API Gravity

Specific Gravity

The term specific gravity, symbolized sp gr, refers to the ratio of the density of a solid or liquid to the density of water at 4 degrees Celsius. The term can also refer to the ratio of the density of a gas to the density of dry air at standard temperature and pressure, although this specification is less often used. Specific gravity is a dimensionless quantity; that is, it is not expressed in units.

API gravity is calculated using the specific gravity of an oil, which is nothing more than the ratio of its density to that of water (density of the oil/density of water). Specific gravity for API calculations is always determined at 60 degrees Fahrenheit.  API gravity is found as follows:

Though API values do not have units, they are often referred to as degrees. So the API gravity of West Texas Intermediate is said to be 39.6 degrees. API gravity moves inversely to density, which means the denser an oil is, the lower its API gravity will be. An API of 10 is equivalent to water, which means any oil with an API above 10 will float on water while any with an API below 10 will sink.

The API gravity is used to classify oils as light, medium, heavy, or extra heavy. As the “weight” of an oil is the largest determinant of its market value, API gravity is exceptionally important. The API values for each “weight” are as follows:

Light – API > 31.1

Medium – API  between 22.3 and 31.1

Heavy – API < 22.3

Extra Heavy – API < 10.0

Redlich-Kwong Equation of State

In physics and thermodynamics, the Redlich–Kwong equation of state is an empirical, algebraic equation that relates temperature, pressure, and volume of gases. It is generally more accurate than the van der Waals equation and the ideal gas equation at temperatures above the critical temperature. It was formulated by Otto Redlich and Joseph Neng Shun Kwong in 1949.

Hagen-Poiseuille equation

Hagen-Poiseuille's Equation can be used to determine the pressure drop of a constant viscosity Newtonian fluid exhibiting laminar flow through a rigid pipe. Non-newtonian liquids do not obey Poiseuille's law because their viscosities are velocity dependent. The assumption of
streamlined (laminar) flow is built in to Poiseuille's law. If turbulence occurs than you must be very careful about using Poiseuille's law to calculate flow rates. If turbulence does occur in the flow then the volume flow rate is dramatically reduced.

Viscosity

Viscosity is the resistance of a fluid to flow. Virtually all fluids have viscosity which generally changes as a function of temperature; although different types of fluids exhibit different types of fluid–shear velocity dependencies.

“When a fluid or semisolid is subjected to a constant shearing force it flows, i.e., it deforms continuously at a velocity that increases as the applied shearing force increases.” Viscosity quantifies the resistance of the fluid to flow

Introduction

Viscosity is a quantitative measure of fluid’s resistance to flow (shear stress) at a given temperature. This resistance arises from the attractive forces between the molecules of the fluid. A fluid will only flow if enough energy is supplied to overcome these forces.

Dynamic Viscosity / Viscosity

The dynamic viscosity (η) of a fluid is a quantitative measure of the resistance it offers to relative shearing motion.
Dynamic viscosity, which is also referred to as absolute viscosity, or just viscosity, is the quantitative expression of a fluid’s resistance to flow (shear). Fluid dynamicists, chemical engineers and mechanical engineers commonly consider the use of the Greek letter mu (µ) as the symbol to denote dynamic viscosity.

Units

The SI unit is pascal-second [Pa.s] or millipascal-second [mPa.s]:

    1 Pa.s = 1000 mPa.s
    The SI unit is named after Blaise Pascal.

Other commonly used units are poise [P] or centipoise [cP]:

    1 P = 100 cP
    This unit is named after Jean Poiseuille

    1 cP = 1 mPa.s = 0.001 Pa.s = 0.01 P

However, the most common expression is centipoise (cP), which is mainly used in ASTM standards.

Kinematic Viscosity

It is defined as the ratio of absolute viscosity to the density of fluid. Kinematic viscosity describes a substance's flow behavior under the influence of Earth's gravity. It is dynamic viscosity divided by density ρ, rho, which is defined as mass per volume. The quantity mass carries the gravitational influence. Kinematic viscosity is sometimes called the diffusivity of momentum.

        ν= η/ρ

Units

The SI unit is square-meters per second  [m2/s] or square-millimeters per second [mm2/s]:

    1 m2/s = 1 000 000 mm2/s

Other commonly used units are stokes [St] or centistokes [cSt]:

    1 St = 100 cSt

This unit is named after George G. Stokes.

    1 cSt = 1 mm2/s

 It should be noted that water (H2O) at 20 degrees centigrade is about 1 cSt.

Relation of Kinematic Viscosity with Dynamic Viscosity

Limitations

The above equation holds only when

1-Fluid is Newtonian

2-Specific Gravity Remains the Same

Application

Liquids are generally considered viscous if viscosity is more then 40 centipoise (cp). Centrifugal pumps are not recommended for fluid having viscosity more then 300 centipoise (cp).

The viscosity of liquids decreases with increase the  temperature. Typically 2% per degree C. For some materials (fruit juices) the Temperature effect follows an Arrhenius relationship. 

Viscosity of gases increases with the increase the temperature.

Hazen-Williams Equation

Introduction

The Hazen-Williams equation is an empirical formula used to model the friction head loss of water flowing through pipe. The accuracy of the Hazen-Williams is less than that of the C.F. Colebrook equation. This equation uses the coefficient C to specify the pipes roughness, which is not based on a function of the Reynolds number, as in other pressure loss equations. 

It is also possible to use Hazen-Williams to model fluids other than water as long as the viscosity is approximately 1.130 centistokes.

Calculating Head Loss

Where:

C      = Friction Factor (Hazan William Constant)

d       = Inside diameter of pipes (in)

Q       = Flow rate in Gallons per minute of water

L      = Length of pipe (ft)

hf     = Friction head loss (ft)

Limitations

1. it should only be used for water between the temperature of 55 degrees Fahrenheit (12.8 deg C) and 65 degrees Fahrenheit (18.3 deg C). The formula is popular with civil engineers who constantly need to make calculations for water flow through pipe in the ambient atmosphere.

2. Hazen William Equation can only be used in equation can only be used when water is flowing within the 'turbulent' flow range

Applications

The Hazen William formula has now become adopted through the world as the pressure loss formula to use for the hydraulic design of fire sprinkler systems and in almost all cases the use of the hazen william formula will provide adequate answers. The Hazen William formula can also be used for the calculation of water mist systems where the system pressure does not exceed 12 bar (low pressure water mist systems) or the water velocity does not exceed 7.6 m/s and the minimum pipe size is 20 mm in the case of intermediate and high pressure water mist systems.

Ideal Gas Equation

Introduction

The three historically important gas laws derived relationships between two physical properties of a gas, while keeping other properties constant:




It is the most basic equation of state, assuming that the particles in the gas behave in such a way that they do not interact with each other at all, and approximate point masses. This makes the math very simple as the extensive state of an ideal gas is only a function of temperature, pressure, and volume.

P V = n R T

Application

There are three common applications of Ideal Gas Equation.

• Calculation of Molar Volume 

• Calculation of Density 

• Calculation of Molar Mass (weight of one mole) 

Properties of the gaseous state predicted by the ideal gas law are within 5% for gases under ordinary conditions.

Peng-Robinson Equation of State

Introduction

The Peng-Robinson EOS has become the most popular equation of state for natural gas systems in the petroleum industry.



In the form of Pressure



Where