CODE OF GOOD PRACTICE FOR THE OPERATION OF LIQUID FUEL FIRED COMMERCIAL, INDUSTRIAL AND DOMESTIC APPLIANCES 1st June 1972 PRINTED BY THE GOVERNMENT PRINTER HONG KONG
94157 140K 7/72
INTRODUCTION CONTENTS Page FUEL 1 FUEL OIL PREHEATERS 1 FILTERS 1 ATOMISATION OF FUEL 2 AlR FOR COMBUSTION 2 FUEL OIL BURNERS 3 Pressure jets or gun type 3 Air pressure atomisers 3 low, medium, high Steam atomisers 3 Rotary cup burners 4 Vapourising burners. 4 BURNER MAINTENANCE. 4 STARTING UP LIQUID FUEL FIRED INSTALLATIONS.... 5 FROM COLD FROM WARM INSTRUCTION MANUALS 6 AUTOMATIC CONTROLS. 7 SHUTDOWN PROCEDURE 7 CARBON BUILD-UP IN FURNACE 7 LIQUID FUEL IN FIRE TUBE BOILERS 8 COMBUSTION CHAMBER VOLUME....... 8 BRICK SETTINGS 8 EXAMINATION OF FLAME 8 v
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INTRODUCTION This code of good practice aims to reduce air pollution in Hong Kong while saving money in fuel bills. Quite simply, if proper methods are used to achieve good combustion in furnaces and boilers, the maximum amount of useful heat will be extracted from a minimum amount of fuel. The result: down go the bills and down goes pollution. If this code achieves its purpose, the colony will become not only more prosperous, but also a cleaner, better place to live in.
CODE OF GOOD PRACTICE FOR OF LIQUID FUEL FIRED INDUSTRIAL, COMMERCIAL AND DOMESTIC APPLIANCES This Code of Good Practice is applicable to any liquid fuel fired boiler, oven or furnace. FUEL Liquid fuels used in Hong Kong are generally either grade 1 or grade 3 fuel oils. Grade 1 fuel oil can be used successfully without any preheat conditioning before the fuel oil burner. Grade 3 fuel must be preheated before admission to the burner to achieve complete combustion, prevent the formation of dark smoke, and reduce fuel costs. The usual range of preheat for grade 3 fuel oil lies between 160 and 170 F (71 to 77 C) but the actual preheat temperature required should be ascertained from the supplier. Some of the grade 3 fuel oils differ in wax content and may need a slightly higher preheat temperature. FUEL OIL PREHEATERS Preheaters for use with grade 3 fuel oil may receive their heat as steam or as electricity. Both types should be fitted with thermostatic controls to ensure a steady preheat temperature. It is preferable to have a thermometer inserted into the fuel supply line between the preheater and the burner, and as close as is practicable to the burner. The thermostat controlling the preheater should also be close to the burner. For start-up periods on a steam installation, an electric preheater should be used for a short time until steam is available from the boiler to operate a steam preheater. If steam is not available, an electric preheater is essential. FILTERS Screen-type filters should be inserted in the fuel supply lines between the fuel tank and the preheaters. However because the viscosity of the fuel is reduced in the preheater, the carrying capacity of the fuel reduced. For this reason it is desirable to have additional screen filters between the preheater and the burner to trap any solid material released by the lower viscosity. 1
ATOMISATION OF THE FUEL The purpose of the fuel oil *, urner is to break down fuel into tiny droplets; this is called atomisation. Heavier grades of fuel cannot be made to burn without atomisation. Small droplets of fuel have three advantages: they heat up quickly to ignition temperature; the surface area is increased and exposes a greater area of the fuel to the air required for combustion; good atomisation also promotes a steady and stable flame. The size of the droplet is affected by the viscosity. High viscosity, as with an unheated fuel, gives large droplets; low viscosity permits the formation of small droplets. Preheated grade 3 fuel oil results in better combustion efficiency with reduced fuel costs, and much less risk of producing of dark smoke. Although it might seem that the higher the fuel temperature, the lower the viscosity and the better the atomisation, there is a limit to how high you can go. Too high preheat temperature can promote a break-down of the fuel in the burner, causing carbon build-up in the burner which will itself interfere with atomisation and may lead to the formation of dark smoke. Too high preheat also wastes steam or electricity on the preheater. AIR FOR COMBUSTION Liquid fuel will not burn unless sufficient air is provided. The theoretical air requirement for liquid fuels is about 185 ft 3 per pound of fuel, but to ensure complete combustion, it is necessary to admit some extra air. This raises the air requirement to about 220 ft 3 per pound of fuel. Too little combustion air will result in dark smoke formation, and a waste of fuel. Too much combustion air may produce white smoke, and again a waste of fuel. A good guide to the correct air to fuel ratio is to see a light brown haze emerging from the top of the chimney. Some of the air for combustion may be admitted through the burner. In some installations part or all of the combustion air may enter the furnace through ports around the burner tip, or alongside the burner mounting. All air admission ports should be kept clean and free of obstruction, but it is desirable to have some form of damper control which can be adjusted to control the air input.
