Blowdown in Boilers

Significance of Blowdowns

Blow down in Boiler is a very important procedure.  The Blow down helps flush the boiler of impurities which may accumulate as the water evaporates.  If the blow down is not carried out, the impurities can reach dangerous levels which can result in the formation of scales in the pipelines and the formation of sediments due to precipitation.  Scaling and deposit formation reduces the heat transfer between the boiler and the water and affects the efficiency. 

Impurities can also cause foaming which results in the loss of water which gets carried away in the steam. 

There are two ways in which blowdowns can be carried out in Boilers.  The Bottom blowdown and the Surface blowdown. 

The Bottom blowdown is done by opening a drain at the bottom of the boiler.  The boiler pressure pushes the impurities and deposits out. 

Surface blowdown is done to remove the impurities which have formed a foam on the surface of water.  The foam needs to be removed for optimum heat transfer.  A pipe placed at the water level in the steam drum is used for this purpose.  Opening the pipe causes the water on the surface to be vented. 

The duration and frequency of blowdown depends on the boiler design and the conditions of operation.  It also depends on the levels of the contaminants in the feedwater and thus on overall water quality.  The boiler blowdown rate should be determined uniquely for each boiler installation. 

Surface Blowdown

Surface Blowdown in boiler is carried out to remove dissolved substances at the surface of the water.  That is, it is used to remove impurities which are in the liquid or dissolved phase.  Impurities which are in solid phase precipitate to the bottom where they are removed by the bottom blowdown.

The impurities and dissolved substances tend to form a layer of foam on the surface of the water.  This layer of foam needs to be removed to reduce the level of the dissolved substances. 

Since the blowdown involves removing water from the surface, it is called surface blowdown.
Surface blowdown is done by a pipe which is made to float a few inches below the water surface.  The pipe is connected to the outlet by means of a swivel joint.  The pipe can thus freely float in the water.  The pipe is held afloat by means of a float.

The pipe has a needle valve at its end.  The size of the valve opening can be adjusted based on the frequency and amount of blowdown required during each session.  Today, Automatic blowdown controllers which can control the rate and volume of the blowdown are also available. 

Bottom Blowdown

Bottom blowdown in boilers is used to remove impurities which have fallen to the bottom as precipitates.  These impurities are in the solid phase.  Bottom blowdown is done by means of a valve connected to the bottom of the valve.  When the valve is opened, the impurities are flushed out by the boiler pressure.

The steam collected during the blowdown can be removed into a steam flasher and a heat exchanger.  The heat can be recovered and the steam can be recirculated after passing through the flash tank and the deaerator.

The duration and frequency of the blowdown is determined on factors such as size of the boiler, water quality and the location and the operating load.

A proper blowdown programme improves efficiency and reduces maintenance costs.

 

Boiler Blowdown Rate

The Boiler Blowdown rate refers to the rate at which the blowdown should occur in an operating boiler.  It describes the blowdown in kilograms per hour.

The Boiler blowdown rate depends on the quantity of the impurities and the limits of tolerance for the employees.  The Blowdown rate is a product of the steam consumption and the ratio of the level of the TDS to the difference between the maximum allowable TDS and the actual TDS. 

q_BD = q_S f_c / (b_c - f_c) 

where

q_BD    the blowdown rate in kg/hour

q_S is the rate of steam consumption in kg/hour

f_c is the total dissolved substances in ppm

b_c is the limit of the total dissolved substances in ppm

Boiler Blowdown Percentage

The Boiler Blowdown percentage refers to the amount of boiler water drained during a blowdown to the total quantity of the boiler feed water.  This is a very useful value

The formula is 

This value is a very important parameter.  The boiler blowdown percentage can range from 1% for high quality feed water to 20 % for low quality feed water. 

Types of Blowdowns

Two types of blowdown can be carried out in boilers. They are,

Intermittent blowdown

Intermittent blowdown, as the name suggests, is the blowdown performed at frequent intervals.  The general rule is to do the blowdown for 2 minutes in 8 hours.

This method requires increases in the feedwater input to the boiler.  Feedpumps of large size may be required for this method.

With each blowdown, a significant amount of energy is lost. 

Continuous blowdown

Continuous blowdown involves a steady discharge of concentrated boiler water and its replacement by a constant input of feed water.  TDS and steam purity are maintained at a given load.

Once the discharge rate of the blowdown and the feed rate are set, it requires no operator intervention.

The heat lost during continuous blowdown can be recovered by blowing it into a flash tank and generating flash steam.

The blowdown which leaves the flash tank will still have heat which can be recovered.  This is done by using a heat exchanger to heat the make-up water.

Package blowdown heat recovery systems which can be customized are available.

Benefits of blowdown control

The benefits of blowdown control are

• Reduced cost of pretreatment.
• The quantity of makeup water required is less.
• The maintenance downtime is less.
• The boiler life is increased.
• The amount of chemicals to treat the water is less.