Air Cooler Tubes Failure

AIR COOLER TUBES FAILURE

Reasons for Turbo charging are:
1. Increased air charge density permitting more fuel to be burned.
2. Increased engine output.
3. Reduced thermal load on engine mechanism.
4. Improved Scavenge efficiency of engine

Turbocharger is compressing the air thus an increase in charge air temperature will transpire. This will trim down the density of the air entering the engine. To reduce this effect the air is cooled. This is the function of the Air cooler. Cooling the air also reduces the thermal load. The volume and density of scavenge air establishes the maximum amount of fuel that can effectively be burned in a diesel engine. An efficient heat transfer across the air cooler will lower the air temperature whereby increasing the density, which ultimately results in an increase in air compression, mean effective pressure and combustion rate. This will eventually also lead to reduced specific fuel consumption and NOX emissions. However, scavenging air drawn from the engine room is tainted with oil vapour, carbon and dust. When this compressed heated air passes through the air cooler, these pollutants deposit on the finned tube surfaces of the Air cooler as a carbonaceous film. This would result in declining efficiency of the charging air system and engine performance. Successful process of turbo charger is thus reliant on satisfactory heat transfer across the air cooler tubes. Thus the Turbocharger coupled with Air cooler enhances the performance and condition of the engine if the system is functioning satisfactorily.

Fouled Air Cooler 
The regularity of cleaning the Air cooler will be reliant on several factors such as the source of scavenge air and extent of contamination. This will also be indicated by an increase in pressure differential across the air cooler and rise in charge air temperature after the air cooler. The vital parameter to check the performance of the Air cooler is the temperature difference between the air outlet and the cooling water inlet. This difference is a direct measure of the cooling ability and as such an essential parameter for the thermal load on the engine.

It is said that an oily film on tubes and fins can also influence heat transmission and this will only show as a minor increase in the pressure drop across the air cooler tubes. Deviations in scavenge pressure is like the exhaust temperatures indicating an overall estimation of the engine condition.

Drop in scavenge pressure for a given load will cause an increase in the thermal loading of the combustion chamber components.

Secondary indications of fouling also include reduced engine power output and increased deposits in exhaust trunking and on turbocharger turbine blades.

However the declining cooling potential of the Air cooler can be summed up due to:
1. Fouling on the airside.
2. Fouling on the waterside.

On the air side the air cooler can be cleaned by the following methods: -
1. Soaking of the Air cooler in-situ in a chemical bath.
2. Removing the Air cooler from place and soaking in chemical bath.
3. Spraying of chemical in place.
4. Closed loop circulation of Air cooler with permanently mounted spray nozzles.
5. Removing from place and cleaning in ultra sonic cleaning tank.

All the above procedures must be carried out at about 60ºC and also would require washing or rinsing with fresh water later.

On the waterside the tubes can be cleaned with special tube brushes. This is ideal for removing soft scale deposits, but if the scale is formed into hard marine growth, it may require cleaning with a high-pressure hose or the air cooler will require to be cleaned in the ultra sonic cleaning tank. It is very important to be careful while piercing tubes for cleaning purpose, as the tubes are thin walled.
Leaking tubes can be a difficulty and will be noticed when the water is found in the scavenge drain system. It must be confirmed that it is seawater and not condensation due to humidity.

The way to find a leaking tube with the Engine running is as follows: -
1. Reduce engine speed and air temperature before turbocharger to be less than 65ºC
2. Cut off the cooling water and remove end covers
3. Cover one end of tubes completely with a thick sheet of jointing. Apply soap water at the other end.
4. Leaking tube will give off bubbles.
5. Plug the leaking tube with wooden plugs or copper tapered plugs.

Another method of finding a leaking tube/ tubes would be to stop engine and remove end covers. Several wooden plugs can be fitted in a row on one end of the tube plate. The other end to be pressurized with water and checked for leaks. This process can be accomplished for all the tubes. The leaking tube can later be plugged.

The materials used for the heat exchanger are normally non-ferrous alloys such as Aluminum, Brass, Cuprous nickel with water boxes and doors of either cast steel or iron. In some case the latter is also made of alloy.

Reason for failure:

The most common cause of corrosion leading to failure of tubes is: -
1. Impingement attack due to excessive turbulence in the circulating water system causing the protective oxide film to be removed These spots become small anodes surrounded by cathodic areas where the protective film remains intact thus causing local corrosion.
2. Continual impingement of high-speed water at the anodes keeps them depolarized, by removing metal ions and corrosion products causing concentrated local attack of metals to take place leading to erosion of the metal. This combination of corrosion and erosion causes pits to be formed in the metal, which leads to perforations occurring in the Air cooler tubes.

