5 replies to this topic

[frpe]

I have a 6.5 Km natural gas (C1 about 78 % ) pipeline (80 Bar) to be depressurized (Atm),

a manually operated valve + RO allows to reduce the pressure of system before maintenance, depressuring time is about 8 hours.

I have simulated the process with a series (5) of PIPE() (pipeline segments) operations available in Prode Properties, each segment keeps in account for the different heat transfers due to soil conditions etc.

due to the relatively long depressuring time the resulting temperature curve is well above pipeline minimum design temperature, no problems there.

the problem concerns predicted gas temperature at RO (Restriction Orifice) which goes well below hydrate formation temperature predicted by Prode Properties, 

with Prode I have estimated the amount of methanol required to avoid hydrate formation : due to relatively high water content in gas (not saturated) the required volume of methanol is not low.

 

My questions are, 

1) are (usually) inhibitors adopted in these cases ?

2) GPSA discusses Hydrate Inhibition in chapter 20 but it seems difficult to compare values, see below 

21) method is based on McKetta and Wehe graph, a problem is that free water estimated with the graph seems different from values calculated with ISO 18453 or GERG 2008 or CPA-PR which don't predict free water while the graph does (I must admit that it is not easy to get accurate values from a graph for a 0.000... water content)

22) GPSA shows (figures 20-61 to 20-64) values which I am unable to compare (I have not acccess to the paper by Chen, Robinson "The Effect of Ethylene Glycol or Methanol on Hydrate Formation in Systems Containing Ethane, Propane, Carbon Dioxide, Hydrogen Sulfide or a Typical Gas Condensate" Gas Processors Association, September 1985), however it seems that different methods may predict different values and I am unable to understand which is the specific area of application of each method to compare values.

 

Could you provide some help  ?

Thanks. 

[marchem]

by design RO works at critical condition, hydrates and ice should form after RO,
if there is some large space immediately after RO (for example a separator) formation of solid phase may have a limited impact
and you could (manually) stop depressuring to allow hydrates to dissociate (providing heat etc.),
instead, if there is some small piping (for example to a flare), you should consider the possibility to have a plug and pressure rising to pipeline pressure....

About inhibitors, different companies adopt different procedures and it is difficult to suggest a rule, for example consider that MeOH is much more toxic than MEG but there could be several reasons to prefer different methods or different products...

About water content in natural gas I would stay with ISO 18453 which gives certified values (within its range of application),
for water contents the ISO 18453, extended Peng Robinson and GERG models available in Prode Properties give similar results.

I have not access to the paper "The Effect of Ethylene Glycol or Methanol on Hydrate Formation in Systems Containing Ethane..." and I can't comment,
however similar methods are discussed in books such as Clathrate Hydrates of Natural Gas (Sloan) or  Natural Gas Hydrates (Carrol).

Finally, I can observe 30-60% (and in some cases higher) differences in amount of inhibitors calculated with different methods,
may be someone will claim that he can estimate the exact values but in practice I never (except by chance) observed that accuracy.
 

[frpe]

thanks marchem,

MEG is an option,

another option is heat tracing,

do you think heat tracing (power 150-200 W/m) could provide a way to avoid hydrate formation ?

[marchem]

sure heat tracing could be a solution,

however to be effective you need to keep pipe wall at a temperature above hydrate formation,

installed power will depend from fluid flow, properties etc. etc.

there are correlations in literature or you may ask the supplier...

[frpe]

thanks,

following your advice I have estimated  the power (W / m) required to avoid hydrate formation,

given gas flow, temperature, density, thermal conductivity etc.  I have calculated the heat required per mt of pipe to maintain the pipe at temperatures above hydrate formation, 

the value is more than two times the value proposed by a manufacturer... 

interesting lesson :-)

[serra]

those differences (your calc's and manufacturer selection) do not seem too unusual to me for heat tracing,

unless you specify exactly the amount of power required manufacturers will apply their own methods which may give different results.