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Why internal gear pumps are considered for viscous liquids
Internal gear pumps are positive-displacement pumps that move liquid in spaces formed between an internal rotor, an idler gear and a crescent divider. DESMI describes ROTAN internal gear pumps as producing smooth, non-pulsing flow and as being suitable across a wide viscosity range. The technology is therefore often considered for oils, resins, syrups, polymers, bitumen and other viscous products. The pump family still has to be matched to the fluid chemistry, temperature, required flow, differential pressure, suction conditions and sealing requirements.
Use viscosity at operating temperature, not a room-temperature label
For viscous-liquid selection, the viscosity value is only useful when it is tied to temperature. Many liquids change viscosity sharply between storage, startup and normal process temperature. Provide the viscosity range at startup and at the expected operating temperature, plus any heating or cooling arrangement. DESMI positions the ROTAN HD family specifically for difficult and high-viscosity applications and lists a family viscosity capability up to 250,000 cSt; that family-level limit is not a substitute for checking the exact size, speed, pressure, temperature and construction for the duty.
Protect the suction side from excessive losses
High viscosity increases friction loss in the suction line and can limit how quickly the pump cavities fill. Keep the suction path short and appropriately sized where possible, minimize unnecessary fittings and confirm the available inlet pressure or static head. Speed should be selected with viscosity and inlet conditions in mind rather than simply using the highest available motor speed. If the liquid must be heated to reach a workable viscosity, the startup condition should be part of the design review, including whether the pump and connected piping need heat tracing or jacketing.
Check differential pressure, torque and pressure protection together
A positive-displacement gear pump can continue to build pressure if the discharge path is restricted, so pressure protection is part of the system design. DESMI lists an integral safety relief valve as a feature on the ROTAN HD family. The selected configuration, relief direction and setting still need to be confirmed for the application, and the external system may require additional protection under the applicable engineering standard. Differential pressure also affects drive torque, so the motor and coupling selection should be checked at the actual viscosity and duty point rather than chosen from flow alone.
Common mistakes when selecting a viscous-liquid gear pump
Frequent mistakes include sending only the product name without viscosity, giving viscosity without temperature, ignoring startup conditions, assuming a family maximum applies to every size, or treating the relief valve as the normal flow-control device. Another common error is overlooking the sealing and material requirements because the hydraulic duty appears straightforward. A useful RFQ should include fluid composition, viscosity-versus-temperature information, temperature range, required flow, suction pressure or tank arrangement, discharge pressure, operating hours, connection standard and any cleaning requirements.
Frequently asked questions
Is a ROTAN HD pump automatically suitable for any liquid below 250,000 cSt?
No. The 250,000 cSt figure is a family-level published limit. The actual selection must be checked for pump size, speed, differential pressure, temperature, inlet conditions, materials and sealing arrangement.
Why is startup viscosity important?
The liquid can be much more viscous before heating. That can increase suction losses and torque demand, so the pump and drive should be reviewed for the worst credible startup condition.
Should an internal gear pump run at maximum speed for viscous service?
Not necessarily. Speed is part of the selection and may need to be reduced so the pump fills properly and inlet losses remain acceptable. Use the manufacturer's curve and application review for the chosen size.
What should I send DFIN for a DESMI ROTAN selection?
Provide the liquid, viscosity at relevant temperatures, temperature range, flow, suction condition, differential or discharge pressure, duty cycle, connection standard, materials and sealing requirements, plus any existing model or nameplate information.
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Technical references
Prepared by the DFIN editorial team as practical identification, selection and maintenance guidance. Confirm the exact product configuration and service procedure before ordering or maintenance.