Basics of Orifice plate pressure transmitter

instrumentation
flowmeasurement

#1

orifice plate

An Orifice plate is a differential pressure transmitter used to measure the flow rate of fluid passes through a pipeline. A differential pressure transmitter makes a restriction in the flow channel which makes a pressure difference in the channel is measured to calculate the flow.

An orifice is a flat disc with a hole in it as shown the figures.

Construction:

The orifice is fixed perpendicular to the pipeline where the pressure should be measured.

The front edge of the orifice plate is sharp and back edge is chamfered.

Tap holes or Flange gaps are drilled in the pipelines just before and after the orifice plate. These holes are provided to connect the differential pressure sensors, such as a manometer or differential pressure gauge.

Operation:

When the fluid comes out of the orifice plate’s opening, its cross section is minimum and uniform for a particular distance and then the cross section of the fluid starts diverging in the downstream.

The pressure at the upstream is represented as p1 and pressure at downstream is p2.

As the fluid enters the orifice the pressure drops because the fluid start converging. There will a maximum pressure at the upstream and minimum at downstream.

This minimum cross-sectional area of the fluid obtained at downstream from the orifice edge is
called VENA-CONTRACTOR.

The pressure difference between the upstream pressure p1 and downstream pressure p2 is measured using a differential pressure gauge. The pressure difference is proportional to the flow rate of the fluid.

Basic types of orifice plate

Concentric:

This is the standard orifice plate. The vent hole is for gases in a liquid flow. The gas bubbles present in the liquid escapes through the vent hole.

The drain hole is for liquids in a gas flow measurement. Liquid present in the gases condensates at the drain hole.

Eccentric:

This is used when the flow of thick dirty liquids is being measured, e.g. engine cooling oil etc.

Segmental:

This is used when the liquids contain solids, e.g. cement. Solid parts in the liquid usually travel through the bottom of the channel due to gravity. Accumulation problems can be avoided by using this plate.

Advantages

  • Simple operation

  • Reliable for a long time.

  • Less expensive

  • No moving parts

Disadvantages

  • Square root relationship

  • Difficult to install

  • Range of measurement small. The operator has to change the plate to change the range.