What is Orifice Plate Flow Meter and how it works?

What is Orifice Plate Flow Meter ?

An Orifice Plate Flow Meter is the name given to a form of flow meter that is used for the measurement of the flow rate of Gas or Liquid, particularly steam, with the help of the principle of Differential Pressure Measurement. It is used primarily for strong applications, given that it is very cheap to buy and reputed for its durable construction. This type of meter is used commonly in steam, gas and clean liquid services. It can be availed for pipes of all sizes, although it is very economical for measuring flows in larger pipes. Many organizations also approve the use of this meter for custody transfer of gases and liquids. Find out how this meter works.

Orifice Meter – What is the Basic Working Principle?

It comprises of an Orifice plate that is its basic element. The bore of the plate can be made in numerous configurations, to deal with different types of flow measurement tasks. The flowing conditions have to be assessed to determine which the most appropriate configuration is for each type of measurement job.

Once this Orifice plate is put in a line, there is development of a differential pressure across the Orifice plate. It is a linear drop in pressure, which is directly proportional to the flow rate of the gas or liquid. Once there is placement of an orifice plate in a pipe that transports the fluid – the flow rate of which has to be measured – the orifice plate results in a drop in pressure that varies with the rate of flow. This reduction in pressure is measured with a differential pressure sensor. When it is adjusted finely, the drop in pressure can help measure the flow rate.

What Is Its Working Mechanism?

Once the fluid comes closer to the orifice, there is a slight rise in pressure and then an abrupt drop as the orifice gets passed by. It goes on dropping until it reaches the “vena contracta”. Gradually, it rises until at around 5 – 8 diameters in the downstream direction an optimal point of pressure is reached. The orifice pressure will be lower at this point as compared to the upstream pressure of the orifice.

As the fluid moves through the orifice, the drop in pressure is due to the higher velocity of the gas that passes through the orifice’ narrowed section. When there is reduction in velocity as the fluid gets out of the orifice, there is a rise in pressure and it finally goes back to its actual level. All the loss of pressure does not get recovered due to losses of turbulence and friction in the stream.

The drop in pressure across the orifice rises when there is an increase in the flow rate. No differential is there when no flow is there. The differential pressure happens to be directly proportional to the velocity square. Thus, it stands to reason that in case the other factors stay constant, the square of the flow rate can be directly proportional to the differential.

What is Venturi Flow Meter and how it works?

What is Venturi Flow Meter ?

A Venturi flow meter refers to a device that is used for the measurement of fluid discharge or flow rate via a pipe. This is a differential producing flow meter and is used for many purposes and in many areas. It is one of the commonest flow meters due to its reliability and simplicity of operations. It does not lead to abrupt restrictions in flow and does not include any moveable components. Thus, this meter is able to measure the flow rate of fluid with a minimal reduction in total pressure.

Working Principle

This mechanism of action of this type of flow meter makes use of Bernoulli’s equation, which states that there is a drop in pressure with a rise in velocity. The principle for this meter was developed for the first time in 1797 by G.B, Venturi although it was not until 1887 with C. Herschel that the principle began to be considered.

According to this principle when there is a decrease in the cross-sectional area of the fluid’s flow, a difference in pressure is developed between the various areas of the flow. This aids in measuring the pressure difference. With the aid of this difference in pressure, the discharge in flow can be measured easily. Bernoulli’s principle mentions that the potential energy of the fluid reduces with a rise in fluid velocity. Reducing the pressure of fluid in those areas where there is higher flow velocity is referred to as Bernoulli Effect.

How Does the Venturi Flow Meter Work?

It has a basic style of working. The flow meter actually operates on the Bernoulli principle – that states that pressure drops when there is a rise in velocity. The same principle is made use of here. The throat’s cross section happens to be smaller as compared to the inlet pipe’s cross-section area. Due to this reason, the flow velocity at the throat area is higher than the inlet section. It occurs as per the continuity equation.

The rise in flow velocity at the throat will lead to reduced pressure at the section. Due to this reason, there is a difference of pressure between the venturimeter throat and the inlet. This variation in pressure can be measured with ease by making use of a differential manometer between the throat and the inlet area or through the use of two isolated gauges at throat and inlet. When you measure the varied pressure at the two varied sections, you can easily calculate or measure the rate of flow through the pipe.

It is important to note that one can install a Venturi flow meter at varied locations inside a pressure reducing flow system. Each of these locations has its own pros and cons. You can set it up as the inlet component to a flow system. Such an arrangement is frequently used as a calibration standard for a Constant Volume System (CVS) flow stand during emissions testing. The Venturi flow meters that are used in such applications are called Smooth Approach Orifices or SAOs.