Introduction
Steam generation ushered in a new era of the industrial revolution. It was now possible for us to power huge factories, dig deeper mines for resources and establish a more reliable transport network.
Since then, the use of steam has been somewhat replaced by electricity but steam generators and boilers still hold a very important place in our industries today.
For instance, auxiliary steam boilers on ships provide steam for COPTs, accommodation heating, fuel oil heating, oil purifiers, jacket water preheating, calorifiers, etc.
In some naval (and non-naval) vessels, steam boilers also provide propulsion power through steam turbines. Steam propulsion provides greater power to weight ratio and maneuverability which is crucial for a combat vessel.
All steam equipment, however, requires timely and effective maintenance for a long and productive life. Boiler water treatment is one such aspect of boiler maintenance.
In this article, we shall take a look at the different types of treatment and their benefits to the boiler feed water system. But let’s start from the beginning.
What is boiler water treatment?
Boiler water treatment is the quality improvement and refinement of boiler feed water to make it fit for use in boilers.
Maintenance teams may use processes such as surface and bottom blowdown, filtration (and ultrafiltration), deaeration, evaporation, membrane processes (nanofiltration and reverse osmosis), etc. for water treatment.
These processes are supported by chemical treatment methods to condition sludge, exchange ions, scavenge oxygen, and reduce hardness and foaming. Timely treatment safeguards the boiler from water-related issues.
In addition to removing and reducing impurities, we also add chemicals that will form a protective lining on pipes to prevent corrosion, scale deposits, etc.
Why boiler water treatment is required?
Although a boiler is capable of handling the harsh conditions it operates in, there is a certain standard of boiler water that must be provided for its reliable operation.
A boiler provides high-temperature high-pressure steam across the system. The constant phase change and exposure of various chemicals already present in the boiler system (such as those in hard water) can cause the boiler components to degrade at an alarming rate.
Using sub-standard quality boiler water can put the system components under a lot of stress. It can result in corrosion, leakages, low efficiency, poor steam quality, sludge deposits, scale deposits, and premature failures.
All of the above issues can result in steep repair costs ranging from a few thousand dollars up to a million dollars. In addition to that, boiler downtime during repairs and replacements costs more. The likelihood of incidents and accidents also increases which may result in loss of life or serious injuries.
The advantages of boiler water treatment far outweigh its disadvantages. It is therefore a necessary part of every boiler maintenance system to keep a close eye on boiler water parameters.
The system must be treated and tested continuously for any chemical imbalances to prevent problems due to fluctuations in the water treatment program and resulting in poor water quality.
Related read: 29 types of boilers explained
External and internal boiler water treatment
Boiler water treatment can be carried out in various steps. It can either be done outside of the boiler or while the water is in use in the boiler. Based on this parameter, we can classify boiler feed water treatment into two main types.
- External treatment
- Internal treatment
External treatment
External treatment is the treatment of boiler water outside of the boiler. We recommend this when the amount of one or more of the impurities in the boiler water is above tolerable limits and cannot be introduced into the system.
In external treatment, the boiler water is treated for various issues such as hardness, alkalinity, suspended solids, dissolved solids, dissolved gases, minerals, dissolved oxygen, iron oxide and other impurities.
The external treatment contains many processes. Some of these are:
- Evaporation
- Deionization
- Softening
- Dealkalinization
- Deoxygenation
- Removal of suspended solids and turbidity
Internal treatment
When the concentration of impurities is within tolerable levels for the system, internal treatment suffices for the water treatment and no external treatment is necessary.
Internal boiler water treatment is a must in every boiler system. It is required even when the makeup water quality is exceptional. For instance, on ships, the makeup water is usually sourced from the fresh water generator which is very high quality as it has negligible impurities and usually less than 10 ppm salinity.
Even this water is conditioned and treated to maintain the various parameters within the recommended range as per the treatment plan to ensure it does not harm the boiler in any manner.
Common water-related issues and their boiler water treatment chemicals
Boiler water is the lifeblood of any boiler system. It reaches every part of the system in one phase or another. It is also responsible for the majority of the issues faced within a boiler. In this section, we shall take a look at the various issues that are a direct result of sub-optimal boiler water and how we can rectify them. These issues are:
- Corrosion
- Water hardness
- Scaling
- Sludge accumulation
- Carryover
- Phosphate treatment
Corrosion and its causes
Corrosion is one of the most difficult issues encountered when maintaining a boiler. Different types of corrosion occur in different areas of the boiler. Multiple types of corrosion can occur in the same area too.
