What Is Process Water? Scaling, Corrosion and Quality

In production, water is usually counted as an input that is “already there”. You open the valve, it flows, it does the job. And yet there is the unexplained drop in boiler efficiency, the unplanned stop at the cooling tower, the surface defects appearing on a coating line and the product quality moving from batch to batch. Behind those pictures there is often the same cause: the quality of the process water being used. The unseen mineral and ion load in the water leaves its mark quietly at every point on the production line.

In this guide we take the idea of process water up with an engineer’s eye: what it means, why it differs from drinking water, what problems it creates and which treatment stage solves which problem. By the end of the article you will recognise the water-related risks at your own plant and know how the right system is engineered.

What Is Process Water?

Process water is the water used in production processes, serving as a raw material component, a heating or cooling fluid, a washing and rinsing medium or a carrier. Its quality is set by the requirement of the process it is used in. Meeting the drinking water standard does not mean it is fit for industrial use.

Water does not do a single job at a plant. In the boiler it turns to steam, in the cooling tower it carries heat, on the wash line it cleans surfaces, and in some sectors it goes directly into the product. Every one of those jobs demands a different property from the water; so there is no single definition of “good water”.

That is the heart of the idea of process water: water is not an auxiliary material but a production input. Just as the properties of a raw material are controlled, the properties of the water should be defined and monitored. Once that view is taken, most water-related problems can be prevented before they appear.

In practice, managing process water can be reduced to three questions: what job is the water used for? What level of quality does that job call for? Does the existing source meet that level? The answers to those three questions make up the water strategy of the plant.

The Difference Between Process Water and Drinking Water

Drinking water is defined so as to be safe for human consumption; process water is defined so as not to damage the equipment and the product it is used with. A water that can be drunk may carry high hardness or conductivity, and that creates a risk of scaling in the boiler and of corrosion in the line.

That distinction corrects the misconception met with most often in the field. The assumption “the mains water is drunk, so it will cause no problem in production” is technically wrong. Drinking water standards are health-focused; industrial parameters such as scaling, corrosion and conductivity are not the main subject of that framework.

A concrete example makes the table clear: a mains water entirely suitable for drinking but high in hardness forms a layer of limescale in a short time when it is fed to a steam boiler. The same water causes no problem at all when it is drunk. So a process water assessment is made on an entirely different scale.

So the first thing to do at a plant is to have the existing source analysed for industrial parameters. Hardness, conductivity, silica, iron and dissolved gas content are the real indicators of process water quality.

Why Does Every Plant Need a Different Quality?

The quality needed varies with the sector, the equipment and the purpose the water is used for. A steam boiler calls for high purity, while a more flexible quality may be enough on a wash line. In water that goes into the product itself, purity standards rise to their highest. Different quality classes can be needed even within one plant.

It is ordinary for more than one water quality to be used in the same factory. In a food plant the water that goes into the product does not have to be to the same standard as the water used to wash the floor; that would create unnecessary cost. The right approach is to feed every point of use to its own need.

The typical need profiles by point of use separate out like this:

That variety takes managing process water out of the realm of “buying a unit” and turns it into a design problem. Investments made without establishing which point gets water of which quality either fall short or cost more than they should.

Scaling: A Quiet Loss of Efficiency

Scaling is the calcium and magnesium in water precipitating on heated surfaces and forming a solid layer. That layer blocks heat transfer, raises energy consumption and shortens the life of equipment. It is one of the most common and most costly problems caused by process water.

The most dangerous thing about scaling is that it advances not as a sudden failure but as a gradual loss of efficiency. The boiler keeps running, the cooling tower does its job; but both begin doing the same work while spending more energy. The bill rises, and the cause goes unnoticed for a long time.

Technically a layer of limescale behaves like insulation. That layer, coming between the heating surface and the water, makes the passage of heat into the water harder. The system runs longer and at a higher load to reach the target temperature. At the same time local overheating points can form on the metal surface, and that prepares the ground for material fatigue.

