8 replies to this topic

[m42364236k]

dear colleagues

in the sizing of the restriction orifice vendor said that

due to high deferential pressure through the orifice we have chock flow

this RO has been installed after the BDV.

AS i think chock flow dos not have any problem for us on operation is not it?

he suggest to use the multi stage type of RO

my question are:

does it have any problem for us if the flow will chock

why the vendor said that he want to use the multi stage ,.

i think vendor wanted to reduce the noise due to this high pressure i think .it is reasonable to use the multi stage

but what about the chock flow

dos it need to prevent from chock flow

 

thanks

mike

[shan]

You may use multi stage RO to avoid possible hydrate or extreme low temperature by allowing the blowdown stream to be warmed up in the piping section between 2 RO's.

[latexman]

Less noise for you.  More $ for him.

[flarenuf]

shan   you wouldnt get any appreciable heat recovery in a short length of piping with the gas velocities in the pipe

 

latexman is correct   its all $$      save money use 1 RO and buy ear defenders .....

[fallah]

Not just $, if there is high differential pressure it can be a matter of safety and health...

[paulhorth]

Mike,

 

Every blowdown orifice is in choked flow. This is because choked flow will occur when the upstream pressure is greater than about 2 x the downstream pressure. This condition will apply to every blowdown orifice, I would think. Choked flow is not a problem operationally, though it will be noisy.

If your vendor was correct then every blowdown orifice in the world would have to be a multi-stage orifice. They are not.

 

When you size the orifice, you should check if it is too small to be practical, or at risk of blockage. This can happen if the blowdown is from a small volume, starting from high pressure, and designed to take the standard 15 minutes. In such a case it is better to use a bigger orifice and have a shorter duration.

 

Acoustic insulation can be applied to mitigate noise, and acoustic vibration checks should be made if the flowrate (hence the energy) is large.

 

Paul

[fallah]

Using shorter depressurizing duration needs the flow rate to be higher which, in turn, will result in:

 

-Larger size of BDV inlet/outlet lines...

-Larger size BDV...

-Higher flare load...

 

Then before deciding to do so, the technical and economical evaluations for mentioned matters should be performed...

[paulhorth]

Fallah,

In the general case, you are right. However, when I suggested a shorter blowdown duration, i had in mind a case where the blowdown flow was small, requiring a small orifice (say less than 2 - 3 mm). I think it is unlikely that such a case would be the governing case for the flare load, so increasing the flow would not have an impact on flare sizing. By the same reasoning, the size of the BDV and inlet line would likely be unaffected, since the gas velocity would be low here. The outlet line downstream of the orifice would need to be checked.

 

Paul

[fallah]

Paul,

 

Agreed...and in many cases your solution might can be applied, but with prior evaluation...