Information and Definitions
Tagname of the instrument. This is the identifier of the field
device, which is normally given to the location and function
of the instrument.
Plant, Area and Notes
Information Referred to the physical installation of the
instrument. Plant and Process Area where the instrument is
installed. Notes about the instrument.
Fluid Name or Composition. Fluid is called a type of
continuous medium formed by some substance whose molecules
have only a weak force of attraction. A fluid is a set of particles that are held together by weak
cohesive forces and the walls of a container; The term
encompasses liquids and gases.
State of the matter. It could be Liquid, Gas or Steam.
Mass of a substance which passes per unit of time. Mass flow
in Kg/s units, flowing through the pipe.
Operating Temperature of the fluid in Celsius units. The
flowing temperature is normally measured downstream from the
orifice and must represent the average temperature of the
flowing stream in degrees Celsius. Temperature has two effects
on volume. A higher temperature means a less dense gas and
higher flows, but when this higher flow is corrected to base
temperature, the base flow is less.
Considering the direction of the fluid, we define P1 as the
pressure (gauge or absolut) existing in the pipeline before
the restriction orifice. Pressure has two effects on volume. The higher pressure makes
the gas denser so less volume flows through the meter.
However, when the volume is expanded to base pressure, the
volume is increased.
Viscosity is the measure of a fluid's resistance to flow.
Dynamic viscosity is a measure of internal resistance.It
measures the tangential force per unit area required to move
one horizontal plane with respect to an other plane.
It is commonly expressed, particularly in ASTM standards, as
centipoise (cP) since the latter is equal to the SI multiple
millipascal seconds (mPa·s).The viscosity of a fluid is
highly temperature dependent.
Density is the relation of mass and volume.The density of a
material varies with temperature and pressure. This variation
is typically small for solids and liquids but much greater for
Ratio of Sp.Heats
Ratio of the heat capacity at constant pressure (CP) to heat
capacity at constant volume (CV). It is sometimes also known
as the isentropic expansion factor and is denoted by γ
(gamma) for an ideal gas or κ (kappa), the isentropic
exponent for a real gas.
Inside diameter of the pipe. All process calculations are
based on the volume of the pipe which is the function of
internal diameter of the pipe. As per standards, any pipe is
specified by two non-dimensional numbers Nominal Diameter (in
Inches as per American Standards or mm as per European
standards) and Schedule (40, 80, 160,...). The outer diameter
of the pipe is the diameter of outer surface of the pipe.
Pressure Ratio at which the discarge coefficient determined
has the value C.
Reynolds (ReD) and Reynolds Flow Regime
The Reynolds number (Re) is an important dimensionless
quantity in fluid mechanics used to help predict flow patterns
in different fluid flow situations.
At low Reynolds numbers, flows tend to be dominated by laminar
(sheet-like) flow, while at high Reynolds numbers turbulence
results from differences in the fluid's speed and direction,
which may sometimes intersect or even move counter to the
overall direction of the flow.
The discharge coefficient is a dimensionless number used to
characterise the flow and pressure loss behaviour of nozzles
and orifices in fluid systems.It depends on the orifice shape.
The discharge coefficient can be obtained for any
differential-pressure meter and any installation by
calibrating it in a flowing fluid: for a particular orifice
meter the discharge coefficient is a function of the Reynolds
Over many years of experiment it has been found that the
discharge coefficient can be predicted within a defined
uncertainty provided that the orifice meter (i.e. the orifice
plate and pipework) are constructed within the standards. If
the discharge coefficient is to be used for an orifice meter
without calibrating it in a flowing fluid, the discharge
coefficient is usually taken from a published discharge
coefficient equation. Therefore, the discharge coefficient
equation is very important for orifice plates: an error of 0.1
% in discharge coefficient gives an error of 0.1 % in many
flow measurements of natural gas. ISO 5167-1:2003 provides an
equation for the orifice discharge coefficient calculation,
Cd, as a function of Beta Ratio, Reynolds number, L1 and L2,
where L1 is the distance of the upstream pressure tap from the
orifice plate and L2 is the distance of the downstream
pressure tap from the orifice plate.
Beta Ratio is the ratio between the line inner diameter to
bore size of the orifice. The flow coefficient is found to be
stable between beta ratio of 0.2 to 0.7 below which the
uncertainty in flow measurement increases. An orifice plate
beta ratio of 0.6 means that the orifice plate bore diameter
is 60% of the pipe internal diameter.