Viscosity controllers Viscotherm

The basic principle of the Viscotherm viscosity monitoring device is shown in Figure 2.24. A continuous sample of the fuel is pumped at a constant rate through a fine capillary tube. As the flow through the tube is laminar, pressure drop across the tube is proportional to viscosity.

In this unit an electric motor drives the gear pump through a reduction gear, at a speed of 40 rpm. The pump is positioned in the chamber through which the fuel is passing from the heater to the fuel pumps or combustion equipment. Tapping points are provided to enable the pressure difference to be measured by means of a differential pressure gauge. The gauge is calibrated directly in terms of viscosity. Parts in contact with the fuel are of stainless stee! for corrosion resistance.

A differential pressure transmitter (Figure 2.25) provides an analogue of viscosity to a pneumatic controller, which regulates the supply of fuel heating steam through a control valve.

Homogenizer

The homogenizer (Figure 2.26) provides an alternative solution to the problem of water in high density fuels. It can be used to emulsify a small percentage for injection into the engine with the fuel. This is in contradiction to the normal aim of removing all water, which in the free state, can cause gassing of fuel pumps, corrosion and other problems. However, experiments in fuel economy have led to the installation of homogenizers on some ships to deal with a deliberate mixture of up to 10% water in fuel. The homogenizer is fitted in the pipeline between service tank and engine so that the fuel is used immediately. It is suggested that the water in a high density fuel could be emulsified so that the fuel could be used in the engine, without problems. A homogenizer could not be used in place of a purifier for diesel fuel as it does not remove abrasives such

Viscotherm Internal Drawing
Figure 2.24 Viscotherm viscosity monitoring device

Steam Controller Recorder

Steam Controller Recorder

Condensate

Figure 2.25 Differential pressure transmitter

Condensate

Figure 2.25 Differential pressure transmitter as aluminium and silicon, other metallic compounds or ash-forming sodium which damages exhaust valves.

The three disc stacks in the rotating carrier of the Vickers type homogenizer are turned at about 1200rev/min. Their freedom to move radially outwards means that the centrifugal effect throws them hard against the lining tyre of the homogenizer casing. Pressure and the rotating contact break down sludges and water trapped between the discs and lyre, and the general stirring action aids mixing.

Figure 2.26

Package boiler combustion system

The elementary automatic combustion system based on a two flame burner (Figure 2.27) is used for many auxiliary boilers. The burner is drawn oversize to show detail. Various different control systems are employed for the arrangement.

The burner has a spring loaded piston valve which closes off the passage to the atomizing nozzle when fuel is supplied to the burner at low pressure, if the fuel pressure is increased the piston valve will be opened so that fuel passes through the atomizer. The system can supply the atomizer with fuel at three different pressures.

The solenoid valves are two-way, in that the fuel entering can be delivered through either of two outlets. The spill valves are spring loaded. When either one is in circuit, it provides the only return path for the fuel to the suction side of the fuel pressure pump. The pressure in the circuit will be forced, therefore to build up to the setting of the spill valve.

A gear pump with a relief arrangement to prevent excessive pressure, is used to supply fuel to the burner. Fuel pressure is varied by the operation of the system and may range up to 40 bar.

Combustion air is supplied by a constant speed fan, and a damper arrangement is used to change the setting.

Upper Cover

Lubrication Nippte

Ringteeder

Upper Rotor Bearing

Homogenizer (Vickers type)

Casing

Homogenizer (Vickers type)

Upper Cover

Lubrication Nippte

Ringteeder

Upper Rotor Bearing

Casing

Viscotherm

lank

Figure 2.27 Elementary automatic combustion system System operation lank

Figure 2.27 Elementary automatic combustion system System operation

Control of the setup may be through various combinations of electrical, electronic or mechanical systems. An electrical control scheme is employed in this description. Electrical circuits are arranged so that when the boiler is switched on (assuming water level and other factors are correct) the system will

(I} heat up and circulate the fuel; (2) purge the combustion space of unburn! gas; and (3) ignite the flame and, by controlling it, maintain the required steam pressure.

When the boiler is started, current is supplied first to the fuel heater. The electric heating elements are thermostatically controlled and when oil in the heater reaches the required atomizing temperature, another thermostat switches in the fan and oil circulating pump. Air from the fan purges the combustion spaces for a set time, which must be sufficient to clear any unburnt gases completely. If not removed an air/gas explosive mixture may be present, so that flame ignition could result in a dangerous blowback. The oil circulates from the pump and heater through the system via the oil circulating valve. This ensures that the oil flows through the burner until it is hot and thin enough to atomize.

When the oil circulating solenoid is operated, the fuel no longer returns to the suction side of the pump but is delivered to the low flame spill through the oil change valve. With the ignition arc 'on', oil pressure builds up sufficiently to push open the piston valve in the burner. The atomized fuel is ignited and once the flame is established, control of the oil change valve and fan damper depends on steam pressure. With low steam pressure, the oil control valve is actuated to deliver the fuel to the high flame spill. Pressure increases until this spill opens and the higher pressure forces a greater quantity of fuel through the burner. When steam pressure rises, the fuel is switched back to the low flame spill The fan damper is operated at the same time to adjust the air delivery to the high or low flame requirement. The solenoid or pulling motor for the operation of the high/Sow flame is controlled by a pressure switch acted on by boiler steam pressure.

Boilers with automatic combustion systems have the usual safety valves, gauge glasses and other devices fitted for protection with additional special arrangements for unattended operation.

The flame is monitored by a photo-cell and abnormal loss of fiame or ignition failure, results in shut down of the combustion system and operation of an alarm. Sometimes trouble with combustion will have the same effect if the protective glass over the photo-cell becomes smoke blackened.

Water level is maintained by a float-controlled feed pump. The float chamber is external to the boiler and connected by pipes to the steam and water spaces. There is a drain at the bottom of the float chamber. A similar float switch is fitted to activate an alarm and shut-down in the event of low water level (and high water level on some installations). Because float chambers and gauge glasses are at the water level, they can become choked by solids which tend to form a surface scum on the water. Gauge glasses must be regularly checked by blowing the steam and water cocks through the drain. When float chambers are tested, caution is needed to avoid damage to the float. Frequent scumming and freshening will remove the solids which are precipitated in f he boiler water by the chemical treatment.

The boiler pressure will stay within the working range if the pressure switch is set to match output. If a fault develops or steam demand drops, then high steam pressure will cause the burner to cut out and the fuel will circulate as for warming through.

Incorrect air quantity due to a fault with the damper would cause poor combustion. Air delivery should therefore be carefully monitored.

Many package boilers burn a light fuel and heating is not required. Where a heater is in use, deviation from the correct temperature will cause the burner to be shut off.

The automatic combustion system is checked periodically and when the boiler is first started up. The flame failure photo-cell may be masked, so to test its operation or some means - such as starting the boiler with the circulating solenoid cut out - may be used to check flame failure shut down. Cut outs for protection against low water level, excess steam pressure, loss of air and change of fuel temperature are also checked. Test procedures vary with different boilers. At shut down the air purge should operate; the fan being set to continue running for a limited time.

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