Air for combustion comes from the room where the appliance is situated, so the room must be adequately ventilated. FUEL OIL BURNERS There are many different makes of fuel oil burners, but they generally fall into the following types: Pressure jets or gun type Fuel is atomised by passing it at pressure through a small orifice at the burner tip. The pressure required may be in the range 80 to 150 lb/in 2 and variation alters the amount of fuel supplied through the burner. It also alters the flame shape. Higher pressures generally tend to give a wider flame. Low pressures tend to increase the size of the droplet of atomised fuel. When a pressure jet has to work at a low throughput of fuel, it is preferable to change to a smaller jet than to reduce the fuel oil pressure below the given limits. Do not increase the fuel pressure on a pressure jet burner above 150 lb/in 2. If more throughput of fuel is required, a slightly larger burner tip should be used. On any pressure jet installation always ensure that there is a range of jet sizes in stock. Low air pressure atomisers Fuel is atomised by an air blast delivered concentrically with the fuel oil at the burner tip. The usual air pressures required are in the range 6 to 30 inches water gauge, which is obtainable from a centrifugal fan. Medium air pressure atomisers These operate on a similar principle, but the required air pressure is in the range 3 to 15 lb/in 2, supplied from a rotary compressor, or a compressed air supply. High air pressure atomisers Again of similar type, but the air pressure required is in excess of 15 lb/in 2. Steam atomisers Steam atomisers operate on the same principle, but using pressures generally higher than 15 lb/in 2. They have the disadvantage that on
boilers which have to be brought up from a cold condition, steam is not available. In these circumstances it is sometimes possible to start up by using air pressure. Rotary cup burners This burner atomises the fuel by centrifugal force. The droplets of fuel are thrown off the edge of a spinning cup. Fuel is fed into the inner surface of the cup, which is spun by an electric motor, or by an air or steam driven turbine. For electrically driven burners the speed of rotation generally lies within the range 2,850 to 3,500 rpm; for turbine driven burners the speed of rotation may be as high as 5,000 to 7,000 rpm. Because the speed of rotation of the cup governs the size of the droplet, and the shape of the flame envelope, it is advisable to check on the rpm frequently. This is particularly important on electrically driven burners incorporating a belt drive. Vapourisiiig burners These should only be used for grade 1 fuel as they are not suitable for grade 3 fuels. The fuel oil vapour and the air for combustion must mix intimately; this can only be effective if all perforations in the pots are kept constantly clean. Air admission to the burner must also be kept clear of obstruction other than control dampers, BURNER MAINTENANCE No fuel oil burner can give effective atomisation for long without careful routine maintenance. Cleaning of all burners other than the rotary cup burner, should be done only when there is carbon build-up in the furnace, when the flame shape is distorted, or when carbon forms on the burner tip. Rotary cup burners should be cleaned more frequently, and at regular intervals, because some of the faults through failure to clean are masked by the rotation of the cup. The burner should be shut down, allowed to cool, and then stripped down to the burner tip. The fuel orifice of pressure jets and air atomising burners, should be soaked in diesoline or kerosene to soften any carbon or gummy material It should then be wiped clean with a soft rag. The orifice should be examined for any deformation or damage. Any carbon inside should be removed with a soft wood probe; hard metals will deform the orifice and make it unfit for further use. Some burner manufacturers provide soft metal "Dollies" which
are a clearance fit in the orifice. These should be used for cleaning. Some burners have a diaphragm or spin plate; these also should be soaked in kerosene to soften any deposit and then wiped clean with soft rag. If spare nozzles, jets, and cups are kept in stock, then the time needed to clean a faulty burner is much reduced. The dirty part can be replaced quickly with only a short shut down, and then cleaned later. On rotary cup burners the nose cone should be removed, and cleaned. Straighten any bent or distorted swirl vanes in the nose cone. If any vanes are broken, replace the nose cone. The rotary cup should be removed from the end of the fuel tube, and soaked in diesoline or kerosene, and cleaned with a soft rag. The edge of the cup should be examined. If the edge is bent over, or if there is any irregularity in the edge, the cup should be replaced. Examine the inner surface; if it is scratched, the cup should be replaced. Some attempts have been made with badly scratched cups to skim the inner surface on a lathe. Although the surface can be restored in this way the inner diameter of the cup has been enlarged, with the result that the flame envelope shape has also been changed. A rotary cup should be resurfaced in this way only if the scratches are very shallow. When a rotary cup burner is stripped for cleaning, the speed of rotation of the fuel shaft should be checked. STARTING UP LIQUID FUEL FIRED INSTALLATIONS FROM COLD 1. Switch on electric preheater and make sure the thermostat is set to the correct temperature and that the fuel oil supply to the preheater is open. Use the thermometer at outlet from the preheater to check that the temperature is correct. 2. Check that all gas dampers in the flues leading to the chimney are open. 3. When the preheater is fully warmed up, and the fuel oil temperature is correct, open up any air supplies to the burner(s) and allow a short time for air purge to clear any inflammable gases. Turn on the fuel supply and light the flame at the burner tip. If the method
