3 replies to this topic

[cea]

Dear all-

On the subject matter, I read few articles on net including some post from Cheresources. Following is my understanding & conclusion.

The concept of self venting line is applicable to two phase flow, especially in vertical line, wherein gravitation force is considerable for liquid particles. Thus Froude number applies here. Following is the correlation for Froude number.

Froude Number = Inertial force / Gravitational force = V /gD


The Froude number is applicable when flow regime is “Annular”. In annular flow, liquid flows across pipe diameter, thus by creating annuli for gas phase to flow.

In case of vertically downward flow, liquid downward flow sucks gas phase.

In case of vertically upward flow, liquid particle are subjected to gravitational force which drags some of liquid in downward direction, which further sucks gas phase thereby causing localized two phase flow of other regime than annular.

In order to avoid this “back mixing”, it is necessary to have low superficial velocity, which can be ensured by ensuring Froude number less than 0.3.

Thus (Conclusion:)way of ensuring self venting in vertical segment is to increase the diameter of pipe so as to have Froude number less than 0.3.

(Conclusion:)For horizontal segment, the minimum recommended slope is 42 mm/m. However, I found detail calculation posted by Mr. Art..

Please confirm / correct my understanding.

Thanks & Regards

Edited by cea, 21 October 2009 - 02:18 AM.

[katmar]

Although I agree with you that self venting flow is a special case of two phase flow, I believe it refers only to those cases where there is no net flow of the vapour phase. In self venting vertical downflow any gas that is drawn downwards will be able to disengage from the liquid and flow back up so that there is no net downflow.

I would not refer to vertical upflow as self venting because the vapour will travel upwards, irrespective of the liquid velocity.

In horizontal (or near horizontal) self venting flow the liquid does not form an annulus and will flow along the bottom of the pipe.

The minimum slope for nearly horizontal pipe is given as 1:40 (i.e. 25 mm/m) by Hills, but this should be regarded as an economic minimum because it is quite feasible to design a pipe with a slope of 1:100 to run part full if it is large enough.

[djack77494]

I concur with Harvey as to the common meaning of "self venting". The most common usage is in reference to a liquid flowing out of a vessel, including column tray drawoffs. As a liquid flows from (say) the bottom of a drum into a pump suction, the engineer wants to ensure that the pump suction is flooded (liquid full). If a vortex were to form and pull vapors into the exiting liquid stream, the pump would run poorly, or worse. So the engineer sizes the drawoff line to be self venting. Then, even if a vortex forms/vapors become entrained, they will separate from the liquid and return to the source vessel.

[fallah]

One of most important applications of "Self Venting" is what may being considered as connection line between condenser and reflux drum through which the condensed vapor being moved as gravity flow .
This line is sized large enough to provide self venting that lead to pressure equalization in both equipment.
This large size line allows to displaced vapor in the downstream drum to flow backward into the upstream condenser at the same time.If this line haven't been sized large enough to provide this requirement a separate equalization line between two drums shall be considered.