Blowdown

[ianfire]

I am an experienced fire and gas detection / protection engineer but a little out of my depth with the following problem - any help would be greatly appreciated.

We have an elderly, but still useful,in house jet fire simulation program which is driven by the depressurisation curves of the local fluid (typically low order alkanes) through a hole in the system - this is the fire term - and any other routes - PSV, B/D valves.

I am uncomfortable that our present approach enforces Vapour & Liquid volumes / levels in a vessel based *solely* on the VLE associated with the fluid composition, system pressure, volume and temperature of that vessel.

My questions are:

Is the liquid level in a gas separator,(for example), set *solely* by the local VLE (I supect not) or is it dependent on external conditions like level and pressure controls.

If a VLE calcualtion does *not* determine the liquid level in a vessel, how should a simple simulator handle flash calculations as the system depressurises through the leak and the liquid vapourises to continue feeding the fire term?

Sorry if this question is trivial / puerile to you experienced process hands, but from where I'm standing it's anything but.

Thanks in advance

 

[EGT01]

Ianfire,

Regarding your first question, for the same temperature, pressure and composition, the VLE properties will be the same whether a vessel has a level of 10% or 90% so I don't see how that alone could determine the liquid level. Certainly, the VLE properties will be a factor in determining how much liquid needs to be removed from the separator but the liquid level will be a function of how the separator is intended to be operated. Maybe it has a simple overflow seal loop or maybe the separator is designed to have a certain amount of liquid surge capacity.

It sounds like your separator may have a level control, if so, you should be able to get information from the level instrument design to know the liquid level operating set point. If it is connected to your process control computer, then you should be able to look at past trend data. There may be additional high level alarms that you should look for that might be considered as upper limits to the liquid level control.

Regarding your second question, I can't speak to how your simulator works or how it is to be used. If a leak develops below the liquid level, then liquid would pass through the hole with the potential for some of the liquid to flash to vapor as it passes through. If the leak is above the liquid level, then only vapor is removed from the separator. I would think your fire term would be greater for the liquid release case.

If you have not seen this before, visit Milton Beychok's website.

http://www.air-dispersion.com/usource.html#top

He has a lot of good info about determining accidental release rates that you find helpful.

 

[ianfire]

EGT01
Thanks for your reply - for the pointer to Milton Beychock’s site. I had come across it before but had forgotten about it (although it does not address the issue I have raised above).

My apologies for not being clearer with my question:

You have confirmed what I suspected; namely that the process control system controls the liquid level, which in turn confirms that we have a serious problem with our fire simulator.

The issue with a gas release from a liquid filled vessel (assuming the liquid is volatile) is that as gas escapes through the 'hole', the liquid flashes to maintain the leak. In order to calculate this decay curve, we need to do a series of EOS and VLE calculations.

The programmer of the fire simulator however was apparently unable to cope with ratios of liquid/vapour that disagreed with the EOS / VLE calculations hence the present program ‘enforces’ an initial liquid volume in the vessel based on the input PVT (apparently on the basis that anything else would not be in equilibrium).

While this has confirmed our problem I confess I am no nearer seeing how it should be done;

Do you/anyone know how full blown process simulation programs such as PRO II (which I know can do this) address the issue of depressurising a vessel where the starting (and subsequent) volumes of gas and vapour differ from a VLE calculation at the working pressure?

Or have I missed something fundamental here perhaps.

Thank you

Many thanks