6 replies to this topic

[smalawi]

Hi,

in services where we have > 4m3 LPG or lighter material we need to consider EDP systems.

My query is where is the best locaiton to install such line in the following system

debutinizer column with heavy oil reboiler, overhead water cooler and reflux vessel (all connected in one system)

1. on top of the reflux vessel

2. on the top part of the vapour line (horizontal section) from column to ohd water cooler hx

3. on top of the column direclty on the column shell

4. any other part i did no indicate ?

let me know your views and why did you select such location. of course safety is the final measure.

Also in sizing the EDP load do you consider all the system ? i.e. refluc vessel, ohd hx, column (trays + bottoms) and anny connected system (e.g. linked hx's)

or you only look at the reflux vessel only ?

let me know your opinions, and thanks in advance,

cheers,

SM

[Art Montemayor]

Smalawi:

I do not agree with you on the priority given to Safety while depressurizing a system. SAFETY SHOULD BE THE FIRST MEASURE by which you judge a solution – NEVER THE FINAL.

And in keeping with the above priority, WHO and HOW was the size and type of system selected to be depressurized at one time? Was the decision made by management – especially a business manager without any knowledge of how the different components operate and are designed? A business manager would naturally try to carry out the total depressurization with only ONE valve and save a lot of money. I would hope that whoever had the final decision in defining the size and type of system to be depressurized was fully qualified in experience, knowledge, and operating responsibility while considering the priority of safety and not minimizing the quantity of depressurization valves.

Every time you introduce or evacuate fluids into a distillation tower you are obliged to carefully scrutinize the amount of vapor/liquid flow distribution that will cause enough pressure drop across the trays (or packing) that will cause internal structural damage due to excessive "lift" or "push" on the trays or other internals. ANY unit operations vessel that has internals is subject to the same criteria. This is not taught in university, but is expected to be common sense learnings. Therefore, you have different criteria in determining the conditions for depressurization rates and conditions for each vessel – depending on the mechanical design criteria of each vessel AND its internals. Any collapse or damage of internals immediately causes a larger pressure drop across the debris during the depressurization time – and this could easily defeat the purpose or safety of depressurization.

Study your system well! You cannot generalize or summarize it all in one, compact, and short Forum query. You have to study the entire system and all its internal components. It is a job for an experienced and proven operational engineer who can be consulted and trusted to give a comprehensive and detailed report with recommendations and reasons for such. The precise location of all (maybe more than one) valves should be carefully studied and confirmed with all the final ramifications and results taken into consideration. This is not a job for a rookie - or recent graduate.

We don't know the pressures, temperatures, alloys, internals, phase equilibria, compositions, method (manual or auto?), targets (atm. or flare?) and the vital quantities and depressurization times involved. We therefore can't seriously give you firm recommendations on the method and location of depressurizing your entire unit. Critical low temperatures may easily be involved with respect to the materials of construction. These low temperatures resulting from depressurization could harm the materials of construction with embrittlement or weakening of the allowable stress.

If you are unqualified to carry out a thorough, detailed depressurization study and subsequent report, then you should obtain the services of a recognized and trusted consultant who has done this in the past and can repeat the performance for you.

Good Luck.

[fallah]

As per what i have seen in similar cases,one BDV is located on the top part of the vapour line (horizontal section) from column to ohd air cooler hx,and another BDV on top of the reflux vessel,both simultaneously cover Debutanizer and its Reflux Drum Area which isolated by SDVs in the case of emergency depressurization,and both activated by one ESD signal.

[smalawi]

Thanks Art / Fallah,

Art, what gives you the impresion I'm designing such system ? or that I'm not qulified ? My question is to understand deferent views on this critical subject as you alreay know. The location as you elaborated is function of many things, but there is always a reason why.

As for the service any "experianced engineer" with understand that debutinizer s in general are most of the time trayed, ~ 30 to 40 trays, designed for high uplift force for EDP. pressure ~ 10 bar, temeprrature of ohd vapor ~ 70 -80 C and all service is desinged for LPG, typically low temerature carbon steel or stainless steel. I'm not talking about specifc system here but more general undertanding of EDP in such service. On safety, its the first, second, third and final measure, of course, we are engineers at the end of the day and we design systems that are practical and cost effictive.

If you can share your views on the location of such system I will really aprecaite it, note that I'm not desinging a system here this is typically the duty of EPC contractor with sufficent expriance and reviewed by outside consultant, but I do want to know in general why.


Fallah,

I understand that you have an SDV valve separating the air cooler + reflux vessel from the column during EDP and two BDV valves one on the column top and another on the reflux vessel top. Not sure, but this implies you may have two systems if your pressure control is via a valve / liquid level in air cooler type, so you anyway have two RV's on the column and relfux vessel. Please correct me if I did not understand it right,

Any idea why do you have this SDV installed ? why you will not depressure from one valve in single location or several valves in one location ?

