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Chemical Process Technology

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Tuesday, March 2, 2010

Two phase gas liquid flow is commonly occur in any oil & gas production and processing system. Several flow patterns can occur in two phase gas liquid flow. There are Bubble flow (with minimum vapor bubble), Plug flow, Stratified flow, Wavy flow, Slug flow, Annular flow and Mist/Dispersed flow (with minimum liquid droplet). Previous post "Problems Caused by Two Phase Gas-Liquid Flow" has discussed about obvious and non-obvious location where two phase gas liquid can present. Problem caused by two phase gas liquid flow such as Impingement erosion, Splashing erosion, Cavitation erosion, Flashing erosion, Flow induced vibration, Surge / hammer, Noise, Decrease process performance and Phase separation possibly lead to severe consequence. Minimizing / prevention of two phase gas liquid becoming important during design phase.

As discussed in "Some Measures To Prevent Problem Caused by Two Phase Gas Liquid Flow", the destructive level of two phase gas liquid flow varies with flow pattern. One shall try to design their system to avoid Slugging flow and/or plugging as slugging/plugging can lead severe vibration and erosion. The destructive level reduce from slugging flow to stratified flow, and minimum at annular, mist and bubble flow. So a process engineer shall design their system to move away from slugging flow.  

Slugging flow in Horizontal pipe
Slugging flow can occur in horizontal pipe and vertical pipe. However, the flow pattern may be slightly difference. You may view slugging flow in horizontal pipe in below video clip.

Slugging flow in horizontal pipe

For slugging flow in horizontal pipe, trapped vapor is main stay at the top of pipe, flowing at high velocity pushing liquid slug move forward. Liquid slug right in front of trapped vapor is accelerated by the vapor flow. As slug velocity is increased and couple with gravity force acting on the slug, liquid slug becomes unstable. Liquid slug will destroy and liquid slug is dropped to bottom of pipe.Trapped vapor at the back of liquid slug loss resistance (due to liquid slug) is accelerated at high velocity. As the vapor is moving at high velocity where it is exceeded the inception velocity, liquid at the bottom of pipe is moving upward to the top of pipe, close the vapor gap and form another liquid slug. This process of slug formation, accelerates by vapor flow and slug destroy due to gravity force and weak surface tension will repeat continuously. The repeated cycle will generate severe vibration, noise and erosion to the pipe.

Below is a video clip for slugging flow in vertical upward pipe.

Slugging flow in Vertical upward pipe

Slugging flow in Vertical upward pipe
The vapor is trapped in the liquid flow and flow upward together. Large trapped vapor is sometime called Taylor bubbles flowing upward and liquid is separating Taylor bubbles. Between two Taylor bubbles, smaller bubbles are following upward as well. These bubble is rather unstable. They may coalesce to form larger bubble and join the large Taylor bubble or they may destroy due to liquid movement. As the Taylor bubble moving upward, liquid is moving at lower speed between Taylor bubble and pipe wall will tends to entrain vapor to form smaller bubble.

Special thanks to fernandoagf1 & tmccorkle6719

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posted by Webworm, 5:09 AM


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