Restriction Orifice Sizing


Thread Starter

Sanjay Varma

Dear Members,

I need to select a Restriction Orifice (RO) to meet the process conditions below.

1) Fluid: Gas
2) Normal Flow: 397111 Nm3/h
3) Upstream Pressure: 152.4 kg/cm2
4) Diff. Pressure: 150.9 kg/cm2
5) Operating Temp.: 56 deg C
6) Density: 21.82 kg/m3
7) Viscosity @temp.: 0.01
8) Molecular weight: 5.196
9) Gas "Z": 1,08
10) Line size: 10", Sch160 CS(A106) pipe
11) Orifice material: SS316

However I have some questions regarding the type and calculation basis of the above particular RO and I would like to proceed the work after verification. Please comment with your valuable experience.

1. Possibility of Single Stage Restriction Orifice (RO).
Whether single stage orifice would be applicable or not. We could ignore big noise due to high velocity since this RO would apply only emergency depressuring of system pressure.

2. In case of Multi-stage RO considering high Velocity:
1) Bore Sizing Basis
- Background of sizing basis ex) AGA, ISO 5167 or any other code/standard.

2) Example (Former executed project...)
- Bore Calculation sheet

Note: Frankly speaking, I have doubt bore size of first and last stage.

In other words, bore size of all stage should be same or bigger down the line in sequence from inlet to outlet up to the last stage considering volume of gas at each stage.

Please do reply... Thanks in advance.


Sanjay Varma
Dear Sanjay,

For this application I suggest you do not try a restricting orifice. Calculation of the discharge coefficient is not straightforward, and the edges will wear relatively quickly making for poor flow accuracy. A much better approach is to use a truncated Critical Flow Venturi (CFV). A CFV discharge coefficient can be approximated to +/-1.5% using traditional standards such as ASME/ANSI MFC-7M-1987, etc. Since the throat diameter is only about 45mm the cost is not excessive. Also, the CFV geometry will not deteriorate significantly over time making for a very accurate long lived device. If you are interested contact me for quotation on a CFV for the application.


John Catch