Viscosity
Viscosity is a medium's resistance to flow: how hard it is to shear one layer of it past the next. It is the third question we ask when picking a duty, after flow rate and the pipework, because it decides the pump type. Thin media suit a centrifugal pump. Thick media want positive displacement.
In more detail
Two figures get quoted and they are not interchangeable. Dynamic viscosity, in centipoise (cP), is the resistance itself. Kinematic viscosity, in centistokes (cSt), is that resistance divided by the medium's density, and it is what pump viscosity correction charts are usually drawn against. Say which one you mean, and give a temperature with it. Liquids thin as they warm, so a bare viscosity figure with no temperature beside it describes nothing in particular.
Design for the worst case, not the typical one
Select against the thickest state the medium ever reaches, not the figure on the datasheet. The question we ask is whether the medium is water, or as water, then how thick it gets and how much it varies. Most media thin out as they warm, so a datasheet figure is usually the easy end of the range. Take the coldest, thickest state the medium sees on your site, including the first start after a weekend shutdown. A pump sized on the typical figure still has to survive the worst one.
What viscosity does to a centrifugal pump
A published performance curve is tested on water. Put a viscous medium through the same pump and flow, head and efficiency all fall, while the power at the shaft and the NPSHr both rise. The Hydraulic Institute's ANSI/HI 9.6.7 is the industry method for correcting a water-tested curve onto a viscous duty, and it applies to Newtonian media only. Select against the corrected curve, not the catalogue one. Watch the suction side while you do it. A viscous duty squeezes the NPSH margin from both ends at once: NPSHr goes up with the viscosity, and NPSHa comes down because the friction losses in the suction pipework go up too.
Where viscosity moves you off a centrifugal
There is no single viscosity at which a centrifugal stops working. It just gets steadily worse at the job, until the corrections are large enough that you are buying a much bigger motor to move the same flow rate. In practice the boundary is the one solids content sets. The trade rule of thumb is roughly 3–4% solids, and above that you move to positive displacement; viscosity pushes you the same way, though solids content is the number a customer usually has to hand. A positive displacement pump will move broadly whatever you can get into it, from water through to thick food product or sludge, and it holds its flow rate as the discharge pressure rises. That is why the thick end of the range is progressive cavity and rotary lobe pumps, and why a high viscosity duty is settled by pump type before it is settled by pump size.
Thick media want pumping slowly
Speed is the constraint that never appears on a performance curve. A viscous medium needs time to fill the pumping chamber, so a positive displacement pump on a thick duty runs slower than the same pump on water and delivers less for it. Two other properties want the same treatment. An abrasive medium wears faster the harder you drive it, and a shear-sensitive one is damaged by the shear itself. The trade-off is capital: running slowly means a physically larger pump for the same flow rate, and a larger pump costs more.
Viscosity is not always a single number
A lot of what we pump has no one viscosity. Sludge, slurries and many food products are non-Newtonian, which means they thin as they are sheared, so the figure measured in a sample pot at rest is not the figure the pump sees. That is where a centrifugal sized on one viscosity figure disappoints. It is also the second reason to select against the worst case: with these media there is no typical case to select against. If your medium behaves that way, say so when you send us the duty, and send a temperature with it.
Frequently asked
What does high viscosity mean?
High viscosity means the medium resists flow. Sludge and a thick food paste are both high viscosity; they are slow to pour and slow to shift. For a pump it means higher friction losses in the pipework, more power at the shaft, and a centrifugal that no longer delivers the curve it was sold on.
What is viscosity measured in?
Dynamic viscosity is measured in centipoise (cP), or in pascal-seconds in SI units. Kinematic viscosity is measured in centistokes (cSt), or square metres per second. Kinematic viscosity is the dynamic figure divided by the medium's density. Always state which of the two you mean, and give the temperature with it.
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity is the medium's resistance to shear on its own. Kinematic viscosity is that resistance divided by density, so it also accounts for how heavy the medium is. Pump viscosity correction charts are usually drawn against kinematic viscosity in centistokes, so that is the figure worth sending with a duty.
Does viscosity increase with temperature?
No. For liquids it falls as temperature rises. Warm most media and they pump more easily, which is why a trace-heated line or a lagged tank sometimes settles a viscous duty without a bigger pump. It follows that a viscosity figure quoted with no temperature beside it is not much use to whoever is sizing the pump.
Is high viscosity thick or thin?
Thick. High viscosity means the medium resists flow and pours slowly. Low viscosity means it is thin and runs freely, and water is the reference point at that end. The trade shorthand for a thin medium is that it is water, or as water, and that phrase is what a pump supplier is listening for.