Control Valve Choking

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Thread Starter

Shailesh C Patel

i have some confusion about control valve choking phenomena.

1> One plant is running there is one control valve which is not choking at any design process condition, if i will replace that control valve with same one but it will choked at normal condition.then what is affect on process dynamics?

2> Fisher say that if you use harden trim then there is no problem, how it is possible because choking define that if your flow will not inc. even if you increase dp!!!!!!!

plz help me

Regards,
Shailesh C Patel
FDC - Control Systems And Instrumentation
 
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Sajid Ali Khan

Dear Patel,
The query is too complex to be resolved simply.
Can you send me the following data for a better analysis.
1. Process conditions of both valves.
2. CV Data Sheet, and calculation sheet.
3. Conditions at which choking occurs, the frequency.
4. The remedial measures you normally take.

Regards,
Sajid
 
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Shailesh C Patel

Thanks sajid
Actually, I want to know that is it any effect of valve choking on process dynamics?

Regards,
Shailesh C Patel
 
B
Your question is very good,

A general rule of thumb is that you will have choking if Pin/dP is greater than 2. Of course, choking will also be a function of the valve geometry: ball valves are easier to choke than globe valve.
Valve choking occurs when the pressure drop across the valve is very large. For compressible fluids, the gas enters at the inlet of the plug and its volume expands to a pressure below the outlet pressure (vena contracta). The specific volume then recompresses to the valve outlet pressure. Choking happens when the expansion is so large that the outlet pressure has no more effect on the throughput flow (large dP). The only variables havings an effect at chocked conditions will be the inlet pressure and the valve position: changes at the outlet pressure will have no effect on flow.
Does chocking has an effect on process dynamics? You bet it does! Let's take an example of a steam header valve:
No-Choked valve: Disturbance downstream of the valve, the inlet pressure increases, the deltaP decreases, the flow throughput decreases, the disturbance is mechanically caught by the valve fluid dynamics before the controls start working.
Choked valve: Disturbance downstream of the valve, the inlet pressure increases, the deltaP decreases, the flow throughput DOES NOT decrease, the disturbance is NOT mechanically caught by the valve fluid dynamics and the controls have to do all of the job :large downstream pressure excustions.
Fisher might be trying to sell you a valve that chokes (ball valve) because it is cheaper than a good old globe valve. If your process disturbances are slow enough (1 min+) you might be okay. However, if the disturbances are fast the valve selection is critical to your process stability.

Hope this helps,

Ben Janvier, Senior Process Consultant.
[email protected]
 
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Shailesh C Patel

Thanks Ben Janvier

Few more quotation for choking if you don't mind.

1> could you say anything about valve opening in both case (same of difference)?
2> is any relationship between mach no. and valve choking?
3> either static or dynamics characteristic affect due to chocking?

Thanks
Shailesh
 
Fisher is wrong.
Harden trim is a material issue.
Choke flow is a process issue --- occurs in gas flow when downstream pressure is reduced below the critial pressure of the gas, usually accompanied with shock waves and vibrations.
See Practical Guide for Measurement and Control, Control Valves, Borden, ISA, 1998.
When x > Xt*Fk, you will have choke flow in the valve. Try to ask your valve Suppliers if they have a valve with Xt = 1 at all flows (at all openings, for all Cv's). The highest Xt in Fisher valves is only 0.6. Many other valve Companies have valves with Xt = 1. E.g. Valtek, Masoneilan, CCI,....More the torturous path in the valve trim, higher the Xt, higher the resistance to flow. This resistance helps to reduce the valve trim exit velocity. Good design is to increase this resistance (i.e. loss coefficient, or K-factor) of the trim so that the trim exit velocity will not exceed sonic velocity (Mach Number = 1).

Hope this helps.
 
Ben, in chocked condition, will the increase in inlet pressure affect flow rate keeping %openning and downstream pressure same?
 
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Dr. R. Murugesan

Dear Mr. Shailesh Patel,

The performance of any control loop largely depends on the true capability of the final control element-control valve, in most applications.