For instance, general corrosion occurs all over the system whereas pitting corrosion is localized and typically occurs on tube surfaces. Multiple agents are responsible for corrosion in a boiler and require different treatment methods.
Moreover, as most boilers are in a closed loop, the iron oxide generated due to corrosion adds to sludge deposits and needs sludge conditioning agents for effective removal.
There are three main reasons of corrosion in boilers. These are:
- Too low or high alkalinity
- Dissolved oxygen and other gases
- Condensate system corrosion
Low and high alkalinity
Acidic environments are more prone to corrosion. Boiler water is maintained alkaline to prevent this. One way is to use alkalinity builders to raise the pH levels of boiler water. In addition to the boiler proper, these chemicals protect the feed line, condensate line and the flue gas economizer.
However, too much alkalinity is also an issue. It can cause foaming, caustic corrosion and embrittlement. In addition to corrosion, alkalinity builders also prevent scale formation.
Alkalinity builders usually contain a 25 or 50% solution of sodium hydroxide or potassium hydroxide. The selection depends on the pH levels and requires regular checks. Some alkalinity builders also contain sludge conditioning polymers which is an added advantage in high-temperature applications.
Dissolved oxygen and other gases
Dissolved gases such as carbon dioxide and oxygen can also cause corrosion in the system. Oxygen causes pitting corrosion of steel surfaces whereas carbon dioxide lowers the boiler feed water pH making it susceptible to acid and galvanic corrosion. Carbon dioxide also has the tendency to form magnesium and calcium carbonate deposits in alkaline conditions.
Oxygen scavengers can effectively reduce the oxygen levels in the boiler water. Some of these chemicals also passivate the surface by forming a thin layer of magnetite (Fe3O4) and preventing corrosion. Tannin, sodium sulphite and hydrazine are some popular oxygen scavengers that reduce/eliminate oxygen and reduce the corrosion risk.
The type of deaerating heater in use determines the selection and dosage of a suitable oxygen scavenger for the system. A deaerating heater can reduce the amount of oxygen, carbon dioxide and other dissolved gases from the system effectively and support oxygen scavengers.
Condensate system corrosion
When steam condenses in the return lines, it has a tendency of absorbing oxygen and other gases causing corrosion in the steam lines as well as the hot well.
Even if we remove the oxygen in the boiler vessel, when the steam condenses, it can absorb it again and reintroduce it into the system.
Condensed steam also causes the formation of carbonic acid due to the presence of carbon dioxide. This lowers the pH of the system and causes acid corrosion.
Volatile neutralizing amines and filming inhibitors are particularly effective against condensate line corrosion. Amines are compounds containing nitrogen and hydrogen. They neutralize the acid, lower the pH and also form a protective film over the surfaces.
Amines are either fed directly through the steam or added to the boiler feed water systems. Experts recommend continuous feeding but intermittent feed can be sufficient in some cases.
Water hardness
Hard water is water with a higher than usual mineral content. It usually refers to the high concentrations of bicarbonates, chlorides and sulphates of calcium and magnesium in water. Rainwater is soft but when it seeps through limestone and other porous rocks, it absorbs minerals and hardens.
Thus, hard water is a more common problem in land based boilers as marine boilers usually run on water from fresh water generators which has very low mineral content.
Hardness can lead to limescale formation on boiler surfaces with high temperatures and bring down the thermal efficiency of boilers. This leads to greater fuel consumption as well as the risk of boiler tube failure if left unchecked.
If your source provides hard water, you may consider changing it if possible. If it is not economical, you will either have to provide external treatment, internal treatment or both to neutralize the minerals.
You may use sodium phosphates, caustic and soda ash among other softening chemicals for treatment. These chemicals remove the high concentration of hardness minerals.
Sodium silicate is preferred for magnesium hardness compounds because we can precipitate magnesium silicate through it. For calcium bicarbonate, we can use caustic soda and convert it into calcium carbonate which is comparatively insoluble. Caustic soda also reacts with magnesium sulphate to precipitate magnesium hydroxide.
