GOOD OPERATING PRACTICES• Preparation for performing work on heat exchangers involves two major elements1. Choosing the proper tools and equipment2. Observing the appropriate safety precautions Choosing Tools and EquipmentThe tools needed to perform maintenance on heat exchangers are divided into 2 categories1. Working Tools2. Safety Equipment
c• The first category includes hand tools and power tools needed to do a maintenance job. The working tools may include wrenches, impact tools , cleaning devices such as high pressure water guns, scrapers, brushes, squeegees etc., shovels, towels, buckets and large containers.• The equipment needed to seal or plug leaking tubes may include plugs, screw drivers, wrenches, hammers , welding equipment, and equipment to roll tube ends into tube sheets.• The second category includes the equipment required to insure personal safety.
• For work on larger components , where work must be done in a wet environment or in a confined space, low voltage safety lighting is needed. If high pressure water guns will be used , or if a water box must be entered, rain suits, boots, and goggles or safety glasses are required.
SAFETY PRECAUTIONS• Work should be never done on a heat exchanger until the system that includes the heat exchanger is shut down and isolated, and equipment such as valves , motors , pumps , switchgear , and controls are tagged out so that they will not be operated while maintenance is going on.• The heat exchanger should be drained and vented before it is opened
• The nature of some heat exchangers necessitates additional safety precautions .Work on large heat exchangers, for example ,, often requires working in confined work spaces. In such situations, air quality checks are performed to make sure that there is enough oxygen to breathe and that no dangerous gases are present• The two man rule is always followed in confined areas so that help is available if a worker encounters trouble :One worker always stays outside of the confined workspace and keeps steady contact with the worker on the inside.
• Additional ventilation may be necessary in some areas for two reasons1. To remove dangerous gases and provide fresh air with enough oxygen to breathe2. To cool a confined space if work is being done on hot equipment, such as a condenser being run under a partial load.• The safety precautions associated with the rigging of large , heavy parts, such as the heads of large heaters as illustrated in figure-2 must be carefully observed when large components are moved.
• The tools and equipment needed to perform maintenance on heat exchangers can be divided into two general categories.• Important practices to be exercised by the operator:1. Always make sure that the heat exchanger has been vented before start-up.Vapor is not a good conductor of heat and will therefore reduce its efficiency.
2. Never increase the pressure or temperature quickly when starting-up. This can cause mechanical damage.3. Report any mechanical rattling noises to your supervisor. Baffles can come loose and wear away the tubes. Moving baffles will rattle.4. Check the inlet and outlet temperatures regularly. They will indicate the efficiency of the exchanger, its efficiency can be calculated by the process engineer. However if temperatures change check that:
• cooling fluid flow is correct and that the temperature is at or below design• The hot fluid flow is correct and that the inlet temperature is at or below design5. If these are not at fault the exchanger may be fouled . This is a term used when a layer of solids is deposited on the tubes. The solids reduce the amount of heat transferred because they are poor conductors of heat. If fouling is suspected report this to the supervisor.
6. Always use minimum number of air fans for air duty. It is often possible in the winter to shut off a fan yet still obtain the required amount of heat exchange. This will reduce the energy costs.7. Regularly check the fins on a finned fan exchanger as they can become covered in dirt. This reduces the amount of heat transferred and should be reported if it occurs.8. The cold fluid valve outlet may be opened slowly until the cold fluid is passing through the exchanger.
9. Start opening the hot fluid inlet valve slowly10. Both valves, the cold fluid outlet valve and the hot fluid inlet valve should be opened slowly until fully open.
THE START-UP AND SHUT-DOWN OF HEAT EXCHANGERS• Heat exchangers must be warmed up slowly and cooled off slowly. This is particularly important when operating temperatures are very high or very low.• The sudden application of extreme temperatures will cause great inequalities of tube and or shell expansion. This may cause leak or other damage
PROCEDURE TO TAKE HEAT EXCHANGER OUT OF SERVICE1. The hot fluid must be shut off before the cold fluid. This should be done slowly to allow the exchanger to cool down. The cold fluid must not be shut off first. Otherwise, the heat from the hot side will cause the cold fluid to increase in temperature and as there is no place for the expansion , the pressure would build up and cause exchanger ruptures.2. After the hot fluid has been shut off, both on the inlet and outlet of the exchanger and the temperature has cooled to that of the cold fluid, then the cold fluid can be shut off on both inlet and outlet valves.
