Why are PSV spools cut on an angle?

[AndrewEngEET]

I was working at a refinery awhile back and had a quesiton that no one could answer.

I was looking up at some of the PSV's on the towers that wer not vented to the flare. All of these atmospheric relief safety valvs had a small spool that came out of the horizontal exit and then made a 90 degree bend upwards and then were cut on an angle at the atmospheric exit.

Does anyone have any idea why this is the case?

Some of the answers I got:

1. So birds don't nest in them
2. So they don't whistle
3. So the pressure is released gradually as it exits at the slash

thanks,
Andrew

 

[EGT01]

Andrew,

You might be interested in the discussion in this thread.
thread135-30609

Otherwise, it will be interesting to see if anyone has any new thoughts on the subject.

 

[Scipio]

How about this one, "If we didn't cut those at an angle so it's obviously open-ended, someone might come along and think it's supposed to go somewhere and the pipe run wasn't finished"

 

[TGS4]

There was a little nugget of truth in the thread that ETG01 pointed you towards. There were a few authors who stated that the intent of the angle is to create a lateral load when the PSV is discharged. With a judicious selection of the angle and the height of the discharge line, the line of action of the resultant force (Vertical + Horizontal) will go right through the PSV discharge flange.

So why would this be important?

If the line-of-action of the resultant force goes right through the flange, then no bending moment will be applied to the PSV discharge flange. Several authors in that previous thread indicated a preference for straight cut discharge lines, hand-in-hand with a vertical support element below the discharge elbow. They indicated that the vertical support would counteract any applied bending moment, resulting in essentially zero bending moment at the PSV discharge flange.

HOWEVER, with a small amount of thermal expansion caused by the line to the PSV heating up fom the discharging flow, the support element(usually a standard elbow stanchion) becomes useless due to lift-off. Then, there has to be an equivalent amout of deflection to re-engage the support element. In the meantime, a substantial bending moment has been applied to the PSV body.

The consequences of excessive bending moments on a PSV body could be:
- damage to the moving parts,
- inability of the PSV to re-seat automatically,
- permanent deformation of the PSV body, which could result in either the PSV not being able to re-seat or the PSV not being able to discharge at it rated pressure.

Needless to say, any of these consequences are unacceptable.

One way around the lift-off of the discharge elbow stanchion would be to place a spring support under it. However, what would be cheaper/easier - cutting the discharge pipe at an angle, or designing/procuring/installing/maintaining a spring?

Hope this helps,
Trevor

 

[Latexman]

I've seen horizontal PSV discharges cut on a 45. Turned the right way, this acts to keep rain, snow, sleet, etc. out of the pipe. Maybe you saw a "carryover" of this wrongly applied to a vertical discharge.

Good luck,
Latexman

 

[AndrewEngEET]

Awesome! Thanks for the in-depth answers! I also appreciate the link to the previous posts. I will make sure I do a more thought search next time.

Andrew