Introduction
Boiler blowdown refers to the process that takes place on board ships to remove dissolved salts, sludge and other impurities from the water in boilers. This process is crucial to maintaining the correct chemistry of the water, preventing scale buildup and ensuring the proper functioning of the boiler. Blowdown is performed periodically, depending on the test results of the boiler water and the operating conditions of the boiler.
Boiler blowdown is the controlled discharge of boiler water from the boiler system to reduce the concentration of dissolved and suspended impurities.
As water is converted into steam, most dissolved solids remain inside the boiler. With continuous steam production, their concentration gradually increases. Blowdown helps keep these concentrations within the limits specified by the boiler water treatment programme.
There are generally two types of blowdown:
A small quantity of boiler water is discharged continuously or at regular intervals to control dissolved solids.
A larger quantity of water is discharged for a short period from the lower part of the boiler to remove sludge and settled impurities.
Regular blowdown is necessary for several reasons:
The exact procedure varies depending on the boiler design and shipboard arrangement. The manufacturer’s instructions and the vessel’s operating procedures should always be followed.
Before carrying out blowdown, check the boiler water test results and operating conditions. Parameters such as chloride level, phosphate, alkalinity, pH and conductivity may be monitored according to the vessel’s water treatment programme.
Determine whether continuous or intermittent blowdown is required.
Blowdown should be carried out with proper communication between the personnel involved. If necessary, inform the duty engineer or other engine room personnel before opening the blowdown valve.
Make sure the discharge arrangement is ready and that no maintenance work is being carried out on the blowdown system.
Inspect the blowdown valve, piping and associated equipment for any visible leakage or abnormal condition.
The blowdown line should be connected to the designated blowdown tank, drain tank or other approved discharge arrangement. Never discharge boiler water into an unsuitable location.
Check the boiler pressure and water level before starting. The boiler should be operating within its normal parameters.
For bottom blowdown, the water level and boiler pressure should be carefully monitored because a sudden discharge can cause a noticeable change in boiler water conditions.
Open the valve slowly according to the ship’s approved procedure. Sudden opening can cause rapid flow, water hammer or excessive stress on the piping and valves.
During the operation, observe the blowdown line and associated equipment for abnormal vibration, leakage or unusual noise.
The duration and frequency depend on the boiler design, boiler water test results and operating conditions.
For intermittent bottom blowdown, the valve is normally opened for a short controlled period and then closed securely. The operation should remove accumulated sludge without unnecessarily discharging a large amount of boiler water.
Continuous blowdown may be adjusted using the designated valve or control system to maintain the required water chemistry.
After the required blowdown has been completed, close the blowdown valve as instructed by the manufacturer’s procedure.
Check that the valve is fully closed and that there is no leakage from the valve, flange or piping.
After blowdown, monitor the boiler water level and pressure. If necessary, make adjustments using the normal feed water system.
Avoid making sudden changes to boiler operating conditions. Continue monitoring until the boiler returns to stable operation.
The blowdown operation should be recorded as required by the vessel’s procedures. Details may include the date, time, boiler condition, duration of blowdown and relevant boiler water test results.
These records help engineers monitor trends in boiler water quality and determine whether the blowdown frequency needs adjustment.
Insufficient blowdown can result in excessive dissolved solids, sludge, scale, foaming and poor steam quality. On the other hand, excessive blowdown can lead to unnecessary heat loss, chemical wastage and increased feed water consumption.
Therefore, blowdown should not simply be performed for a fixed duration without considering boiler water test results and operating conditions.
Boiler blowdown is a relatively simple but critical process on board ships. It helps to ensure the quality of the water in the boiler and protect it from the formation of scale and corrosion as well as impurities. Thus, it is important to carry out blowdowns at the right time and in the right proportion, taking into account test results, boiler characteristics and manufacturer recommendations.
It is done to remove concentrated boiler water, dissolved materials, and sludges.
It is a short periodic blowdown made from the lower part of the boiler.
It depends on the results of water analysis, boiler design, boiler operating conditions and manufacturer’s recommendations.
There is wastage of hot treated water, chemicals and heat and consequently poor efficiency.
To avoid high pressure differences leading to water hammer and extra strain on the blowdown piping and valves.
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