Quite often, the final control element is not adequately sized to meet operating requirements of the plant.

Issues like 'cavitation' and 'flashing' need to be addressed correctly while sizing the control valve, no matter whatever is the kind of trim. Fisher Control Valve Handbook is quite a good material. Yet, individual process dynamics need to be evaluated while sizing the control valve.

Please furnish the process parameters of the fluid to be controlled by 'this control valve'. I would like to offer my suggestions.

Regards,

Dr. R. Murugesan
 
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Ronald Deepak

Dear All,

Good to see a discussion going on here about choked flow. I would like to add a few points which you might know but have not been discussed here so far. When we talk about choked flow, we need to be aware of whether the fluid is compressible (gas/vapor) or incompressible (liquid) and why choking causes problems.

1) A valve passing a liquid begins to choke when the downward pressure swing toward the vena-contracta forces the liquid pressure to a value near or below its vapor pressure. This is the point where the liquid starts to form bubbles. The formation of bubbles causes the liquid molecules in the flow stream to become crowded or 'clustered' at the vena-contracta and this crowding prevents any further increase in flow with decrease in the outlet pressure. The vapor bubbles collapse if and when the liquid pressure recovers above the outlet pressure (after crossing the vena-contracta), or remain in the vapor phase if the pressure does not recover above the vapor pressure. The point to note here is - Occurrence of choked flow is not determined by the valve alone. The liquid's vapor pressure, inlet & outlet pressure also determine whether the valve will choke or not. The pressure drop at which a particular valve chokes is often referred to as "Choked flow pressure drop". Incipient choking in itself is not a major problem. However, when it becomes severe i.e., when the actual pressure drop is much higher than the choked flow pressure drop, it causes severe damage due to cavitation or flashing (depending on outlet pressure). So, as such choking is not a big concern. While cavitation can be prevented through appropriate anti-cavitation technology, flashing cannot be eliminated no matter what (it's a process dictated phenomenon). The focus should be on reducing the flashing damage by suitable selection of valve style & trim. Cavitation if it does occur can cause noise and subsequent vibration. A valve with anti-cavitation trim technology can eliminate cavitation by having low recovery characteristics & staging the pressure drop thereby ensuring that the fluid pressure does not fall below the vapor pressure when the fluid transitions from the inlet to outlet pressure.

2) When a compressible fluid (gas/vapor) undergoes pressure drop when it flows through a valve, the velocity of the gas increases as it loses pressure (bernoulli's principle). We also know that at the vena-contracta the fluid flow takes the least flowing area, which means to maintain the same flow rate, the gas has to travel at a much higher velocity (Flow rate = Velocity * Area). So when we keep increasing the pressure drop across the valve, the fluid velocity at the vena-contracta keeps increasing. However, the sonic velocity defines the maximum rate at which the fluid can travel. So when the velocity at the vena-contracta reaches sonic velocity, its not possible to achieve a higher flow rate through the valve by increasing the pressure drop any further, because the gas is already travelling at its maximum possible velocity (at the vena contracta). This is the point where the gas/vapor flow begins to choke. Any additional pressure drop will produce a small increment in flow which occurs due to the enlargement of the vena-contracta (slight increase in the flow area at vena-contracta). Any pressure drop beyond that, the gas/vapor flow begins to produce a lot of noise i.e., the energy from the pressure drop is dissipated as noise. Increased noise levels can cause significant vibration in the valve and piping system downstream of the valve. The occurrence of choked flow in the case of compressible fluids, is determined by the ratio of pressure drop to inlet pressure, the fluid's specific heat ratio factor Fk and the valve's Xt. Again, flow beginning to choke is not a big concern. Only extreme levels of choking causes excessive noise and vibration. A valve with noise attenuation technology can eliminate noise by either shifting the noise's frequency to higher levels where it cannot be heard and subsequently also reducing vibration. Other trim technologies produce staged pressure drops which reduce the noise levels by reducing the energy dissipated due to the pressure drop taken at the last stage (which dissipates as noise through the downstream piping).

