Boilers are critical pieces of equipment on any vessel, providing heat and steam for a variety of applications. Despite their importance, boilers are among the most dangerous pieces of shipboard machinery. It is critical to perform a variety of maintenance tasks on a regular basis, including blowing down the boiler, to ensure the safe and efficient operation of your boiler.
The blow down of a Boiler Repair Spare Partsboiler is extremely beneficial in extending its working life.
Why Boiler blowdown?
The water that circulates inside the boiler tubes and drum contains Total Dissolved Solids (TDS) as well as other dissolved and undissolved solids.
Water is heated and converted into steam during the steam making process, i.e. when the boiler is in operation. These dissolved solids, however, do not evaporate and separate from water or steam, and they tend to settle at the bottom of the boiler shell due to their weight. This layer will prevent heat transfer between the gases and the water, eventually overheating the boiler tubes or shell.
Scaling, corrosion, and erosion are all caused by different dissolved and undissolved solids. Solid impurities will be carried over with the steam into the steam system, causing deposits on the heat exchanger surface where the steam is the primary heating medium.
To alleviate all of these issues, boiler blowdown is performed, which aids in the removal of carbon deposits and other types of impurities.
Boiler Blow Down Process
The boiler is blown down to remove two kinds of impurities: scum and bottom deposits. This means that blow down is done either for scum or for bottom blow down. Furthermore, the following factors contribute to boiler failure:
- To remove the precipitates that form as a result of the chemical addition to the boiler water.
- To clean the boiler water of solid particles, dirt, foam, or oil molecules. This is primarily accomplished through the use of a scum valve, and the process is known as “scumming.”
- To reduce water density by lowering the water level.
- In case of an emergency, to remove excess water.
The blowdown arrangement in a marine boiler is divided into two levels: the bottom level and the water surface level known as “scum blowdown.” As a result, when the bottom valve is used, the procedure is known as boiler blowdown, and when the Scum valve is used, it is known as “scumming.”
Depending on the type, design, automation, capacity, and characteristics of the boiler feedwater system, the boiler water blowdown can be accomplished in one of two ways:
● Intermittent Or Manual Blowdown
Manual blowdown occurs when the boiler operator performs blowdown manually at regular intervals in accordance with the established operating program. This type of blowdown is beneficial in removing sludge or suspended solids from the boiler.
When there is an oil ingress in the boiler water as a result of a leak in the heat exchanger, this type of blowdown comes in handy. The oil on the water’s surface can be removed using manual scumming.
The primary disadvantage of manual blowdown is heat loss from hot water exiting the water drum. A small amount of water enters the blowdown because the valve is slightly opened. Even so, heat and pressure are lost in significant quantities.
● Continuous Blowdown
Blowdown automation is now standard on many new boilers. It enables continuous boiler water blowdown, which aids in keeping dissolved and suspended solids within boiler operating limits. Continuous blowdown is the name given to this system.
The automation in this system continuously monitors the blowdown, which in turn checks the quality of feed water and the quality of water inside a boiler shell for dissolved and undissolved impurities. As a result, if the boiler water TDS exceeds the allowable operating limit, it will automatically open the blowdown valves.
Since the blowdown valves are precisely controlled, the water discharged from the blowdown removes the maximum amount of dissolved impurities from the boiler water while causing the least amount of heat and water loss, thereby maintaining boiler efficiency.
Most boilers with continuous blowdown automation are equipped with heat recovery systems, which means that the hot water from the boiler blowdown is first sent to a heat exchanger unit, which uses the heat of the water (for example, to preheat the feedwater by installing a heat exchanger or heat recovery equipment in the path) before it is discharged.
The blowdown system, whether manual, continuous, or automatic, will be determined by a variety of factors, and the blowdown valves will be outfitted with appropriate system accessories.
How to determine the blowdown percentage:
Quantity blowdown water/Quantity feedwater X 100 = % blowdown
Procedure for Scumming and Bottom Blow Down
The procedure for blowing down the boiler using the blow down valve located at the bottom of the boiler is outlined below.
Note: Instead of blowing down the bottom, the scum valve must be opened to perform scumming.
The following are the steps in the blow down procedure:
When a modern boiler is steaming at high rates, it should never be blown down. When performing a blowdown, always open the shipside valve first, followed by the blowdown valve. In the event that a pipe bursts, the operator will have control.
