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Vent stack sizing

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bird57

Chemical
Jun 19, 2008
2
Hello Everyone,

I am process engineer and working with oil and gas consultant company.For my experience is not much.it's around 1 year for this job.This is my first time for posting.if there are some mistake,please forgive me.

Now I'm sizing vent stack for natural gas metering and regulating station.I so confuse about criteria for sizing.I want to know about Much No. at tip of vent stack used in stack.My boss told me that using 0.99 Much No. for sizing.for this case is choke flow.pressure at tip of vent stack is more than atmosphere and gas velocity equal sonic velocity.if used this case,gas diffusion will be very good.This I misunderstand or not? but I see some work don't used right this.They use Much no less than that.Assumed it's 0.80 . If calculating with value, it's not choke flow so pressure at tip of stack will be atmosphere pressure. This point make me very confuse.this can used or not. In my opinion if there is not pressure difference between tip of stack and surrounding which cause gas velocity.why gas velocity can be 0.80 much No. This make me don't understand for a long time.what point i misunderstand.Please suggest me.

Thanks in advance for every recommends
Best regard,Sakkrit
 
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On a vent stack with flammables you want a high velocity for hydrocarbon dispersion.
The intent is to prevent ignition of the gas stream and turning the vent stack into a flare.
The traditional target is 500 ft per second although consult API-521 for refinment.
Given good separation from ignition sources, 200 ft per second can be used.

One does not want to have sonic velocity inside the vent stack.
Limit the base pipe diameter to obtain less than 75% of sonic velocity.
Depending on the system geometry, the tip of stack may be at sonic velocity.
When going from high pressure gas to atmosphere, some device must create pressure drop.
If not, all the pressure dissipates at the tip.
 
Thank you,Yitbos for your answer

I would like to ask you further detail.what point I should do to start when calculated.Usually I start with tip of stack and calculate backward to blowdown valve.If back pressure not more than critiacl pressure,it's ok.my sizing base is using 0.99 much No.and choose pipe size that give back pressure less than critical pressure.Then I get oulet pressure at tip.

however I calculated like this.I will not find outlet pressure when Much No less than 0.99.If I want to calculate less than 0.99 Much No in my spread sheet I must reduce pressure to atmosphere at tip.

I want to know how do I calculate for pressure in case much no less than 0.99.or the way i used is not correctd.please suggest me for this point.


Thanks in advance
Best regard,Sakkrit


 
The backpressure at the blowdown valve and whether the exit at your vent stack is at sonic flow are separate issues.

Your gas flow, MW, temperature and compositions sets the Ma number at the tip exit since you have to move so much volume across the tip with such and such open area. The pressure inside the tip will be the pressure needed to move that volume/mass of gas through the tip (K =1 if it's just an open tip) into the atmosphere plus the velocity head fator.

The back pressure at your blowdown valve is then what pressure you need at the valve to move the gas to the flare tip at the above calculated pressure. That's set by the flow rate as well as the size and length of the piping between the flowdown valve and flare tip and any equipment (flare stack, KO drum, etc).
 
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