3. Both shell and tube side should now be pumped out to slop or drain down.4. Both inlet and outlet lines should be blanked off for safety.5. If the exchanger is in sour oil service or any iron sulfide scale is expected, the exchanger should be washed before opening to the atmosphere.
PROCEDURE TO PLACE A HEAT EXCHANGER IN SERVICE1. Check the exchanger carefully to ensure that all plugs have been replaced and that all pipe work is ready for the exchanger to be placed in service.2. All valves should be in shut position.3. Open hot and cold fluid vent valves.4. Crack open cold fluid inlet valve- vent all air when liquid is full. Close cold fluid vent valve.5. Crack open hot fluid inlet valve and vent all the air, then close hot fluid vent valve. At this stage, the exchange is liquid full of both hot and cold flowing fluids-open cold fluid inlet valve and hot fluid outlet valves fully.
PREVENTIVE MAINTENANCE OF HEAT EXCHANGERS• Whenever the plant’s systems are shut down, or whenever a system’s load is reduced, it is possible to schedule inspection and maintenance routines to ensure the proper operation of the major components in the system. In many systems, two heat exchangers are installed in parallel with each other.• Inspection and cleaning allow early detection of developing problems before they contribute to problems throughout the systems.
• Preventive maintenance cannot catch every failure before it occurs.• Troubleshooting is a responsibility shared in three ways:1. By operators who monitor the plant’s operation2. By lab people who keep checks on the environmental and plant chemistry and3. By maintenance personnel who are responsible for keeping individual components of plant system in proper order.
• Operating Personnel- can tell if a system, or some part of a system, is not functioning properly by checking gauges and recorders and observing alarms that warn of malfunctions. For example , an operator might notice a change in the temperature and flow of cooling water through a condenser.• If the flow of cooling water must be increased to keep the flow and temperature of the condensate within the normal operating ranges, this is an indication of probable blockage in the condensers water side from contamination that has bypassed filter screens.
• Lab Personnel- check samples of system fluids for signs of contamination . Oil in the feed water , for instance may lead the lab to suspect a leak in a fuel oil heater.• Personnel responsible for maintenance- perform simple checks during the daily routine of other duties. Bearing temperature is easily checked on components that require cooling of lubricating fluids. Any rise beyond the normal temperatures may indicate failure of a cooler to fulfill its job. Such impurities can cause failure in critical machine components.
• Reporting any problems that are found to a supervisor ensures that proper steps are taken to prevent potential problems from affecting the system as a whole.• These two checks – temperature and sight glass readings are examples of early warnings that can indicate potential problems before they spread.
CATEGORIES OF HEAT EXCHANGER PROBLEMS• Maintenance problems associated with heat exchangers can be grouped into two basic categories:1. Blockage and2. Leakage• The causes of specific maintenance problems also depend on the fluids involved and the temperatures and pressures at which the two systems operate.
BLOCKAGE• Blockage is caused by impurities in the fluids passing through the unit.• It is a gradual build up of impurities on heat exchanger tube walls.• Any blockage causes a loss in the efficiency of the heat exchanger.• Impurities range from chemical impurities that can cause corrosion deposits to build up, to silt and mud and marine life that may accumulate in a system.• Mud and marine life such as plants, fish and eels may build up so heavily that the flow through the tube is lessened or stopped
• Blockage can also cause problems in direct contact heat exchangers. Gases trapped in the water can be corrosive, and chemicals can cause a build up of deposits in the inlet and spray nozzles.
METHODS OF REMOVING BLOCKAGE FROM HEAT EXCHANGER1. CLEANING OUT BLOCKAGE2. PLUGGING LEAKS• The two basic categories of heat exchanger maintenance problems are blockage and leakage.• Blockage is accumulation of foreign material, impurities or buildup from chemical action within a heat exchanger that interferes with its operation.• Leakage is generally internal leakage- one fluid leaking into the other, external leakage- leakage of liquid out of the heat exchanger(rare problem)
• Blockage can interfere with heat exchanger operation in two ways :1. As an accumulation on a heat exchanger’s heat transfer surfaces, blockage can prevent efficient heat transfer and2. As an accumulation that blocks the flow paths of the fluids in heat exchanger, causing reduced flow .• Blockage is a common problem with both surface heat exchangers and direct contact heat exchangers.• Leakage problems are generally associated only with surface heat exchangers- condensers and shell and tube heat exchangers.