of ignition is by torch through a port at the front of the burner, do not stand close while watching for ignition. A minor blowback can cause bad burns to face or eyes. Check after igmition that the flame is stable. 4. Adjust the air supply to the burner for complete combustion without dark smoke. Control the air supply so that there is a light brown haze at the top of the chimney; remember that white smoke indicates too much combustion air. For a short time when starting up from cold there may be an emission of smoke darker than Ringlemann L 5. Do not open up the burner to its full capacity. Let the boiler, furnace or oven warm up slowly, allowing ample time for thermal expansion to take place. This avoids cracks in brickwork, or undue distortion inside the appliance. 6. When the normal fuel is grade 3 fuel, it is sometimes helpful to start up from cold using grade 1 fuel. This may mean that only a steam heated preheater need be installed. When this is possible, it is preferable to use grade 1 fuel for a short time before shutting down to ensure that the fuel supply lines between the preheater and the burner are full of the lighter grade fuel. STARTING UP FROM WARM 1. With a steam boiler installation, turn on the steam preheater. If no steam is available, use the electric preheater. 2. Follow normal procedure as for cold start. 3. When relighting under hot or warm conditions, ensure that the combustion air is turned on first, and allow sufficient time for air purge. NOTE In some cases where both electric and steam preheaters are installed, it is possible to set the thermostat controls on the preheater to ensure that the steam heater is at work when steam is available, and the electric heater out of commission. INSTRUCTION MANUALS All burner manufacturers, and all suppliers of package boilers can supply instruction manuals. These should be carefully read before any fuel oil burner is started up.
AUTOMATIC CONTROLS Many installations, particularly package boilers are fitted with comprehensive automatic controls. Again the instruction book should be read carefully until the controls are fully understood. Many automatic installations include a light-up sequence which may repeat if ignition fails at the first attempt. If ignition fails on the second attempt the automatic control will go over to lock-out. When this happens the entire system should be examined before any attempt is made to ignite manually. Automatic controls, in good condition, will always give a better performance than manual control. They should be checked at frequent intervals by a competent person. The safety of the appliance rests with the control. If an appliance has a pressure control on the steam pressure of the boiler, the relative times when control opens up the burner or shuts it down should be noted. If the shut down time is greater than the run time, it may be necessary to fit a slightly smaller burner control. SHUTDOWN PROCEDURE When a fuel oil burner is to be shut down for more than a few minutes the burner should be retracted from its housing so that a heat shield can be placed between the burner tip and the furnace. Heat radiated back from the furnace to the burner will cause a break down of any fuel left in the burner creating a carbon build-up which will interfere with atomisation when the burner is restarted. Any air ports should be shut, or at least closed down to reduce the amount of cold air drawn into the furnace. If the shut down is for more than a few minutes, turn off the fuel oil preheater; this saves heat expenditure, and prevents any break down of the fuel in the heater which might produce carbon or gum and reduce the effective heating surface. CARBON BUILD-UP IN THE FURNACE Examine the furnace tubes in any boiler, or the walls in ovens or furnaces for any build-up of carbon. This build-up will interfere with the correct development of the flame and may cause dark smoke.
If carbon build-up persists in one area, for example at one side of the furnace tube, check the alignment of the burner in the tube. If the alignment is correct, then the build-up is probably due to a distorted burner, and the part should be replaced. The flame should never impinge against the wall of the furnace. This also causes carbon build-up. LIQUID FUELS IN FERE TUBE BOILERS Any boiler of the economic type, or any modern package boiler, uses a large number of fire tubes to increase the heat transfer surface. These fire tubes tend to pick up deposits of soot and dust, which reduce the heat transfer rate. Whenever possible for example during a temporary shut down the tubes should be cleaned. Always use an unworm, correct size cleaning brush. COMBUSTION CHAMBER VOLUME The combustion chamber volume is critical and controls the amount of fuel burned in a given time. The volume is already decided by the manufacturers and any attempt to install a larger burner on an existing combustion chamber as or increase the throughput of the fuel oil burner may result in a waste of fuel and the formation of dark smoke. BRICK SETTINGS Furnaces or boilers with a brickwork setting should be examined for cracks when running at operating temperature. Any cracks should be filled in with fireclay to prevent ingress of air which will reduce the thermal efficiency, and the chimney draught. EXAMINATION OF FLAME Never try to examine a flame without using a blue glass screen, The light emitted from the flame can cause lasting damage to the sight.