Orifices A plate is a hollow plate inserted into a pipe and placed perpendicular to the flow stream. When the fluid flows through the orifice plate it causes increased speed and decreased pressure. The pressure difference before and after the orifice plate is used to calculate the flow velocity. Instruments used to measure the flow is commonly used differential pressure transmitter.
Picture 1, Working Principle of the orrifice plate
Calculation of Orifice Platform
Calculations refer to Bernoulli's law.
The law of conservation of energy states: if there is no heat transfer and work done, the fluid energy at each point along the pipe will remain constant. From the principle of conservation of this energy can be derived Bernoulli equation.
The unity volume energy flow equation for incompressible fluids is:
½ ρ V² + Ps + ρ g h = constant
A fluid stream passing through an orifice plate bar will have a pressure drop on the orifice. This change can be used to measure the flowrate of a fluid.
To calculate the flowrate of a fluid stream passing through an orifice plate, then as long as the fluid flow velocity is below the subsonic velocity (V <mach 0.3), then the above Incompressible Bernoulli’sequation can be used, so that:
Where location 1 is upstream of the orifice, and location 2 is
downstream of the orifice.
this equation states that the number of fluids entering the 1st section (Q1) is equal to the number of fluids coming out of the cross section 2 (Q2), which means:
V1 . A1 = V2 . A2
From the equation of the Bernoulli Equation and the Continuity Equation we can derive the equation connecting the flow of flow (Q) with the static pressure difference between upstream and downstream (p1-p2). The total heads in both places are the same.
For uncompressed fluids, the relationship between the flow rate (Q) measured by different pressure (p1 - p2) are:
the solving for the flowratevolumetric is (Q) :
The above equation can only be applied to perfect flow (laminar, inviscid or non viscous). For real streams (such as water or air), viscosity and turbulence characteristics have an effect and result in the conversion of kinetic energy into heat. For the effect, a discharge coefficient (Cd) is introduced into the above-mentioned equation to outline the flowrate (Q).
Since the real flow profile at the 2 (downstream) location of the orifice is very complex, it is thus made an effective value for replacing uncertain A2, ie, flow coefficient (Cf),
Where Ao is from orifice. As a result, the volumetric flow rate (Q) equation for real flow is:
Flow coefficient (Cf) is obtained from the experiment (can be seen in the table that has been arranged in the reference books), Cf value extends from 0.6 to 0.9 for most types of orifice. It therefore depends on the orifice and pipe diameter (as does Reynolds Nomors). Usually in table Cf is the ratio of orifice diameter and pipe inlet diameter, sometimes defined as β, ie:
Mass flow rate (Q mass) can be determined by multiplication of volumetric flow rate (Q) with fluid density (ρ), ie:
- General Process Flow Measurement:
β ratio = d / D, within the range 0.25 - 0.75
- Custody Transfer Flow Measurement:
β ratio = d / D, within the range 0.4 - 0.6 (preferably close to 0.5)
For compressible fluids, the following relationships apply:
G = mass flow rate
Y = Expansion factor, depending on the type heat and fluid pressure.
Types of Orifice Plate
1. Concentric Orifice
The location of the concentric holes with a pipe cross section. Used to measure the volume of gas, liquid and steam in large quantities.
· Can be used on various pipe sizes (wide range).
· Accuracy (accuracy) good, if the plate is installed properly.
· Prices are relatively cheap.
· Pressure loss (pressure drop) is relatively high.
· Can not be used to measure slurry flow rate, as it tends to be blocked.
In Orifice Eccentric type, the center point of the barrier is not a central line with the center of the pipe cross section. Installation of the non-concentric hole is intended to reduce the problem if the measured fluid carries various solids (solid).
3. Segmental Orifice
This segmental orifice plate is applied to measure the flow of mild slurries and liquids with high solids concentrations. The segmental hole design eliminates the containment of foreign objects and provides more complete drainage than the eccentric orifice plates.