Calcium sulphate is removed by using sodium. We use sodium carbonate, sodium phosphate and sodium silicate to form calcium carbonate, calcium phosphate and calcium silicate. Sodium sulphate requires evaporation to exit the water as it has high solubility.
Scaling
Scaling affects the heat transfer properties of boiler tubes and ultimately causes tube failure. Silica, calcium, magnesium, aluminum and iron compounds are generally responsible for scale deposits.
When the boiler water containing these compounds evaporates on the tube surface, it leaves behind these compounds in a solid state. Typically, the scale has 1/48th the heat transfer capacity of steel and it acts as an insulator.
This causes local hotspots to develop and the tube fails even due to a thin layer of scale. Corrosion may also take place under the scale surface.
The most efficient solution to preventing scale is therefore to supply good quality demineralized water. Whatever compounds still get into the system can be effectively neutralized using internal treatment.
The corrective action in the case of scaling is to separate the hardness cations of magnesium and calcium from the scale-forming minerals, replacing them with sodium ions in the process.
Silica scaling is rare in boilers as it is usually found in small quantities in the boiler water. If found, silica is removed by increasing the frequency of boiler blowdown until the concentration drops to tolerable levels. It is important to find the source of silica contamination and address it for a complete solution.
Sludge accumulation
Sludge can accumulate in a boiler system through various sources. Suspended solids in the feed and make up water can accrue over time as the steam exits from the system.
It consists of oils, minerals, rust particles, salts as well as all other solids carried by boiler water. This sludge builds up over time and sticks to the metal surfaces. Some of it may harden and even clog the lines.
It may also result in corrosion as well as, like scale formation, reduce the heat transfer capacity and cause ruptures in the boiler tube metal. The chemical cleaning of sludge is done by sludge conditioners.
Sludge conditioners work in two different ways. If the amount of sludge is high, we use coagulants to coalesce and remove it from the system through blowdown. If, on the other hand, the sludge quantity is within permissible limits, we use dispersants to spread it evenly across the system.
Coagulants are usually polyelectrolytes. We can also cause coagulation by meticulously adjusting the alkali, organics and phosphates used for treatment. Some other popular coagulates are aluminum-based such as alum and polyaluminum chloride.
Common dispersants include tannin and starch. Their main objective is to prevent the coagulation and the formation of sizeable crystals of sludge salts such as MgSO4.
Some sludge conditioners can be used at a higher dosage to remove hardened scale in established boiler systems over a period of time. This may, however, start leakages when the scale dissolves.
Carryover
Carryover refers to the transport of impurities with the steam. This causes solid, liquid and gaseous impurities to reach the after-boiler components and affect various components such as superheater, non-return valves, control valves and turbine.
The turbine is especially vulnerable as carryover particles can cause shock damage and erode turbine parts. Foaming is the primary reason of carryover and occurs due to the high concentrations of chlorides, oil, silica, TDS and suspended solids. High alkalinity can also lead to foaming and eventual carryover.
High salinity can also cause carryover as it allows bigger bubbles to form and carry foam with it when the steam exits the boiler.
To reduce the carryover, the most efficient method is to increase the boiler blowdown frequency. If the carryover keeps increasing, use of anti foam agents can economically reduce carryover.
Phosphate treatment
Phosphate treatment of boiler water removes leftover hardness compounds from the boiler water.
When it reacts with calcium-based hardness, it forms calcium phosphate and solves the problem of calcium carbonate scaling. Calcium phosphate is insoluble in water and can be removed easily by boiler blowdown. It, however, requires an alkaline environment to remain non-adherent within the boiler.
When the phosphate compound reacts with the magnesium portion of the hardness, it is preferred that it precipitates as magnesium silicate. This compound is non-adherent. If there isn’t enough silica, the reaction will form magnesium hydroxide. But in the absence of an alkaline environment, it will form magnesium phosphate which is an adherent compound.
The alkalinity of boiler water is hence an important part of an efficient phosphate treatment program.
The effectiveness of a phosphate treatment can be further improved by using organic supplements such as starches, lignins and tannins. These organic materials help the formation a fluid sludge which can be removed by blowdown.
However, in recent years, synthetic polymers are being used increasingly as dispersants in the place of organic supplements.
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