOULD NOT be cleaned using this method.
3.8.5 Service personnel (operator) shall have minimum III level of electrical safety qualification and a permit to work with electrical installations with voltage above 1000 V.
3.9 Ultrasonic method
3. 9.1 This method is used to clean tubes of hot water supply and heating heaters with straight and U-shaped tubes from carbonate deposits up to 2.5 mm thick without heaters shutdown and disassembling for a period of cleaning.
3.9.2 The method is based on the excitation of ultrasonic vibrations on the surface of tubes and deposits.
Due to the difference in physical and mechanical properties of the tube metal and deposits, the ultrasonic vibrations result in generation of fatigue cracks in the deposits and their further separation from the metal.
3.9.3 The advantages of this cleaning method compared to those described above are as follows:
- no heater shutdown for a period of cleaning;
- minimum unit power (0.2 kW);
- no water drains and other media (compressed air) and materials;
- prevention of new deposits formation in tubes.
The ultrasonic method is the only nonchemical method, that may also be applied to clean a tube outer surface, particularly with relation to heating heaters, extremely subjected to deposits formation upon partial make-up of the secondary heating circuit with raw tap water.
3.9.4 When cleaning the tubes from the inside of hot water supply sectional water heaters with U-tubes using this method, the detached deposits in the tubes then fall out into the U-tubes and do not enter the system.
Periodic purging or flushing of intertubular space is required to remove detached deposits in heating heaters of similar design, where the deposits are usually formed from the outside of the tubes.
As for steam-water heaters, the detached deposits should be collected into special devices such as mud traps downstream heat exchangers. The deposits shall be periodically removed from the mud traps by purging (draining from a lower point).
3.9.6 The disadvantages of this method include the following:
- relatively slow (2.5-3 months!!!) process of tube cleaning from deposits;
- generator de-energization at emergency power outages;
- inability to clean tubes completely clogged with deposits or locally plugged.
4 COMPARATIVE ASSESSMENT OF VARIOUS HEATER TUBE CLEANING METHODS
4.1 Quantitative characteristics of the basic known cleaning methods are given in Tables 1 and 2, Section 3. A quantitative assessment of the requirements specified in Section 2 has been accepted based on the best performance indicators achieved at various cleaning methods.
4.2 A Summary Table 3 has been drawn up based on all the indicated characteristics. In this table, the quality of each method as to various indicators is defined by "+" and "-" signs.
The "+" sign means that the method satisfies the indicator requirements specified in Section 2. The "-" sign means that the method does not satisfy these requirements.
4.3. Considering the Tables 1, 2 and 3, it follows that the hydromechanical method is the OPTIMAL method for cleaning tubes from deposits that satisfies most of the requirements of Section 2.
This method is suitable for cleaning both large heating surface heat exchangers at TPPs and small heating surface heat exchangers in boiler houses and at heat points.
The cost of Krot unit is significantly lower compared to Tubecleaner-2000 unit (Austria).
4.4 The electric hydro-pulse cleaning method is somewhat inferior to the hydromechanical one due to the imposed restriction regarding cleaning of heat exchangers with long service life tubes. This method is recommended with restrictions for cleaning heat exchangers at TPPs, in boiler houses and at heat points.
The additional advantages of the hydromechanical method over the high-pressure hydraulic, hydro-cavitation, and thermo-abrasive methods are as follows:
- ease of maintenance, allowing semiskilled personnel to work with the units;
- small unit dimensions and weight.
In case there is no source of compressed air with 0.6 MPa pressure at heat points, the hydromechanical cleaning method shall only by applied, if there is a compressor capable of producing the required pressure.
4.5 The hydro-cavitation and thermo-abrasive cleaning methods should be used at power plants and in boiler houses along with the hydromechanical and electric hydro-pulse methods. The high-pressure hydraulic method is allowed to be used on heaters with brass tubes in case of 0.5-3 mm thick NON-HARD deposits.
The hydro-cavitation and thermo-abrasive methods are inferior to hydromechanical and electric hydro-pulse methods in many features and requirements.
Units for cleaning using these methods are single products, they are not mass-produced, and are maintained by the developers of these methods. Moreover, they are power-consuming and difficult to operate.
4.6 The high-pressure hydraulic method is inferior to the hydro-cavitation and thermo-abrasive methods due to the high labor intensity and lower tube cleaning performance.
The ultrasonic cleaning method is the only method that allows cleaning during operation without heat exchangers shutdown. It is most effective for cleaning hot water supply heat exchanger tubes.