Understanding offline boiler corrosion prevention, protection and control is essential for businesses that regularly rotate their steam boilers, or periodically take them out of service for other reasons. In this article the industrial steam generation specialists at Water Treatment Services consider the most common techniques for protecting boiler systems from the damaging effects of corrosion whilst offline.
Offline boiler corrosion prevention and control
If you take your steam boiler offline as part of a rotation process or for maintenance purposes this can often result in oxygen entering the system as the fabric of the boiler begins to cool. Oxygen which is dissolved in the boiler water can quickly cause metal to corrode through pitting, and in the most serious cases lead to premature failure.
This type of boiler corrosion problem is particularly acute in steam generating systems using multiple boilers, where individual units are taken out of service on rotation, or left standing unused for extended periods of time.
Unless you’re using specialist boiler water treatment chemicals such as oxygen scavengers and other corrosion inhibitors, and taking steps to mix the boiler water thoroughly when it is offline, the end result can be serious corrosion of critical components.
There are however a number of ways in which you can prevent boiler corrosion in a unit which has been taken off-line or remains idle for some time. Two of the most common techniques used are wet storage and dry storage.
Wet storage involves flooding the boiler system with water once it is offline.
Dry storage, as its name suggests involves removing all water and moisture from the boiler.
Both techniques can protect offline boilers from the effects of corrosion, there are however a number of issues to consider before deciding which one to use.
Boiler corrosion protection using oxygen scavengers and alkalinity control
To control corrosion in boilers that are taken out of service many operators will add an additional “feed” of a sulphite oxygen scavenger and, or chemicals to control alkalinity into the boiler water.
This is usually done manually, checking and rechecking the levels of key parameters in the boiler water to make sure that they are within acceptable ranges.
Maintaining sulphite levels to control corrosion in boilers
Before you hit the switch to take a steam boiler offline, make sure the boiler water sulphite levels are increased. If you rotate the use of the boilers frequently enough, the sulphite concentration may never get the chance to drop to levels which cause corrosion problems.
Always check levels regularly, if sulphite levels drop too low, corrosion pitting can quickly occur.
Remember that if you keep your boilers on stand-by or offline for long periods, you might have to reconsider your rotation strategy to minimise potential boiler corrosion issues.
Consider wet storage for idle or offline boilers
Wet storage describes the process of adding treated water (typically sulphite and alkalinity builders) to the boiler when the system is off-line. The boiler is flooded with the treated water ensuring that the blowdown lines closed. Close the steam header valve to stop any water getting into the steam header. This also prevents condensation getting in and affecting the levels of treatment chemicals.
Wet storage has the advantage that it is easier to bring a boiler back into service quickly when needed. Simply drain the boiler to the normal levels and open the steam header valve before bringing it back in to service.
How cascading blowdown can be used to protect offline boilers
The idea behind the cascading blowdown technique is to use treated water from boilers which are already operational. The treated water from the online boilers flows through any offline boilers to provide protection against corrosion.
As the water entering from the operational boilers has already been treated with corrosion inhibitors and oxygen scavengers, there’s usually no need to add extra water treatment chemicals in the way of additional feeds.
The cascading blowdown method only works when there is enough heated and treated water to keep the offline boilers flooded. It is most effective when the hot blowdown water flows into the boiler near the bottom and leaves through the opposite end. This gives the best distribution of the boiler water treatment chemicals.
Periodic re-circulation and monitoring when using wet storage
When using wet storage techniques it is important that the re-circulation pumps are run regularly to ensure any boiler corrosion inhibitors or oxygen scavengers added remain thoroughly mixed. Once the re-circulation cycle is completed you should also test the boiler water for important parameters including sulphite and pH to ensure they remain within an acceptable range.
Preventing offline boiler corrosion using dry storage techniques
Preventing boiler corrosion using dry storage involves opening up any boiler which is not in use and draining the water to leave surfaces dry. Additionally, special desiccants are placed in to the boiler to keep the inside surfaces perfectly dry. The fireside has to be kept dry too.
Dry storage is a popular strategy used when the steam generating plant has a seasonal shutdown, or when steam boilers are going to be offline for a year or more. The downside of using the dry storage approach is that it can take a lot longer to get the boiler system fully charged with water and operational again if needed.
Using vapour phase corrosion Inhibitors
Another method of controlling boiler corrosion when a system is offline is to use vapour phase corrosion inhibitors. Once the boiler has been prepared, simply place the vapour phase pack inside the boiler and expose the contents of the pack to the atmosphere. The contents will vapourise inside the boiler and coat the tube surfaces to protect them.
Using nitrogen gas for boiler corrosion prevention
Another novel technique employed to protect offline boilers from corrosion is to use pressurised nitrogen gas. The nitrogen gas is introduced in to a sealed boiler to create a layer or blanket of nitrogen which works to protect boiler surfaces when it is offline. The layer of nitrogen gas works by stopping air getting into the boiler and causing corrosion.
Preventing boiler corrosion
Taking boilers offline can lead to increased corrosion resulting in additional maintenance and repair costs, and potentially reducing the operational life of the boiler itself. However, with careful planning a boiler can be taken offline and protected from the effects of corrosion using either wet storage or dry storage techniques. For further specialist advice speak with the industrial steam boiler specialists at Water Treatment Services.
Expert industrial water treatment and corrosion control
Water Treatment Services offer a comprehensive range of industrial water treatment solutions for the management and treatment of corrosion in boiler systems. Our experts can help you optimise costs and operational efficiencies, reduce downtime, achieve water and energy savings, and increase plant reliability and safety.
With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians, in-field specialists and consultants we can offer professional, cost effective boiler corrosion prevention and protection, water treatment and engineering services throughout the UK and Internationally.
Contact us today to learn how our industrial water management solutions can help you.
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