8 replies to this topic

[Anup87]

Dear All,

Will you please tell me the meaning of following message in Flarenet:

"Possible slug flow in piping segment"

Why this message comes and how flarenet detects the possibility of Slug flow. And how much important is this message.

Thanks.

[fallah]

Anup87,

This message tells about the possibility of the condensation/slug formation in the piping segment and Flarenet would detect it by considering the saturation conditions as a reference. It is very important issue in a flare network might lead to catastrophic shocks and vibration and liquid carry over from flare tip, especially if would occur after relevant KO Drum.

[Anup87]

Thanks Fallah.

I am getting this message in tale pipes for many of the valve in my network.

Will you please guide me, How do we mitigate this issue?

[fallah]

Anup87,

Depending on the conditions, heat tracing or methanol injection may mitigate the issue.

[flarenuf]

Anup

The first thing to realise is that Flarenet is performing a steady state analysis based on the information you have provided. In reality you may not have slug flow depending on the duration of the relief flow and the backpressure in the header system. You also say that you have these messages in the tailpipes, indicating possible two small a pipe diamter. One thing to remember is that if you do have a slug in a tailpipe then when this flow enters into the downstream header the slug , if any will collapse as the header will have a larger diameter.
Providing the tailpipe has a clean run into the header ie no 90 deg bends then you should be Ok in regard to slug hammer.
if this is a new design then the easiest way to solve this is to increase the tailpipe sizes. i would also look at the composition of the fluids you are relieving , if you are using MW basis then FNT will generate the composition itself and this may be a cause of your problem

flare e nuf

[Anup87]

Thanks Flare n nuf.

Compositions have been manually entered on mole basis. Most of the components are Hydrocarbons (C2 to C8). I am doing revalidation study for existing flare header. I did not understood the meaning of:

Providing the tailpipe has a clean run into the header ie no 90 deg bends then you should be Ok in regard to slug hammer.

Will you please elaborate.

Also, slug flow message vanishes as flow enters the header. I think there again flashing is occurring and flow becomes single phase. Please correct me if this is not true.

[flarenuf]

hi
sorry if i didnt make this clear.
2 things FNT uses the Baker chart to identify the flow regime that you are in based on the liquid and vapour flow. In your case it has identified it as slug flow. These correlations are based on long lengths of piping where the gas , flowing over the liquid, pushes a wave forward, for want of a better term , until the wave fills the pipe cross sectional area forming a slug . In short tailpipe there probably isnt sufficient time for the liquid to condense out and form a slug before it drops into the larger header.
If slugs are present they cause problems when they come up against a 90 bend(s) as the momentum can be sufficiently high enough to actually distort or rupture a pipe. Hence keeping a straight line, no bends with a 45/30 deg tee piece into teh header will reduce any possibility of this happening.
Slugs will collapse when the flow passes into a larger header simply because the T,P changes and vaporisation occurs or due to the increase in cross sectional area . What fills a 4" pipe will not fill a 12" pipe

hope this is clear now

[Bobby Strain]

Slug flow occurs at low velocity, i.e. large pipe diameter. Check the Baker map as suggested.

Bobby

[Anup87]

Thank you very much Flare n nuf for guiding me. Now I am bit confident in dealing with the Slug Flow message in Flare net.

Will you please tell me is the same chart (Bakers Chart) is applicable for sizing of two phase piping and if possible could you provide good reading material to clear this fundamentals to next level.

Also does Beggs and Brills follow the same chart for pressure drop calculation.