Valve, Gas fluid flow question 2

[robin72]

Hi all,
I am new here and need your help.
After sudden opening of a ball valve with cross section area of 10 mm come to an blow up (explosion) of the 2" pipe.
(before opening) Up stream the valve was the gas pressure 190 bar during downstream the pressure was 0 bar, however it was air inside (s.attached).

Downstream of the valve the distance to the orifice plate is 1,5 m, there was upstream the OR an blow up of the pipe about 50%.
Could you explain me the physical phenomenon, mathematical equation would be appreciated.
best regards
robin

 

[zdas04]

The industry averages about one fatality per month in purging related incidents.

I'm going to assume that the 190 bar stream was natural gas? If it is air, then the heat of compression discussion below still reaches the same temperature, but there must be something like grease or residual hydrocarbons to have an explosion.

When the valve opens, the high pressure gas begins to flow at sonic velocity. A sonic stream is very dense and will not mix with a static volume. Instead it will compress the gas in the low pressure system until that gas is about 1/2 the pressure of the upstream. So you've taken the air from 1 bara to 86 bara or 86 compression ratios. The adiabatic heat of compression (see the FAQ's) says that if the air started at 15C it would end up at around 750C. The autoigintion temperature of methane is 538C. In the vernacular, you "dieseled" the line. It happens in meter runs several times a day around the world. Most of the time there either isn't enough gas to explode or the operator got lucky and didn't get killed. Sometimes the operator doesn't get lucky.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[robin72]

Thank you very much for your replay, but could you tell me where you got this statment ,,until that gas is about 1/2 the pressure of the upstream,,
thank you again.

 

[zdas04]

Take a look at faq378-1201. Milton has an equation for "critical pressure" that describes when choked flow can occur. The result of that calculation is generally a downstream pressure around 60-68% of upstream pressure. To be conservative (and to keep the calculation something I can do in my head) I say "1/2". Actually, the compression goes on a bit longer than that, but a rigerous analysis rarely yields different results (e.g., in my calculations above I get 750C, doing the math right I would have gotten around 780C which is still WAY above the autoignition temperature). That is always the case. If I do the calc and find that the heat of compression is within 30-40C of autoignition then I know that doing it right might give me a different result and I do the full calc.

David