Thanks all for your help again,

cheers,

sm

[fallah]

Fallah,
I understand that you have an SDV valve separating the air cooler + reflux vessel from the column during EDP and two BDV valves one on the column top and another on the reflux vessel top. Not sure, but this implies you may have two systems if your pressure control is via a valve / liquid level in air cooler type, so you anyway have two RV's on the column and relfux vessel. Please correct me if I did not understand it right,

Any idea why do you have this SDV installed ? why you will not depressure from one valve in single location or several valves in one location ?

Thanks all for your help again,

cheers,

sm

The whole Debutanizer Unit should be divided into networks, i.e. unit portions which can be isolated by shutdown valves (SDV’s) and/or “failure closed” control valves.Number of divisions are depended on dimentions of Unit Area, the max operating pressure,the isolated volume to be depressurized,....that in this case there are two area which each of them would be depressurized by its own BDV.

Generally,three different depressurisation procedures are considered:
-Depressurisation in case of external pool fire (“Hot fluid depressurisation”)
-Depressurisation after prolonged shutdown (“Cold fluid depressurisation”)
-Spurious depressurization
In 2nd and 3rd procedures it may possible to depressurize whole network in single location by one or several BDVs,but considering 1st procedure as worst case, each vessel shall has its own BDV to avoid having sudden pressurizing of the vessel due to external heating and probable bottlenecking of single blowdown location.
Hope this clarified.

[Art Montemayor]

Smalawi:

I am responding to your specific questions, in the order you issued them in order to clear up some issues:

Art, what gives you the impresion I'm designing such system?
You opened your post, in clear English, with "in services where we have > 4m3 LPG or lighter material we need to consider EDP systems". Since you failed to specifically state that you merely want to open a philosophical discussion on generalities, I am forced to understand that you are designing the subject system as you describe it. I am sorry if I assumed the wrong thing, but since you don't make a specific statement as to what you want to discuss – a real design or an engineering philosophical discussion – I wasted a lot of my time trying to help you out.

….or that I'm not qulified?
I am NOT under the impression that you are unqualified. I cannot take that stance or assume it – simply because I neither know you personally, nor do I know your qualifications. You could be the world's leading expert on depressurization for all I know. The exact statement I made is: "If you are unqualified to carry out a thorough, detailed depressurization study and subsequent report, then you should obtain the services of a recognized and trusted consultant who has done this in the past and can repeat the performance for you." Note the qualifier "IF". The statement is true, as stated, for everyone - including myself. You should read carefully what others write --- especially if you don't have complete knowledge of the English language. Please do not let your personal sensitivities take over your engineering logic. You asked for help in the form of a query and stated, "let me know your opinions, and thanks in advance".

I have responded to your request for opinions. I gave you mine, which obviously is not to your liking. In my real-life role as an engineering consultant, this is a common result. My clients seldom "like" what I have to tell them. But they keep on asking for my help – because they realize that they call on me because I hold the answers to their problems, whether they "like" the answer or not.

The most important point I tried to make is the first one, which strictly contradicts your original remark. Your said: "of course safety is the final measure". And I corrected that with "SAFETY SHOULD BE THE FIRST MEASURE by which you judge a solution – NEVER THE FINAL".

Now, you contradict your original statement with "On safety, its the first, second, third and final measure, of course".

I frankly don't know what your statements mean since they contradict each other. This is what happens when a conversation involves generalities instead of specifics.

I have gone to great lengths to tell you that you CANNOT constructively discuss any location or quantity of Emergency Depressurization Valves without revealing or sharing the intricate and detailed information of your process – which normally you are not free to do. That is why an experienced and capable engineer is assigned to do the study and issue a formal report. This type of engineering study is similar to the importance and complexity of a Relief Vent + Flare study. There are many process and mechanical details that are related to a proper and safe design. We can simply generalize when we don't have the facts and details before us. And generalizations are not my kind of engineering. I deal in specifics.

[smalawi]

Hi Fallah / Art,

Fallah, you are right, you need to partition the system but I think one EDP will do the job but in your case you will have possibly liquid Lpg going to the EDp valve if you only have EDP on location 1 (see list), simply because you will condense vapour Lpg in the air fan cooler. The SDV is to prevent this issue and you end up having two EDP's on SIL 1 class, more compilcated but practical solution still.

By the way, the Lpg might flash and give you very low temperatures in the flare line, so you have to deisgn for that, not sure if it economic or possible.

Art,

Away from all what was said, Lets talk details, here is a system I made up ! its not an actual unit or unit on design but lets condiser few things,

1- a new design, can be updated

2- exsisting unit (not much to do and you have only EDP 1 not EDP 2 and no SDV)

3 - you dont have any EDP at all

4 - unit meterail is fit up to -50 deg C ( say lines 316 300# and vessel fine grain CS only)

By the way, you are welcome to fill in the gaps as you wish, I dont know everything there is to know.

Thanks all for your time

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