Restriction Orifice Plate Thickness Calculation

While calculating the thickness, it is advisable to follow 'Flowel' specified thickness for computer calculation or to use R.W. Miller's equation for manual calculation, highlights Meghdut Manna.

Thickness of orifice plate is generally selected as per the guidelines given by ISO-5167 / ISA-RP-3.2 / API-2530 / IS-2952 / ASME-MFC-3M. In fact, these are meant for measurement orifice plate. In general, these standards specify the following thickness depending on the line sizes only(Note 1).

Line Size in Inch	Thickness in mm
< 6 > 8 < 16 > 18	3.18 6.125 9.53

None of the above standards give any clear guideline for the thickness of the restriction orifice plate, which leads us to follow the same, as given for measurement orifice plate. This is an attempt to highlight the importance and calculation guidance for the restriction orifice plate thickness.
Importance of Thickness
Calculation Orifice plate thickness should be sufficient to prevent the plate from undue stress under operating conditions for restriction orifice and should not exceed the deflection required from the metering standards for measuring orifice plates. If the differential pressure across the orifice increases, the plate tends to deform elastically. For measurement orifice plate, beyond a certain point, this deformation will result in a shift in the characteristics of the meter, resulting in increased measurement uncertainty. AGA -3, Appendix- 2-F (American Gas Association) provides the guidelines for using high differential for measurement natural gas with orifice plate. This maximum limit is dependent upon the thickness, diameter and b ratio. The relationship(Note 2) demonstrates that for a given b ratio, the allowable differential pressure is a function of the diameter and plate thickness. For a given line size, there is always a maximum allowable differential pressure on the plate e.g. for 2" size, the maximum allowable DP is 1000 MPa in WC ("2.5 bar) with minimum thickness of 3.2 mm.

Sonic Flow and Restriction Orifice Thickness
Thickness of the restriction orifice plate needs careful consideration as the differential pressure across the orifice plate is generally higher as compared to measurement orifice, particularly for the sonic flow application where high differential pressure is desired to be maintained to achieve the sonic condition(Note 3).
Sonic flow through restriction orifice plate in quite common in chemical process plants as well as oil and gas industries. The design of the restriction orifice plate will be such that sonic flow is maintained(Note 4). Restriction orifice bore selection(Note 5) for sonic conditions are generally done on trial and error basis in process simulation software. Upstream pressure of the orifice plate is calculated in such a way that flow through the restriction orifice is sonic until the vessel pressure has reached 50% of its design pressure. When a gas flows from one point to other through an orifice, it is obvious that as the down pressure is reduced, the flow rate steadily increases until the velocity at the orifice reaches sonic velocity. Any further reduction of downstream pressure, the flow rate is unaffected and the excess pressure is dissipated in shock waves downstream of the orifice. It should be noted that square
root formula used for normal orifice flow measurement is not valid for sonic flow.

Thickness Calculation Formula
Thickness of any orifice plate, as defined in the R.W.Miller's handbook, depends on the following parameters l Line size
Differential Pressure
Yield Stress of Plate
ß Ratio

cont....

To Read Further Subscribe Your PRINT COPY Today.