Cage in globe valves 2

[Maddyp]

Hi all,
I want to automate the process of cage modelling using Inventor, for this I have few questions.
- What does stage indicate in cage?(Does it mean a complete one cycle of pressure drop and recovery, or just single pressure drop, or does it one stage means when the fluid passes through one cylindrical cage, no matter how many times the pressure drop and recovery is taking place inside that one cylinder.)
- When is the cage designed? Before or after or while same time while designing the valve?
- How is cage designed? What are the only input parameters that a designer has while designing this cage and how he calculates the remaining parameters of the cage?
- As the Cv value also changes with the use of cage(one or many), if one cage is used how should be the hole placing such that we get different valve characteristics(linear, quick opening, equal percentage)?
- Also where can I find all material to understand the complete design procedure of cage design?

 

[SPDL310]

I think that you will have a hard time generating an automated process for a broad range of cage designs. I would suggest starting in the narrow range of single stage linear opening and trying to go from there. Successful multistage designs are based upon many years of organizational experience, physical testing, and extensive CFD simulation.
[ul]
[li]When valve designers mention the number of stages in a trim it generally refers to the number of pressure letdown stages the fluid goes through.[/li]
[li]The cage is designed first and the valve is designed around the cage.[/li]
[li]The cage design will depend upon the fluid it is used for as well as its intended pressure drop and desired flow rates. At a minimum you will need to calculate the forces/stress of the cage, required thickness, size and number of holes. Again, this depends on the intended fluid services and flow parameters.[/li]
[li]The characteristic curve will change on the area exposed for fluid to flow through. So if your plug is traveling and the same amount of area is exposed for every increment of travel you will have a linear characteristic. If you wand a certain characteristic you will need to adjust the area exposed to match that characteristic.[/li]
[/ul]

For a better understanding of these concepts and the sizing equations you will need to use take a look at some of the major manufactures websites and published information. I recommend the Emmerson/Fisher and ISA control valve handbooks as well as Crane TP410.

 

[LittleInch]

This sort of design is what the control valve companies have done for the last century and guard quite closely as it is their IP.

As SPDL310 says, this is now subject to 3D CFD analysis.



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[Maddyp]

@SPLD310 and @LittleInch thanks so much for your valuable answers.

As you said number of stages in a trim refers to number of pressure letdowns. I understand it as pressure letdowns means pressure drops.(please correct me if I am wrong) i.e. how many times the pressure is dropping that many will be the number of stages.

Let us look at this pic

, in this pic according to me the fluid pressure is dropping two times that is at the starting point of the orifice or hole and second time at the point where the convergent nozzle starts. So if the pressure drop occurs twice then why it is called as one stage trim?

Also if you have any related resources related to the general design procedure of cages in industries then please provide it to me.

Thanks

 

[LittleInch]

Because the first "stage" as you call it is negligible compared to the second so most people would view this as a single stage trim.

multi stage trims often create a labyrinth path from one side to the other with effectively many stages, or some have series of metal cages inside each other.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.