Tube failure can also occur as a result of deposits in the tubes, giving rise to partial local blockage, turbulence and impingement attack.Failures may also be due to stress corrosion, corrosion fatigue and local overheating.

The aspects leading to this impingement attack are: -
1. Poor design of equipment
2. Improper piping layout
3. Incorrect operation
4. Poor maintenance

The resistance of corrosion for all non-ferrous metals and alloys relies upon the formation of an adherent and resistant oxide film. The resistance to corrosion depends upon the formation of this supporting oxide film, but it is said that new systems are virtually susceptible to attack until an oxide film is formed. Aluminum Brass and Cupronickel offer a high resistance to corrosion by good value of their ability to form protective oxide films. The limit to the speed of water without suffering impingement attack for Alumiuium Brass is 3 m/sec, Cupro nickel 90:10 is 3.7 m/sec and cupro nickel 70:30 is 4.5 m/sec.

It has been found that presence of iron compound in the salt water system has positive effects on copper alloy tubes. The protective oxide films are by and large remedial in formation. They will only fail if the attack of turbulence is severe. Normally a good oxide protective film formed in clear water is orange brown in colour and contains a percentage of iron oxide. The iron is consequent from large areas of ferrous compounds which are unprotected i.e. pipelines, valves, water boxed etc. The iron corrosion products resultant from these products plays an important part in the growth of good protective oxide film in Air cooler tubes. Where there is no source of iron corrosion products, untimely failure will occur.

The natural oxide film can be further helped by the introduction of iron compounds in the water. This will form a film of hydrated ferric oxide, which helps to resist impingement attack. Sacrificial anodes, in the form of iron or mild steel blocks in Air cooler water boxes provide cathodic protection and in addition by releasing iron compounds as they corrode, help to protect the whole length of the tube.

The anodes do however have three drawbacks: -
1. The sacrificial anodes deposit a cathodic scale, which can prevent the formation of natural oxide film in the tubes of Aluminum brass or cupro nickel. Moreover this cathodic scale might become unstable if the anode itself becomes unproductive through increased depletion or being insulated from the tube plate because of the corrosion products.
2. The second drawback is that the sacrificial anodes might hamper the smooth flow of water when fitted in the water boxes.
3. Corrosion products might break away from the anodes and partially block the tubes, which can result in turbulence and impingement attack.

To compensate for these disadvantages, ferrous ions can be added to the water without the disadvantages associated with iron blocks. A 10% solution of ferrous sulphate FeSo4.7H2O is injected into the water stream to give a concentration of 1 ppm. The normal dosage required is 5 ppm for one hour each day. The solution of ferrous sulphate is injected into the water salt water system at a point close to the seawater inlet.

In addition to the treatment of the seawater cooling systems to protect non-ferrous components, the use of biocide to prevent marine fouling of the Air coolers and heat exchangers is very important.
The most common treatment is Chlorination for the prevention of marine fouling. In this event, it is generally advisable to carry out the two treatments at different times, if possible, since chlorination oxidizes the ferrous ions to ferric and renders the iron ions additions less effective. Marine growth prevention unit produces chlorine and hypo chloride by electrolyzing seawater and protect the equipments from marine growth by blending them to cooling seawater. Even if the residual chlorine in cooling seawater is at very small density of around 0.05 ppm, it will be effective and also it will not corrode the equipment nor pollute the seawater.

Replacing Tubes:
It is important that the tools for removing / inserting and expansion of tubes are available on board.
After identifying the damaged tube, the expanded part of the tube is shaved off on both sides with a drill bit leaving about 5 mm in the tube plate. The tube-setting tool is inserted into the shaved part of the tube and hit with the hammer. This way the tube can be pushed out of the tube plate till the tube end is grasped with a wrench. The whole length of the tube is pulled out with the help of a wrench. In case the tube cannot be removed by this method, then expanded parts of both ends are shaved off completely. The tube-setting tool is then used.

A spare tube is inserted into the tube hole and adjusted such that it exceeds the face of the tube plate by 1.5 mms on the cooling water inlet side. The projected part of the tube is expanded with bell expansion tool. This part of the tube would not be inserted into tube plate when the tube expander is used. The tube expander is used with the help of a drill machine or manually. The tube can be held on other side if the tube turns while expanding. Pushing forward the tube expander to about 20/25 mms expands the tube. When reversing the drill machine, the tube expander must be drawn out of the tube. After expanding both ends, the tubes can be pressure tested.