Flow meter sizing for orifice


What is orifice?

Orifice plate produce a constriction in the flow channel which results in a differential pressure across the constrian. The differential pressure is measured to measure the flowrate.

Flow rate:

The flow rate can be derived from the Bernoulli’s principle:

Where prefix number represents the location 1 as the upstream and 2 as the downstream. From continuity, the velocities can be replaced by cross-sectional areas of the flow and the volumetric flow rate Q,


Solving for the volumetric flowrate Q gives,


Since the actual flow profile at location 2 downstream of the orifice is quite complex, thereby making the effective value of A2 uncertain, the following substitution introducing a flow coefficient Cf is made, and Cd is the discharge coefficient:


where Ao is the area of the orifice. As a result, the volumetric flow rate Q for real flows is given by the equation,

The orifice plate can be made of any material, although stainless steel is the most common. The thickness of the plate used (1 / 8-1 / 2 ") is a function of line size, process temperature, pressure and differential pressure. The traditional hole is a thin circular plate (with a flange for handling) and for data), inserted into the pipeline between the two flanges of an orifice joint. This method of installation is cost-effective but requires closing the process whenever the plate is removed for maintenance or inspection. , an orifice connection allows the orifice to be removed from the process without depressurizing the line and closing the flow. In such accessories, the universal orifice plate, a circular plate with no tab, is used.

Orifice Plate Sizing:

QM = Maximum flow in GPM
Gb = Base S.G. [(S.G. of liquid @ 60°F (Water @ 60°F = 1)]
N = 5.667 for GPM
D = Pipe ID in inches
GF = Flowing SG of liquid @ flowing temperature (see Crane A-6)
hM = Meter differential in “WC
S = Orifice ratio (reference Spink pg. 167 Table 12 for corresponding β)

Steam or Gas Orifice:


basic steam equation:


Basic Equation Gas:


W = Flow in lbs / hr
SW = Specific Weight of vapor in lbs/ft3 = 1  Specific Volume
For Steam, reference Crane A12 thru A18 (use 1/specific volume)
For Gas, reference Crane A-8, column rho ‘ρ’)
hM = Meter differential in “WC
D = Pipe ID in inches
S = Orifice ratio