PSV Discharge Piping

[bluemax89]

I know that you shouldn't design a PSV discharge for more than .75 mach, however I don't know where to find this in the code. Is it code? ...or just a recommended practice. Can anyone help in this area.

Thanks.

 

[TD2K]

It's a recommended practice though API 520 or 521 might discuss this.

There are a couple of reasons, one is that the max flux gets high enough that vibration can be an issue. The other is that your pressure drop calculations become more sensitive to small variations in process conditions/gas properties. You don't want to inadvertally get a higher backpressure that begins to impact the capacity of the PSV to less than required.

 

[EGT01]

TD2K's summary about says it all. Unfortunately, my searches through API RP-520 Part I, II and API RP-521 have never revealed much about velocity limit for the outlet piping.

In API RP-521, section 5.4.1.3 Design of Relief Device Discharging Piping you'll find "Since laterals in relieving systems can often be sized at velocities approaching sonic, pressure losses or recoveries caused by velocity change can become a significant factor in system analysis. These densities can produce large reaction forces."

API RP-521 also has some discussion about design velocities for flare stacks that may be only somewhat related. In the section Disposal by Flaring you'll find "Designs may be based on velocities of 0.5 Mach or higher, if pressure drop, noise, and other factors permit."

One company I've worked with had in-house design practices suggesting a limit of 0.5 Mach but under some conditions allowed up to 0.75 Mach.

Recently, I found similar practices stated in a text book that you could purchase or find in a library. "Pressure Safety Design Practices for Refinery and Chemical Operations" by Nicholas P. Cheremisinoff, Noyes Publications.

There were several places where maximum line velocity was discussed but for relief valve outlet piping "The PR valve riser should be sized for a maximum exit velocity, under the design contingency release, of 75% of sonic velocity. This is to limit noise problems and avoid choked flow. If it is necessary to install a section of enlarged diameter riser piping to limit the maximum exit velocity, its length should equal at least ten diameters of the enlarged piping."

In another section covering flare header system design, it seems the discussion was downstream velocity greater than 50% sonic would trigger a concern for potential vibration problems. I'm sure there are other text book references that may list such practices.

Now, when it comes to rupture discs as a stand alone relief system, I can't say that I've seen recommendations to limit the velocity to below sonic.