Auxiliary Steam Plant

Silencer boiler (Spanner)

Spanner exhaust gas boilers can be provided as steam raising boilers or hot water boilers or economisers. These boilers recover from 20% to 50% more heat at a higher working pressure than some other designs.

They utilise the heat contained in the exhaust gases from diesel engines and gas turbines. Spanner boilers of this type have tubes expanded and then seal welded into the tube plate thus designed to be able to be run dry or with varying water levels giving varying steaming rates. In the dry condition access doors are opened to allow air circulation.

They may be fitted with an integral by-pass with either manual or automatic control so that steam output can be controlled by varying the amount of gas passing over the heating surface.

The boiler is designed to offer the minimum of resistance to gas flow with a back pressure less than 150mm water.

There are therefor three ways of controlling steam pressure/generation rate

via a gas by-pass valve

via a steam dump system

via changing water levels(thus changing generating surface)

There is no capability for oil firing therefore a donkey package boiler is required for use in port.

SWIRLYFLO tubes are used

Stone vapour boiler.

The steam generating part of the unit consists of sets of coiled tubes nested and connected in series to form a single tube several hundred feet long. Water is pumped into the coil inlet and converted to steam as it progresses through the coils. Heat is furnished by combustion of diesel fuel oil which is sprayed by compressed air through an atomising nozzle in the fuel sprayhead into the firepot above the coils. Here fine oil spray mixes with air supplied by the fan and is ignited by a continuous electric spark. The heat flow is first downward, then outward through the nest of coils.

The fuel supply is regulated such that the steam generated is equal to 80% to 90% of the water supplied, and any scale or sludge formed is carried out of the system by the water. And deposited in a separator. This ensures that the inner surfaces of the tubes are kept clean, the outside is cleaned by filling the chamber with water and bubbling steam through the water for about 12 Hrs.

In the separator the sludge etc, settles to the bottom and is removed periodically by blowdown. When water level reaches overflow level it is removed via a steam trap back to the water supply tank but some of its heat energy is first used to heat the incoming feed water. Feed pumps are driven at constant speed, regulation is by a by-pass valve. When demand for steam falls, the steam pressure at outlet increases, this acts on the feed regulator in such a way as to increase the proportion of by-pass. Fuel and air control is by feed flow into the boiler, thus if the by-pass is opened and feed flow to the boiler reduced, so the fuel/air flow is adjusted accordingly.

Water pressure relief valve        40bar

Fuel pressure regulator             10.9 bar

Atomising air pressure             4.9bar

Max evaporation at                  7bar 1250 Kg/Hr

Fuel consumption                       114 litres hr

Water capacity                         77 litres

The advantage of this type of boiler is a very short warm up time, the disadvantage is of course the disastrous consequence of a perforated pipe

Composite boilers-(Clarkson thimble tube)

This type of boiler will generate steam by means of exhaust gas and oil firing, many different arrangements are possible, the one shown is the Clarkson composite boiler which makes use of thimble tubes for heat steam generation. The only restriction is that it must not be possible to have oil firing and exhaust gas firing on the same heating surface at the same time. If this were the case there would be the possibility of exhaust gas entering the engine room through the burner section or of blowback due to the action of exhaust gas and oil firing. There would also be a risk of poor engine performance due to exhaust back pressure, in order to avoid problems and ensure that full use is made of exhaust gases even at slow speeds it is usual to have completely separate oil and exhaust fired heating sections.

Boiler tubes

In order to promote better heat transfer between gas and water it is necessary to provide as large a surface area as possible. Specially shaped tubes will allow for this. Heat transfer is also improved by giving the water a swirling action and the tubes shown will achieve this, for plain tubes twisted strips of metal known as retarders were sometimes inserted in order to cause turbulence in the water flow

The Swirlyflo tube is formed from plate and rolled into seamless tubes. The Cochran sinuflo is straight when viewed from the side, to prevent deposits building up in the depressions.

Swirlyflo tubes may be inserted anyway around but the sinuflo tubes must be inserted as shown. In addition they can only be fitted through one tube plate and hence only removed the same way.

With any tubes deposits and corrosion can occur on the water side. The nature of deposits and the degree of corrosion if any will depend upon the quality of the feed and the effectiveness of treatment. The amount of treatment is important too, too or too much both causing problems. Circulation of water will also influence corrosion especially if stagnant flow areas are allowed to develop. Stresses induced in the tube and the tube plate due to expansion can cause cracking in the area of the tube plate connection. Vibration can also weaken this expanded connection.

On the gas side the nature of the deposits and there quantity depends upon the quality of the fuel burnt and the combustion process. Vanadium, sulphur and carbon can all form deposits with the sulphur leading to corrosion. Carbon will impair the heat transfer as will Vanadium which can also cause corrosion.

Severe corrosion on either side of the tube means that it must be replaced but the degree of corrosion must be considered in terms of thickness, operating conditions and history. I.e. has the corrosion been reduced.

Tubes should be expanded if signs of leakage are evident. Over expansion damages the tube plate and may cause problems when replacing the tube. A leaking tube can be plugged as a temporary measure but only if that leakage is from the tube itself and not from the expanded connection.

Replacing a tube requires the old tube to be removed usually by cutting three grooves through the tube at the expanded connection and collapsing the ends and then punching the tube out

Motor ship steam plant

Were a large quantity of exhaust gas is available for extended periods of time it is economical to make use of the exhaust gas to improve the quality of the steam generated by superheating. This allows for the use of a turbine which connected to a generator can provide electricity. Diesel generators are required as back up in the event of failure. For in port a boiler with an independent superheater will be required.

The steam generator part of the waste heat unit does not generate 100% dry steam at out let. 10% remain as water which carries back any sludge to the boiler where it can be removed. The waste heat unit should have sufficient reserve capacity to allow several minutes of operation after engine load reduction to allow the in port boiler to be flashed