👉 " Three Times Better (Engineer) "

The 3 Things You will learn are as follows.

👉 One Engineering Question

👉 One Engineering Standard

👉 One Career Tip from Top Professionals

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Before we start :

If you would like to access PDF of the "Instrumentation Engineers guide to Orifice Flowmeter Sizing"

then reply "Orifice Sizing PDF"

And I will send it to you.

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#1 -Engineering Question

👉 "Hi Asad,
​
How do we perform Orifice Plate Sizing ? I am confused since there are multiple parameters - Is there a Step by Step Process ?"

Orifice plate sizing is an iterative process.

• I start with the process data.
• I select an initial differential pressure.
• I calculate the orifice bore.
• I check the beta ratio and permanent pressure loss.

If the result is not acceptable,

I change the selected DP and repeat the calculation.

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Step 1: Collect the Input Data

I begin with the latest approved documents.

From the P&ID

I collect:

• Flow instrument tag number
• Line number
• Nominal line size
• Design pressure
• Design temperature
• Flow direction
• Available straight-pipe length

From the Pipe Class Document

I collect:

• Pipe schedule
• Pipe material
• Flange rating
• Flange material
• Actual pipe internal diameter

The actual pipe internal diameter is important.

I do not use only the nominal pipe size.
Different pipe schedules have different internal diameters.

From the Process Datasheet

I collect:

• Fluid name
• Fluid phase
• Minimum flow
• Normal flow
• Maximum flow
• Operating pressure
• Operating temperature
• Density
• Dynamic viscosity
• Vapour pressure for liquids
• Molecular weight for gases
• Compressibility factor,
• Specific heat ratio,
• Allowable permanent pressure loss
• Base pressure and temperature, if applicable

The fluid properties must correspond to the flowing conditions.

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Step 2 : Select the Calibrated Flow Range

I establish the calibrated maximum flow.

A typical starting point is:

This provides a 20% margin above the expected maximum flow.

However, the margin must follow the project specification. (If it is specially mentioned)

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Step 3 : Check the Required Rangeability

I calculate the required rangeability:

For example:

​
Max Flow = 100 GPM

Min flow = 20 GPM

A conventional orifice flowmeter normally performs best over a limited flow range.

above 4:1 or 5:1 the orifice rangeability is not good.

Due to sqaure root relationship.

Hence if higher rangeability then we might need 2 flow transmitters or a different flow technology.

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Step 4 : Select the Orifice Standard

Then I confirm the applicable project standard for sizing.

Common standards include:

• ISO 5167
• ASME MFC-3M
• AGA Report No. 3

The standard determines:

• Orifice plate construction
• Pressure tapping arrangement
• Discharge coefficient
• Beta-ratio limits
• Reynolds-number limits
• Straight-run requirements
• Calculation uncertainty

The selected calculation method must match the actual installation.

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Step 5 : Select the Initial Differential Pressure

I select an initial maximum DP.

A common starting value is:

Other trial values may include:

• 1000 or 1250 mmH₂O
• 2500 mmH₂O
• 3000 mmH₂O
• 5000 mmH₂O

The initial value is only a starting point.

The final DP depends on:

• Required flow range
• Minimum-flow accuracy
• Allowable pressure loss
• Available process pressure
• Calculated beta ratio
• DP transmitter capability

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Step 6. Calculate the Orifice Bore

I enter the process data into the approved sizing software.

The software calculates the required orifice bore.

The basic mass-flow equation is:

Where:

• = mass flow
• = discharge coefficient
• = expansibility factor
• = orifice bore area
• = upstream fluid density
• = differential pressure
• = beta ratio

The orifice bore area is:

The beta ratio is:

Where:

• d = orifice bore diameter
• D = pipe internal diameter

The sizing software performs an iterative calculation.

This is necessary because depends on:

• Beta ratio
• Reynolds number
• Pressure-tap arrangement
• Applicable calculation standard

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Step 7 : Check the Beta Ratio

After calculating the bore, I check the beta ratio:

For this sizing workflow, I use the following preferred range:

This is a practical project range.

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Case A: Beta Ratio Is Acceptable

If:

I continue to the permanent pressure-loss check.

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Case B: Beta Ratio Is Greater Than 0.70

If:

the orifice bore is too large relative to the pipe diameter.

The generated DP signal may be too low.

My action

I increase the selected DP.

For example:

A higher selected DP requires a smaller bore for the same flow.

Therefore:

I then repeat the sizing calculation.

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Case C: Beta Ratio Is Less Than 0.20

If:

The orifice bore is too small relative to the pipe diameter.

This may create:

• High local velocity
• High permanent pressure loss
• Greater plugging risk
• Erosion risk
• Cavitation risk in liquid service

My action

I decrease the selected DP.

For example:

A lower selected DP requires a larger bore for the same flow.

Therefore:

I then repeat the sizing calculation.

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Step 8. Check the Permanent Pressure Loss

Once the beta ratio is acceptable, I check the permanent pressure loss.

The measured DP is not the same as permanent pressure loss.

Some pressure recovers downstream of the orifice plate.
The unrecovered portion is the permanent pressure loss.

I compare the calculated permanent loss with the process allowable limit.

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If the Permanant Pressure Loss Is Acceptable

I continue with the remaining sizing checks.

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If the Pressure Loss Is Too High

I reduce the selected maximum DP.

A lower DP normally gives:

• A larger orifice bore
• A higher beta ratio
• A lower permanent pressure loss
• A weaker measurement signal

I then repeat the calculation.

The new beta ratio must also remain acceptable.

