Managing boiler carryover in steam generating systems is an essential engineering process required to maintain steam purity and boiler system performance. In this expert guide the boiler specialists at Water Treatment Services review the management of industrial steam generating systems looking specifically at the impact of carryover, what causes it, why it needs to be managed and how to control it.
What is boiler carryover?
Carryover in steam boiler systems occurs when the steam leaving the boiler becomes contaminated with unwanted solids, liquids or vaporous materials. If not dealt with correctly, boiler carryover can have a significant effect on the performance of the system.
The main causes tend to be either mechanical or chemical in nature. Mechanical causes can include poor boiler design, fluctuating boiler loads and high water levels. While chemical issues often involve high levels of suspended or dissolved solids, high levels of alkalinity or other sources of contamination.
What are the effects of carryover in steam systems?
Carryover of boiler water in steam generating systems can cause a range of significant problems affecting the operation and efficiency of the system. The main adverse effect is a build-up of deposits in parts of the system such as turbines, heat exchangers, superheaters and valves. If the problem is allowed to continue unchecked, large amounts of deposits can impact on the efficiency of the turbine, heat transfer properties, or both.
Boiler carryover can also start to affect the protecting magnetic layer found on steam lines. It may also be a main cause of corrosion, and can cause a wide range of problems in manufacturing and other processes which use steam.
One problem caused by boiler water carryover which is often forgotten is loss of thermal efficiency. Water at any pressure is cooler than steam. If the system is creating wet steam, which contains a higher concentration of water, it will develop a lower heat content than dry steam leading to reduced heat transfer efficiencies. This can be a common problem in low and medium pressure boilers operating at below 100 bar.
What causes boiler carryover?
The causes of boiler carryover can be split into two main categories.
The way in which the boiler has been designed has a direct affect on the purity of the steam created. Elements of the design which can affect boiler carryover can include the size of the reservoir or steam drum, pressure, circulation rate, the rate at which the steam is generated, and the choice of any mechanical separating equipment. The way the boiler is used can also affect the purity of steam produced. If operators place too heavy a load at the recommended operating range, or the load is suddenly increased, then this can greatly increase the risk of potential for boiler carryover.
The two main chemical related causes of carryover are foaming and selective vaporous carryover.
Foaming inside the system can be caused by suspended solids, or solids which have become dissolved in the water. Other causes of foaming include high levels of alkalinity or contamination from the process. Contamination from grease, oil or other contamination also cause foaming.
Increased alkalinity in the water can trigger a chemical reaction which results in a type of soap which in turn causes foaming.
Vaporous carryover tends to occur because of the solvent properties of steam. High pressure steam can dissolve some of the minerals and salts which are often present in boiler water. This doesn’t usually cause a problem with common minerals unless the boiler pressure rises above 1000 psi. However, silica can cause problems at pressures as low as 400 psi. It is therefore vitally important to monitor the levels of silica in systems operating at higher pressures. This is even more important if the system uses turbines.
How to prevent steam boiler carryover
Just as with the causes of carryover, the ways of preventing them can be divided into chemical and mechanical factors.
Prevention – Mechanical factors
Firetube boilers, and other boilers which are low capacity and low pressure, operate using the principle of gravity to separate water from steam. Higher pressure systems, such as water tube boilers, typically employ mechanical steam separation devices to deliver steam at acceptable levels of purity.
A wide range of components can be used. Some of the most common are screens, baffles, centrifugal separators or centrifugal separators. Whatever is used to control carryover, it is essential that the equipment is secured closely together and kept clean. A few millimetres gap between sections of cover baffles can stop them working properly.
Similarly, if you allow deposits to build up on mesh demisters or screens, these can also stop working. If the boiler system regularly runs above recommended capacity, then fit a steam flow regulator on the steam drum. The best place for this is between the steam drum and the non-return valve.
Prevention – Chemical factors
The main chemical factor which contributes to boiler carryover are dissolved solids in the water. These could be from suspended solids, silica or oil. The simplest method of dealing with these chemical factors is to limit them getting into the boiler water in the first place, or managing the concentration at acceptable levels. The optimal levels of solids will differ in each system, depending on individual boiler system design.
Using boiler water antifoam chemicals
One potential solution to unwanted foaming in a boiler is to add a chemical treatment to the boiler water to limit foaming. These types of chemicals are called boiler water antifoams. Antifoams are designed to control carryover at acceptable levels without needing any additional treatments. Using boiler antifoams at the right concentrations can even reduce the amount of fuel you are using, while still producing high quality steam. Antifoams won’t control every incidence of foaming, but they are the obvious solution when the problem is first detected.
Steam quality monitoring and condensate testing
The easiest way to identify the potential for boiler water carryover is to monitor the condensate. Every boiler differs, but in general terms, measurements of less than 25 µmhos indicate that the steam is high purity. If measurements are higher, this could indicate a carryover issue.
It’s essential that operators monitor boiler water chemistry on a regular basis, testing the condensate for total dissolved solids, alkalinity, silica and organic contamination. These parameters should be kept within the manufacturers recommendations or those identified in BS 2486:1997 – Recommendations for treatment of water for steam boilers and water heaters.
On its own, high conductivity doesn’t always mean that solids are getting into the steam. If the intake water is very alkaline, then the breakdown of the carbon dioxide in the water can form carbonic acid. If the acid is neutralised with amine, then the reaction can greatly increase the conductivity of the condensate, even without the presence of water solids. If you are finding no other evidence of carryover, and the makeup water is very alkaline, then you probably don’t have a problem with carryover.
Increasing boiler blowdown
When carryover issues result from high water concentrations a simple solution to the problem is to increase the rate of boiler blowdown. However, if increasing blowdown fails to resolve the issue or becomes too expensive then a boiler antifoam should be considered.
Understanding boiler carryover tolerances
However well maintained and well-designed your steam boiler system, it can still produce low levels of carryover. The most important factor is knowing the limits of tolerance for boiler carryover in the system, and how to keep the boiler running well to limit carryover.
Specialist boiler water treatment solutions
Water Treatment Services offer a comprehensive range of industrial boiler water treatment solutions for the management and treatment of commercial and industrial boilers and steam generating systems. Our experts can help you optimise costs and operational efficiencies, reduce downtime, achieve water and energy savings, maintain steam quality, 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 treatment and engineering services throughout the UK and Internationally.
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