The source of scaling is clear: hardness. So in process water treatment, hardness control is usually the first and basic link in the chain. We took up how hardness is measured and which solutions suit in our softening system selection guide .

Figure 1: Scaling on a heat transfer surface and the loss of efficiency

Corrosion and Material Damage

Corrosion is the wear that occurs on metal surfaces because of gases dissolved in the water, unbalanced water chemistry and high conductivity. It leads to thinning of the pipe wall, pitting and perforation. Unlike scaling it advances more insidiously and is generally not noticed until a leak appears.

Corrosion has three main triggers. The first is dissolved oxygen; it starts electrochemical wear on the metal surface and causes serious damage in closed circuits in particular. The second is the chemical balance of the water; a water tending to be acidic prevents the protective layer forming. The third is conductivity; as the dissolved ion load rises, the electrochemical processes speed up.

Conductivity holds a special place in managing process water, because it is an indicator both for corrosion and for deposits. We took up in detail what that idea says and how it is interpreted in our dissolved solids and TDS article .

The cost of corrosion is not limited to the pipe or equipment replaced. Corrosion products breaking away from the system are carried round the circuit, form deposits at other points and lead to secondary problems. So corrosion control is not a single-component problem but a whole-system process water matter.

Boiler Feed Water and Steam Lines

Boiler feed water is the group calling for the most sensitive quality within process water. Hardness leads to boiler scale, dissolved oxygen to corrosion, and a high dissolved solids load to carry-over with the steam. So softening and, where needed, advanced treatment stages become essential.

In steam production the water evaporates continuously and the minerals in it stay in the boiler. That concentrating effect turns even a small mineral load in the inlet water into a serious build-up over time. So the tolerance in boiler feed water is far narrower than in other uses.

In low-pressure systems a softening stage generally makes the basic answer; once the hardness is removed, the main cause of boiler scale disappears. Built for that purpose, industrial softening systemsare the first link in a boiler protection strategy.

At plants running without a break another requirement arises: no hard water should pass into the line while the softening system is in regeneration. So in businesses in continuous production, twin-tank tandem systems are preferred; while one tank is being renewed the other carries on working.

As the pressure rises, softening alone is not enough; the mineral and conductivity load has to be brought down as well. That is where reverse osmosis and deionisation come in. So a process water build deepens in stages according to the working class of the boiler.

Cooling Water and Circulating Systems

Because cooling water circulates continuously, the minerals concentrate over time. Evaporation reduces the water while the dissolved substances stay in the system, and that raises the risk of scaling and deposits. Correct pre-treatment and control of the feed water preserve the efficiency of circulating systems.

In open-circuit cooling towers the mechanism works like this: the water takes up heat, part of it evaporates and cools inside the tower, and it returns to the circuit. Because the water that evaporates leaves in pure form, the mineral concentration in the water left behind rises continuously. When that concentration is not managed, scaling advances quickly in the pipework and the exchangers.

The second risk in circulating systems is biological growth. Warm water in contact with air can make a suitable environment for microbiological development. That both lowers heat transfer and leads to blockages in the system. Where necessary, solutions such as UV (ultraviolet) treatment are added to the build to reduce microbiological risks.

On the cooling side the quality of the feed water is decisive. A feed water high in hardness and mineral load enlarges the load entering the tower from the start. So in managing process water the cooling line is a point to be taken as seriously as the boiler.

Applications That Bear Directly on Product Quality

In some sectors water is directly a component of the product or touches the final surface. Food and drink, cosmetics, chemicals and coating and paint are in that group. In those applications a fluctuation in process water quality shows up directly in the product as a change in taste, an unstable formulation or a surface defect.

In those fields water is not an auxiliary fluid but a raw material. So when water quality is not controlled, the same recipe starts producing different results. The effects by sector can be summed up like this:

At plants in this group the quality needed rises above an ordinary process water level and comes close to the pure water class. We examined in detail which sector calls for which level of purity in our uses of pure water article .

