Boiler Type
CARRYOVER CONTROL
TYPES OF CARRYOVER
Carryover is caused by two mechanisms, priming and foaming.
Priming is the sudden violent eruption of boiler water, which is carried along with steam out of the Boiler, usually caused by mechanical conditions.
One of the major causes is a sudden increase in the steam demand. This creates a rapid lowering of pressure on the water surface.
The surface erupts, much like the water in a bottle of soda. Nothing can be done about this chemically. It is controlled mechanically, either by using slow-opening valves, or by moderating charges in steam load.
PRIMING
Priming can cause deposits in the main steam head, and around main steam header valve in a short period of time, since it usually involves the deposition of relatively large amounts of solids when it happens.
FOAMING
Boiler water foaming causes carryover by forming a stable froth on the boiler water. This is easily carried out of the boiler along with the steam. Foaming increases with the amount and nature of the impurities in the boiler water.
Higher alkalinity, higher conductivity and impurities, which affect the surface tension of the water, all increase foaming. Over a period of time, foaming can completely plug a steam or condensate line.
FIRETUBE BOILERS
In a fire-tube boiler, the fire or hot gases are directed through the inside of a large number of tubes within the boiler shell. The modern fire-tube boiler is a very efficient unit, with maximum heat transfer per square foot in mind.
These high efficiency boilers are not as tolerant of waterside deposits as less efficient units. Therefore, more attention than ever before must be given to the water treatment given for these units.
WATER TUBE BOILERS
In this tube of boiler, the hot gases are directed around the outside of boiler tubes, while the boiler water is inside the tubes. The tubes in water tube boilers are generally connected to two drums, the steam drum, where steam escapes and the mud drum. The mud drum is at the bottom of the boiler and collects boiler sludge and other materials, which are removed through blow-down. Water tube boilers are commonly larger than fire-tube boilers, and operate at pressure up to and above 1000 psig, depending on their service. Normally larger boiler installations are water tube boilers.
PROBLEMS AFFECTING STEAM GENERATING SYSTEMS
The problems associated with water in steam generating systems are classified under these categories.
1. Corrosion.
2. Scaling and deposition.
3. Carryover.
Any competent water treatment program can control these three problems.
SCALE CONTROL
As water heats and is converted into steam, contaminants brought into the system with makeup water are left behind in the boiler. The boiler acts like a distillation unit, taking pure water out as steam and leaving behind the minerals and other contaminants in the boiler. Scale forms as a result of the precipitation of normally soluble solids from solution because of heat that makes them become insoluble. Some examples of scale are calcium carbonate, calcium sulphate, and calcium silicate.
OXYGEN PITTING CORROSION
Oxygen gets into the boiler system through a variety of sources, but primarily as a contaminant from the makeup water. Under the conditions of pressure and high temperature seen in boiler systems, very small amounts of oxygen getting into the boiler can cause corrosion. Oxygen pitting corrosion will cause isolated, deep craters in the metal surface. The deep pitting caused by oxygen corrosion can lead to sudden and early failure of boilers. Dissolved oxygen is removed either by mechanical deaeration, or by chemical oxygen scavenging.
Even with a deaearator working perfectly, oxygen scavenger is necessary to ensure that corrosion is minimized. Without a deaerator, generous amount of an oxygen scavenger may be necessary to assure complete oxygen removal. Sodium sulphite is by far the most common oxygen scavenger used. In a boiler system that operates without a deaerator, it is the oxygen scavenger that is of choice. In high-pressure systems, over 450 psig, alternative oxygen deaerator may be desirable.
STEAM AND CONDENSATE SYSTEM CORROSION
Corrosion in the steam and condensate system can be due to either oxygen attack or low pH corrosion. Low pH corrosion is due to the presence of carbon dioxide in the steam. This reacts with the Condensate to form carbonic acid, which attacks the piping.
The pH of untreated condensate can be as low as 3.5-4.0 which is extremely acidic. This acidic condensate attacks the metal in the piping, particularly along the bottom of horizontal runs, or in areas where the piping is thin, such as at threaded joints.
Corrosion is increased at weld locations, where the metal is heat stressed from the process of being welded.
Low pH corrosion is controlled by the use of neutralizing amines.
The neutralizing amine functions by reacting and neutralizing the acid in the condensate water. It elevates the pH above the corrosive range. Maintaining pH levels between 7.5 and 8.5 in the returned condensate controls neutralizing amines. If the pH is allowed to fall bellow these limits, corrosion is likely to occur. If condensate pH reaches 9.0 or above, there is a possibility of the formation of amine carbonates, which can cause temporary deposit problems.
CARRYOVER CONTROL
TYPES OF CARRYOVER
Carryover is caused by two mechanisms, priming and foaming.
Priming is the sudden violent eruption of boiler water, which is carried along with steam out of the Boiler, usually caused by mechanical conditions.
One of the major causes is a sudden increase in the steam demand. This creates a rapid lowering of pressure on the water surface.
The surface erupts, much like the water in a bottle of soda. Nothing can be done about this chemically. It is controlled mechanically, either by using slow-opening valves, or by moderating charges in steam load.
PRIMING
Priming can cause deposits in the main steam head, and around main steam header valve in a short period of time, since it usually involves the deposition of relatively large amounts of solids when it happens.
FOAMING
Boiler water foaming causes carryover by forming a stable froth on the boiler water. This is easily carried out of the boiler along with the steam. Foaming increases with the amount and nature of the impurities in the boiler water.
Higher alkalinity, higher conductivity and impurities, which affect the surface tension of the water, all increase foaming. Over a period of time, foaming can completely plug a steam or condensate line.