Orifice Performance

Principally, an orifice plate is a precision instrument. In best circumstances, the inaccuracy of Orifice plates can possibly fall in the range of 0.75-1.5% AR.

Orifice

However, there are numerous error causing conditions which can terribly affect the accuracy of an Orifice plate…

Following factors are used to judge the performance of an Orifice plate:

  • Precision in the bore calculations
  • Quality of the installations
  • Condition of the plate itself
  • Orifice area ratio
  • Physical properties of the fluid flow under measurement

Further class of installation depends upon following factors

  • Tap location and circumstance. Generally, there are three ways to position a pressure tap.
  • Provision of the process pipe
  • Competence of straight pipe runs
  • Gasket intervention
  • Misalignment of pipe and orifice bores
  • Lead line design

Extra detrimental conditions consist of

  • Dulling of the sharp edge or nicks caused due to corrosion or erosion.
  • Warpage of the plate because of waterhammer and dirt.
  • Grease or secondary phase deposits on any of the orifice surface.

Any of the above said conditions has the tendency to affect the discharge coefficient of an orifice plate to a large extent

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In case of metering systems with compliance to AGA 3, other measurement errors can occour due to internal surface roughness (i.e. as per AGA 3, if Beta ratio is 0,6 for a 10" line, minimum roughness shall be 34 micro inch and maximum 250 micro inches).

Apart from fiscal metering application, other things that can create measurement errors are relevant to installation: quantity of pipe curves before and after the element, lenght of upstream and downstream pipe, valves or other inline equipments, flow conditioners ect.

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