Viscosity in Industrial Pump Selection

1. What is viscosity

Viscosity is the measure of how much a fluid resists flow. Water flows easily (low viscosity), honey flows slowly (high viscosity), and asphalt cement (CAP), semi-solid or solid at room temperature, becomes fluid when heated. Physically, viscosity is caused by intermolecular cohesion forces within the fluid — large molecules and long chains (like polymers and heavy oils) generate more resistance than small molecules (like water and solvents).

There are two types of viscosity: (1) Dynamic (or absolute) viscosity — measures flow resistance as a function of applied force. SI unit: Pa·s (pascal-second). Industrial unit: cP (centipoise). Reference: water at 20 °C = 1.0 cP. (2) Kinematic viscosity — is dynamic viscosity divided by fluid density. SI unit: m²/s. Industrial unit: cSt (centistokes). Reference: water at 20 °C = 1.0 cSt. The relationship: cSt = cP / density (g/cm³).

In the Hydraulic Institute definition, viscosity is the property of a liquid that makes it resist shear stress, such as that caused by flow, mainly at the pipe wall. Absolute viscosity is the shear stress divided by the shear rate; expressed relative to density, it becomes kinematic viscosity.

2. Viscosity units and how to convert

cP and cSt represent different quantities and require density for conversion. SSU is an efflux time measured by the Saybolt method at a specified temperature, not an exact multiple of cSt. Use a correlation with declared range and temperature or fluid-supplier data; do not apply one linear factor across the whole scale.

SSU (Saybolt Seconds Universal) is the time a measured quantity of liquid, at a specified temperature, takes to drain through a calibrated orifice in the bottom of a container. In Hydraulic Institute references, water is about 31 SSU at 60 °F (15.6 °C); light lubricating oils may have 100 or 200 SSU; heavier lubricating oils, thousands of SSU; and extremely viscous fluids such as heavy tar reach 1 million SSU.

The FBE tabulates speed in SSU (MTEC-01/01 and catalog). The FBE 2026 catalog (p.5) converts as follows: cSt = cP ÷ density in g/cm³, and above 300 SSU the approximation SSU = cSt × 4.635 applies. Below that, use the catalog’s own conversion chart, which relates SSU and cSt to scales such as Saybolt Furol, Redwood, Engler degrees and Ford and Zahn cups.

Example with an illustrative value: an oil of 900 cP and a density of 0.9 g/cm³, both measured at pumping temperature, has 1,000 cSt, or about 4,635 SSU. In the FBE speed table (section 5), that value falls in the 2,500 to 7,500 SSU band: 850 rpm, direct drive, models 1/2″ to 6″.

3. Viscosity table: fluid, value and temperature

A useful table identifies product, concentration, temperature, method and unit. ISO VG 32 oil has a viscosity grade referenced to 40 °C; that does not mean 32 cSt at 20 °C. SAE grades alone also do not supply one process viscosity. For molasses, resin, emulsion or grease, also check rheology and measurement conditions.

The table below gathers values from primary sources and shows why a product name does not replace process viscosity: each value comes with its temperature and its source. CAP 50/70 asphalt, for example, is named after its penetration at 25 °C, and the Petrobras asphalt manual links workability, the consistency at handling temperatures, to pumping, storage and application.

4. How viscosity influences selection

Selection combines viscosity, flow, pressure, temperature, composition and suction. Higher viscosity can reduce gear-pump slip and increase volumetric efficiency while also increasing friction and inlet requirements. Compare performance and power for the same duty without a fixed 200 cSt boundary.

A centrifugal curve obtained with water requires assessment for the process liquid. The Hydraulic Institute (HI) describes viscosity- and pump-dependent corrections to flow, head and efficiency. They also affect power and can affect suction; one cSt value does not prove the solution is uneconomic.

With viscous liquids, HI summarizes the effect relative to the water test: head and flow drop, and input power rises markedly because efficiency falls. The same text points to two further concerns covered in ANSI/HI 9.6.7: the increase in required NPSH (NPSH3) and in required starting torque.

In the Hydraulic Institute example, the efficiency of one centrifugal pump drops from near 80% on water to about 50% on a 1,000 SSU liquid with a specific gravity of 0.9, using the empirical correction of ANSI/HI 9.6.7. HI concludes, without setting a value, that centrifugal pumps may be limited when viscosity results in unacceptable efficiency and that, in those cases, a positive displacement pump may offer a better solution.

In rotary positive displacement pumps, HI notes that flow and volumetric efficiency typically increase with viscosity; internal slip, the difference between theoretical and delivered capacity, depends on viscosity and pressure. On the other hand, friction losses raise input power above theoretical power, and the net positive inlet pressure required (NPIPR) increases with viscosity.

On the FBE, the 2026 catalog (p.5) gives efficiency (η) by viscosity and pressure. This η is not the volumetric efficiency mentioned above: it falls with viscosity and rises with pressure. At 7 kgf/cm², it goes from 40% at 1,000 SSU to 14% at 100,000 SSU. The figure applies to the FBE under that condition and is not directly comparable with the HI centrifugal example, which is a different pump.

For choosing the commercial motor, the same catalog recommends, by pump shaft power band (BHP), a power reserve of 50% up to 2 cv (metric horsepower), 30% from 3 to 5 cv, 25% from 6 to 10 cv, 15% from 11 to 25 cv and 10% above 25 cv.

5. FBE speed-by-viscosity table

Table 2 of the FBE (MTEC-01/01) and page 5 of the FBE 2026 catalog give the same relationship between viscosity band, speed, drive and models. Up to 7,500 SSU, the drive is direct; above that, speed drops to 700 to 150 rpm, through a pulley or gear reducer, and, in the table, the 50,000 to 100,000 SSU band lists only models 3″ to 6″.

FB Bombas Engineering extends the table to models A, CA and CA-A; per the 2026 catalog, these variants apply only to models in the current line, and the FBE 1/2″ A is no longer supplied. The table guides speed; selection also checks flow, pressure, power with the catalog reserve and suction conditions. The FBE viscosity-selection article details the maximum suction pressure in Table 5 (MTEC-01/01) and the model nomenclature.

6. Temperature and viscosity at startup and duty

Check startup and duty using the selected fluid’s property curve. Cold and duty viscosity values of one thermal oil do not carry over to another without its own data. Confirm power, speed, suction and heating procedure for the circuit; an auxiliary pump or automatic technology change requires its own design.

Even water changes measurably: according to NIST data, its dynamic viscosity falls from 1.00 cP at 20 °C to 0.65 cP at 40 °C and 0.28 cP at 100 °C. For viscous products, ask the supplier for the viscosity-temperature curve and give engineering the minimum, startup and operating temperatures.

When the product must be kept hot inside the pump, the FBE can be fitted with a heating chamber (CA variant, an accessory in the catalog) from 1.1/2″ to 6″ (MTEC-01/01), and the FBE 2026 catalog (p.3) gives a temperature of 10 °C up to 500 °C for the series. Confirm the configuration for the temperature, pressure and viscosity of each duty.

7. Not sure about viscosity? Talk to the engineer

Send trade name, datasheet, composition and minimum and maximum temperatures. Identify missing viscosity or rheology data for review with the fluid supplier. The technical proposal should confirm input data and analysis scope. Contact comercial@fbbombas.com.br or WhatsApp +55 11 97287-4837.