Restriction Orifice.

[nasroh]

During depressuring a portion of pipes more than 1h and half is taken. A Restriction Orifice (RO) is used to depressurize the system to avoid reaching -46 deg-C (pipe design temp.)The RO pressure drop was designed for 300 bars.
In fact the pressure drop is about 200 bars, how can we determine the new orifice diameter (should be enlarged)to allow a short depressuring time (30 minutes for example) and avoiding to reach the pipe design temperature ?


Best Regards.

 

[hacksaw]

piece of cake, the flow is proportional to the orifice area.

simple ratio exercise...

 

[krsmani77]

Can you clarify wheher you are talking about upstream(to orifice)temperaures OR downstream pipe temperature. What is the volume hold up of the piping system whose temperature should not fall below -46 deg C? Is it being depressurised to atmosphere without any piping d/s of the orifice? Is the piping insulated? what is the insulation and thickness? The problem is one of un-steady state compressible fluid flow and not as simple as "hacksaw" feels. There is an old thread in this ( or is it fluid flow forum?) that has a lot of people answering a very similar query? Did you try the "search" of this site with "all forums"selected? Best of luck

 

[nasroh]

Thanks to you both hacksaw and krsmani77.


krsmani77, I am talking about downstream pipe temperature.
The fluid depressurized thru a piping system then flare system.

Thanks again for your help with my best regards.

 

[25362]

For critical flow conditions of compressible fluids, the maximum flow depends, in fact, on the upstream conditions, and on the area of the orifice (hacksaw), but also on the orifice discharge coefficient. The later is a very non-linear function of the Reynolds number and the beta (orifice to pipe diameter) ratio. See Perry VI.

 

[CHD01]

Why don't you simply control the downstream pressure in the low pressure piping with a pressure control valve to prevent a temperature below the pipe design temperature. Is the pipe on the low pressure side not rated high enough to do this?

Otherwise, develop correlation for enthalpy each side of orifice to calculate temperature drop of fluid and % flash (if a liquid). The harder part is to set up an equation for the heat loss to atmosphere. Once done, then set these equations up in an EXCEL spreadsheet and examine smaller time intervals (100 intervals?) to get a good apporximation. Of for a quick estimate average calculations done at initial conditions and at final or equilibrium conditions.

The more you learn, the less you are certain of.

 

[krsmani77]

Check out this old query in one of the Engineering forums"Time required to exhaust air through an orifice"-use advanced search with "all forums" and "any date" selected. It has some 44 threads and is quite informative, Best of luck.

 

[krsmani77]

Further details are:
Mechanical Engineers Area - Piping & fluid mechanics engineering Forum
cbuck (Mechanical) 11/11/02 (posted 09/10/02) 44 replies
"Time required to exhaust air though an oriface "