Onboard vessel, in general, saturated steam is produced into the oil-fired boiler or the exhaust gas boiler. There are special type of systems, where steam is used for propulsion or electrical generation into the steam turbines. On these cases steam need to be superheated and at high pressure and steam generating systems are more complex with bigger boilers, large economizers, superheaters, large steam condensers and hotwells.
On the majority regular vessels a normal pressure of 6.0kg/cm2, saturated steam is led from the oil-fired boiler or the exhaust gas boiler, through pipe lines connected to a common 6.0kg/cm2 steam line from which steam is distributed to consumers. The steam is produced by boiling water using fuel in oil-fired boiler or by using the energy from the main engine exhaust gases during vessel cruising or in some cases, when large diesel generators are in play, the energy of their exhaust gases, as they are fitted with economizers on their exhaust manifold as part of the heat recovery systems.
Steam has a lot of use onboard vessel, like: heating fuel, oil and sludge, heating cargo, heating water, crew accommodation heating and climatization, de-icing of sea chests, pipe’s heating through steam tracings, etc.
In the close proximity of boilers, on the the common steam line there is a branch to the pressure-controlled steam dump valve, which discharges surplus steam to the dump condenser. Here below there is an example of dump valve operating principle (onboard vessels they are pneumatically actuated valves):
The correct procedure of supplying steam to the consumers is as follow:
- Ensure that the boiler is operating correctly and that the desired steam pressure is being generated.
- Ensure that cooling water (fresh or sea water) is open and flow is present through steam dump condenser cooling tubes. This can be checked by opening the vent valve or through a manometer port on the outlet side of the steam dump condenser.
- Open all steam line drain valves and drain the section of steam pipe of condensate as the pipe warms up. Where drain traps are fitted, line drain valves may be left open after the pipe has warmed through, otherwise the drain valve must be closed when the pipe is warm and steam comes out from the drain valve.
- Slowly crack open the steam valve and allow the pipe line to sufficiently warm up and expand.
Water hammer in steam lines can be an issue since it can damage the pipe system and potentially cause the steam line to fail, injuring the crew members. It is critical to drain all steam lines of condensate and to gradually supply steam to cold lines with line drain valves open. This enables for proper warming of the steam line and drainage of the condensate. Drain valves are positioned throughout the steam system, and they must be opened prior to opening the steam valve for that section of line. Drain valves for all systems are normally left open when a drain trap
is fitted, however, if a system is shut down for maintenance the drain valve should be shut as well as the steam valve after the line has been allowed to cool down. The cooling down period is required to ensure that a vacuum is not formed in the steam line by the remaining steam in the heating coil/heater condensing.
Steam traps enable condensate but not steam to flow through, allowing for more efficient use of heating steam because the latent heat of evaporation is recovered as the steam condenses. If a drain trap fails, steam can travel through, resulting in low efficiency.
Auxiliary steam service drains are routed to the hotwell via the steam dump condenser and the observation tank and after that, the condensate is returned to the feed water system. Because there is a risk of contamination from hydrocarbons from oil heating services, the condensate is tested in the observation tank before being returned to the system.
The condensate from the steam dump condenser is gravity-fed to the observation tank and finally to the hotwell. In order to maintain the desired steam system pressure, the dump condenser condenses excess steam from the exhaust gas boiler. An inverted ‘U’ bend in the water output line keeps the condenser water level stable.
A three-way valve at the condenser’s drain line entrance bypasses the atmospheric condenser, allowing condensate to flow straight to the observation tank and by allowing hot condensate directly into this, the system allows the temperature of the water in the observation tank to be adjusted. The three-way valve is temperature controlled, with a set point temperature of 85 – 90 deg. C at the observation tank. When the atmospheric condenser is being serviced or is malfunctioning, the condenser bypass can also be employed.
Water travels from the observation tank to the hotwell, which has a steam heating coil to help heat the boiler feed water if necessary and the level in the hotwell is maintained either directly from the discharge line of the fresh water generator into it or indirectly from the home fresh water hydrophore system. The make-up water should ideally be distilled water from the fresh water generator.
The observation tank has observation windows as well as hydrocarbon monitoring equipment. If any oil is found, an alarm will trigger, and precautions can be taken to prevent oil from being pumped into the boilers. The observation tank has a big reserve of water, and the flow from the observation tank to the hotwell is from the bottom of the observation chamber. This decreases the possibility of oil carryover to the hotwell and, as a result, to the feed pump suction. Any floating sediment or oil in any portion of the observation tank can be emptied to the sludge tank via a scum line.
If oil contamination occurs every effort must be made to avoid pumping oil into the boilers. The scum valve should be opened in order to remove oil from the surface of the observation tank and the temperature in the observation tank should be maintained in order to assist in the oil removal operation. If oil is present in the observation tank, the drains from the drain traps on all the steam services should be checked until the defective service is located and this must then be isolated for repair. After repair the drain line and drain trap from the defective service must be cleaned to remove all traces of oil. The observation tank and the oil content monitor probe must also be cleaned.
Water from the hotwell gives a positive inlet head of pressure to the pump suction for the auxiliary boiler and exhaust gas boiler feed pumps. A salinity sensor is fitted in the feed pump suction line or between steam dump condenser and observation tank (in case of sea water cooled steam dump condenser) and is linked to ECR alarm monitoring system and its purpose is to detect any salt water contamination in the system but it can be isolated if required.
The dump condenser’s condensate outflow temperature should be sufficient to keep the observation tank at 85 – 90°C. The three-way bypass valve, as previously mentioned, permits direct passage from the drains to the observation tank.
There are two boiler feed pumps, one for the oil-fired auxiliary boiler and one for the exhaust gas boiler. Each boiler has two feed water lines, with the main feed line to each boiler having a controlled valve that is controlled by the boiler’s water level. There is a return connection from the feed pump output to the hotwell to ensure that water flows through the feed pump even when the feed control valve is closed. This line has four orifice plates to control the flow of water returning to the drain tank. If the feed control system fails, the auxiliary feed valve on each boiler provides manual control of the water level.
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