In order to maintain a high thermal efficiency of the exhaust gas economizers and avoid excessive soot accumulations periodical soot blowing and cleaning must be carried out.
The goal of cold cracking is to cool and then heat the economizer steel structure. Because the coefficients of expansion of the boiler steel and the hard deposits differ, the deposits come loose during cold cracking. The carbon particles released during the process are burned off when the economizer is heated afterwards. This burn-off must not take place near coastal areas or in congested areas.
The economizer must be cleaned by soot-blowing on approach to the selected port for the cold cracking to be carried out, to minimize the subsequent burn-off.
The correct procedure for cold cracking process is, usually, as follow:
- the cooling down process may be started on or during arrival. If the economizer is equipped with a by-pass damper than the cooling process can start during arrival when operating on the exhaust gas economiser bypass damper; if no by-pass damper then immediately after finish with engine.
- close the steam supply to the heating coils in the drum if supplied.
- keep the circulation pumps running during the procedure. If small soot fires occur, the circulating water will keep the exhaust gas economiser cool.
- as an option to accelerate the cooling process, dumping steam can be carried out, but this must be done with care, and carried as below:
- the steam can be dumped by opening the steam dump valve manually. The pressure drop must not exceed 0.3 bar/minute, to prevent the exhaust gas economiser circulating pumps cavitating, with the resulting pressure drop causing the standby pump to start.
- open the vent valves on the steam drum.
- stop the vacuum pumps if the vessel is equipped with the heat recovery system and turbo-generators.
- open the hopper for the economiser to be cleaned. To increase the differential pressure across the economiser, the funnel fire flaps can be closed.
- as an option, water spraying can be carried out as the water will accelerate the burn-off process. Water can be supplied from the hopper, or by using the water washing nozzles for a short period.
- open the heating coil in the steam drums, if equipped, before vessel departure for slow warm up of the economizer.
- close the hopper.
- close the steam dump valve and keep the vent valve open.
- after the start of maneuvering, open the exhaust gas damper (if equipped) to heat-up the economiser slowly and monitor the steam pressure and check if steam comes out through vent valve.
- close vent valve.
Water washing is recommended to be carried out during dry docking as frequent water washing
causes an external corrosion layer of steel on the economiser tubes, which with regular water washing will eventually cause tube leakage.
Water washing must be carried out when a main engine is stopped, but the economiser should be warm enough for the water to evaporate so that the tubes and fins will not remain moist after washing. Economizer washing must be done with fresh water only and where deposits are highly corrosive or bonded, a soaking spray with a 10% soda ash solution is advisable before washing.
When the water washing has begun, it must be continued until the heating surface is thoroughly washed and all deposits are removed, due to the fact that some types of coatings harden and get very difficult to loosen when they have been saturated and then dried out.
It must be ensured that all the washing water is drained away so that water will not enter the turbochargers. The dirty washing water is collected in a tank which is located beneath the economiser.
The economiser must be dried out immediately after water washing. This is because soot formations produced by the combustion process in the engine contain sulphur compounds. Any residual soot and water will therefore react chemically to form highly corrosive sulphuric acid.
The water washing is done utilizing a lance water pressure jet, but on some vessels there is a water washing system inbuild into economizer and the system consists of a number of water washing tubes installed on top of the evaporator sections inside the economiser. Each tube is equipped with spraying nozzles designed in such a way that the whole area of the heating surface will be covered.
Water washing must be carried out when the main engine is stopped and the economiser has been cooled down. However, the economiser should be warm enough for the water to evaporate, so that the tubes and fins will not remain moist after washing.
It is important to note that when the heating surfaces are water washed, there is a possibility of generating steam. To avoid a risk of scalding, the operators must remain outside the economiser casing.
The amount of water used for washing depends on the amount and condition of deposits and should be based on experience. The following procedure should be followed:
- Open the economiser drain valves to the soot collecting tank and make sure that there is free drainage for the dirty water.
- Open the drain valves at the main engine turbochargers and make sure that there is a free drainage from the casings.
- Open the inspection doors above and below the economiser.
- If there is a risk that the washing water will run into the exhaust gas pipe and down to the turbochargers, the exhaust gas pipe must be covered. Alternatively, the main exhaust gas damper (if equipped) may be closed, but this will not provide complete shut-off and washing water may also accumulate on the outlet side of the damper.
- Switch off the operational and standby water circulation pumps and close the outlet valves.
- If the economizer is equipped with washing nozzles, open one of the water washing nozzle supply valves.
- Open the main fresh water supply valves to the water washing system.
- Crack open one or two water wash nozzle valves on the economiser. The flow should be sufficient to ensure that the drains are working properly. This will also result in having only a small amount of the soot deposits coming down. If a large amount of water is used, large amounts of soot deposits will loosen, which might block the drain system.
- There is a time delay from washing water entering the economiser and to draining into the hopper tank. Under these circumstances, care must be taken to control the flow of washing water so that the hopper tank does not overflow if the drain becomes blocked. It is important to check that the drain and soot collecting system is working properly during the whole water washing procedure.
- When it has been established that the washing water is running freely into the drain system, the water supply can be slowly increased to the full pressure for the water washing system. The nozzle supply valves should be opened as required to ensure that all the heating surfaces are covered. Check the nozzle spraying pattern and that it covers the whole heating surface.
- When the whole surface has been cleaned, close the main fresh water supply valves and the nozzle valves.
- When the water washing process has begun, it must be continued until all deposits have been removed and the heating surfaces are clean. This is due to the fact that some types of deposit will harden if they are saturated and allowed to dry out.
- The soot-blowers may additionally be used during the water washing process. This will enhance the washing efficiency.
- A fresh water hose may also be used to enhance the washing process.
- Monitor the water level in the soot collecting tank.
- When the economiser has been cleaned, it must be ensured that all the washing water is drained away so that water will not enter the turbochargers. The washing water supply valves should be closed and the soot collecting tank drain valves closed.
- The economiser must be dried out immediately after water washing. This can be done by just having the natural air circulation through the economiser or by heating it up by circulating warm boiler water.
- If the main exhaust damper was closed during water washing to prevent water from running back in the turbocharger, the damper should be inspected. Any water and soot accumulated at the damper should be removed and the damper properly dried. The condition of the damper and the soft seal can be checked at the same time.
- Start the circulation pumps and hot water from the separator drums will slowly enter the economiser.
- Slowly increase the flow through the economiser by opening the outlet valve for the operational circulation pump.
- Close the inspection doors, close the drains, remove the covers for the exhaust gas pipe if used.
- The economiser can now be brought back into normal service.
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