Removing Blockage• The procedure that is used to clean a particular heat exchanger depends on three factors:1. The type of heat exchanger2. The physical size of heat exchanger3. The types of fluids flowing through the heat exchanger.• The main cause of blockage in a direct contact heat exchanger is scale formation.• Corrosion problems in a direct contact heat exchangers come from some of the non- condensable gases that are released during heat exchanger operation
• The method used for cleaning surface heat exchanger depends on the physical size of the heat exchanger and the type of fluids that flow through it.• If river water is used for cooling, blockage may be caused by accumulation of mud, water plants and marine life.• The method of cleaning a particular surface heat exchanger also depends on whether the tube side, the shell side or both sides must be cleaned. Water boxes and tube sheets are generally cleaned by brushing or picking and scraping.• For deposits that are harder to remove, rubber squeegees, wire brushes, or metal scrapers can be shot through the tubes of straight –tube heat exchangers.
LEAKAGE• Leakage in a heat exchanger is generally internal leakage, external leakage is rarely a problem.• For example any impurities in the feed water going to a boiler can quickly lead to failure of boiler tubes. In a lubricating or hydraulic system, cooling water in oil can cause failure of machine components.• Leakage is usually caused by failure of the metals due to 1. erosion from the flow of fluids passing by or 2. cracking caused by corrosion and vibration.
Leakage caused by erosion is affected by several factors:1. Design2. Temperature and pressure3. Type of fluid involved• Erosion is wearing away of a material caused by the physical(not chemical) action of another substance.• By design, the flow of fluids in a surface heat exchanger is directed in some way to minimize erosion of tubes and tube sheets.• Temperature and pressure affect the rate at which metal erode• The type of fluid passing through the heat exchanger also affects the rate of erosion of metal parts, because different fluids have different physical properties.
• The second cause of leakage in a heat exchanger is corrosion. Corrosion is a chemical ‘eating away ‘ of the materials used in a heat exchanger caused by impurities in the fluids.Using sacrificial Anodes:• Heat exchangers that use non-purified water for cooling are particularly subjected to corrosion problems.• The zinc plates used in this way are called sacrificial anodes or sacrificial zincs, because they are gradually eaten away by the corrosive action of chemicals.
Stopping leaks• Internal leakage in a surface heat exchanger either a condenser or a shell and tube heat exchanger – generally occurs in one of two places:1. Leaks develop in the tubes or2. Leakage occurs through the joints where the tubes pass through the tube sheets.3. Leakage within tubes is usually stopped by plugging both ends of leaking tubes so that there is no flow through them.4. Leakage where the tubes pass through the tube sheets can be stopped by welding the affected tubes to the tube sheet.
• Tubes are typically set into tube sheets in one of the two ways: 1. Rolling 2. Welding• Rolling tubes is a method of expanding the tubes into corrugated holes in the tube sheets.• Welding is one when temperature, pressure, vibration, or a combination of these factors make rolling the tubes insufficient or impractical.Sacrificial Anodes – are large pieces of zinc that are used in some heat exchangers to minimize corrosion of the metals that make up the heat exchanger. The zinc plates are more susceptible to corrosion than heat exchanger metals are called sacrificial anodes as they are gradually eaten away
LOCATING AND SEALING LEAKS IN LARGE HEAT EXCHANGERS1. VACUUM TEST2. DYE TEST• Vacuum test is a common check on condensers and other heat exchangers that run under a vacuum. With the component’s water boxes open to the atmosphere, a vacuum is drawn on the shell side. Pumps are used to evacuate the air in the shell, however if a condenser is being run at reduced load with one half open, a vacuum already exists on the steam(shell) side.
• To make the test, material such as kitchen plastic wrap is placed over both tube sheets so that it covers the openings of the tubes at both ends. The tube sheets are wetted first so that the plastic wrap clings to the surfaces. Sheets of the wrap are overlapped and pressed tightly to make sure that the sheets adhere to each other.• Soap suds and foam can also be used for this type of leak testing. Suds or light foam is brushed over the tube sheets.• The dye test is used on condensers as well as on other types of surface heat exchangers. The shell side of the component is flooded with water that contains a fluorescent dye. Leaks in the tubes or where the tubes pass through the tube sheets will become evident when exposed to black light from a ultraviolet lamp
Sealing leaks in heat exchangers• The two methods used are 1. Plugging and 2. welding.• Plugs for leaking tubes are available in several types. The type most commonly used is a tapered plug, which is shaped like a cork or rubber stopper for a bottle or jar and is used in a similar fashion.• Some metal plugs are threaded so that they can be screwed into the ends of leaking tubes.• There are also two general styles of expandable plugs. One is a rubber doughnut sandwiched between two large washers with a bolt through the middle. When the bolt is tightened, the rubber expands to make a tight seal in the tube end.
