5 replies to this topic

[mosa]

To purge a pipeline till hydrocarbon free of certain 'x ' Kms length and 'y inch 'pipedia carrying HC of 'z 'density using scraper tool pushed by 'p' bar pressure nitogen.What are the steps involved to calculate Nitrogen qty in NM3/HR and lits/hr and the approx 't' time taken to free the line from HC.

[djack77494]

mosa,
Since you are dealing with a pipeline your length is >>> diameter. Thus if you admit nitrogen at one end and displace your hydrocarbon vapors at the other end, the nitrogen can be very effective in purging. Assuming a reasonable purge rate (and I won't hazard a guess as to what that would be) you might need little more than the volume of the pipeline itself. Practically speaking, you could determine when your purging has been effective by looking for HC vapors at the discharge. Hopefully the nitrogen would act sort of like a piston and very effectively displace the HC. If so, you'd see a period of only HC flow followed by a rapid drop in HC concentration and then just nitrogen.
HTH,
Doug

[mosa]

mosa,
Since you are dealing with a pipeline your length is >>> diameter. Thus if you admit nitrogen at one end and displace your hydrocarbon vapors at the other end, the nitrogen can be very effective in purging. Assuming a reasonable purge rate (and I won't hazard a guess as to what that would be) you might need little more than the volume of the pipeline itself. Practically speaking, you could determine when your purging has been effective by looking for HC vapors at the discharge. Hopefully the nitrogen would act sort of like a piston and very effectively displace the HC. If so, you'd see a period of only HC flow followed by a rapid drop in HC concentration and then just nitrogen.
HTH,
Doug

Dear Doug

Thank you for the information, however i require to know the Nitrogen quantity for purging to confirm wheteher our Nitrogen production plant is adequate or we have to procure N2 bottles.

regards
mosa

[sgkim]

Mosa,

I would like to recommend to use a vacuum pump if the pipe is bearable under vacuum.

(1) Time required for vacuum purging from P1 to P2 would be approximately

Priming Time required, t = (Total Pipe Inside Volume,V)/(Vacuum Pump Capacity, v)*ln (P1/(P2-Pu))
Pu = Ultimate Pressure of the vacuum pump.

Once the pipe is pureged up to P2, the concentration of residual hydrocarbon (HC) in the first N2 charged to replace "one system volume" in the piping would be:

Conc of HC in one system volume of N2 = P2/P
P = Absolute Pressure of N2

(2) If, for example, P2 = 50 torr, P1 = 760 torr, Pu = 20 torr, d = 4 inch or D ≒ 0.1m, L = 5 km, v = 0.5 m3/min, P = 7600 torr (10 atm) then,

System Volume, V, m3 = (π(0.1^2/4)(5*1000) = 39.25 m3
Time Required to Purge from 760 torr to 50 torr
= (39.27/0.5)*ln(760/(50-20)) = 253.7 min. or 4.23 h

Residual HC centration in the first one system volume = 50/7600 = 0.66 v%.

(3) If the concentration is not satisfactory, then repeat the vacuum purging after breaking the the vacuum by N2.

The residual HC concentration after N-times' repetition of vacuum purgings,would be

C after N-times of vacuum purging = (P2/760)^N*(760/P)*(100 v%)
The units of pressures P2 and P are in torr.

The 0.5 m3/min vacuum pump is a very small one.

Stefano

[ashadiazwan]

To purge a pipeline till hydrocarbon free of certain 'x ' Kms length and 'y inch 'pipedia carrying HC of 'z 'density using scraper tool pushed by 'p' bar pressure nitogen.What are the steps involved to calculate Nitrogen qty in NM3/HR and lits/hr and the approx 't' time taken to free the line from HC.

Based on my experience normally it takes 3 times batch N2 purge for hydrocarbon free. Last time I just used ideal gas just to have some initial guess on N2 required. What u can do is calculate volume of pipeline to be purge and determine the mass of N2 required at purge pressure (for Temp u can just assume to be ambient,normally it will be lower than that) and multiply by 3(if purge 3 times) .Based on calculated mass than determine the volume in whatever cond.u like such as Nm3/hr .standard cond..or etc. and times purge)

[Technocrat]

dear mosa,

to size purge lines, the following criteria should be used:
1. calculate volume of the equipment or piping to be purged.
2. multiply volume by four.
3. assume 10-hrs purge period.

purge line should not be designed to be permanently connected to a high pressure process vessel. a piping spool piece should be removed from the purge line after the plant is on-line. this is required to prevent high pressure hydrocarbons from backing into the purge system. (reference: process design for reliable operations, by norman lieberman, second edition, gulf publishing company, chapter 16, page 201)

to avoid ga spocketing the nitrogen flow should be turbulent for better mixing between nitrogen and the hydrocarbons and hence good purging. you have to cross-check effectiveness of purging by portable explosimeter or a chromatographic gas analysis.

regards.