Why viscous fluids need a positive displacement pump
Centrifugal pumps are designed for water-like liquids. As viscosity rises they lose head, draw more power and, above a certain thickness, stop moving the product at all. Thick liquids also refuse to prime: they will not run into the impeller on their own, so the pump sits dry and overheats.
A self-priming external gear pump turns the problem on its head. The fluid is trapped between the teeth and the body and carried round to the discharge, so the flow is constant and proportional to speed, almost free of pulsation and independent of downstream pressure. With viscous products the fluid itself lubricates the internal surfaces and seals the clearances: more viscosity means better volumetric efficiency, not worse. Gear pumps are the right choice from medium viscosity, indicatively from 50–100 cP, up to products that are solid at room temperature.
Our four series of high viscosity pumps
- NAV series — heavy fluid transfer at high flow and high discharge pressure: nine models from 54 to 2,770 l/min, flanged ports DN 40 to DN 125 PN 16, cast iron base, stainless steel versions for aggressive fluids, continuous duty in petrochemical plants.
- CR series — jacketed pumps for fluids that solidify at room temperature: ten models from 30.7 to 2,812 l/min at 950 rpm, jacket on thermal oil, steam or superheated water, electric cartridge and hot-spot alternatives.
- ZV series — small flows and dosing: ten models from 0.47 to 102 l/min, flanged directly to the motor, volumetric efficiency close to 100% with inverter control.
- FX series — hygienic stainless steel pumps in AISI 316 machined from solid, six models from 9.2 to 247.1 l/min, CIP configuration and optional heating or cooling jacket.
Thick liquids we pump every day
- Bitumen, pitch, bituminous compounds and bitumen emulsion for road and roofing plants.
- Heavy fuel oil, bunker fuel, dense naphtha, heavy lubricants and thermal oil.
- Resins, adhesives and hot-melt, polyurethane glue, isocyanate and polyol, silicone, high-density paints and inks.
- Molasses, syrups, glucose, honey, chocolate, creams and fats with the food-grade FX series.
- Glycerine, soaps, caustic soda, waxes and paraffin, molten polymers and polystyrene in solution.
- Water-glycol mixtures, landfill leachate, fire-fighting foam concentrate and concrete admixtures in ZV or NAV execution.
If your product is not in this list, send us its name and viscosity: the same families cover most thick liquids found in industry.
Heating, materials and abrasion
Some viscous fluids simply need a bigger pump; others need help to flow at all. For products that set at room temperature we use the CR heating jacket, which wraps the whole pumping chamber and leaves no cold spot where the product can crystallise. Where no thermal plant exists, electric cartridges bring a standard pump up to temperature in about two minutes, and the self-contained hot-spot circuit heats a CR jacket with electric elements of up to 900 W and a small 8 ZV circulation pump.
Cast iron is the standard body. AISI 316 stainless steel is used for aggressive chemicals, food, cosmetics and pharmaceuticals. For strongly abrasive fluids, such as filled bitumen, the SEF version places the support bearings outside the pump body, with external timing gears in an oil bath and four replaceable gland packings. ATEX documentation is available for hazardous areas.
Data we need to size a viscous fluid pump
Gear pumps are selected, not picked from a chart. The data that matter are: the fluid and its viscosity at the working temperature, the temperature range, the flow in l/min or m³/h, the discharge pressure, the suction conditions (tank below or above the pump, length of the suction line), the presence of solids or abrasive filler, and the drive you prefer. With this information we calculate the model, the speed and the motor power, and we tell you whether a jacket, a stainless steel execution or an inverter is worth having.
Write to us on WhatsApp or fill in the quote form. Our Bologna factory, building self-priming external gear pumps since 1955 under ISO 9001:2015, replies with a technical proposal and a price, not with a brochure.
Frequently asked questions
From what viscosity should I use a gear pump instead of a centrifugal pump?
Indicatively from 50–100 cP upwards. Above that threshold a centrifugal pump loses efficiency quickly, while an external gear pump gains volumetric efficiency as the fluid gets thicker and keeps a constant flow.
Are viscous fluid pumps self-priming?
Yes. All our series are self-priming external gear pumps: they draw the product even when the suction line is not full, which is essential with thick liquids that do not flow into the pump on their own.
What are viscous fluid pumps made of?
Cast iron or AISI 316 stainless steel. Cast iron suits most industrial fluids, while stainless steel is used for aggressive products such as acids and caustic soda, and for food, cosmetic and pharmaceutical products.
Can I dose a viscous fluid with these pumps?
Yes. Flow is proportional to speed, so an inverter on the motor turns the pump into a volumetric dosing unit. The ZV series, with volumetric efficiency close to 100%, is designed for this duty.
Do you make stainless steel high viscosity pumps?
Yes. The FX series is made entirely of AISI 316 stainless steel for food, cosmetic and pharmaceutical products, and the NAV and ZV series are available in stainless steel for aggressive chemicals.