Guidelines for sizing of Restriction Orifice for single-phase fluids

Of many kinds of flow restriction devices, restriction orifices (RO) are frequently used, because they are simple and economical devices. RO is applied to regulate the flow rate or pressure. This article will provide the guidelines for the sizing of Restriction Orifice. It should be noted that this standard practice is applicable to single-phase fluids only.

Inputs Required for Restriction Orifice Sizing

The following is a summary of input data to be prepared for the design of RO:

(1) Operating Conditions

  • Flow rate
  • Upstream & downstream pressure
  • Temperature (for vapor)
  • Line Size

(2) Physical properties

  • For liquid service: density, vapor pressure
  • For vapor service: molecular weight, Cp/Cv, Z-factor, viscosity

(3) Minimum allowable value of cavitation index for liquid service

Output from the Restriction Orifice Design

(1) Single orifice

  • Orifice diameter
  • The pressure at vena-contracta
  • Velocity at orifice
  • Calculated cavitation index (for liquid)
  • Critical or non-critical (for vapor)

(2) Multi-stage orifice

  • Required stage number
  • Orifice diameter of each orifice
  • Distance between adjacent orifice plates
  • Inlet and outlet pressures of each orifice
  • The pressure at vena-contracta of each orifice
  • Velocity at each orifice
  • Calculated cavitation index of each orifice

Principle of RO Sizing Calculation

Flow restriction orifice calculation for Gas Service

Critical Pressure Ratio:

The critical pressure ratio, rc can be obtained from the following equation.

Critical Pressure Ratio for RO Sizing

Orifice Diameter:

The equation for orifice diameter should be selected using equation (2), depending on whether the flow is critical or non-critical.

(A) Critical flow (sonic region)- When the ratio of downstream pressure to upstream pressure, r4, is smaller than or equal to critical pressure ratio, rc, the following equation of orifice diameter for a critical flow should be used.

Orifice Diameter Equation for Critical Flow

(B) Non-critical flow (subsonic region)- The following equation of orifice diameter can be used for non-critical flow region.

Orifice Diameter Equation for Non-Critical Flow

Restriction orifice calculation for Liquid Service:

Orifice Diameter:

Orifice Diameter Equation for Liquid Service

Cavitation Index

In order to avoid cavitation problem, the minimum allowable value of cavitation index, Kd, should be selected based on the following:

(1) Cavitation index Kd=0.37 shall be used for the usual case. At this critical cavitation condition, the noise is steady but still light. No erosion will occur. (Once the orifice chokes and supercavitation occurs, no damage by erosion will exist near the orifice. This is because the damage is caused by the collapse of the cavities and the collapse occurs far downstream during supercavitation)

(2) On some occasion such as the following cases, use Kd=0.93 as an incipient cavitation condition in order to avoid severe economical risk.

  • Material is high grade such as stainless steel or higher and pipe size is larger than 12”.

Interval of Orifices

In the case of the multi-stage restriction orifice, the minimum distance, as shown below, should be provided between orifices, to avoid the reduction in RO performance.

  • For concentric orifice: L ≥ 5.4*D*(1-β)
  • For eccentric orifice: L ≥ D

Deflection Ratio of Eccentric Orifice

The deflection ratio of the eccentric orifice “e” is 0.75.

e = 2δ /(D − d0 ) = 0.75 (14)

Where,

  • δ: Pipe center to orifice center length (m)
  • dO: Orifice hole diameter (m)
  • D: Pipe inside diameter (m)
Image showing Deflection Ratio for Eccentric Orifice
Image showing Deflection Ratio for Eccentric Orifice

Restriction Orifice Plate Sizing Calculation Procedure

A. Single RO system-

① Decide the minimum allowable value of the cavitation index to meet a given situation.

  • Kd = 0.37: for usual case
  • Kd = 0.93: for conservative case

② Assume dO and calculate β by equation (8).

③ Incompressible coefficient of contraction CCi shall be calculated by equation (7).

④ Orifice diameter (dO) shall be calculated by equation (13).

⑤ Compare the assumed d0 with the calculated dO in step ④.

⑥ Calculated dO shall be replaced with previous dO and repeat step ② to step ⑤ until the dO agrees with calculated dO.

Calculate cavitation index, Kd by equation (15), and compare with minimum allowable value.

If cavitation index ≥ 0.37 (or 0.93), then the orifice diameter is acceptable.

If the cavitation index < 0.37 (or 0.93), then a single orifice is unable to accommodate the required pressure drop. In that case, a multi-stage orifice system should be applied.

B. Multistage Restriction Orifice Calculation

① Decide the minimum allowable value of the cavitation index to meet a given situation.

  • Kd = 0.37: for usual case
  • Kd = 0.93: for conservative case

First stage orifice: located at the outlet

② Assume upstream pressure of the first stage orifice.

③ Assume dO and calculate β by equation (8) for the first stage orifice.

④ Incompressible coefficient of contraction CCi shall be calculated by equation (7).

⑤ Orifice diameter (dO) shall be calculated by equation (13) using assumed upstream pressure.

⑥ Compare the assumed d0 with the calculated dO in step ⑤.

⑦ Calculated dO shall be replaced with previous dO and repeat step ③ to step ⑥ until the dO agrees with calculated dO.

⑧ Calculate cavitation index, Kd, using Equation (15), and compare with minimum allowable value.

If cavitation index > 0.37 (or 0.93), increase the upstream pressure and repeat the steps from ③ to  ⑧.

If cavitation index < 0.37 (or 0.93), decrease the upstream pressure and repeat the steps from ③ to ⑧.

If the cavitation index equals to or slightly bigger than 0.37 (or 0.93), the design of the first stage RO is completed and go to step ⑨.

n-th stage orifice

⑨ Set the upstream pressure of (n-1)-th stage orifice for the downstream pressure of n-th stage orifice.

⑩ Assume the upstream pressure of n-th stage orifice.

⑪ Repeat the steps from ③ to ⑧, until the cavitation index is equivalent to the minimum allowable value.

Special Consideration of RO Design:

(1) The minimum hole diameter of RO-

To prevent the plugging problem with RO caused by debris, the hole diameter should be greater than the following values:

  • For the clean liquid service: 2mm
  • For the clean Gas service: 1mm

When the diameter smaller than the above values is required, the strainer or filter to remove debris should be provided upstream of RO.

(2) The necessity of minimum straight run length-

Basically, the objective of RO is rough control of flow rate and should not be used for strict control of flow rate. Therefore, it should not be necessary to take a straight run of piping both upstream and downstream of RO to keep performance. However, for the erosional services such as slurry or flush services, countermeasures for erosion shall be considered.

(3) The calculated hole diameter of RO should be rounded to the conservative size for easy manufacturing.

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

3 thoughts on “Guidelines for sizing of Restriction Orifice for single-phase fluids

  1. Thanks for the very useful info.

    I have a doubt for Multistage RO, Is the flow rate changes after each stage or will it be the same at final stage?

    In case If any changes occur, how to calculate the flow rate at each stage?

    Regards,
    Jagan

  2. Thank you for posting this. Can you define what “k” and “C1” are referring to exactly? Also, is this a dimensionless equation? If not, what are the units for length and pressure?

  3. Hi. Thanks for posting.
    I have started using RW Miller flow consultant. I need some help understanding the sizing orifice bore in cavitating flow and thick orifice in cavitating flow. How are these method differs in sizing the bore in the sizing of restriction orifice?

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