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Terminology Explained: What is multiphase flow?

Multiphase - Karen Tay

Karen Tay

Because of its importance to the industry, in this edition of Terminology Explained, we cover the topic of multiphase flow.  And to do that, we have the writings of one of our experts.  Karen Tay is a Senior Pipeline Consultant with our Software Consulting organization in the UK.  She has extensive experience in the industry, and has a great skill in explaining complex concepts in straightforward terms.

Coupling the expertise from people like Karen, with the capabilities of DNV GL’s Multiphase module of Synergi Gas, is quite a powerful combination.  So take it away Karen!

Simply put, multiphase flow is the simultaneous flow of materials (gas, oil and water) with very distinct velocity fields in each phase. This is common in the oil and gas industry, especially in gathering systems where fluid drawn out from wells has not yet been processed.

ultiphase flow - Phase Diagram

Phase diagram: the pressure and temperature determines the phase

Complexity of multiphase flow explained – using a motorway analogy

Operating pressures and temperatures change as fluid flows along the pipeline. It is typical to send out saturated gas upstream, and transport it to distant receiving facilities downstream. Whilst it is important to maintain a single-phase flow throughout, there are occasions where it inevitably enters a two-phase region, depending on the fluid phase behaviour.

DOES IT MATTER? Try to imagine the following analogy:

Pressure  ⇒  Traffic
Pipeline  ⇒  Motorway
Gas  ⇒  Car
Liquid  ⇒  Truck

A motorway analogy for a pipeline: trucks are the slower, heavier liquids, and cars are the lighter and faster gases

For an equal number of cars and trucks entering the motorway, as the traffic decreases, the ratio of trucks to cars increases. This is because the truck is heavier and has more drag compared to a car resulting in “truck holdup”.

multiphase flow - liquid holdup

Multiphase flow – liquid holdup

Same applies to a two-phase flow. The lower density and viscosity of the gas phase causes it to flow at a higher velocity, relative to the liquid phase.  This is a characteristic known as slippage. This multiphase slip flow results in what is known as liquid holdup.

Unlike single-phase flow, pressure drop across a pipeline at low flow rate is dominated by hydrostatic pressure drop (gravity). This is due to the high liquid holdup, whereby gas is unable to sweep out the liquid effectively.

As flow rate increases, pressure drop decreases as more liquid is swept out. When the flow rate is high enough, the pressure drop then becomes proportional to the flow rate, due to the dominance of frictional losses.

Multiphase flow – a variety of flow regimes

When the two phases are forced to travel along the pipeline, flow regimes which are dependent on superficial velocities are developed.  Visualising the following event as a horizontal tank with liquid at the bottom and gas at the top may help. At low velocities, each phase appears to be flowing separately (stratified).

Flow regimes, illustrated

As gas velocity increases, waves start to form as the liquid is being dragged by the gas phase (stratified wavy).  When the waves get high enough, the gas phase can be throttled.  The flow then becomes intermittent (slug), which is generally undesirable.

At high gas or liquid velocities, the intensifying turbulence rips the liquid surface, forming droplets entrained in the gas phase (annular), or mixes the gas into fine liquid bubbles.

Hence, it is important that pipeline operators take note of the highlighted points: pressure and temperature change, phase behaviour, liquid holdup and flow regimes.  This will help to create an optimised operating envelope to avoid any damage to facilities and operational instability.

Now that we’ve piqued your interest, want to learn more about multiphase flow?  Learn about our full scale multiphase laboratory, sign up for training on the topic of multiphase, or a more specific software training for the gathering and midstream market!

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