4 replies to this topic

[modgasman]

Happy new year all (although technically speaking I am 17 days overdue)

Please I need your expertise with regards to the following:

I need to size an RO to be fitted downstream a blowdown valve in a natural gas processing facility. The initial pressure is 1640 psia, and the RO empties into a flare KO drum which vents to the atmosphere via a flare stack.

1 - When doing my calculations, if I take 1640 psia as the pressure P1 upstream the RO, would I be correct to take the pressure down stream the RO P2 as atmospheric? (Since it vents to atmosphere).

2- If my P1 and P2 assumptions above are correct, then P2/P1 (14.7/1640) is less than 0.75. In this case, according to ISO and API 14.3, the equations given in these standards for expansion factor Y can not be applied. Is there another equation for expansion factor I can use for P1/P2 less than 0.75 or do I take it out of the calculations entirely?

3 - In API 14.3, what value should I use for the term M1 (this is associated with the tapping positions in of the RO). It says in API 14.3 page 54 that M1 = max(2.8 - (N4/D), 0.0). What does this mean?

Thank you all in advance for your valued contributions.

Edited by modgasman, 17 January 2013 - 07:55 AM.

[Bobby Strain]

If you search this site, you can find a program for downloading at Mediafire. Append my name for a successful search.

Bobby

Edited by Bobby Strain, 17 January 2013 - 11:39 AM.

[breizh]

http://www.pipeflowc...ns.com/orifice/

Consider this resource to support your query.

Breizh

[kkala]

1. Flow would be choked due to high (absolute) pressure ratio P1/P2. No difference in flow rate is anticipated if P2 = 0 barg or 2 barg (for example); yet in such case we specify P2 in orifice data sheet by calculating downstream ΔP (for the flow rate of choked flow) up to a point of known pressure, probably just to know downstream flow conditions.
2 / 3. Instrument Dept undertakes the rest till bid processing, so I do not have experience. Searching the web, attached "AFT-Modeling-Choked-Flow-Through-Orifice.pdf" is found useful, though a bit hard to study. Following is based on this attachment and subject to comments by members with expertise.
α. For precise measurements in choked flow, the system design should incorporate a flow nozzle in lieu of an orifice (p. 13), http://www.engineeringtoolbox.com/flow-meters-d_493.html#Flow_Nozzles.
β. ASME formula (p. 6) is not applied for choked flow, even though expansion factor Y is used. Cunningham (p.3) seems to apply for chocked flow (not explicitly noted). Check API 14.3 for any clarification on M1, Y, concerning choked flow.
Note: Probably local Instrument Dept specifies flow nozzle in case of choked flow, but we do not follow the procedure.

Attached Files

•  AFT-Modeling-Choked-Flow-Through-Orifice.pdf   164.57KB   576 downloads

Edited by kkala, 21 January 2013 - 07:52 AM.

[modgasman]

Thanks Kkala,
The document was somewhat usefel; hard to follow, but useful. It provides the equation to be used for expansion factors where P2/P1 is less than 0.63.
Y = Y0.63 - 0.3051(0.63-r); where r = P2/P1. What it doesen't say clearly is the source of the equation.
I found another equation Y = Y0.63 - (0.49-0.45beta^4)*(0.63-r/k) on this site for Y where P2/P1 < 0.63. Again, the seorces have not been clearly stated.
Note that Y0.63 is the value of Y calculated using the ASME equation for Y when P2/P1 = 0.63.

Edited by modgasman, 22 January 2013 - 04:52 AM.