If the pressure-loss and beta requirements cannot both be met, I consider another solution.

Possible alternatives include:

• Changing the meter-run diameter
• Using a flow nozzle
• Using a Venturi meter
• Using a vortex flowmeter
• Using an ultrasonic flowmeter
• Using a Coriolis flowmeter

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Step 9. Check the DP at Normal and Minimum Flow

I calculate the expected DP at different flow rates.

The relationship is

​

Example

Assume:

• Maximum flow = 100 m³/h
• Normal flow = 70 m³/h
• Minimum flow = 20 m³/h
• Maximum DP = 2500 mmH₂O

DP at normal flow

DP at minimum flow

​
​
​

I verify that the transmitter can accurately measure the minimum DP.

I check:

• Transmitter calibrated span
• Transmitter range limits
• Reference accuracy
• Zero stability
• Static-pressure effect
• Ambient-temperature effect
• Impulse-line effects
• Required flow uncertainty

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Step 10. Check the Reynolds Number

I check the Reynolds number at:

• Maximum flow
• Normal flow
• Minimum flow

The minimum flow is often the critical condition.

At low flow, Reynolds number decreases.
The discharge coefficient may also change.

If Reynolds number falls outside the standard’s limits, the expected calculation accuracy may no longer apply.

A sizing result is not acceptable only because the beta ratio is correct.

The Reynolds number must also comply with the selected standard.

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Step 11: Perform Fluid-Specific Checks

For Liquid Service

I check:

• Vapour pressure
• Upstream pressure
• Downstream pressure
• Vena-contracta pressure
• Cavitation risk
• Flashing risk
• Erosion risk

The lowest pressure occurs near the vena contracta.

This pressure may be lower than the downstream tapping pressure.

If the local pressure approaches the liquid vapour pressure, cavitation or flashing may occur.

In that case, I consider reducing the selected DP.

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For Gas and Steam Service

I check:

• Upstream absolute pressure
• Operating temperature
• Fluid density
• Compressibility factor
• Isentropic exponent
• Expansibility factor
• DP-to-static-pressure ratio
• Velocity or Mach number, when required

The expansibility factor corrects for the change in density through the restriction.

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Step 12. Check the Installation Requirements

A correct bore does not guarantee accurate measurement.

The physical installation must also comply with the selected standard.

I check:

• Upstream straight-pipe length
• Downstream straight-pipe length
• Elbow locations
• Control-valve location
• Reducer and expander locations
• Thermowell location
• Pressure-tap type
• Pressure-tap orientation
• Impulse-line routing
• Plate orientation
• Flow direction
• Drain or vent requirements

If the available straight length is inadequate, I consider:

• Relocating the meter
• Installing a flow conditioner
• Using a conditioning orifice
• Selecting another meter technology

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Step 13. Select the DP Transmitter

I select the DP transmitter after finalising the orifice DP.

The transmitter must cover:

• Maximum calibrated DP
• Expected operating DP
• Startup conditions
• Process overrange
• Maximum static pressure

I avoid selecting an unnecessarily wide transmitter range.

An oversized range can reduce performance at low DP.

I also check:

• Maximum working pressure
• Static-pressure rating
• Overpressure limit
• Wetted-part material
• Diaphragm material
• Process connection
• Manifold arrangement
• Environmental protection
• Hazardous-area certification

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Quick Decision Guide

Beta is greater than 0.70

Action: Increase the DP.

Expected result:

• Bore decreases
• Beta decreases
• Signal increases
• Permanent pressure loss may increase

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Beta is less than 0.20

Action: Decrease the DP.

Expected result:

• Bore increases
• Beta increases
• Signal decreases
• Permanent pressure loss normally decreases

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Permanent pressure loss is too high

Action: Decrease the DP.

Then:

• Recalculate the bore
• Recheck beta
• Recheck minimum-flow DP
• Recheck transmitter performance

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Minimum-flow DP is too low

Possible actions:

• Increase the selected maximum DP
• Use a lower DP transmitter range
• Use two DP transmitters
• Reduce the calibrated maximum flow
• Select another flowmeter technology

Any increase in DP must be checked against the allowable permanent pressure loss.

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Final Summary

I perform orifice sizing as a calculation loop.

1. I collect the process and piping data.
2. I establish the calibrated flow range.
3. I select an initial DP.
4. I calculate the orifice bore.
5. I check the beta ratio.
6. I check the permanent pressure loss.
7. I check the DP at minimum flow.
8. I check Reynolds number.
9. I check fluid-specific limits.
10. I check the installation.
11. I select the DP transmitter.
12. I repeat the calculation if any check fails.

The main DP adjustment rules are:

• The DP signal must be large enough to measure.
• The permanent pressure loss must remain acceptable with Process requirements.

#2- Engineering Question

For Orifice Flowmeter which tapping should i select for line size of 1/2 to 1 "

Option 1 :

• Corner Taps
• Flange Taps
• Pipe Taps

Right Answer gets feature next week to 10,000 Engineers in the newsletter.

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#3 -Career Tip

If you had to choose between.

• The Passion to learn.
• And IQ (or Smartness)

If woudl always choose Option 1.

If you have the Passion to learn you can outlearn and smart engineer.

Period.

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Please learn them in the series presented for better understanding.

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Until next week,

Happy Learning and Keep Smiling.

Asad Shaikh

Your Well Wisher and Instrumentation Friend!! 😊

See you next week!

PS: If you would like to access "Instrumentation Engineers guide to Orifice Flowmeter Sizing"

Then reply "Orifice Sizing"

And I will send it to you.