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Slug Catcher Sizing with Symmetry Dynamics

One of the most challenging problems in flow assurance is dealing with slugging, particularly terrain or severe slugging. Without mitigation measures, these slugs will affect the process equipment downstream, potentially causing production upsets, piping and equipment damage. We have been able to predict the onset of these kinds of slugging for quite a while, and our ability to integrate pipelines with processes has facilitated analysis of different mitigation strategies. But this was often tedious and cumbersome because it required a user to supply a vessel size and level control strategy up-front. However, that information is not known at the Conceptual Design stages, where a “Required Surge Volume” is expected as an input to the vessel sizing step.  Therefore, VMGDynamics required manual iteration from the user to converge to an acceptable slug catcher size. But in Symmetry, we have added a built-in tool in the Pipe workspace to estimate peak surge volumes and slug catcher sizes.

Example

Our system is a single pipe with a feed flow spec and an outlet pressure spec. There is a dP orifice and a control valve so that we can explore that kind of slug mitigation strategy later. But for this example, the valve will be fully open.

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The pipe has a slightly declined section flowing into a vertical riser—the classic configuration for severe slugging:

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If we plot the holdup along the pipe with time, we can see the cyclic pattern of liquid building up and then slugging out the top of the riser. 

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The slug catcher sizing utility is built into the sink unit operation. To enable it, check the “Do Shortcut Slug Catcher Sizing” box on the sink form.

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This exposes the Slug Catcher Sizing tab. If we open that tab, we will see that it is currently empty. Once we run the integrator, it will record the necessary data.  After running the integrator for 2000 seconds, and clicking the “analyze” button, we get the following information:

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The utility has computed a required drain rate, a surge volume, and then a volume for the slug catcher. Unless specified, the drain rate is calculated as the average liquid flow over the time period. The surge volume is the maximum volume that accumulates after subtracting out the drain rate each time step. Finally, the slug catcher volume uses the liquid level and L/D ratio data to convert the surge volume into a vessel volume. 

That’s it! Or is it? As far as the sizing utility is concerned, that is the end. But there is more to sizing slug catchers than just a volume.

For example, the control strategy of the slug catcher can significantly affect its effectiveness; if your level controller is too aggressive, then the slug catcher won’t actually catch the slugs but rather just propagate them downstream. Symmetry makes it easy to take the results from our slug catcher sizing utility, and then simulate what it suggests. To do this, we can add a separator to the outlet of the pipe, with the volume that the sizing utility suggested. And we can add a level control valve which has been sized from the calculated or specified drain rate. Once we take these few simple steps, it is easy to run our pipeline slugging scenario again and validate this more realistic setup. 

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In summary, Symmetry’s new slug catcher sizing utility provides a streamlined methodology for calculating surge volumes and slug catcher sizes, which are important for initial design. At the same time, Symmetry’s integrated workflow makes it easy to transition to a more detailed analysis.  

Please contact your local VMG office for more information.

Kyle Macfarlan, Dynamic Simulation Developer, VMG Houston

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