Multiphase Flow regimes

[dreidpc]

My SAGD plant uses flash treaters to drive off residual water from our oil product. The pressure drop is induced by a pressure control valve upstream of the flash treater vessel. We experience severe erosion issues in the spool pieces downstream of the control valve. There is very likely sand in this stream that is being moved at high velocities once flashing in the line occurs.

The liquid and vapour velocities through the line place the flow regime as Slug Flow. I’ve read that this is an undesirable flow regime but I’m not sure if it’s for erosion issues or other various reasons. Increasing the amount of water in the oil would result in greater gas velocities and would move the flow regime into the Annular region. Would this help or perhaps make things worse?

 

[BigInch]

Slug flow is generally avoided due to the high changes in pressures and flowrates that occur as slugs travel the system. It can cause control problems as all target setpoints are typically run outward to the extreme ranges making recovery time consuming if not totally unstable altogether. In pipelines, extreme variations can be encountered as pressures build as liquid flows uphill only to reduce to vapor pressure as liquid cascades downward (at sometimes slope-critical flowrates). It is true, as slug volumes are building, gas velocities across the top may increase significantly thereby increasing erosion, if sand is in the gas stream, but I believe that would be a secondary consideration in addition to the control problems.

Annular flow will not cause such wide variations in control parameter swings and usually result in more stable and constant operations, but may invite other problems if the erosive particles or corrosive compounds are mostly contained in the liquid as opposed to the gas, etc. however annular flow is otherwise and generally a more desireable process control regime. I think annular flow would be the better option, simply due to the inherent control difficulties with slug flow, but wouldn't think that it would be any better in the long run over slug flow for erosion control purposes.

The best treatment for erosion is to either remove the offending particles, or if that is impossible, slow the velocity, but then you may get back into slug flows. Slower velocities are usually much more cost effective than increasing the hardness of materials, although I suppose you could find some specific examples to the contrary. This might be one of them. Evaluate your production rate in the annular flow regime, your maintenance cost and see if one pays for the other. Otherwise slow down and live with the wide variation in control parameters.

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