Steam in Boilers

Superheated Steam

When water is heated to the boiling point at a given pressure, its temperature rises.  However, once the boiling point is reached, the temperature stops to rise.  The energy which is fed to the water is used to convert the water to vapour. 

The heat which is given to the water converts it into the vapour state.  This is known as the latent heat of vaporization.  The steam produced in this stage contains water droplets.  The temperature does not rise till all the water has been converted into steam. 

The steam which contains suspended droplets of water is called saturated Steam.
Saturated Steam is used widely in the industry for drying, heating.  It is also used in cooking as it has a high energy transfer coefficient. 

If the steam is heated further, all the water is converted into steam.  The steam, at this stage, is called superheated steam. 

     

Saturated Steam

Superheated Steam is steam which does not have any water droplets.  When saturated steam is heated,  the water droplets which are suspended get converted into steam.  Superheated steam is also called dry steam.  Superheated Steam is used to drive turbines.  

Superheated steam has a lower density and higher temperature. 
The main advantage of superheated steam is that there are no droplets.  In turbines, where the steam moves at high velocity, any water droplet which hits the turbine blades can seriously damage the blades or cause corrosion. 

Superheated Steam can store large quantities of internal energy and can release them during expansion.  This is utilized in turbines.  Superheated Steam has higher change in specific volume when it cools.  This enables better efficiency in turbine operation. 

Superheated Steam has low oxygen.  Hence, there is a reduced risk of corrosion in components using superheated steam. 

    

Steam Washing

Steam Washing in boilers refers to the "washing" of the steam with fresh water or steam with condensate.  The objective of steam water is to remove the impurities in steam such as silica.  Silica in steam is mostly in the vapour state.  Silica can deposit in the blades of turbines and affect the efficiency and the operation.

When water which is at a colder temperature than steam is sprayed on the steam, the silica condenses and gets carried away by the water. 

The washing is usually carried in many stages for better efficiency. 

 

Steam Separation in Boilers

The steam that is generated from the boiler is wet steam.  This steam has water droplets suspended in it.  The water in steam can damage the components of a boiler system by way of corrosion.  It has lower energy carrying capacity.

Steam Separators are devices used to separate the suspended water from the steam.  There are many different methods to achieve this.  One method is the use of baffles which are placed in the path of the steam.  The baffles collect the water from the steam. 

Another method is by using the centrifugal principle.  The steam is passed through a chamber where there is a rotary device which spins the steam.  Due to the centrifugal principle, water which has a higher mass is separated from the steam and collected. 

Another method is by passing the steam through a wire mesh known as a demister.  The water particles tend to collect in the mesh while the steam alone passes. 

Steam separators usually use more than one method to separate the water droplets from the steam. 

Carryover in Boiler Steam

Carryover is the escape of moisture and impurities such as silica, copper, sodium, etc. along with the steam.  These impurities which are carried away by the steam can affect the piping and other equipment in the system.

Carryover occurs in two ways.  The first is due to mechanical action wherein the high velocity steam carries with itself small droplets of water which carry the impurities .  This is called mechanical carryover.  The second way in which carryover occurs is called vaporous carryover. 
In vaporous carryover, the impurities are carried over by steam. 

The sum of the mechanical and the vaporous carryover gives the total carryover. 
Causes of Carryover

The causes of carryover in boilers  are

• Boiler Design
• Fluctuating load on the boiler.  This causes the water level to rise and fall owing to the change in steam pressure.  This results in small droplets of water being ejected from the water surface into the steam above.
• High quantity of impurities and dissolved substances
• Overloading of the boilers
• High water level

Use of efficient steam separation devices can minimize carryover.  A well designed blowdown schedule will help drain impurities and greatly reduce carryover.  Antifoam agents can also reduce carryover. 

 

Condensate Slug in Steam Lines

A Condensate slug occurs when condensate which collects in a steam line and moves through it at high speed encounters a point of resistance such as a bend or a closed valve. 

The condensate travels at the speed of the steam in the line.  When this condensate meets a point of resistance such as a bend in the pipeline or a pressure reducing valve, a tremendous impact occurs.  The shock wave travels through the system getting reflected many times.  This results in hammering.  This can cause damage to the pipelines and even injury or death to personnel.

Hence, adequate precautions to prevent such a scenario. 

Steam Traps should be fitted at points where the line makes sharp turns.  Steam Traps should also be placed before pressure reducing valves and valves which are mostly closed and at the end of steam lines. 

Function of Flash Tank

The Flash tank is used to flash the condensate at high temperature into steam. The primary function of the Flash Tank is to reduce the pressure of the condensate.  Some of the condensate "flashes" into steam in the Flash Tank.This low pressure condensate can be reintroduced into the boiler or condensate through the low pressure lines.  The steam can be used in place of live steam in the boiler system.

The Flash Tank serves to preserve the heat content of the condensate while reducing the pressure to a safe value.  The Flash Tank is a very important component of closed systems.  In fact, the flash tank is what makes closed systems possible