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How NOT to Corrode a Boiler

Written by Janice M on . Posted in Technical Tips

A boiler is a closed container in which water under pressure is transformed into through the application of heat. This steam is then used to provide energy for heating and processes in a factory.

The combination of water, heat and a mix of various substances in the water can create a corrosive and potentially explosive situation in a factory, if not regularly monitored and corresponding corrective action not taken.

Two main types of industrial boilers:

  • Fire tube boilers

  • Water tube boilers

Common Problems with boiler water:

1. Scaling / deposition – a build-up of solid material from the reactions between the impurities in water and metal in the pipes.
Reduces heat transfer and decreases boiler efficiency, which leads to fuel wastage and equipment damage

2. Corrosion-Oxygen attack – the most common cause of corrosion in boilers. Oxygen promotes the formation of hematite or red iron oxide, which can cause deep pitting and damage. To prevent oxygen corrosion, keep oxygen out of the boiler.

3. Acid corrosion – feed water with a pH of less than 8.5 can cause an acid attack, as carbonic acid can form under this pH

4. Boiler water carryover – substances such as oil, alkali, grease, organic matter or other solids can cause foaming and contamination of the steam.
A sudden surge of boiler water caused by a rapid change in load causes priming. Priming can result in carryover of boiler water solids into the steam, by allowing small droplets of water to be released into the steam space. Such carryover causes contamination in the many processes for which steam is used.

Boiler Water Control:

ASME provides Water Quality Guidelines for Boiler Water. The requirements become more stringent as pressure increases.

ASME Guidlines for Water Quality in Modern Industrial Water Tube Boilers

Boiler Feed Water

Drum Pressure (psi) Iron (ppm Fe) Copper (ppm Cu) Total hardness (ppm CaCO3)
0 – 300 0.100 0.050 0.300
301 – 450

0.050

0.025 0.300
451 – 600 0.030 0.020 0.200
601 – 750 0.025 0.020 0.200
751 – 900 0.020 0.015 0.100
901 – 1 000 0.020 0.015 0.050
1 001 – 1 500 0.010 0.010 0.0
1 501 – 2 000 0.010 0.010 0.0

 

Boiler Water
Silica (ppm SiO2) Total alkalinity (ppm CaCO3) Specific conductance (micro-ohms / cm) (un-neutralised)
150 700 7 000
90 600 6 000
40 500 5 000
30 400 4 000
20 300 3 000
8 200 2 000
2 0 150
1 0 100

The most important instrumentation control parameters in a boiler are Conductivity and pH. These should be measured in different streams – starting from Boiler feed water, then Boiler Water and then Condensate recovery.

1. pH

Maintain close to alkaline:

  • Boiler feed water 8.0 – 9.0
  • Boiler water 9.0 – 11.0, depending on boiler pressure
  • Condensate 7.0 – 8.0, to avoid carbonic acid corrosion

2. Conductivity

Conductivity in water needs to be controlled in the following areas, as it relates directly to energy savings. There can be too much conductivity in the feed water when there are too many dissolved solids.

  • Boiler feed water – may indicate carryover
  • Boiler water – used to monitor blowdown requirements
  • Condensate recovery – check purity

Controlling the amount of ions present ensures a longer life for boilers and coolers by minimizing rust, corrosion and scale build-up.


 

Instruments suited to testing Boiler Water Quality:

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