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 hot asphalt flows only under elevated pressure and temperature (extreme viscosity). 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).
2. Viscosity units and how to convert
In the pump industry, the three most used units are cSt, cP and SSU. The SSU (Saybolt Universal Seconds) scale is the most common in American and Brazilian pump catalogs — including FB Bombas manuals. SSU measures the time in seconds for 60 mL of fluid to flow through a standard orifice (Saybolt viscometer) at a specific temperature. Approximate conversion: for SSU > 100, cSt ≈ SSU × 0.2158.
For more precise values: cSt = 0.2253 × SSU - (194.4 / SSU) for SSU between 32 and 100; cSt = 0.2158 × SSU - (13.56 / SSU) for SSU > 100.
3. Industrial fluid viscosity table
Typical values at 20°C (reference): Water = 1 cSt / 31 SSU. Diesel = 3-6 cSt / 36-45 SSU. Hydraulic oil ISO 32 = 32 cSt / 150 SSU. Hydraulic oil ISO 68 = 68 cSt / 315 SSU. Lubricating oil SAE 30 = 100 cSt / 465 SSU. Lubricating oil SAE 90 = 200 cSt / 930 SSU. Honey = 2,000-10,000 cSt. Glycerin = 1,500 cSt at 20°C. Melted chocolate (40°C) = 500-2,000 cSt.
Asphalt CAP 50/70 (180°C) = 200-400 SSU. Epoxy resin = 5,000-30,000 cSt. Grease = > 100,000 cSt. Note: temperature drastically affects viscosity — CAP asphalt at 25°C is solid, but at 180°C has pumpable viscosity (~300 SSU). Always specify temperature along with viscosity.
4. How viscosity determines pump type
Viscosity is the cutoff criterion between centrifugal and gear pumps. FB Bombas rule of thumb: below ~200 cSt (1,000 SSU), use FBCN centrifugal — efficiency is high and cost is lower. Above ~200 cSt, use FBE or FBEI gear — gear volumetric efficiency increases with viscosity (viscous fluid seals better between gear and casing, reducing slip). In the intermediate range (100-500 cSt), both can work — the decision depends on flow, pressure and other factors.
Impact on centrifugal: when an FBCN centrifugal pump operates with viscous fluid, three things happen simultaneously: flow drops (fluid resists movement more), head drops, and power consumption increases. Manufacturer performance curves (flow × head × efficiency) are valid only for water — for viscous fluids, Hydraulic Institute correction factors must be applied. Above ~500 cSt, centrifugal efficiency drops so much it becomes uneconomical.
5. Maximum speed by viscosity table — FBE Manual
Data extracted from FB Bombas FBE technical manual (MTEC-01/01) — this table is the official selection reference: 30–250 SSU → 1,750 rpm, direct drive, models 1/8" to 1"D. 250–2,500 SSU → 1,150 rpm, direct drive, models 1/8" to 5". 2,500–7,500 SSU → 850 rpm, direct drive, models 1/2" to 6". 7,500–10,000 SSU → 700–500 rpm, belt or gearbox, models 1.1/2" to 6". 10,000–50,000 SSU → 500–300 rpm, belt or gearbox, models 1.1/2" to 6".
50,000–100,000 SSU → 300–150 rpm, belt or gearbox, models 3" to 6". Operating above maximum speed for the viscosity range causes cavitation — the fluid cannot fill the gear tooth chamber at rotation speed, generating partial vacuum and bubble collapse.
6. Temperature × viscosity: the most expensive mistake
The most frequent mistake in industrial pump selection is specifying the pump for viscosity at normal operating temperature — and forgetting the cold startup condition. Real example: a thermal oil transfer system (FBOT) operates at 280°C with 2 cSt viscosity (trivial pumping). But at 20°C startup, the same oil has 500 cSt — and the centrifugal pump sized for 2 cSt simply cannot move the fluid.
Solution: use FBE gear pump for cold startup (pre-heating), then transfer to FBOT centrifugal when temperature reaches operating range. FB Bombas designs systems with automatic transfer switches for exactly this scenario.
7. Not sure about viscosity? Talk to the engineer
If you don't have the exact fluid viscosity, don't guess. Send the fluid trade name, operating temperature range and application to FB Bombas application engineering. We maintain a viscosity database of over 500 industrial fluids, built over 82 years of experience. Model selection, speed and transmission are based on real data — and the quotation includes installation NPSH analysis. Contact: comercial@fbbombas.com.br or WhatsApp +55 11 97287-4837.