• The second kind is an expandable metal plug- a tapered plug of soft metal with a hard metal pin in the center.• A fourth type is an explosive plug, which is used in heat exchangers run under extremes of temperature and pressure, such as steam generators in nuclear power plants. This type of plug contains an explosive charge that is detonated to expand the plug into the tube end.
TUBE REPLACEMENT• When heat exchanger tubes are replaced, the old tubes must be cut away carefully to avoid damage to the tube sheet. After the tube has been cut loose, it is carefully withdrawn and a new tube is inserted to replace it. Tubes may be cut away using several tools such as1. Safety ripping chisel2. Round nose gouge or3. General purpose cutter
CLEANING LARGE HEAT EXCHANGERSBLOCKAGE• Accumulation of mud, water plants, and marine life in the water boxes and tube sof heat exchangers that use river water as a source of cooling water.• Tube side cleaning- the method used to clean the tube side of a large heat exchanger depend on several factors including the following:1. The amount of work space available- cleaning must often be done in confined areas2. The tube configuration- U tubes in a shell- and – tube heat exchanger may limit the choice of cleaning techniques.
3. The kind and amount of deposits – different approaches are required for cleaning heavy accumulations of mud and debris than removing light scale buildup.• Cleaning out the tube side of a component such as a condenser typically includes the following steps:1. Ranking, scraping, or picking accumulated blockage off the tube sheets.2. Shoveling or troweling accumulations out of water boxes.3. Shooting the tubes with a high pressure water gun to clear them of built up slime, silt and crushed deposits, rakes, scrapers and picks are typically used to remove heavy accumulations from the tube sheets
• After the blockage is removed from the tube sheets , the next step is usually shoveling or troweling any heavy accumulation out of water boxes.• A variety of implements can be forced through the tubes by the water from the water from the gun; the choice of implements depends on what has to be removed. These implements are often referred to as “bullets”, they may be hard rubber squeegees that just fit the insides of the tubes, wire brushes with rubber squeegees that just fit insides of the tubes, wire brushes with rubber squeegees on each end , or meatl- balanced scrapers.
Shell side cleaning• Shell side cleaning is done only on shell and tube heat exchangers.• Rigging for tube bundle removal involves separating the large flanged connection between the shell and the header.• Solvents may be used to remove oil from the tube surfaces.• Automatic cleaning methods• Automatic cleaning systems operate while heat exchangers are in service to reduce the accumulation of deposits. These systems can be divided into two general categories1. Mechanical systems and2. Chemical systems
• In one mechanical system sponge balls with rough surfaces are circulated through the tubes in a condenser. These balls remove deposits from inside the tubes. The balls are caught by a screen trap and pumped back to the inlet for recirculation.• Automatic chlorination is one type of chemical cleaning system chlorine is periodically introduced into the incoming water to kill algae and thus prevent the buildup of algae growth on tube walls.
MAINTENANCE OF SMALLER HEAT EXCHANGERS• Five major steps are involved in servicing small heat exchangers in the shop1. Testing for leaks2. Disassembly3. Cleaning , inspection and repair4. Reassembly and5. Final testing
MAINTENANCE PROCEDURES• Testing of leaks is usually done prior to disassembly, because it is helpful to identify what needs to be done to the component and what parts will be necessary for the job.• Leak testing is accomplished by pressurizing the shell in one of two ways. One method illustrated in fig 10 is to plug the outlet and feed compressed air at moderate pressure into the inlet.
• Another means of testing for leaks is to fill the tube side with water and plug the outlet. The application of pressure from the inlet water source makes it possible to look for leakage into the shell. Any water found in the shell side is an indication of leakage. Care must be taken to avoid exceeding the maximum rated pressure of the component being tested.• Rodding – a method of cleaning tubes in a smaller heat exchanger is as shown in figure.
• When the heat exchanger has been successfully tested, it can be returned to service or put into stock for future use. If the component fails to pass this final test, it will be necessary to look further for the source of leakage, this inspection should include the tubes, the tube bundle, and gasket connections.