Choked regulating valve 1

[Fabio360]

Hi,

I'm sizing a pneumatic test stand for the testing of aeronautical components. I have the need to use pressure regulating valves.

There is a concept i still don't understand and i would appreciate your help.

I have a P1 pressure of 30 bar and a flow of let's say 50 Kg/min.

I need to have a P2 pressure of 3.5 bar or 5 bar, for the same flow.

In this conditions a pressure regulating valve will be chocked.

My question is that if a can control the P2 pressure, the confusion is that for both conditions i have the same Kv.

Regards,
Fábio Silva

 

[LittleInch]

The issue here is for the same mass flow rate and volumetric flowrate, something else must change to allow the pressure drop to change. If the downstream system is the same you won't be able to have the same flow at a lower pressure through the same valve. As a reduction in downstream pressure only in choked flow results in the same mass flow rate, nothing changes. However the control of the downstream pressure must then be controlled by another control element (valve, fan, different piping layout etc).

If you don't change anything, to get a lower pressure with your main control valve means you will have less flow and hence a different Kv.

If you draw out your system and post it, it might become clearer what you are doing.

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[Fabio360]

Thank you so much for your help.

I've attached the test requirements and the diagram of the test facility

http://files.engineering.com/getfil...c-92e3-47b7-bb9e-5077773ae007&file=tests.xlsx

http://files.engineering.com/getfil...0482-4fff-858d-2cf5f4e74f5a&file=Completo.jpg

 

[Fabio360]

To be more clear. This facility is intend to test Pneumautic Valves and Cooling turbines.

 

[LittleInch]

Hmmmm,

I'm not actually sure this is going to work as shown. E.g. your spread sheet shows test 1 and test 4 with same p2 pressure, but different flow rates (one double the other). This will simply not be possible to achieve with a single control valve. To do this sort of thing you might need two or three different control valves to mix and match OR have some sort of additional control valve downstream the equipment to be tested to create a variable back pressure (p2) which then needs to be varied along with the primary valve which could be used to vary mass flow or keep mass flow the same. Other than test 6, all your P2 pressures appear to be in the choked flow region so mass flow should stay the same if you don't change the valve opening. There is quite a large range of mass flow here so one valve might not be able to do this entire range with any accuracy.

As it stands, you could vary pressure and flow at the same time, but couldn't keep one constant and change the other.

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

 

[Fabio360]

I think that if i guarantee a certain pressure p2 the cooling turbine will only "pull" a certain mass flow of air. Do you agree?

 

[Fabio360]

I simplified the schematic. I'm thinking of having two regulating valves working in parallel.

 

[LittleInch]

I can't readily envisage what is meant by a "cooling turbine". If you want to vary (or fix one and vary the other) the pressure and mass flow rate by the amount you're talking about, you might need three or more valves in parallel of differing sizes, but the key which I something I can't see is how does the downstream system - your equipment under test - manage to handle different mass flows for the same input pressure???

This implies some sort of control over flow or pressure within the equipment under test which isn't visible and I'm not sure you've quite got this, but I could be wrong.

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

 

[Fabio360]

Cooling turbine is basically just a turbine, that wotks in the air plane air conditioning. The turbine receives compressed air and expands it to create cooler air to insert in the cabin of the airplane

 

[LittleInch]

Ok, but that's my point - if the turbine spins at a certain speed at a certain inlet pressure with a certain mass flow, with outlet pressure constant (the cabin air pressure) how are you going to get more mass flow for the same inlet pressure? Doesn't make sense to me / physicaly impossible

Your turbine in effect has a flow pressure curve associated with it, the key word being curve, not flat line....

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

 

[Fabio360]

I get the confusion. I don't have to have different conditions for the same cooling turbine. Each test is for different cooling turbines witch have different working conditions and specs

 

[LittleInch]

You didn't mention that earlier... You still have listed 24 tests, all at different flow rates and pressures and temperatures. Are you testing 24 different turbines? A few are identical flow and pressure, but most are different.

To get one valve to provide this range of pressures and flows is not feasible, especially with differing equipment.

I would go for at least three different sized valves in parallel to even get close to your range of pressures and flows.

That's also a fairly significant sized heater for a "test" rig (max 216 kW). I'd love to see a picture of how big this is - I'm thinking about the size of a small hangar....

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

 

[Fabio360]

Actually it's quite small. It's of a new technology called Open Coil.

 

[Fabio360]

I have used this sizing software for the valves:

http://www.schubert-salzer.com/en/download-cs/software.html

It's fort a type of valve with a 40:1 rangebility in Kv

 

[LittleInch]

Looks interesting, but I still think you're trying to do too much here if this is the same piece of test equipment. In terms of actual air velocity d/s your valve you've got extremes of 1 bar (presume 1 barg) at 100 kg/min ad then 10 barg at 9.1 kg/min. That's a variance in actual gas velocity of nearly 100:1. I think your equipment won't like that sort of variance.

Using software like you have is all well and good to get you an idea, but you really need to talk to a few vendors to get some advise on what is actually practical.

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

 

[Fabio360]

Thank you so much for your help. In my complete drawing i have two different lines of compressed air with 2 valves each working in parallel. I hope the back pressure created by the cooling turbine is enough to ensure the test conditions.