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**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.

**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.

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

### Restriction orifice calculation for **Liquid Service:**

**Orifice Diameter**:

**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)

## 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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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

Mass flow will always remain same at every stage. Law of conservation of mass will apply.

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?

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?

Hello,

I am a mechanical engineer that was worked on steam turbines with ABB, ALSTOM & SIEMENS. I graduated 30 years ago.

I was asked to calculate the size of an orifice to reduce steam pressure from 600 psig to 20 psig. Reviewing my old Fluids Book from college I only find stuff for fluids, not steam. Can you direct me to something that deals with steam?