- First, open the overboard or ship side valve.
- Turn on the blow down valve , which is a non-return valve.
- The blow down valve next to the boiler should be fully opened to avoid cutting the valve seat or the “Wire drawing effect.”
- The valve regulates the rate of blow down.
- Close the valve in reverse order after blowing it down.
- Even when all valves are closed, a hot drain pipe indicates a leaking blow down valve.
If the boiler is blown down for inspection, the firing must be stopped and the boiler allowed to cool. Allow natural cooling at atmospheric pressure by opening the boiler vent plug.
Ensure that the overboard valve (non-return) is properly functioning so that no seawater enters the boiler pipeline; otherwise, the sudden steam cooling will create a vacuum, resulting in a pipe burst.
When the boiler blowdown is finished, open the belly plug to remove the remaining content in the engine room bilges.
Make sure to perform a ‘Scum Blowdown’ before a ‘Bottom Blowdown,’ or else the scum will settle on the water surface or, if there is any oil content, it will be agitated, contaminating the boiler water.
Advantages Of Boiler Blowdown:
- The regular blowdown of boiler water keeps the total dissolved solid impurities within the rated limits.
- The process aids in corrosion prevention by removing impurities that speed up the corrosion process.
- It aids in the prevention of scaling on boiler tubes and the internal surface.
- It prevents impurities and contaminants from being carried over with the steam, resulting in pure steam.
- It keeps the internal parts of the heat exchanger where pure steam is used as a heating medium from scaling.
Disadvantages of Boiler Blowdown:
● If the procedure is not carried out correctly and on time, the blowdown of boiler water tends to increase heat and pressure losses.
● Heat and pressure losses caused by boiler water blowdown reduce boiler efficiency.
● Additional work hours required to complete the operation if the blowdown arrangement is manual.
Requirements and Regulation:
- If there is visible oil sheen in the boiler gauge glass or hotwell inspection glass. Because the oil will be on the water’s surface, avoid scum blowdown at all costs, as this will result in oil pollution.
- The oil leakage inside the boiler water must be stopped, and every effort must be made to clear the oil from the hotwell by filling it with freshwater and removing the oil-water.
- Ensure that the operator is aware of the Vessel General Permit areas, that he or she is in compliance with Chapter 12 of the VGP, and that no wastewater from the boiler blowdown is discharged in the restricted areas other than for safety reasons.
- The vessel must make certain that no boiler water is discharged into port waters as a result of boiler blowdown. This is due to the fact that the water contains various chemicals or other additives that are added to reduce impurities or prevent the formation of scale.
- The blowdown of the boiler must be performed as far away from the shore as possible.
- Before beginning the blowdown operation, the Master and the bridge duty officer must be notified.
- The boiler blowdown procedure must be documented in the Engine Room Logbook, along with the starting and stopping times.
- If the boiler blowdown or hot well water is transferred to the bilges, the ORB and engine room logbook must be updated.
Only under the following conditions can the boiler be blown down in territorial water or a harbour:
- If the ship is entering dry dock, the boiler must be blown down.
- For reasons of safety
How can boiler blowdown be minimised?
Boiler blowdown is estimated to account for 2-3% of total energy consumed during a typical boiler operation. As a result, reducing boiler blowdown has the potential to result in significant energy savings. There are several methods for reducing boiler blowdown, which are discussed further below.
The primary goal of a boiler blowdown is to reduce dissolved impurities in the boiler water, which causes scale formation.
The formation of scale will directly cause heat transfer within the internal surface of the boiler, resulting in a decrease in boiler efficiency.
If the boiler water is tested on a regular basis and effectively treated using various chemicals in the hot well, the feed water will have fewer impurities and will be suitable for use.
The need for boiler blowdown will be reduced, resulting in water savings as well as a reduction in heat and pressure losses.
Boiler Water Blowdown Reduction
The water and fuel consumption of boiler water will increase as boiler blowdown increases. The manual blowdown system should be removed, and an automatic boiler water measurement and blowdown system should be installed.
As explained in the continuous blowdown system above, this system will effectively monitor impurities in the boiler and open the discharge blowdown valve as needed.