ORIFICE PLATE METERING STATIONS
The orifice plate is small and inexpensive and the simplest flow-path restriction used in flow detection and measurement in pipe systems. Measurement of pressure drop across an orifice plate is one of the most widely accepted methods of determining flow.
Normally, orifice plates are flat plates mounted between flanges where the thickness is small in comparison to the pipe diameter but when the orifice plate is provided with its own integral carrier as a single piece item, it is frequently referred to as a metering station, particularly in the HVAC industry sector.
There are no moving parts and since the design is based on the fundamental physical Bernoulli principle, high reliability is assured. The pressures on both sides of the orifice are measured resulting in a differential pressure which is proportional to the flow rate.
As the fluid passes through the sharp edged orifice the fluid converges and velocity increases to a maximum value at which point the pressure is at a minimum value. As the fluid diverges downstream to fill the entire pipe area, the velocity decreases back to its original value and the pressure increases to about 60%-80% of the original input value. The pressure loss is irrecoverable therefore the output pressure will always be less than the input pressure. It is assumed that the flow is steady and incompressible through the orifice metering station, velocities upstream and downstream are uniform and there are no friction losses.
An orifice plate metering station is a very simple device installed in a straight run of pipe where the orifice bore is smaller than the pipe bore. Traditionally, a tab handled type orifice plate is mounted between two flanges and tapping points for pressure measurement are provided in the flanges but alternatively, the orifice plate can be fitted with its own carrier plates which have pressure tappings and installed between pipe flanges.
The orifice plate metering station has an advantage over traditional orifice plates such as the tab handled type because it is a single piece item with corner type tapping points included as integral features. Installation of the orifice metering station is very simple and is mounted wafer style between two flanges and within the bolt circle thereby centralising around the circumference of the metering station body. Two screw in extension pieces provide ease of access to the pressure tapping holes outside the main flange diameter.
Orifice plate metering stations should be fitted in straight pipe runs to avoid disturbance of flow patterns. For best accuracy the recommended installation is with a minimum 20 pipe diameters upstream and 10 diameters downstream of smooth straight pipe. 12D upstream and 6D downstream will give a typical accuracy of ±2% while 6D and 3D will give ±4% depending on the pipe layout. Localised installation conditions will determine what is practically achievable but 5D and 2D should be regarded as the absolute minimum which will typically give ±5%.
The orifice plate metering station is designed and manufactured to meet the requirements of British Standard BS 1042 and International Standard Organisation ISO 5167 and the performance requirements of BS 735