Water in Boiler Operation

Water Level in Boiler

Maintaining the proper level in a Boiler is a very important aspect of boiler control.  A boiler should have a reliable water level controller.  There are many automatic controllers which automatically keep the level of the water within the specified limits.  The boiler can be seriously damaged if the level of water is not maintained.

Boilers will also have alarms which alert the operator if the water level falls or exceeds the safe limits. If the water level rises above the safe limit, it can result in priming and carryover.  If the water level increases, it can exceed the horizontal limit of the boiler, the surface area for steam generation will then reduce.

If the water level falls below the safe limits, the tubes of the boiler will be exposed.  This will result in the tubes getting overheated and they can rupture.

The level of water in the boiler is also dependent on the pressure.  If the steam pressure is reduced, the water level rises.  If the pressure is increased, the level can fall.  

 

Boiler Feed Water Pumps

Boiler Feed Water Pumps are used to supply water into the boiler.  The water may be fresh water or the water from the condenser.    The size of the Boiler Feed Water Pumps depends on the capacity of the boiler.

The operation of the Boiler Feed Water Pumps is dependent on the water level.  A level switch is used to switch the pumps on or off.

There are generally two pumps with one running and one as a standby.

Boiler Feed Water Pumps are of the centrifugal types.  They are usually multistage pumps.    The pumps are driven electrically or by a turbine.  Turbine driven pumps are preferred as the cycle efficiency of the boiler increases.  

Foaming in Boilers

Foaming refers to the formation of froth or bubbles in the boiler.  Foaming is caused by high concentration of dissolved solids in boiler water.  When a bubble forms in water that has high levels of dissolved solids, the dissolved solids surround the bubble and make it tougher.  The bubble does not break.  When more bubbles collect, foam is formed. 

Foaming can be prevented by maintaining good water quality.  The level of dissolved solids should be maintained as low as possible. 

The presence of oil in the water can also cause foaming.

Anti-foaming agents are available which prevent the formation of bubbles and foam. 

 

Boiler Water Sampling

Boiler Water sampling involves taking a sample of boiler water to analyse for dissolved substances. This is necessary to determine the level of TDS (Total Dissolved Substances) in the water and, consequently, the quantity of the blowdown.

Samples taken from points such as the level gauge glass, inlet for the feed water or the level control mechanisms are usually inaccurate.

It is dangerous to take a sample from the boiler shell as the water will be at pressure and may flash into steam causing injuries to the operator.

The safe method of taking boiler water samples is by use of a small heat exchanger.  In this method, cold water is used to cool the sample being taken.  This eliminates any risk of flashing.  The sample is also more acccurate.

Another method is to use a TDS sensor.  The sensor reaches into the shell of the boiler and can continually monitor the TDS in the water boiler. 

 

Priming in Boilers

Priming refers to the carryover of small droplets of water along with the steam.  Prime is undesirable as it causes corrosion to the steam line and other equipment.
 
Priming is caused due to many reasons such as high water levels, poor boiler design and fluctuating loads.

When the load in the boiler suddenly increases, the steam pressure in the boiler drops.  This causes the level of water to surge.   This surge of water level causes priming.  Small droplets of water are thrown up into the steam above the water.  This water can carry with it dissolved substances such as chloride, silica, copper, etc.  This is known as carryover.

Priming can be prevented by operating the boiler at steady loads and by maintaining good water chemistry which prevents foaming.

Kettling in Boilers

Kettling refers to the sound of boiling water coming from the boiler.  It is similar to the sound coming from a tea kettle.  Hence, the name. 
Kettling noises can have a number of causes.  Some of them are

Deposits

If the water quality is not properly maintained, deposits can form on the bottom of the boiler.  This is particularly in areas where the water is hard, resulting in limescale deposits.  The deposits prevent proper heat transfer between the water and boiler.  This uneven heating can cause kettling.

Flow rate

If the flow rate of the water is not proper, the water can get overheated as the "dwell time" will be more. this can result in kettling.  The flow rate of the boiler should be reviewed. 

Improper Thermostat Settings

If the thermostat settings are not proper, they may result in overheating. 

Improper Burner functioning

If the burner of the boiler is not properly function, kettling can result to uneven heating. 

Improper Installation

Improper installation can also cause kettling.  If the noise occurs soon after installation, do contact the manufacturer.

Kettling Noises should always be investigated as they affect efficiency and may point to other problems