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Steam Boiler Water pH Control

Modern Steam Boilers have a high heat transfer rate: impurities in the feedwater can cause scale or chemical deposits to form and reduce the rate of heat transfer from the tube walls to the water, overheating the metal surfaces whilst reducing the steam output.

The maximum steam output and fuel efficiency of a boiler at any given pressure is governed by the rate at which heat can be supplied and transferred in the boiler.

Chemical deposits or scale inhibit or restrict heat transfer, boiler efficiency falls and the possibility of failure increases.

Boiler feedwater is treated to prevent scale forming on the heat transfer surfaces and to prevent water side corrosion. The treatment should be monitored and adjusted relative to the actual steam evaporation rate throughout the course of the day.

pH maintenance of the feed, boiler and condensate system is essential for corrosion control.

The boiler’s operational pH depends on the boiler pressure, system metals, feed water quality and type of chemical treatment in use and is generally dictated by the water’s p-Alkalinity.

pH Control

For effective, economic boiler operation, close control of the boiler water pH is a necessity whichever way you do the maths.

The safe pH range for steam boilers is between 8.5pH & 12.7pH, whilst the recommended level is between 10.5pH & 12.0pH.

Boiler water with a pH of 10.5 contributes 12.6mg/l towards the boiler TDS, whilst at a pH of 12.0 it increases to 400mg/l and at 12.5pH the value is 1264.9 mg/l a 100 fold increase.

Take a 5T boiler, containing approximately 8,200 litres of water, the amount of caustic required to maintain a pH of 10.5 would be 0.7 litres whilst at 12.5pH that rises to 69.1 litres of caustic contributing 1264.9mg/l towards the boiler TDS level.

Controlling the pH towards the bottom end of the safe range significantly reduces the quantity of chemicals used and the effect the pH has on the boiler blowdown rate.

Stable pH levels are a prerequisite for accurate TDS control.