Pumping Thin Liquids

Many processes involve thin liquids which provide low NPSH and lubrication

Pumps for Thin and Low-Viscosity Liquids

Thin and low-viscosity liquids flow easily, but they can still present demanding pumping challenges. Volatile fluids such as LPG, liquid ammonia, liquid CO₂, refrigerants, methanol and light hydrocarbons may operate close to their vapour pressure, increasing the risk of flashing, gas formation and cavitation. Other thin liquids provide limited lubrication or require secure containment because they are toxic, flammable or environmentally hazardous.

AxFlow supplies specialist pumps for thin liquids, including centrifugal, side-channel, regenerative turbine, magnetic drive, canned motor and rotary vane technologies. Our engineers select each pump according to the fluid properties, NPSH conditions, flow, pressure and containment requirements of the application.

Pumps For Thin Liquids

What Is a Thin Liquid?

A thin liquid is a fluid with relatively low resistance to flow, more formally described as a low-viscosity liquid. Dynamic viscosity is commonly expressed in centipoise (cP) or millipascal-seconds (mPa·s).

“Thin liquid” is an application term rather than a fixed engineering classification. The suitability of a pump cannot therefore be determined from viscosity alone. Density, vapour pressure, temperature, lubricity, gas content and chemical properties must also be considered. Viscosity and density are separate fluid properties and should not be treated as interchangeable.

Common thin and low-viscosity liquids include:

Why Are Thin Liquids Difficult to Pump?

Low viscosity itself is not always problematic. Clean, water-like liquids are often handled efficiently by conventional centrifugal pumps. The greater challenges arise when a thin fluid is also volatile, poorly lubricating, hazardous or contains entrained gas.

Limited NPSH and Cavitation Risk

Liquefied gases, refrigerants and hot condensate can have high vapour pressures. If pressure at the pump inlet falls too close to the fluid’s vapour pressure, vapour bubbles can form and subsequently collapse inside the pump.

This can cause:

Cavitation
Reduced flow and head
Noise and vibration
Unstable operation
Damage to hydraulic components
Loss of prime or vapour locking

Reliable operation requires the system’s NPSH available (NPSHa) to exceed the pump’s NPSH required (NPSHr) by an appropriate margin. Pump selection must therefore consider suction pressure, liquid temperature, vapour pressure, pipe losses and operating range rather than viscosity alone.

Poor Lubrication

Some pumps rely on the process liquid to lubricate internal bearings, bushings or contacting surfaces. Water-like liquids, solvents and liquefied gases may provide less lubricity than oils.

Bearing materials, internal clearances, circulation arrangements and operating limits must therefore be suitable for the actual liquid. In canned motor pumps, for example, part of the pumped liquid commonly cools the motor and lubricates the internal slide bearings.

Internal Slip

In a positive displacement pump, some liquid can pass backwards through internal clearances from the discharge side to the suction side. This is known as slip.

Lower viscosity can increase slip, reducing volumetric efficiency and delivered flow, particularly as differential pressure rises. A positive displacement pump for a thin liquid must therefore have a hydraulic design and internal clearances suited to low-viscosity operation.

Leakage and Containment

Many thin liquids are volatile, toxic, flammable or environmentally sensitive. Examples include ammonia, LPG, methanol and industrial solvents.

Mechanical seal arrangement, material compatibility and hazardous-area requirements must be assessed carefully. Where containment is critical, a magnetic drive or canned motor pump may remove the dynamic shaft seal and reduce potential leakage paths.

Entrained Gas and Phase Change

Thin volatile liquids can contain dissolved or entrained gas, particularly during tank unloading, condensate recovery and refrigeration processes. Gas can reduce the performance of a conventional centrifugal pump and may interrupt liquid flow.

A pump specifically designed for liquid-gas mixtures, such as a suitable side-channel pump, may be required where gas content cannot be eliminated from the process.

Applications for Thin-Liquid Pumps

Industrial Refrigeration

Industrial refrigeration systems circulate ammonia, liquid CO₂ and other refrigerants under conditions where the fluid may be close to its boiling point.

Pumps must provide reliable circulation while controlling:

Vapour formation
Low NPSH conditions
Leakage risk
Internal cooling
Refrigerant containment

Canned motor and specialist magnetic drive pumps are commonly considered for these duties.

LPG and Light Hydrocarbon Transfer

LPG, butane and other light hydrocarbons are stored as liquids under pressure. Changes in pressure or temperature can cause rapid vapour formation.

