Answer MEP Question 17

Q. With regard to keeping the gas side of boilers in good condition; discusses EACH of the following 

A. the mechanism of combustion, stating the factors which are important to good combustion; 

B. Oil fuel treatments; 

C. Soot removal equipment; 

Combustion is the burning of fuel in the air, in order to generate heat energy. For complete and efficient combustion the correct quantities of fuel and air must be supplied to the furnace and ignited. About 14 times as much air as fuel is required for complete combustion. The air and fuel must be intimately mixed and a small percentage of excess air is usually supplied to ensure that all the fuel is burnt. When the air supply is insufficient incomplete combustion the result is black smoke and soot formation. The fuel must be break up into very tiny particles known as atomising to mix with the air to make a homogeneous explosive mixture. The atomisation of the fuel is done with the help of burners which are of different types. The atomisation of the fuel is achieved by forcing the under pressure oil through an orifice at the end of the burner the pressure energy in the fuel is converted to velocity. Spin is given to the fuel prior to the orifice, imparting a centrifugal force on the spray of the fuel, causing it to atomise.

1) Air supply- The flow of air through a boiler furnace is known as draught. Marine boilers are arranged for forced draught, which means fans that forces the air through the furnace. The mixing of air and atomised fuel is carried out with the help of an air register. The register is generally based on a circle or hexagon with the burner mounted axially at the centre. The air register employing pressure atomiser consists of two principal parts. The diffuser and the airfoils (vanes). Use of two streams the primary air through the diffuser and the secondary through the air vanes. The air is given an angular rotation (swirl) by vanes, on its way to the furnace, in order to assist in mixing the air and oil. The diffuser institutes the primary mixing of the droplets with air and prevents blowing off the flame from the atomiser the air vanes guide the major quantity of air to mix with oil particles after they leave the diffuser and to envelope the flame. The construction of the diffuser, air vanes and register's door is such, that oil and air are given a clockwise motion.

2) Fuel oil- Marine boilers currently burn residual low-grade fuels. This fuel is stored in D.B tanks from which it is drawn by a transfer pump to settling tanks which is being heated with the help of steam coils provided inside here any water in the fuel may be settled out and water can be drained off. Before or just after receiving the bunker fuel, various chemicals according to the grade and quality is added in a calculated measured amount to disperse sludge, suspended particles (catalytic fines or cat fines) known as fuel oil treatment (FOT) chemicals. Fine mesh filters are in the suction line before the pump, to collect the sludge and solids particles, enquired to clean preferably before every pump operation. The oil from the settling tank is filtered and purified through a purifier if the grade is such that cat fines are more, out of two purifiers one to be used as a purifier, the other as a clarifier in parallel and stored in a service tank provided with steam heating coils. The temperature is maintained around 75-80 deg cel. From there the oil is pumped to a heater through a fine filter. Heating the oil reduces the viscosity and makes it easier to pump and filter. Care must be taken for heating up, must be carefully controlled to avoid gassing of the pump or cracking or breakdown of fuel. A supply of light oil is available for an initial operation or continuous use, from the fine filter the oil passes to the burner.

3) Unburnt fuel particles or the residual carbon particles will accumulate on their way along with the flue through the uptake trunking at places, known as soot. Soot is a very dangerous problem, not only it is hindering the heat transfer but can catch fire in suitable conditions which are omnipresent in that environment. The thin layer of soot will decrease the thermal and fuel efficiencies. It is better to disperse the soot by using steam jets. This operation is carried out with the help of soot blowers, soot blowers are the equipment that is fitted at different positions in the uptake and coils. The appliance is consist of a long hollow shaft, closed at one end and a number of holes are provided at one-fourth of the peripheral surface all along. The shaft is opened at one end and connected with a wheel and worm. The shaft can rotate along its axis. The shaft is connected with a steam line through the stuffing box and gland. When the steam valve is opened the steam will come through the holes as jets and clean up the loose soot which will go away with the flue. The operation should be carried out after informing the bridge to check out the wind direction. The EGE or auxiliary boiler must on full steaming condition. But never use when there is an uptake fire.