5 replies to this topic

[Ibnu84]

Hi All,

 

Most of us have known the Mach Number criteria for sizing a PSV tail pipe and the flare header. Each company would have their own criteria, but normally maximum mach no of 0.7 and 0.5 are respectivelly used for PSV tail pipe and flare header sizing.

 

My questions are;  what is the philosophy behind this criteria? Why the mach number of flare header is limited to 0.5, and PSV tail pipe allows higher value? what would be the consequence if the mach number at flare header is 0.6?

 

many thanks for any feedbacks

 

 

Regards

 

[fallah]

Hi,

 

As far as i know, there is a phenomena in compressible flow in a pipe named "transonic flow" in which it might having local condition of mach number equal to 1 while the overall mach number is lower than 1. This phenomena might be created due to change in flow direction, reduction in cross sectional area inside the valves,.... 

The limit value to not reaching such condition is around 0.7 mach number higher than which there might be the possibility of creation such phenomena would lead to shock waves and localized choked flow...

 

As per above criteria the PSV tail pipe is being sized based on maximum 0.7 mach number and the flare header on a little bit lower (0.5 mach) due to its higher criticality and size enable to be supported and controlled easier against lower vibration and displacement...

 

Hope above helps you out...

Edited by fallah, 23 June 2013 - 03:26 AM.

[ankur2061]

Ibnu84,

 

To explain briefly, Mach number limitations are provided for piping to control noise and vibration. Excessive vibration can cause damage to piping and pipe supports whereas noise causes noise pollution.  

 

Several studies exist related to noise and vibration control in piping and one of the major factors attributed to noise and vibration is high velocities or in other words high Mach number. Som links re provided below:

 

http://www.iitr.ac.i...ow in pipes.pdf

 

http://www.xtrados.c...s/Tute3_AIV.pdf

 

http://proceedings.a...ticleid=1628282

 

http://webwormcpt.bl...raints-for.html

 

Hope this helps.

 

Regards,

Ankur.

Edited by ankur2061, 23 June 2013 - 04:15 AM.

[MI_Sidd]

Mach no in main headers is kept lower than in tail pipe and limited to 0.5 to keep margin for future expansions/ addition of safety valves..

[Ibnu84]

Hi Fallah, Ankur and MI_Sidd,

 

Thanks for your explanations and Links.

 

 

Regards

Ibnu84

[Lowflo]

I have a different opinion. Mach number criteria are commonly discussed, but in most cases I think such discussions are way too non-specific, and too simplistic to be of any value to an engineer.

 

It's arbitrary to say that the Mach number should be limited to 0.5, 0.7, or any other value. Ultimately, all that matters is whether the relief devices function properly, the flare isn't overloaded, and the piping design is mechanically sufficient (e.g. resists AIV failure). Mach number limits are an indirect way of preventing these specific problems, but they're still arbitrary. For new designs, Mach limits are an easy and acceptable way to allow for future capacity. But, when used, the limiting value is entirely a case-by-case decision, based on the owners specific needs, plans, and budget.

 

For existing flare systems we want to know whether it will work as-is, or whether changes are needed to address problems (deviations from the criteria listed above). Any Mach number is OK if those criteria are satisfied.