To summarize, choking is not a phenomenon which causes problems. But rather severe levels of choking can lead to cavitation or flashing in liquids & noise and/or vibration in gas/vapor flow streams.

Hope this helps.

Ronald Deepak
jronalddeepak [at] gmail.com
 
Adding to Ronald,

Use of hardened trim is a way of offering protection when the liquid is choking & (cavitation/flashing) when the pressure drop is not huge for the valve. These cases would not typically require a anti-cavitation trim. For high degree (high pressure drop) cavitations, anti-cavitation trim is the solution. Whereas, since flashing is process dictated, we can only minimize the damage to the valve by offering stronger body and trim parts in case of globe valve. A solution such as angle valve would isolate flashing by converging the fluid flow at outlet.

For gases in choked flow we do not offer hardened trim. This is because, when a liquid undergoes choked flow, there is a phase change and it is a non uniform liquid + vapor phase. The damage is caused due to increase in velocity at vena contracta where the vapors at high velocity drag the liquid along with it, strike the trim and/or body. The liquid being dense cause damage (liquids such as water). In case of cavitation, the bubble implosion on trim causes damage.Since vapor/gas chocked flow does not involve any of these phenomena,no hardened trim is required. It would be required to reduce the gas turbulence to reduce noise.

Hence,your statements are generalized on the use of hardened trim for choked flow.
 
dear ben,
as u commented that choking occurs when Pin/dP is greater than 2. But as pressure drop across valve is not much more, then every control valve will choke.

please clarify.

Thanks
Tushar
 
N
Just my two cents worth:

Noise and vibration due to chocking in gas/vapor passing CV is the result of converting critical pressure (just after VC) to existing down pressure (usually lower than critical pressure).

Regards

 
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Ragin Ravindran

Dear,

choking is a critical flow condition in which severe cavitation occurs. Hardened trim is not an optimal solution but an economical solution. Hardened trim will increase the trim life only. for choked application, multipath trims are well suited, which are provided by the leading valve manufacturers. you can find the severity of choking by your self by this equation very easily.

p= fl^2(P1-(0.96-0.28(root of(Pc/Pv))Pv)
where fl= pressure recovery factor of valve
P1= inlet pressure
Pc= critical pressure
Pv= vapour pressure

if P obtained is less than the pressure drop of the valve then choking occurs, and if the difference is more severity level is more.Severity also depend on valve outlet velocity

hope this helps u

ragin
[email protected]
 
> what is a mach number?

Mach number is the ratio of the actual fluid velocity to the velocity of sound in that fluid medium at the flowing conditions.
 
Dear Ronald Deepak,

Thanks for your great and complete knowledge. I have two questions:

1-What is the sign(s) and symptom(s) of a choked flow valve.
2-what should we do for solving this problem(not only noise and vibration). I try to select a control valve using Fisher Software, but the valve is choked what should I do? (whether increasing in size of valve is the solution or not?)

Regards
Ebrahim
 
Dear Otised

Thanks for your replying. As I mentioned, I need a solution for choked flow in control valve for GAS flow. The document you referred to, is about liquid flow.

I do appreciate your attention.
 
Ebrahim,

For compressible fluids (gas, steam) see pages 133 to 139 of the same document. In particular, look at the note on critical pressure drop on page 134.

I recommend you download the handbook - it is considered the control valve bible by many control engineers. While there is much that is specific to Fisher valves, there is also much that is applicable to any control valve.
 
Ebrahim,

>As I mentioned, I need a solution for choked flow in control valve for GAS flow. The
>document you referred to, is about liquid flow.

You mentioned this is a <b>DIFFERENT</b> thread:

http://www.control.com/thread/1375111314

Your post in <b>this</b> thread (in response to a post dated June, 2009) did <b>not</b> mention you were interested in either gas- or liquid flow.
 
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