water content in natural gas 3

[kloroform]

hi all,

i have a confusion about water content in the natural gas. the amount of saturated water in natural gas usually follows Mcketta and wehe chart (fig 20-3) in GPSA book. i have gas analysis data, the sampling pressure was at 344 psig,and 82 F. according to Mcketta and wehe chart, the amount of water in the natural gas shall be around 65 lb/mmscfd. but the amount of water in the gas analysis report is different, (it is just 13 lbs/mmscfd). the difference between actual and from the chart is 52 lb/mmscfd. the natural gas from the well is saturated with vapour, there is no dehydration unit in the gas processing plant (it is only separator, gas scrubber, utility gas filter).

i dont understand why the actual water amount in the natural gas is far below the amount of saturated water at the given pressure and temperature according to Mcketta and wehe chart. is it possible to remove water vapour by coalescing process in the gas scrubber and gas filter ?, i think gas filter or gas scrubber is just to remove the water liquid droplets, not water vapour.

thanks

 

[zdas04]

There is no mechanical method to separate water vapor from other gases. Your coalescing elements won't touch it.

When I was working on my masters thesis, the project was the impact of water vapor on natural gas measurement so I spent a fair bit of time trying to quantify water content of gas. The biggest sources of uncertainty were:
[ul]
[li]Knowing the point where the gas was saturated. Gas will be at 100% RH for the temperature and pressure of the last place it was in contact with a coherent gas/liquid interface. You just do not know where that point was and from that point on everything is just a guess.[/li]
[li]Instrument uncertainty. Draeger tubes are notoriously difficult to get the right amount of gas through and they are hard to read with any repeatability. Electronic instruments become unstable in the presence of the smallest amount of liquid water.[/li]
[li]Samples to the lab arrive at a very different temperature than they left the wellsite. Since all of the GC equipment has very low tolerance for liquid water the lab techs often drain the liquid (even though they are not supposed to) prior to heating the sample. That is a major problem both for water and for heavier hydrocarbon products.[/li]
[li]The ASTM data and the charts are all based on a binary water/methane mix. A bit of CO2 has a huge impact, a trace of H2S or heavier hydrocarbons are game changers.[/li]
[/ul]

I have the ASTM data that goes into the chart and assuming sea level, then 358.7 psia and 82F is 81.65 lbm/MMSCF. That makes your sample 16% RH, I've never seen a competent sample much less than 50%, so my guess is you were bitten by one or more of the problems that plagued my project.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[gaswell]

the Mcketta and Wehe chart gives maximum content of vater in vapor phase at specified temperature and pressure, additional water will condense as free water.
The reported difference may depend from several reasons

1) gas not saturated with water
2) errors in sampling, measurements etc.
3) errors reading Mcketta and Wehe chart
4) etc.

as noted by zdas04 small amounts of CO2, H2S etc. can influence results.

To get accurate values for water contents in natural gas instead of Mcketta and Wehe chart you may use the procedures discussed in ISO 18453 or GERG (2008), both these are reported to be very accurate, however they are complex and you may need a sofware as Prode Properties for calc's.

 

[racookpe1978]

You claim you are surprised the water vapor is "less than the chart" for saturated nat. gas?

Why could that particular sample of nat gas simply be "dry" that particular day and hour- there actually was not very much water in it at the time it was sampled?