Control Valve - Choked Flow

[dragos12]

What could be the consequences of choked flow by a ball control valve?
Valve to be used by butane gas with a silencer to decrease the noise.
Inlet pipe size 24", outlet is 32", valve to be sized for 16"

 

[LittleInch]

Need more details. "Ball control valve"??

Line sizes look odd. if you've got choked flow in your 16" valve / line, velocities in your other lines will be very high also

consequences - fixed mass flow rate, potential for erosion and damage for the valve and pipework.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dragos12]

Yep it is V-Port Ball Control Valve.

 

[LittleInch]

OK, but still a bit in the dark as to why you ask. Any sort of choked flow not intended (like a restriction orifice) is normally bad news and should be avoided.

Are you asking out of interest or trying to solve or improve a particular situation, in which case you need to supply much more info.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dragos12]

I was asked to quote a control valve and it is resulting with choking conditions.
Beside changing process conditions, do you see a way to solve it? I mean to have flow with choking.

 

[LittleInch]

either a different type / size of valve or multiple valves. A control valve that is exhibiting choked flow isn't a control valve anymore, it's just a restriction orifice by another name.

you're going to have to open up a bit if you want more assistance.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dragos12]

I am sorry, I just did not want to ruin your time, Thanks to your request, pls. see sizing page.
Biggest size is 20 inch and does not matter if I quote 16" or 20" , does not have impact on results.
I am thinking that the process is asking for choked flow. (for some lines they ask for flashing valves)
Any further comments?

 

[dragos12]

Sorry , here you go...

 

[LittleInch]

To me it just looks like you're using the wrong type of valve for the duty. To get the pressure drop and the actual pressure downstream (appears to be about 2 to 2.5 barg) with this type of valve for the flow rate you're trying to get.you have really quite a small hole which is then choking and producing a huge noise (122db!)

You need a different control valve with inbuilt multiple pressure drops to eliminate the choking flow.

The critical pressure drop figure is some way from your actual and hence this valve just won't work properly. You'll either get the pressure you want downstream but not the flow or the flow you want but at a higher pressure. At least that's my reading of it, but control valve gurus might see it differently.

what is the controlling aspect? Flow or pressure downstream?

Is this confirmed as gas upstream?

You need to really talk to a control valve technical engineer who really understands this.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dragos12]

Littleinch, thanks for your time and wording. Finally I learnt that choked flow for gas is not a big issue.Only negative result is noise which should be reduced.

 

[LittleInch]

That's a big negative though. "silencers" won't make much of a dent in 120dB and a lot of the noise travels a long way down the pipe.

No one will thank you for introducing a source of noise this loud and it is probably not allowed by environmental laws and protection of workers regulations.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[moltenmetal]

A control valve exhibiting choked flow is still a control valve because it's a variable orifice whose size you can control. Many control valves operating in gas service have choked flow in the trim all the way from 1% to 100% open.

As to whether or not it's tolerable from a noise perspective at this huge size, I have no idea- that's a matter for very careful design and indeed may determine the correct valve selection. A noise-attenuating trim is likely not possible in a vee ball control valve, but if you have choked flow in your situation and that's acceptable, you have plenty of pressure drop available to satisfy the needs of noise attenuating trim.

If the flow also chokes in the upstream or downstream piping as well, you can have controllability problems. The control valve needs to be the "controlling resistance" in order to work.

This thing must be dissipating an enormous amount of energy. Is this an intermittent service? If it's continuous, you should consider a turboexpander- get some of that energy back...

 

[dragos12]

Thanks a lot, it is a turbo expander application, correct.

 

[davefitz]

There are 2 types of choked flow; acoustically choked ( as in this case) or frictionally choked, as with use of a CCI drag valve or with use of capillary tubes ( eg Fanno flow) .

A 16" valve , acoustically choked, at the outlet of a 24" pipe implies the velocity at the end of the 24" pipe is at a 0.44 Mach number, which is not terrible but could be better. Any bends in the pipe would result in noise from oblique shock waves, and of course there will be jet noise at the 16" ball valve. A multistage valve or a CCi style Drag valve would eliminate the noise issue, and a turbo expander would produce usable power and earn money over time.

The flowrate that is limited by acoustic choking is defined by the location of minimum flow area, in this case the 16" ball valve vena contracta.

"Whom the gods would destroy, they first make mad "