2 replies to this topic

Hi,

First time poster, long time reader. Recently I have been working on sizing safety devices for pressure vessels, particularly in external fire cases. One of the tools I have been using is a software that requires a two-phase flow specification (bubble flow, all vapor, all gas, homogenous, and churn).

Previously I have had the help of a senior member in determining the flow type during, but I haven't really grasp the difference between homogenous flow and churn flow. I understand the churn flow is a non-foamy type.

My question is: How exactly can I determine whether the solution I am using will behave in the churn flow manner or the homogenous manner during venting?

I've looked around the manual of the sizing software, and even tried finding a concise article from DIERS. I'd appreciate the input.

[pleckner]

There are two things going on. The first is what happens in the vessel and the second is what happens in the pipe/relief device. In the vessel, we need to determine which region we are in. This is where the terms Churn-Turbulent, bubbly, gassy come from. The term homogeneous flow (or not) is determined for the pipe/relief device.

If your system is water-like, then you will most likely have Chrun-Turbulent behavior in the vessel. This is characterized by fairly easy vapor/liquid disengagement such that for large heat input scenarios (e.g. fire), you will have all vapor venting after the liquid volume in the vessel drops below about 33% of full (saying another way, the vessel is at about 67% full).

To really find out how your system behaves you need to do a lab study (calorimetry testing) as it can only be determined experimentally. But again, if the system is water-like, you probably have Churn-Turbulent behavior in your vessel.

Whether flow is homogeneous or not has to be determined experimentally. The good news is that using homogeneous flow for your relief device sizing calculation will be conservative for the sizing portion of the exercise. BUT the bad news is, homogeneous flow is NOT conservative for your effluent handling system.

I hope this helps.

Thanks. The this really helped clear things up. Based on the what you mentioned to look at, I know have a better idea of the criteria I should be looking for.