Chapter 216
The absolute viscosity of a liquid is measured in lb-s/ft
2
in English units
and pascal-s in SI units. Other commonly used units of absolute viscosity
are the poise and centipoise (cP).
The kinematic viscosity is defined as the absolute viscosity of a liquid
divided by its density at the same temperature.
ν=µ/ρ (2.7)
where
ν=Kinematic viscosity
µ=Absolute viscosity
ρ=Density
The units of kinematic viscosity are ft
2
/s in English units and m
2
/s in SI
units. See Appendix A for conversion of units. Other commonly used units
for kinematic viscosity are the stoke and centistoke (cSt). In the petroleum
industry, two other units for kinematic viscosity are also used. These are
saybolt seconds universal (SSU) and saybolt seconds furol (SSF). When
expressed in these units, kinematic viscosity represents the time taken for a
fixed volume of a liquid to flow through an orifice of defined size. Both
absolute and kinematic viscosities vary with temperature. As temperature
increases, liquid viscosity decreases and vice versa. However, unlike
specific gravity, viscosity versus temperature is not a linear relationship.
We will discuss this in Section 2.4.1.
Viscosity also varies somewhat with pressure. Significant variation in
viscosity is found when pressures are several thousand psi. In most
pipeline applications, viscosity of a liquid does not change appreciably
with pressure.
For example, the viscosities of Alaskan North Slope (ANS) crude oil
may be reported as 200 SSU at 60°F and 175 SSU at 70°F. Viscosity in
SSU and SSF maybe converted to their equivalent in centistokes using the
following equations:
Conversion from SSU to centistokes
Centistokes=0.226(SSU)-195/(SSU) for 32ⱕSSUⱕ100 (2.8)
Centistokes=0.220(SSU)-135/(SSU) for SSU>100 (2.9)
Conversion from SSF to centistokes
Centistokes=2.24(SSF)-184/(SSF) for 25<SSFⱕ40 (2.10)
Centistokes=2.16(SSF)-60/(SSF) for SSF>40 (2.11)
Copyright © 2004 by Marcel Dekker, Inc.