26 replies to this topic

[kkala]

A. After the clarifications by irprocess, following scheme is assumed.
1. Fuel gas is directed to KO (knock out) drum, which feeds LP fuel gas header through a 3" line.
2. Mentioned 3" line has a 2" branch line to continuously send surplus fuel gas (exceeding gas demand) into LP flare header through the referred PCV. The latter keeps constant pressure in the 3" line by diverting proper part of gas flow to flare all the time of operation (or at least long enough to consider flow of 2" line as non relieving / venting).
3. Max operating pressure Pmax of flare header at 2" line connection point (β) is 0.5 barg. Total geometric length of existing 2" line is 50 m.
4. PSV of KO drum is set at 8.7 barg and discharge is sent to LP flare through separate 4" line.

Comments on the above are expected from irprocess, particularly on bold points. PCV data sheet specifies a valve outlet pressure of 0.5 barg, but this would result in (say) ΔP~15-18 bar in the 2" line from PCV to header. Pmax=0.5 barg (or a bit higher) is judged common in flare headers, please try to find what Pmax has been specified.
Presently assumed non intermittent flow through PCV does not allow sizing it as a relief or venting line (generally smaller diameter), but as an ordinary operating line transferring gas.

B. Attached "PCV.xls" presents indicative pressures & velocities in the line, from PCV to header, for several line diameters, as well as required data for the PCV, notes, etc, all preliminary. The intent is to estimate pressure at PCV outlet (at max or normal flow) starting from the "given" 0.5 barg at (β), then estimate required PCV characteristics. Rough conclusions support 6" line connecting PCV to flare header (under above assumptions) and that existing PCV may be suitable for the service. Keeping existing 2" line does not seem possible for the flow rates reported in existing PCV data sheet, even if PCV is replaced. Looking into PCV.xls can reveal further info, despite the "primitive" method used in fluid flow calculations. That is why recalculation is needed by a more rigorous method for compressible fluids, along with more precise data (fittings, check of data sheets, etc), eventually after commenting the "PCV.xls".

Attached Files

•  PCV.xls   36KB   76 downloads

Edited by kkala, 15 November 2011 - 03:08 PM.

[kkala]

Though not of much importance to main topic, compressibility factor of fuel gas was touched. Since gas composition has been given, attached "FGCFr0.xls" (conservatively) estimates a compressibilty factor of 0.969 at 31 oC and 8.7 barg, along with some comments and explanations.
A rather quick way is applied, not calculating pseudocritical conditions of the gas mixture, only the compressibility factor of each component. Same way was used on a slightly different gas mixture on 25 Oct 11, giving a factor of 0.9752 (FGCF.xls). If pseudocritical conditions of whole mixture were used on same mixture, resulting factor would be 0.9782 (difference insignificant).
Basis of calculation is the known spreadsheet offered by breizh. Individual compressibility factors are taken from there.

Attached Files

•  FGCFr0.xls   903.5KB   60 downloads