6 replies to this topic

[linamus]

Hi All,
I was asked to calculate the RO size for purge gas to flare header. The purge gas consumption is 10.5 scfm and the purge gas line is 1" (DN25). My final result gave me 2mm RO size.

However, this is so small compared to the usual 6mm or 11mm RO size. I could not increase the size, since it will increase the purge gas consumption.

Is there such thing as "minimum RO size"? I heard 4mm but could not recall where did I read it.

Please advise.

[JoeWong]

Linamus,

Why not share you basic input data and how you reach the RO size so that CheJedies can verify your calculation for you ? Just suspect you made common mistakes in your basic parameters i.e. downstream pressure and co-efficient of discharge.

[linamus]

Joe,

My input data:

Flowrate: 10.5 scfm
MW: 18.17
Density: 7.472
Viscocity: 0.011 cP
z: 0.9692
Cp/Cv : 1.324
Discharge coefficient: 0.83932
Pin: 120 psig
Pout: 5 psig

From this data, I obtained 2mm RO size. Is it wrong? Pls advice.

[Art Montemayor]

Linamus:

You asked us to advise you and Joe Wong did so, requesting that you submit your calculations so that members could follow your calculation logic and even check your decimal points.

Instead, you sent us basic data input (which you should have submitted in the first place) and a statement about the calculation print out.

Don't you value the availability of having a peer check your detailed calculations? Right now, in my company a peer check is worth US$75 to $90/man-hour. I would think that any professional engineer would jump at the opportunity to have his/her calculations checked by a peer. I detect, but don't understand, the reluctance to have one's calculations looked at and checked.

We still don't know how you arrived at your result. And that leaves us with little to comment on.

[linamus]

Art,
I'm sorry you feel that way. There's a reason for me not posting the calculation. It's because, the spreadsheet calculation is my company's copyright. I have no right distributing it.
However, you are right. There is no way for someone to say whether my calculation is right or wrong without the spreadsheet. Hence, I've attached the calculation sheet for your review.

Please take note also, even tho formulas are shown, but the calculation is not. Only the final figures are shown (calculated values are in red). You should come to the same conclusion if you follow the formulas shown there.

Attached Files

•  ro_xx.xls   35KB   155 downloads

[ankur2061]

Joe,

My input data:

Flowrate: 10.5 scfm
MW: 18.17
Density: 7.472
Viscocity: 0.011 cP
z: 0.9692
Cp/Cv : 1.324
Discharge coefficient: 0.83932
Pin: 120 psig
Pout: 5 psig

From this data, I obtained 2mm RO size. Is it wrong? Pls advice.

Linamus,

Your flow rate for the purge gas is very low hence your orifice size is bound to be small. I inputted your data considering a 2 mm (0.0787 inch) orifice size in my spreadsheet and I get a flow rate of 32.087 lb/h (0.53 lb/min) which is what corresponds to 10.5 scfm and a standard density of 0.05 lb/ft3. The calculated critical pressure ratio is 0.541 for your specific heat ratio and for your case the flow is "Critical" since your P2/P1 ratio is less than the critical pressure ratio.

Hope this helps.

Regards,
Ankur.
Ankur.

[JoeWong]

I have a quick check on your calculation.

The discharge coefficient used is good. The equation used looks for a critical flow. My calculated orifice is pretty inline with yours. OK. You have avoided the common mistakes.

In view the 2.0 mm (infact slight smaller than 2mm, 1.5-1.6mm ) is rather small (pretty inline with Ankur results), i would suggest (i have seen in specification) 3mm as minimum to avoid blockage due to welded slag, rust cause by galvanic corrosion, etc. Slightly higher purge flow will provide additional margin and reduce negative uncertainties and calculation accuracy.