Typical applications include:

Tank transfer
Loading and unloading
Pressure boosting
Process circulation
Refrigeration
Fuel systems

Pump selection must account for vapour pressure, suction conditions, hazardous-area classification and containment.

Process Condensate

Process and steam condensate can be hot and close to saturation pressure, leaving limited NPSH available.

Low-NPSH centrifugal, side-channel or regenerative turbine pumps may be selected depending on the flow, head, temperature and gas content.

Solvents, Alcohols and Methanol

Solvents and alcohols are generally low in viscosity and may also be volatile, flammable or chemically aggressive.

Suitable pumps can include:

Magnetic drive centrifugal pumps
Canned motor pumps
Regenerative turbine pumps
Rotary vane pumps
Chemically resistant centrifugal pumps

Materials, seals, elastomers and hazardous-area requirements must be matched to the specific chemical and concentration.

Chemical and Petrochemical Processing

Thin process chemicals are used throughout refining, petrochemical production, surface treatment and general manufacturing.

Common duties include:

Transfer
Recirculation
Pressure boosting
Reactor feed
Sampling
Dosing
Tank unloading

The required pump may differ significantly depending on whether the priority is high flow, high head, low NPSH, accurate delivery or leak-free containment.

High-Purity Fluid Transfer

Ultra-pure water, pharmaceutical ingredients and semiconductor chemicals require low contamination risk and compatible wetted materials.

Pump selection may need to consider:

Surface finish
Particle generation
Metallic contamination
Elastomer compatibility
Drainability
Cleaning requirements
Sealless containment

How to Choose a Pump for Thin Liquids

Choosing the right pump for a thin or low-viscosity liquid requires a complete review of the fluid and system.

Identify the Fluid Properties

Confirm:

Chemical composition and concentration
Viscosity at operating temperature
Density
Vapour pressure
Boiling point
Lubricity
Corrosiveness
Toxicity and flammability
Solids or contamination
Dissolved and entrained gas

Fluid properties should be based on actual start-up, normal and upset conditions.

Define the Required Duty

Establish:

Normal, minimum and maximum flow
Differential head or pressure
Inlet pressure
Operating temperature
Duty cycle
Minimum continuous flow
Required control range

The pump should be selected for the expected operating range, not only one nominal duty point.

Calculate NPSH Available

NPSHa should be calculated using the lowest expected suction pressure, highest liquid temperature and actual suction-line losses.

The selected pump’s NPSHr must then be reviewed across the full operating range with an appropriate margin. Simply choosing a pump marketed as “low NPSH” does not remove the need for this calculation.

Assess Lubrication and Internal Clearances

Where the pumped liquid lubricates bearings or internal components, confirm that the fluid provides adequate lubrication under all conditions.

For a positive displacement pump, determine whether low viscosity will cause excessive slip at the required differential pressure.

Select the Correct Containment Method

Consider whether the application requires:

Single or double mechanical seals
Seal support systems
Magnetic drive containment
Canned motor construction
Leak detection
Secondary containment

The appropriate arrangement depends on the fluid hazard, pressure, temperature and site requirements.

Confirm Materials and Compliance

Pump casing, impeller, internal bearings, shaft, containment shell, seals and elastomers must all be compatible with the liquid.

Where applicable, also consider:

ATEX hazardous-area requirements
API 685 sealless pump requirements
Site engineering standards
Material certification
Testing and documentation
Environmental and emissions requirements

When to Consider a Different Pump Technology

A pump selected for clean, low-viscosity service may not be suitable when:

The liquid becomes highly viscous at start-up or low temperature.
Large or abrasive solids are present.
Prolonged dry running is expected.
Accurate metering is required at extremely low flow.
The process fluid changes between liquid and gas phase.
The duty involves high solids loading or slurry transfer.

Depending on the application, a progressive cavity, peristaltic, AODD, metering or specialist solids-handling pump may be a better solution.

A gas compressor rather than a liquid pump is normally required once the process medium is entirely in the vapour phase.

Pumping Thin Liquids? Maximise your process efficiency with our Mag Drive & Caanned Pumps!

When it comes to handling thin liquids like LPG, CO2, and methanol, you need pumps that can conquer low NPSHa challenges with ease. Our specialized Mag Drive & Canned pumps are engineered to excel in precisely these conditions, ensuring smooth, reliable, and safe fluid transfer. Say goodbye to cavitation and hello to optimum performance. Ready to supercharge your processes? Contact us today and discover how our low NPSHa pumps can elevate your efficiency and peace of mind in handling these critical fluids.