Differential Pressure Flow meters calibration

Differential Pressure Flow meters calibration

Differential pressure type flow meters provide the best results where the flow conditions are turbulent. Some of the most common types of differential pressure flow meters are:

•Orifice plate

•Venture tube

•Elbow 

•Pitot tube

Advantages

•They are simple to use

•They have no moving parts

•They are sturdy

•They are available in a wide selection of ranges models 

Disadvantages

•They tend to have low accuracy due to the wear of the primary element.

•Some have a high permanent pressure loss.

Orifice Plates

Orifice plates in various forms are the most widely used primary elements and consist of a flat piece of metal with a sized hole bored in to it.

When fluid through the orifice its velocity increases, resulting a drop in pressure and an increase in turbulence. After the fluid has passed through the orifice its velocity decreases again, causing an increase in pressure although only some of the pressure loss is recovered. The amount of pressure recovery can be up to 50% of the total pressure drop across the orifice plate. 

The flow of liquid through the orifice plate creates a differential pressure across it, in such a way that the faster the flow the larger the pressure drop.

The components of a typical orifice plate installation are:

•Orifice plate and holder 

•Orifice taps 

•Differential pressure transmitter 

•Flow indicator / recorder 

Orifice plates and holders

Orifice plates are usually installed between special flanges in a horizontal pipe run. The flanges are thicker than normal to accommodate two small bore tapings for connection to a DP cell. The plate is positioned concentrically within the flange bolt circle with the tap protruding near the top of the flanges. The tab has a hole in to indicate that it is an orifice plate and not a pipe blank. Orifice plates have information engraved on them to indicate the correct upstream / downstream orientation. 

Orifice taps

There are 4 common arrangements of pressure taps: 

•Flange taps

Flange taps are the most popular because their distance from the orifice plate is precisely controlled.

•Vena Contracta

Vena contracta taps are located to obtain the maximum differential pressure across the orifice.

•Corner taps 

Corner taps are located at each side of the orifice plate and are good for pressure measurements in pipes less then 50 mm diameter.

•Pipe taps 

Pipe taps measure the permanent loss of pressure across an orifice.