Built for needs at that level, industrial deionised pure water systemssecure repeatability in product quality by bringing the conductivity to its lowest level. The critical point here is establishing the purity needed correctly; purity higher than necessary means an unnecessary investment, and purity that is too low means a risk to quality.

The Stages Used in Process Water Treatment

Process water treatment works in stages: pre-filtration holds the sediment, activated carbon removes the chlorine and organic load, softening lowers the hardness, reverse osmosis separates the dissolved substances, and deionisation brings the conductivity to a minimum. Which stages are needed is established from the water analysis and the target quality.

Each stage targets a particular problem and protects the next one. The table below sums up which problem is solved by which stage:

Pre-filtration is the unseen but most decisive link in the chain. On sources with a high particle load, sand filters with automatic backwashing come into play and extend the life of every stage behind them. Where chlorine and an organic load are involved, industrial carbon units take on the protective job.

At high-flow plants a reverse osmosis stage is built in to bring the mineral load down as a whole; at that scale high-capacity reverse osmosis systems are sized to the flow of the plant. In that way the process water is carried to the quality aimed for in stages.

Figure 2: A staged process water treatment line and the order of the equipment

How Is a System Engineered for Your Plant?

The engineering is done by assessing the water analysis, the flow requirement, the daily hours of operation, the target quality, the installation space and the level of automation together. Without those data, the choice of capacity and stages rests on guesswork. The right process water system is not a standard product but a design specific to the plant.

The process runs in six steps, and every step makes the input to the next:

1. Water analysis

   The profile of the source is worked out by measuring hardness, conductivity, silica, iron, turbidity and microbiological load.

2. A map of the points of use

   The boiler, cooling, washing and product lines are listed separately; a target quality is defined for each.

3. Flow and the pattern of operation

   The instantaneous and daily consumption and the shift arrangement are established; the need for uninterrupted running is clarified.

4. Building the stages

   The analysis and the target quality are matched to establish which stages will be installed and in what order.

5. Space and infrastructure

   The layout, the drainage, the power supply and access for service are checked and the installation plan drawn up.

6. Automation and monitoring

   The level of continuous monitoring and automatic control is set by the discipline of the operation.

None of those steps should be skipped, because each affects the others. A softening system chosen without establishing the need for uninterrupted running, for instance, can interrupt production during regeneration. In the same way a choice made without checking the space conditions creates extra cost at the installation stage.

For a general framework on managing industrial water quality, the US Environmental Protection Agency document on water quality considerations at commercial and industrial facilitiesstresses that plants should take up their water quality equipment by equipment and process by process.

In the end the right process water system is not a unit chosen from a catalogue but an engineering solution that starts with an analysis, is built around the points of use and is adapted to the conditions of the operation. A turnkey approach lets those steps be carried out from a single source.

The Importance of Monitoring and Continuity

Once the system is installed, continuity depends on monitoring. Following the hardness, conductivity and pressure values regularly shows a loss of performance early and prevents unplanned stops. Managing process water is not a one-off investment but an operating discipline calling for continuity.

The installation is not the end of the process but the beginning. Resins saturate, membranes tire, filter beds block. Because those changes are gradual, they are only noticed through regular measurement. A system that is not monitored looks one day as though it has “suddenly” gone wrong; yet the signals have been there for months.

A monitoring programme is set up under three headings. The first is outlet quality: hardness is followed at the softening outlet and conductivity at the reverse osmosis outlet. The second is the pressure difference; the pressure loss between stages is the earliest indicator of blocking. The third is the consumables calendar: the renewal periods for resin, membranes and filters are run to a plan.

When that discipline is in place, the process water system turns into an unseen but reliable part of the plant’s infrastructure. When it is not, even the best designed system stops producing the performance expected over time.

Frequently Asked Questions