6 replies to this topic

[Ryan Shin]

Hi.
I have a question..

When sizing the RO you have to consider how quickly you need to reduce the pressure in the gas processing equipment by releasing gas into the flare system. API has some guidence on this:

"…provide depressurising on all equipment that process light hydrocarbons and set the depressured rate to achieve 100 psig (690kPag) or 50% of the vessel design pressure, which ever is lower in 15 minutes."

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In my case, there is a quite big difference betwwen 50% of the vessel design pressure and 690kPag(100psig).
(Ex. Design pressure is higher than 40 barg)
Which one should I choose in this case?

Ryan

[fallah]

Ryan,

The statement you mentioned was included in old rev of API 521. As per API 521 Fifth Edition you should reduce vessel pressure from initial conditions to 50% of vessel design pressure within approximately 15 min.

Depressurizing to a pressure of 690 kpag is considered in order to reduce the consequences from a vessel leak.

Fallah

Edited by fallah, 15 June 2012 - 10:31 AM.

[Lowflo]

The bottom line is that you want to depressure the vessel as quickly as it can safely be done, without risking brittle failure due to auto-refrigeration. Cracking the protected vessel or the flare header will only make a bad situation worse.

[paulhorth]

Ryan,

I recommend that you read carefully Section 5.20 in the API RP 521 Fifth Edition. This makes clear that depressuring design is more than a matter of obeying simple rules, and places the responsibility on the process engineer to select the appropriate depressuring basis for each case. The 50% of design pressure criterion is stated as applying to vessels with wall thickness of 25 mm and above, and "vessels with thinner walls generally require a somewhat faster depressuring rate", without giving any guidance. The aim should be to reduce the internal pressure to keep the vessel wall stress below the rupture stress, as the wall temperature rises due to fire exposure. Because this is not easy to calculate, I favour retaining the older and conservative guideline of 100 psig in 15 minutes if this is lower than 50% of design pressure.

I am of the opinion that the process engineer is responsible for taking into account all aspects of his system design and should not expect his professional input to be limited to following a few rules extracted from international standards.

Paul

[Art Montemayor]

To all who may come calling:

What Paul Horth has written is, in my opinion, the true essence and basic meaning of the engineering profession.

WE are responsible and accountable for the design, fabrication, installation, and operation. How we bring about the correct and successful results of a problem is on our shoulders. No court or jurisdiction will prosecute or hold liable the API standards or recommended practices. If anything goes wrong or bad, we will be called upon to testify if we are involved in the design, fabrication, installation, or operation. That is why we are “paid the big bucks”.

And that is also why it is so important to always bear in mind the basic truths and facts expounded by Paul. We must rely on OUR ingenuity and engineering judgment – not on someone else’s recommendations. WE ultimately must weigh and evaluate the recommendation(s) and make the final decision. No one else will.

Thank you, Paul, for reminding us of who we are supposed to be and what we are supposed to be doing.

[fallah]

Dear Art,

Always we all ask members of this site to specify their requests (by submitting the adequate information, sketch,...) if they want to get the right and exact responses. On the opposite side, i think we should try to avoid recommending members with general recommendations may not be applicable for their matters and also may cause they fall into wrong directions instead solving their problems.

IMO, the OP asked a question due to using the old revision of API and seems he/she got his/her response. Then, Paul did submit some general cautioning statements most of us are aware of them due to complexity of the depressurizing phenomena in nature and you approved totally these statements without any comment on them.

Now, please specify if we should suppose you did approve the Paul recommendation of: "...Because this is not easy to calculate, I favour retaining the older and conservative guideline of 100 psig in 15 minutes if this is lower than 50% of design pressure..." and following that if Ryan can use this recommendation while there are some issues as follows:

-The vessel thickness was not specified by OP.
-Using 100 psig (if it is lower than 50% of design pressure) instead 50% of vessel design pressure as final pressure increases the peak flowrate of depressurizing with the same time duration (15 min).
-The relevant flare capacity is unknown, therefore it is not specified if the flare network can accept that higher flowrate.
-In emergency depressurizing, as worst depresurizing case the relevant BDV would normally continued to be opened after, let say, 50% of vessel design pressure as main depressurizing step and the vessel could be depressurized in any desired level.

Fallah

[Ryan Shin]

Dear Fallah/Lowflo/Paul/Are Montemayor

Thanks for your comments. I've just started to work as an beginner in engineering company since the beginning of this year after graduation.
So I'm just studying hard theories and practices these day.
I will keep in mind those you mentioned.

Thanks again for your advice.

Ryan
Beginner in Process Engineering