Boiler feedwater and Total Dissolved Solids (TDS)

If you use a large volume of live steam as a key part of your manufacturing or production process, then you’ll be aware that high levels of Total Dissolved Solids (TDS) in your boiler feedwater can cause problems. These include foaming, wet steam and contamination of boiler and pipework surfaces, which then require extra blowdowns to resolve wasting energy and water. In short: TDS costs you money.

What are Total Dissolved Solids?
The term Total Dissolved Solids in water refers to the concentration of all the substances that are dissolved in a sample of raw water. These substances might include some organic matter but are primarily made up of dissolved inorganic salts, such as magnesium, calcium, potassium, sodium, nitrates, chlorides and sulfates.

Dissolved solids can originate from different sources. Naturally occurring minerals will be absorbed by water as it permeates through rock and soils, while run-off from farming and industry can introduce a range of minerals and other potential contaminants.

Why are Total Dissolved Solids a problem in pressurised boiler steam?
In its simplest form, an industrial steam generating boiler operates by using heat energy to change the phase state of water from a liquid to a gas. By applying heat, the temperature of water is increased until, at a given pressure, it reaches boiling point and bubbles of vaporised gas begin to form. At this stage, the temperature of the water stops rising and the energy applied is used to increase the vaporisation of the water.

The vapour rises to the surface of the water and each bubble of gas bursts as it is released as steam. Impurities in the water will either boil off with the steam, or becoming increasingly concentrated within the water.

As the concentration of dissolved solids increases, the gas bubbles become more stable and fail to burst as they reach the surface of the water. Depending on boiler pressure, size and steam load, this will reach a point where a substantial percentage of the steam space in the boiler becomes filled with bubbles. These coagulate into foam that is subsequently carried over into the steam main. This carryover is both extremely wet and contains high concentrations of impurities, which will subsequently contaminate downstream pipework, heat exchangers, valves and steam traps.

TDS and blowdown
There is a direct connection between the purity of boiler feedwater and the number of boiler blowdowns required. Blowdowns are essential to remove impurities and maintain the efficiency, operating life and safety of the boiler. However, every blowdown increases wasted heated water and uses extra energy, leading to ever higher operating costs. It’s not unusual for blowdowns to consume up to 10% of heated boiler water – and thus additional energy – and for blowdowns to be required at frequent intervals.

Typically, raw water sources used for supplying industrial boilers are treated using some form of softening system – typically ion-exchange – to minimise the risk of boiler scaling. However, these systems do not reduce levels of TDS; indeed, softening can increase the concentration of dissolved solids.

Boiler feedwater treatment systems
The solution is to consider an integrated feedwater treatment system that normally comprises:

• Filtration and ultrafiltration
• Ion exchange (softening)
• Reverse osmosis
• Deaeration/degasification
• Chemical treatment (e.g. coagulation)

Depending on the raw water source, filtration may be required to remove suspended matter such as particles of sediment or organic material, to reduce turbidity and protect downstream water treatment devices. It may also be necessary to eliminate chlorine before the feedwater is passed through a reverse osmosis (RO) system.

An RO system functions by passing pressurised feedwater through semi-permeable membranes. These separate contaminants including bacteria and the dissolved ions and solids that contribute to boiler scaling and foaming; typically, up to 99% of these contaminants will be removed by an RO system.

It’s worth noting that RO systems are the preferred option for use with high-pressure boilers, where the concentration of suspended and dissolved solids needs to be extremely low; they may not be required for other types of industrial boilers.

Boiler feedwater and energy prices
Energy costs have risen fast in the last year and look set to remain high for the foreseeable future. This is putting many businesses under considerable pressure, especially those companies that use large volumes of live steam.

Although an investment in additional boiler feedwater treatment systems may not be the most appealing option, it’s worth considering that payback can be remarkably fast – often within months. For example, the blowdown costs for a standard 8 tonne/hour boiler in a live steam application with just a softening system in place, could be reduced by more than £50,000 p.a.