A pump that performs perfectly on water can behave very differently with heavy oil or a hot chemical. The reason is usually viscosity and temperature. In process plants, liquids rarely stay at one temperature, and their thickness changes along with it. Ignoring this is a common cause of low flow, high power bills and early wear, even when the pump looked right on paper.
Viscosity and the Link with Temperature
Viscosity is a liquid’s resistance to flow. Lubricating oil, for example, is far more viscous than water. Temperature changes it sharply: most liquids thin out when heated and thicken when cooled. A fluid that flows easily at 80°C may turn sluggish on a cold morning, so one pump can face very different conditions in a single day.
Impact on Flow, Head and Efficiency
As viscosity rises, friction inside the pump increases. Flow and head both fall, and efficiency drops because more energy is lost to fluid friction instead of useful work. The best efficiency point also shifts, so the water curve printed in the catalogue no longer applies.
Impact on Power Consumption
Thicker liquids need more shaft power for the same duty. The motor works harder to deliver less flow, so electricity use per unit pumped goes up. A motor sized only for water can easily overload.
Cavitation and NPSH
Temperature also affects suction conditions. Hotter liquids have a higher vapour pressure, which reduces the Net Positive Suction Head (NPSH) available. When it falls below what the pump requires, vapour bubbles form and collapse, damaging the impeller. Thicker liquids add friction in the suction line, which reduces NPSH further.
Mechanical Seal Considerations
Seals are sensitive to both factors. Very viscous fluids can stick or crystallise at the seal faces, while hot liquids can flash into vapour between them. Choose seal materials and flushing arrangements that suit the full temperature range, not just the normal one.
Viscosity Correction During Pump Selection
Pump curves are drawn using water. For viscous liquids, selection should apply viscosity correction factors, which adjust the published flow, head and efficiency to realistic values. The pump and motor are then chosen on this corrected data. Done properly, it avoids both underperformance and wasted energy.
Why Actual Operating Conditions Matter
Always share the real temperature range, viscosity at minimum and maximum temperature, and specific gravity with your supplier. Designing for an average or datasheet value is risky. A split case pump selected with these details runs closer to its best efficiency point, even when conditions change. For horizontal split casing pumps industries in Mumbai can rely on, VEMPL offers guidance from selection to servicing.
VEMPL has over 77 years of expertise in electromechanical engineering solutions and industrial pumps offered by leading water pump manufacturers. Reach out to us on +91-8976951701, +91-9987099448, 022-43117133, or email marketing@vempl.co.in.
FAQs (Frequently Asked Questions)
1. Does a higher temperature always improve pump performance?
No. Heat thins the liquid and reduces friction, but it also raises vapour pressure, which can cause cavitation.
2. Why does a viscous liquid increase power consumption?
Greater friction means the pump needs more shaft power to deliver the same flow.
3. What is viscosity correction?
It adjusts water-based pump curves so that flow, head and efficiency reflect the actual liquid.
4. Can a split case pump handle viscous liquids?
Yes, within limits, provided the pump and motor are selected using corrected performance data.
5. What details should I share when selecting a pump?
Share the temperature range, viscosity, specific gravity, flow and head required.