๐ " Three Times Better (Engineer) "
The 3 Things You will learn today are as follows.
โ One Engineering Learning
Methods for 2 Phase Control Valve Sizing.
โ One Engineering Question
What Standard to Use for 2 Phase Sizing.
โ One Career Tip from Top Professionals
A common thing between Bill Gates & Elon Musk that we can learn.
I am super excited for today's learnings.
So let's get started.
#1 -Engineering Learning
A student asked a great question.
Inside the Advanced Control Valve 2025 cohort.
How do we handle Two-Phase Sizing?
The conventional approach seems easy:
โ Calculate the Liquid Cv.
โ Calculate the Gas Cv.
โ Add them for Cumulative Cv.
Is this really the best method?
It sounds logical on paper.
But fluid dynamics is tricky.
Btw : ACV 2026 will open door on 1st July, If interested join here).
Okay lets explore this 2 Phase Sizing below
Is there a Standard for 2 Phase Sizing ?
Sizing two-phase flow is an engineering nightmare.
ISA and IEC standards officially surrender here.
They do not provide a unified mathematical equation.
Why?
The Answe is Slip Velocity.โ
Because liquid and gas refuse to cooperate.
Gas expands rapidly as pressure drops.
Liquid maintains its volume.
Gas travels much faster than the liquid.
We call this phenomenon Slip Velocity.โ
When phases mix....
The Speed of Sound is lower in Mixed Phase.
(It means a bad thing for us engineers).
It drops lower than both individual pure phases.
This causes premature, destructive Choked Flow.
By the words itself you can understand.
This is not good, my friend.
โ
Standard single-phase equations cannot predict this accurately.
2 Phase Sizing (3 Methods)
Method 1: The Sum of Cvs Method
This is the most common, fundamental approach.
Total CV = Liquid CV + Gas CV
Many basic sizing software tools default to this.
The Concept:
โข You calculate the Liquid CV separately using liquid equations.
โข You calculate the Gas CV separately using gas equations.
โข You simply add both values together to get total Cv.
The Danger:
โข It completely ignores thermodynamic phase interaction.
โข It assumes gas and liquid do not disrupt each other.
โข It almost always dangerously undersizes the valve.
If you read the control valve sizing guide you will realise that this is not good.
Thus use this only for rough, preliminary field estimates only.
Method 2: Homogeneous Equivalent Density
This is mathematically much more rigorous.
Top valve manufacturers utilize variations of this theory.
The Concept:
โข 2-phase mix is treated as one single pseudo-fluid.
โข You calculate an Effective Density of the entire mixture.
โข You combine mass fractions and individual specific volumes.
โข You plug this new density into a modified sizing equation to get CV.
The Danger:
โข It assumes perfect mixing without phase separation.
โข It assumes both phases travel at the exact same velocity.
โข It fails if the flow regime becomes stratified in piping.
โข It requires highly accurate upstream process compositions.
Method 3: The Omega Method
This is the absolute gold standard for rigorous sizing.
It originated in relief valve sizing but applies here.
The Concept:
โข It tracks the exact thermodynamic expansion path.
โข It calculates a specific Omega Parameter.
โข This parameter measures exactly how much the fluid expands.
โข It is the most accurate way to predict two-phase Choked Flow.
Elite sizing tools / engineers use Methord 2 or 3
(But rarely Method 1).
This would be too complex to discuss more here but we will try to have a dedicates session in ACV 2026 to discuss in depth.
#2- Engineering Question
Question of last Week
If I have a Dirty Service (Particle Size greater than 30) and Differential Pressure of 80 Bar and its green field project.
What Type of Control Valve Should I select
- Angle Valve
- Butterfly Valve
- Multi Path Control Valve
- Multi-Stage Control Valve
- Diaphragm Type Control Valve
The Fastest right answer was given by Jayant Parateโ
โLinkedin Profile.โ
Here's the Correct Answer from him.
Angle Valve - to be used, since it will handle the large pressure drop plus will not get clogged while in service, thereby ensuring process continuity and control desrised.
โ
โButterfly Valve - not to be used, will not get desired controllability plus suspended particles may also damage the valve disk.
โ
โMulti Path Control Valve - not to be used, will handle pressure in small interval of complete pressure drop, but will not be able to handle the big suspended particles.
โ
โMulti-Stage Control Valve - same as above for multi path valve
โ
Diaphragm Type Control Valve - not to be used, since not able to handle such a big pressure drop. Also over a period of time the suspended particles will wear out the diaphragm.
Question of this Week
Is valve Outlet Velocity in Control Valve Sizing
Only to be considerd for Liqid services or also applicable for gases ?
If you think it applicable to Gas then what numbershould it be.
Want to be a Control Valve Subject Matter Expert in your company.
ACV is currently sold out.
But you can Hop on the Waitlist of ACV 2026 here.โ
#3 -Career Tip
Bill Gates and Elon Musk had one thing in common.
They would say many times in their biographies that would not know a particular stuff and that it is figureoutable.
Figureoutable.
This seems a small word,
But the impact is very heavy.
Whenever you want to learn something new.
Say to YOURSELF.
I dont know this YET but this is FIGUREOUTABLE.
Proud of you !!
Keep learning and Growing to be World Class Engineer!!
How was today's learning
- Highly Valuable : Reply 1
- Somewhat Valuable : Reply 2
- Needs Improvement : Reply 3
I see each feedback reply as a gift.
Until next week,
Happy Learning & Keep Smiling.
Asad Shaikh
Your Instrumentation Friend!! ๐
See you next week!
PS: If you would like 23 Point Control Valve Sizing Excel Checklist then please reply "Sizing Checklist" and I will share the same with you instantly.
โ