21 replies to this topic

[vinod]

Hi all


I have to decrease the pressure of nitrogen from 6 bar to 0.65 bar...... ..the temperature is 20 deg C.........what is the best method............and how can i avoid choking?

Regards

Vinod

[Hariharasudhan.D]

Hai Vinod,

Why can't you go for pressure reducing valve?

Regards,
D.Hariharasudhan.

Hi all


I have to decrease the pressure of nitrogen from 6 bar to 0.65 bar...... ..the temperature is 20 deg C.........what is the best method............and how can i avoid choking?

Regards

Vinod

[vinod]

Hi

No...........tahts where i have a doubt.........if i go for a pressure reducing valve..wont choking pose aproblem..........since i am crossing the critical pressure ratio.......


Regards

Vinod

[jaychemical]

Size a restriction orifice to decrease pressure drop.


Thank You

Jay A&M

[wnovaes]

Vinod,

We need more detailed information regarding your system and application for leading you in a correct way. For example, range of operation: maximum, normal and minimum nitrogen flow, temperatures, pressures and pressure drop expected; piping diameter and service intended for.

In order to use of restriction orifice, it could not be a good choice if you have a service where flow modulation is expected.

Kindes Regards,
WSN.

[JoeWong]

Hi

No...........tahts where i have a doubt.........if i go for a pressure reducing valve..wont choking pose aproblem..........since i am crossing the critical pressure ratio.......


Regards

Vinod

Pressure reducing valve (control valve) will work in your case. Just make sure you provide sufficient information in the control valve datasheet and pass it to control valve vendor, they will select a good control valve(s) for you.

[vinod]

Hi Sir.......

So do you mean that i can use only a pressure reducing valve and achieve the final condition...............

regards
Vinod

[Radhakrishnan]

Hi all


I have to decrease the pressure of nitrogen from 6 bar to 0.65 bar...... ..the temperature is 20 deg C.........what is the best method............and how can i avoid choking?

Regards

Vinod

[wnovaes]

Vinod,

For sure only a pressure reduction control valve will work properly. For example, in nitrogen blanketing applications the pressure drop available cross the nitrogen intake control valve is quite close of pressure drop available in your application. Here in our tank farm, we have many control valves working by pressure control reducing nitrogen pressure from 7,5 kgf/cm2 to ~50 mmca in nitrogen blanketing application.
The only worry for this king of application is regarding the possibility of critical flow through orifice restriction. However, like Mr. JoeWong said, if you provide sufficient information to a control valve vendor, it will select a control valve that best fits for you application.

Kinds Regards,
WSN.

[proinwv]

Yes all you need is a PRV and the valve sizing will deal with what happens in the valve. Almost all gas flow va;ve applications are choked. Very common.

The use of a fixed orifice is not advised because it is only properly sized at one flow rate.

[djack77494]

Please note the important point mentioned by Paul -

Almost all gas flow va;ve applications are choked. Very common.

What is your concern? Choked flow is common and you shouldn't be obsessed with trying to avoid it. I think you're envisioning a problem where there isn't one.

Doug

[vinod]

Sirs...........

Do you mena to say as such there is no problem in having a choked flow..............only thing is that there is loss of pressure enrgy and no gain in flowrate after it reaches the choked velcoity.............

Can i put in this way....................like as the pressure drops across the valve from 6 bar to the critical pressure there will be gain in the flowrate from teh bernoulli's theorem ...................whereas the pressure drop from the critical pressure to 0.6 bar(in my case the desired downstream pressure)will be aloss of pressure enrgy as sound .....turbulence................and this is a waste of energy...........


Am i correct?

Regards

Vinod

[proinwv]

When you reach sonic flow the fluid flow can be increased with increases in inlet pressure, but not with decreases in outlet pressure. However, in a valve, it opens further, in response to the decrease in outlet (ie. controlled pressure) and flow increases.

The orifice is changing to accomodate the change in pressure.

If you had a fixed orifice you would loose control.

No problem as long as your valve has sufficient Cv to accomodate the worst condition and adequate turndown to accomodate the least flow condition.

[fallah]

However, in a valve, it opens further, in response to the decrease in outlet (ie. controlled pressure) and flow increases.

Can we say :anyway because the valve opens further , flow increases , eventhough still pressure ratio be lower than critical value?
Thaks
Fallah

[chemicalkannan]

dear all,
my small obinion is you use needle valve it will be reduce the pressure from high to low

Hi all


I have to decrease the pressure of nitrogen from 6 bar to 0.65 bar...... ..the temperature is 20 deg C.........what is the best method............and how can i avoid choking?

Regards

Vinod

[proinwv]

Fallah,

Your question is, "Can we say :anyway because the valve opens further , flow increases , eventhough still pressure ratio be lower than critical value?"

Yes the flow is increased by the larger internal valve opening, ie: Cv increases. The valve responds to decreases in downstream (controlled) pressure in this manner.

As to the suggestion to use a needle valve, this is only possible if the process conditions do not change.

Further, you need not worry about sonic flow ("choking") in a gas valve. If you had a liquid valve it is an entirely different matter and very different from gas flow.

Just go and use a PRV, stop worrying and take an hour off.

[vinod]

I am clear with one of the issue that choking is not a problem...........................

But how will i be sure that the pressure fall from 6 bar to the critical pressure and then reaches the final downstream pressure of 0.6 bar...............this is the pendding issue for me

Regards

[djack77494]

I am not clear about your concern. If you tell us that your upstream pressure is 6 bar and your downstream pressure is 0.6 bar, then I don't understand your concern.

You WILL get to whatever the downstream pressure is. Choking is not a problem (as you've come to realize), and choked flow would behave pretty similar to non-choked flow, with one notable exception. IF you have a given orifice (or valve Cv) and a given upstream pressure, then you will initiate flow as you impose a pressure differential across your orifice or valve. Zero dP = zero flow. Start dropping the pressure on the side of the valve we'll call the downstream side, and you start a flow of fluid from the higher to the lower pressure side. Keep dropping the downstream pressure while everything else remains unchanged, and the fluid flowrate will keep increasing.

---Up to a point!------

That point is called the critical pressure ratio. If you decrease the downstream pressure even lower, you do not cause an additional increase in fluid flow. That is because you have "choked flow". Now if the downstream pressure is 0.6 bar or 0.1 bar, it doesn't matter. The flowrate remains unchanged. You can increase the flowrate by getting a larger orifice (or opening your valve) or by increasing the upstream pressure. But, decreasing the downstream pressure will not result in increased flowrate.
HTH,
Doug

[proinwv]

Djack is correct.

The word choking or critical seems to be worrying you. It only means that the velocity of the gas in the vena contracta (smallest flow path) becomes sonic and that is when the flow only can increase with increases in inlet pressure or with increases in the orifice size, i.e. the valve open more to satisfy the demand. See what Djack said.

I have designed and produced many valves doing exactly what you want to do. They do work. Just get a PRV from someone like Richards Indus, Fisher, Cashco or whomever you wish and put it into service.

[vinod]

Thanks djack and proinwv..............now i am 100% clear.........thanks a lot

[proinwv]

Vinod,

You are welcome. Best wishes.

[fallah]

In PRV while valve is getting closed,there is the possibility of passing the critical pressure(cp), and because the target is setting the downstream pressure, fixing the flow due to passing (cp) is not an issue for worrying.
Regards