The feed water system of the boilers is the component of the steam producing plant that circulates feed water from the cascade tank into the oil-fired auxiliary boiler and the exhaust gas boiler via the boiler feed water pumps and feed water regulators.
The feed water regulating valve automatically controls the feed water flow to each boiler in line with the change in water level in the boiler, in order to keep the water level constant. The regulating valves are, generally of pneumatic actuated globe valve type. In the video below you can see how these valves actually works.
Two boiler feed pumps take suction from the cascade tank and provide main and auxiliary feed lines to the oil-fired auxiliary boiler and the exhaust gas boiler. Each boiler’s main lines are equipped with a feed water control valve, which automatically regulates the flow of water to the boiler in order to maintain the proper water level. The auxiliary feed pipes allow for direct feed input in which case human control of the boiler’s water level is required.
Before the feed pump discharge valve, a little amount of water is diverted back to the cascade tank from each feed pump output, thus the discharge line to the drain tank features a number of aperture plates to minimize water pressure.
This water discharge ensures that even when the boiler feed control valve is closed, water flows via the running feed pump.
The boiler feed water is sampled and treated to prevent corrosion and scale formation in the auxiliary and exhaust gas boilers, as well as the degradation of steam quality. Incorrect or insufficient boiler water treatment will severely damage the boilers, necessitating frequent testing and treatment to prevent the risk of damage. Even when distilled water is utilized for boiler feed, there is a risk of corrosion. In service, the pH of the water fluctuates, and oxygen might dissolve in the water where the feed system is exposed to the atmosphere. Although keeping the feed water temperature reasonably high, above 60°C, will reduce the amount of dissolved oxygen, the problem is always present.
Water sampling connections are provided on the auxiliary and exhaust gas boilers, with the outlet from these being sent to a sample cooler that is chilled by water from the service cold water system. Usually, the sample cooler is placed in the workshop or in the close proximity of the boiler. In order to acquire a fully representative sample of water from the boiler, the water must be allowed to run from the boiler for a minute before being sampled. The boiler’s sampling valve is positioned to produce a representative sample, but old water in the pipes and cooler must be purged before the testing sample is drawn. Every day, the boiler water must be tested. To guarantee that the boiler water is properly treated, the directions provided by the water treatment test kit vendors must be strictly followed.
The procedure of taking a water sample from the boiler can be described as follow:
- Check that the cooling fresh water is available for the water sampler.
- Open the sample cooler cooling water outlet and inlet valves and check the flow of fresh cooling water through the sample cooler.
- Open the water sample outlet valve on the sample cooler and
- Slowly open the sampling valve on the boiler from which a water sample is required and allow boiler water to flow through the sample cooler. Ensure that water is leaving the sample cooler outlet and not a mixture of steam and water. If the temperature of the boiler water leaving the sample cooler is too high, reduce the flow of boiler water to the sample cooler.
- After the boiler water has been flowing for one minute, collect a sample of the boiler water for analysis.
- Close the boiler sampling valve and then close the sample cooler cooling water valves and the sample inlet and outlet valves.
- Analyze the sample of boiler water in accordance with the instructions of the chemical treatment supplier and record the information. Add chemical treatment to the boiler feed water as required.
Analytical tests and chemical treatment must be carried out in line with the chemical manufacturer’s recommendations. To keep the chemical levels within an acceptable range, the treatment must be added, but caution must be exercised, as excessive treatment can frequently cause more severe harm than minimal treatment. The results of the chemical analysis on the boiler water are recorded, and the effects of the added treatment can be tracked over time.
Following the analysis of the boiler water, a decision must be made regarding the amount and type of chemicals to be added to the boiler feed water, if any.
The treatment is, usually, added to a chemical injection tank and from there, it is routed to the boiler feed water lines. Chemicals for direct injection into boilers are combined with water in chemical injection tank. The combination is injected into the boiler water feed line immediately after the feed water control valve by a pump unit and the auxiliary boiler and the exhaust gas boiler share the same feed tank and treated feed water. Chemicals can also be put to the chemical injection tank and pushed from the tank into the boiler feed water lines using pressurized feed water.
The addition of chemicals must be done in line with the manufacturer’s recommendations.
If the level of boiler water dissolved solids is too high, these can be removed on a regular basis using the scum valve on each boiler, whereas dissolved solids can be minimized by blowing some of the water out of the boiler and replacing it with fresh distilled feed water. This is known as boiler blowdown, and it is achieved by opening the boiler blowdown valve for each boiler. The scum and blowdown lines link to the same blowdown pipe, which connects to an overboard discharge placed below the waterline of the ship.
The blowdown procedure is as follow and must be performed during boiler low load:
- Check with the bridge that it is safe to blow down the boiler if the ship is in port.
- Open the ship’s side blowdown valve
- Ensure that the boiler is filled to the high water level.
- Slowly open the boiler scum valve and reduce the water level to the normal position, then close the scum valve.
- Refill the boiler to the high water level position and blow down the boiler using the blowdown valve. After the blowdown of the boiler, close the boiler blowdown valve and then close the line and ship’s side valves.
- Test the boiler chemical concentrations and adjust as necessary.
In conclusion, the following precaution must be taken when you are dealing with boiler feed water and treatment chemicals:
- Caution must be exercised as the sampling lines from the boiler are under boiler pressure and the temperature of the water being drawn from the boiler is high.
- Care must be taken when operating the sampling equipment. The cooling water supply must be confirmed to be flowing before the boiler sample valve is opened and valves must be opened slowly.
- Care must be taken when handling boiler water treatment chemicals. Protective equipment must be used.
- When blowing the boiler down the overboard discharge valve must be opened before the boiler or scum blowdown valves, as opening the boiler valves first will subject the blowdown line to full boiler pressure.
- When turning down one of the boilers for a short length of time (for example, the exhaust gas boiler while the ship is in port), it is critical to ensure that the water in the boiler has been properly treated. After a shutdown boiler is restarted, a water test should be performed as soon as practicable.
- It is essential that details of water analysis are recorded together with details of the treatment added. Only with detailed information is it possible to determine the cause of possible future problems.
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