Boiler Blowdown Control | Water Treatment Services

Controlling boiler blowdown in industrial steam generators is an important process used to manage the build-up of suspended solids and so control the formation of scale, corrosion, and wet steam carryover. In this guide the experts at Water Treatment Services consider the operation of steam generators looking specifically at blowdown, what it is, how it can be used to improve operational performance and energy efficiency, and factors that need to be considered to optimise blowdown rates.

What is boiler blowdown?

Boiler blowdown in a steam generating system involves the removal of a certain volume of the concentrated water from the boiler and replacing it with feedwater. The aim is to lower the concentration of suspended solids to ensure the water chemistry in the boiler stays within specified limits, helping to control the formation of scale, corrosion, carryover and other problematic issues. Feedwater is typically a mix of good quality makeup water and condensate.

Why control blowdown?

Controlling blowdown is an essential element in all good boiler water treatment processes. It can improve steam quality and system reliability, reduce maintenance costs and optimise boiler efficiencies. In industrial steam boilers, blowdown is made up of feedwater which has been treated and heated, but is then flushed to drain rather than going through the conventional heating process where it is converted into steam.

Boilers use blowdown to control the levels of suspended or dissolved solids in the water which the boiler uses. If the levels of these contaminants becomes too high, the system may start to experience problems with corrosion, scale deposits or wet steam carryover.

Learn more about boiler water treatment

On the other hand, if there is too much boiler blowdown, this can increase fuel costs and water consumption making the steam generating process less efficient.

Two types of boiler blowdown

There are two distinct types of boiler blowdown:

Surface Blowdown
Surface blowdown is a process which happens continuously or at specified intervals. It is designed to get rid of dissolved solids which are most commonly found on the surface of the liquid. A graduated needle valve or a motorised valve is used in this method of blowdown.
Bottom Blowdown
Bottom blowdown is a manual process. The aim is to get rid of any sludge or sediment which has settled at the bottom of the boiler. The most common way of doing this is to clear the base of the boiler once a shift. In smaller boilers, once a day might be enough.

Boiler blowdown calculation

Boiler operators and engineers can calculate the blowdown rate as a percentage of flow water, taking into account the cycles of concentration within the boiler.

The cycle of concentration is a very important idea in maintaining optimum boiler water conditions. It measures the way in which impurities contaminating the feedwater accumulate (or concentrate) within the boiler. Simply the calculation is expressed as follows:

Feedwater Cycles = Feedwater/Blowdown

Percentage Blowdown = 1/Feedwater Cycles

What this shows is that the higher the feedwater cycles, the greater the level of feedwater impurities concentrated within the boiler and the lower the rate of blowdown. Usually, the cycles of concentration are estimated by looking at the levels of a soluble contaminant in both feedwater and boiler water.

Optimising blowdown to ensure boiler efficiency

In order to keep steam boilers running efficiently, operators need to ensure boiler blowdown takes place only when it is required. Too often and boiler efficiencies can be reduced, too infrequently and poor steam quality or system damage can occur.

Usually, blowdown rates are anything between 1%, when a very high quality feedwater is available, and up to 10% where the feedwater quality is low.

If the quality of the feedwater water is critically low, or the condensate return is significantly reduced, blowdown rates could be much higher than this. In order to work out the optimal boiler blowdown rate for individual steam generating systems, operators should consider a range of factors that include:

Design of the boiler
Quality of the makeup water
Operating conditions
Control equipment
How onsite monitoring is carried out
Type of water treatment products

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.