Pressure variation in a vent pipe

[Ferex]

Hi everybody, how does pressure varies in the vent pipe connected to a PSV? I think it should go from the opening pressure of the valve to 1 ATM or 100 KPa (atmospheric pressure). If I am right is this variation linear during the lenght of the pipe? Does it happen suddenly at the open side of the pipe??
Many thanks

 

[psafety]

No more than 10% of the opening pressure for Section VIII applications.

 

[ggordil]

I don't think the pressure drop will be linear. It depends on your outlet piping. For example, there is a higher pressure drop at bends than straight pipe.

To find out where most of the pressure drop is, you need to calculate the pressure drop in the outlet pipe and then see which segments have the highest pressure drop.

One thing to keep in mind, the pressure on the outlet of a PSV is the constant back pressure, in your case it atmospheric, plus the pressure needed for the fluid to get through the pipe, the pressure drop.

 

[Ferex]

Getting into the details of an example, let's say that I have after valve discharge a straight pipe, an elbow and than it goes up vertical to atmosphere. Let's say valve discharges at 500 KPa (5 ATM). If I understood well, pressure in the vent pipe goes from 5 atm to 1 atm. There's the elbow that causes a pressure drop. So, in my opinion, in the vent pipe the gas will decrease its pressure for the elbow (and for the flow in the pipe which causes a drop too), then it arrives at the outlet section and it suddenly goes from 5 - (all the p.drops)to 1 atm. All the pressure energy transforms in velocity.
Is it a correct description??

 

[Chance17]

I looked at some actual line loss numbers this morning.
For a conventional valve, the safety valve takes greater than 90% of the pressure drop.
In your example, the exit flange pressure would be about 0.5 atm.
From there, line loss is approximately linear with pipe length and normal amounts of elbows.
The typical velocity is between Mach 0.3 to Mach 0.8.
Expansions really cause significant line loss due to localized velocity increase.
The Mach numbers are locally high at expansions.
Of course the remaining pressure dissipates at very end of pipe with exit velocity approaching 1.0.
The software I use routinely calculates Mach numbers higher than 1.0 at end of pipe.
All my co-workers say this is impossible.
But I have Goggled the topic and find references with greater than Mach 1.0 inside pipe.

Using a balanced bellows safety valve allows greater than 10% exit pipe loss.
Some designers limit the loss to 30% of set -- although 50% of set is not uncommon.

 

[Ferex]

Many thanks for the answers, I need a comment on this sentence "then it arrives at the outlet section and it suddenly goes from 5 - (all the p.drops)to 1 atm. All the pressure energy transforms in velocity." that I wrote inmy post. Is it right??

 

[4Pipes]

Yitbos,

You have probably not discovered the perpetual motion machine or some equally new phenomena. Of course, if you have you should patent it pdq.

 

[Ferex]

I only need an answer to my final post, please tell me if I am right or not.

 

[psafety]

No you are not right. Yitbos informs the valve takes greater than 90%.

I early on told you NO more than 10%.

 

[ggordil]

You are not right because the pressure on the outlet of your valve is not 5 ATM, that is your set pressure.

You can think of the relief valve as an orifice in a pipe. It will cause most of the pressure drop but not all. Therefore, the pressure at the outlet of your valve would equal the pressure needed push the flow through the rest of the outlet piping.

 

[CMA010]

You can estimate the back pressure with equations 26, 27/28 and 29/30 of API STD 521 (May 2008).

Yitbos: What are you references for > Mach one for safety valve tail pipes and what software do you use?