5 replies to this topic

[Jon K]

Guys,

I'm struggling with a realistic approach to calculate die flowrate for a vacuum relief valve of a stripping column (sour water).

 

#1

First approach was to consider the amount of steam inside the column to be equal to the volume of the column. It is assumed that steam supply stops but water is still fed to the column. The (cold) water will immediately (within seconds) condense the steam. The vacuum breaker would need to have an extremely high (not practical) capacity.

 

#2

Second approach was to take the interlocking into account that stops the water supply if the pressure drops below a certain value. The vacuum breaker would then only be calculated for a further cooling (and condensing) of the steam due to heat loss (through the insulation) to the atmosphere. Assumed conservative heat transfer coefficient is 12 W/m²K. By doing this the capacity of the vacuum breaker would around 40 times lower than for apporach #1.

But....is it ok to do it like this? Interlocking might fail. And furthermore there is still liquid holdup inside the column. How to deal with this?

 

#3

Third approach. Just an idea so far: Similar than approach #1. The cold water will condense the steam inside the column. BUT it is assumed that the steel (some tons) of the column is hot. So the assumed dT/dt is heat_cap, water * delta T * flowrate, water / (heat_cap, steel * mass, steel)

 

Is this a realistic approach? Or wouldn't the heat inside the steel help at all because the heat transfer (and condensing) of the steam would be much much faster?

 

#4

Any other realistic approaches you would recommend?

 

 

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Please have a look the sketch. If seen in the past that the PRV and VRV are both connected to the low pressure steam line which is connected to the column (no valves in between).

What do you think? Is it ok to place PRV and VRV there?

For PRV I would say yes because high pressure inside the column can only result from high pressure in the steam line.

For VRV I am not so sure. But to be honest I have problems of imagination here. For low pressure (p2 < 0 barg) inside the column is the pressure p1 the same as p2? Or is it possible to reach the design pressure of -0.5 barg inside the column while the pressure p1 is higher? My best guess here is that p1 is p2 plus liquid level.

 

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Thanks for your help!

Jon

[breizh]

Hi,

To me, to protect from vacuum due to condensation you need to install a vacuum breaker on the top of the column, preferably a rupture disk. The realistic scenario is #1.

Note: you should check for videos and reports showing column and tanks collapsing due to steam out.

https://simpliengine...zle-sizing/3045

My view 

Breizh

[Jon K]

Exactly, I'd like to protect from critical vacuum pressure.

 

I've seen several stripping columns where the vacuum breaker size was pretty small (2" or 3"). I can't believe that above mentioned scenario #1 was considered.

 

I totally understand that steam out of equipment can result in collapsed equipment. However there's a huge difference between an uninsulated tank and an insulated column. My hope was to find some kind of rule of thumb for this topic.

 

Btw.: Why does the vacuum breakers needs to be on top of the column (assuming the pressure drop inside the column is low and equal pressure can be assumed)?

[Pilesar]

If scenario #1 is real, then it must be protected against. Columns in condensing steam service should be rated for full vacuum. Perhaps an instrumented protective system might be designed for this, but I do not see it in my head. If the column could not be rerated for full vacuum, I would consider replacing it.

[breizh]

Hi Jon K,

Check where the highest vacuum is in the column and mitigate the risk accordingly. Pressure and Vacuum reliefs should not be at the same location if the scenario of concern is steam out.

 

Breizh

[shvet1]

@Jon K

 

#1

First approach was to consider the amount of steam inside the column to be equal to the volume of the column. It is assumed that steam supply stops but water is still fed to the column. The (cold) water will immediately (within seconds) condense the steam. The vacuum breaker would need to have an extremely high (not practical) capacity.

 

Reflux in your stripper is not overcooled, correct? It has saturation temperature or slightly overcooled (2-3°C), correct? You can mix as much saturated liquid with saturated vapors as possible and no condensation will accrue as those are [...drums...] in equilibrium. 

 

Why do you suppose this saturated liquid has enough energy to condense the whole steam on a vessel? The whole steam, this is a huge amount of energy comparing to energy incoming with reflux! Calculate energy required by yourself and compare it with liquid temperature drop - you will find out that liquid will freeze.

 

Looks like your logic is incorrect. The only equipment I have met that was refluxed with overcooled liquid was a barometric condenser - a method to create vacuum, outdated now.

 

Vacuum/pressure relief deals with energy or mass imbalance or combination of those. There is no magic. You only need to find this imbalance and recalculate energy flow to mass flow. This is might be challenging issue but it is still a simple final goal.

 

Normally relief/vacuum relief of a fractionator deals with energy imbalance caused by a stop of vapor/reflux incoming accordingly plus heat loss to atmosphere minus heat input through external heating coils and similar. Seems this logic is quite obvious.

 

Note that after vacuum relief once got in a vessel through a relief valve decreases steam partial pressure => saturation temperature => relief rate. This is an important point that affects location of relief device and some considerations (e.g. gas buffer) are able to significantly reduce relief device capacity and associated cost.

Edited by shvet1, 19 April 2023 - 02:51 AM.