Introduction

Flow Rate With Orifice MeterBackground and Theory

Flow meters are used in the industry to measure the volumetric flow rate of fluids.

Differential pressure type flow meters (Head flow meters) measure flow rate by introducing a constriction in the flow.

The pressure difference caused by the constriction is correlated to the flow rate using Bernoulli's theorem.

Rate of discharge from the constriction can be calculated by knowing this pressure reduction, the area available for flow at the constriction ,the density of the fluid and the coefficient of discharge Cd.

What is orifice meter?Orifice meter is a flow measuring device used for calculating the pressure drop,flow rate and behaviour of the fluid moving through the pipe.

What is an orifice meter?A restriction in a pipe between two pressure taps; by attaching a pressure gauge the airflow can be determined.

Figure of orifice meter for lab test

Figure of orifice meter in fieldOrifice plates and flanges

Description of Orifice Meter

The main parts of an orifice flow meter are as follows:

A stainless steel orifice plate which is held between flanges of a pipe carrying the fluid whose flow rate is being measured.

The pipe carrying the fluid should be straight in order to maintain laminar flow conditions.

Openings are provided at two places 1 and 2 for attaching a differential pressure sensor (U-tube manometer, differential pressure gauge etc.) as shown in the diagram.

Working principles of the orifice meterAn obstruction (orifice) is placed in a pipe filled with fluid. The pressure of the fluid is measured at two different points.

At the upstream of the orifice, before the converging of the fluid takes place, the pressure of the fluid (P1) is maximum. As the fluid starts converging, to enter the orifice opening its pressure drops. When the fluid comes out of the orifice opening, its pressure is minimum (p2) and this minimum pressure remains constant in the minimum cross section area of fluid flow at the downstream.

The differential pressure sensor attached between points 1 and 2 records the pressure difference (P1 P2) between these two points which becomes an indication of the flow rate of the fluid through the pipe when calibrated

Basic Principle of Orifice MeterWhere, Qa =Flow rateCd = Discharge coefficientA1 = Cross sectional area of pipeA2 = Cross sectional area of orificeP1, P2 = Static Pressures

The orifice plate causes a pressure drop which varies with the flow rate.

Orifice Plate / Restriction Orifice

Orifice plate works based on Bernoullis principle.

Bernoullis equation states that pressure drop across the orifice plate is directly proportional to the volumetric flow rate passing through the orifice plate.The main difference between orifice plate and restriction orifice is their usage. Restriction orifice is used for killing pressure in a pipe by increasing fluid velocity. Restriction orifice and orifice plates are similar in structure and both are based on Bernoullis equation.

What is the Calibration curve of orifice meter ?The calibration curve for an orifice meter will depend on the size of the orifice, the size of the pipe and the pressure loss over the meter.

Typical calibration curves have pressure (or head) loss on the vertical (y) axis and flow rate on the horizontal (x) axis.Calibration Calculation

C - Orifice coefficientD1 - Pipe diameterD - Orifice diameterP1,P2 - Pressure dropV - Orifice volume - densityApplications of Orifice MeterTo measure the flow rate of gases and fluidsThe concentric orifice plate is used to measure flow rates of pure fluids and has a wide applicability as it has been standardized.The eccentric and segmental orifice plates are used to measure flow rates of fluids containing suspended materials such as solids, oil mixed with water and wet steam.

Orifice MeterAdvantages It is very cheap and easy method to measure flow rate.

It has predictable characteristics and occupies less space.

Can be used to measure flow rates in large pipes.Limitations The vena-contracta length depends on the roughness of the inner wall of the pipe and sharpness of the orifice plate.

In the upstream straightening vanes are a must to obtain laminar flow conditions.

Gets clogged when the suspended fluids flow.

The coefficient of discharge is low.

Adds on informationMany factors associated with the pipe, orifice and fluid affect the measurement.

Satisfactory measurement requires steady-state, homogeneous, turbulent flowing fluids.

Other properties which affect the measurement include: the ratio of pipe diameter to orifice diameter and the density, temperature, compressibility and viscosity of the fluid.

Conclusion The largest contribution to the uncertainty in the measured coefficient is due to the time measurement.

The main disadvantage of this meter is the greater frictional loss it causes as compared with the other devices and hence causes large power consumption.

Within the limit of the experimental uncertainty and the Reynolds number range investigated, the result obtained for the discharge coefficient through an orifice plate with the empirical relation