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Free water content in heavy fuel oil

Free water content in heavy fuel oil

Free water content in heavy fuel oil

(OP)
In 2007 we worked on a heavy fuel oil (max s.g.~0.96, it could be higher) transfer to boilers by use of centrifugal pumps. To reduce its viscosity, the oil had to be heated to about 60 oC in the suction atmospheric tank. The latter received fuel oil from a bigger similar tank and this from ships.
The question was whether water vapor pressure at 60 oC (~0.2 bara) should be added to (rather low) fuel oil vapor pressure for the calculation of pump NPSHa. Eventually water vapor pressure was not considered, on the ground that all free water content of fuel oil will settle down in the bigger tank upstream. But this assumes that all emulsions between fuel oil and water will brake down upstream of the pump. I do not know under what conditions this is realizable, it seems that these emulsions are not broken down easily. Besides s.g. difference between the specific fuel oil and water is quite small, while heating in the bigger tank (receiving oil from ships) would be somehow lower than 60 oC. Is it feasible to assume that fuel oil has got rid of free water on its arrival to the centrifugal pumps? I think it is mainly a matter of mentioned emulsion stability, that is why the question is placed here.
On the other hand no consideration of water vapor pressure has been detected on several seen fuel oil pump data sheets. Advice or opinion on this matter would be appreciated.
It is noted that water and fuel oil are practically immiscible, so vapor pressure of the mixture is the sum of the two vapor pressures, at any proportion between them. NPSHa issue has been previously discussed in <http://www.cheresources.com/invision/topic/9571-np...; .

RE: Free water content in heavy fuel oil

Dear, Somehow I am of the opinion that if any mixing is affected in these tanks for homogeneous intake for pumps and indirectly supporting uniform calorific value.
Then the water/water vapors are bound to find their way into the storage tank's vapor space.
Having said that,it is also a plausible idea that such mixing devices if employed do create some emulsification effect on the storage tanks contents i.e. free water getting physically dispersed into the of heavy bulk fuel oil which sometimes supported as per sulfur, nitrogenous and any metallic compounds impurities presence.

Hopefully it will throw some light on the issue!

Best of Dear kkala.

Best Regards
Qalander(Chem)

RE: Free water content in heavy fuel oil

(OP)
Thanks, Qalander(Chem), I have also read of such mixing (not seen in practice). In this case fuel oil vapor pressure at pump suction would be the sum of water and oil vapor pressures. But this has not been detected in the fuel oil pump data sheets locally seen.
Can fuel oil - water separation be so effective in all cases? My impression (from reading) is that fuel oil - water emulsions are common and not easily "broken". Can they be "broken" just in a settling tank of (say) 1-2 days retention time?

RE: Free water content in heavy fuel oil

(OP)
In addition to above post, <http://www.cheresources.com/invision/topic/16670-e...; and mentioned web references indicate it infeasible to break crude - water emulsions at temperatures as low as 60 oC in a short time. Mentioned fuel oil settling in tanks would occur in few days (longer time), but at temperature 60 oC (max). How can effective separation of water occur in this way? Fuel oil vapor pressure does not include water vapor pressure, at least in local fuel oil pump data sheets seen (centrifugal or rotary pumps). No problem has been heard of. Some explanation would be appreciated.

RE: Free water content in heavy fuel oil

For a heavy fuel oil having higher viscosity

we may anticipate settling out faster in case storage temperature stays around 80-85 celcius.

But should be around 2 days I assume.

Best Regards
Qalander(Chem)

RE: Free water content in heavy fuel oil

(OP)
Thanks Qalander (Chem), higher temperatures and longer settling time will promote water sedimentation. Will 2 days at 85 oC be able to brake even emulsions? Probably yes; more data on it (available by anybody) would be desirable.
Here stored fuel oil is not heated more than 92 oC (starting from lower temperatures) to avoid boiling of its water content (boil-over).

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