Why Demineralised Water Changes Pump Selection

Absolutely. I’d spell out demineralised water and deionised water every time, and avoid the DM/DI abbreviations altogether. I’d also correct the bearing point and make the technical claims more defensible.

Here’s the tightened version:

“It’s just water.”

That sounds like one of the easiest pump enquiries in the world.

Until it isn’t.

A customer recently came to us with a pumping problem.

The first question was simple:

“What are you pumping?”

“Water.”

A few more questions changed the whole picture.

It wasn’t mains water, rainwater or washdown water.

It was demineralised water.

And that small detail matters.

Demineralised water isn’t ordinary water

Demineralised water has had most of its dissolved minerals and ions removed.

Depending on the treatment process, it can have very low conductivity and very low dissolved solids.

It may look exactly like ordinary water in a clear container.

But from a pump’s point of view, the details matter.

Demineralised water has very low viscosity and very limited lubricating properties compared with many process fluids. In pump designs where the liquid contributes to cooling or lubrication of specific wetted components, this can affect the way the pump should be selected and operated.

That can make pump type, seal selection, materials, clearances and operating speed important considerations.

Seals deserve attention

Mechanical seals rely on a very thin liquid film between the seal faces for cooling and lubrication.

If operating conditions are marginal — particularly with frequent starts, poor priming, air entrainment or dry running — seal life can suffer.

For demineralised water service, the seal arrangement should therefore be considered alongside:

• Seal face materials

• Temperature

• Pressure

• Operating speed

• Start-stop frequency

• Flush or quench requirements

• Dry-running risk

A duty that looks simple on paper can still be demanding on a mechanical seal.

Materials matter too

High-purity water can interact differently with materials than ordinary mains water.

Depending on the water quality, temperature, chemistry and operating conditions, it can contribute to corrosion, metal ion release or contamination.

Stainless steel is commonly used in high-purity water systems, but “stainless steel” isn’t a complete material specification.

Grade, surface finish, fabrication and passivation can all matter.

High-purity water systems can also experience rouging, particularly in hot-water applications.

What about positive displacement pumps?

This is where pump design becomes particularly important.

Some positive displacement pumps operate with close internal clearances or have specific wetted components where the characteristics of the pumped liquid affect lubrication, cooling or wear.

Depending on the pump technology, demineralised water service may require consideration of:

• Different materials

• Different clearances

• Lower operating speed

• Appropriate seal selection

• Elastomer compatibility

• Dry-running protection

• Or potentially a different pump technology

This doesn’t mean positive displacement pumps cannot handle demineralised water.

It means the pump needs to be selected for the actual liquid and duty.

For example, with a progressive cavity pump, the rotor, stator, elastomer compatibility, seal arrangement, operating speed and dry-running protection all need to be considered.

Don’t just tell your pump supplier “water”

For a demineralised water application, useful information includes:

Water quality — demineralised, deionised, reverse osmosis, distilled or another specification

Conductivity or resistivity — helps indicate the water’s purity

Temperature — important for seals, elastomers and materials

Flow rate

Discharge pressure

Suction conditions

Continuous or intermittent duty

Starts per hour

Dry-running risk

Cleaning chemicals

Required wetted materials

Required water purity

A specification such as:

Demineralised water | low conductivity | 25°C | 10 m³/h | 6 bar | continuous duty | 316L wetted parts

is far more useful than:

Water pump required.

The lesson is simple

When someone says:

“It’s just water.”

The next question should be:

“What kind of water?”

Because water can be clean, clear and have almost no viscosity — and still present some very specific pump-selection challenges.

The right pump isn’t simply the one that can achieve the required flow and pressure.

It’s the one that can do it reliably, with the right materials, seals, speed and protection for the actual liquid.

Because when it comes to pumps, “just water” is rarely enough.