18 replies to this topic

[iplan]

Dear Fellow members,

We need to increase the feed throughput through a refinery unit more than the design. As a part of adequacy checking, we are checking the relief devices adequacy. Since all the vessels ( there are no columns in the unit & vessels handle only liquid) are designed to the take care of the shutoff head of the feed pump, all the relief devices are for external fire case.

In such case, we have ascertained that the external fire case is the governing case in the increased feed rate also.

Since the vessel contents, dimensions, insulation and relief set pressure remain same, I tend to estimate that there will not be any change in the required releiving rate. Is it correct to state so or it is missing something? kindly help.

Thanks in advance
iplan

[fallah]

Dear Fellow members,

We need to increase the feed throughput through a refinery unit more than the design. As a part of adequacy checking, we are checking the relief devices adequacy. Since all the vessels ( there are no columns in the unit & vessels handle only liquid) are designed to the take care of the shutoff head of the feed pump, all the relief devices are for external fire case.

In such case, we have ascertained that the external fire case is the governing case in the increased feed rate also.

Since the vessel contents, dimensions, insulation and relief set pressure remain same, I tend to estimate that there will not be any change in the required releiving rate. Is it correct to state so or it is missing something? kindly help.

Thanks in advance
iplan

iplan,

It is assumed that the vessel is isolated (e.g. by SDVs) during a fire and the wetted area of the vessel remain unchanged, then it could be said that the required releiving rate wouldn't be changed.

Of course, IMO assuming a PSV wouldn't usually prevent a vessel against failure in fire case, any probable change (which can not be considerable) could be ignored.

Fallah

Edited by fallah, 08 December 2011 - 01:05 AM.

[ankur2061]

iplan,

When you say "all the relief devices" is it only relief valves you are talking about or you also have emergency depressurization of the vessels besides the relief valves which are anyway mandatory if the vessels are designed as per ASME Section VIII, Div. I.

Emergency depressurization of vessels containing large hydrocarbon inventories is a better way to protect the vessel from rupture and failure than using conventional relief valves.

Regards,
Ankur

[fallah]

Emergency depressurization of vessels containing large hydrocarbon inventories is a better way to protect the vessel from rupture and failure than using conventional relief valves.,

Ankur,

Emergency depressurization isn't an alternative of a PSV to protect the vessel from rupture and failure in fire case at all. It would usually be considered besides the PSV and to be manually activated in the beginning of a fire (while the relevant vessel is in maximum operating pressure) within a fire zone and then PSV would be opened later at its set pressure (design pressure) if depressurization couldn't be able to decrease the pressure against pressure increasing by fire's heat rate and also if the vessel would have not been failed before the pressure inside the vessel reaches the PSV set pressure!

Fallah

Edited by fallah, 08 December 2011 - 02:09 AM.

[ankur2061]

Fallah,

You have misunderstood what I am trying to say. I am not suggesting an alternative to PSVs because that is the final over pressure protection for the vessel and most codes and standards sya that the final overpressure protection should be by relief valves . What I am implying is that an emergency depressurization can act faster and quicker to prevent vessel wall rupture due to the combination of high temperature and pressure on the vessel walls during a fire by reducing the yield or failure stress values of the vessel material quickly.

Regards,
Ankur.

[fallah]

Ankur,

You compared the ability of ED with PSV for protecting the vessel in fire case against failure and i meant they aren't comparable in this regard because each of them has its own responsibility/functionallity.

Anyway, if i misunderstood your meant i would apologize for it.

Fallah

Edited by fallah, 08 December 2011 - 03:19 AM.

[iplan]

The relief devices I meant was Pressure safety valves provided in each vessel . The unit doesnot have an emergency relief valves which could be operated manually or by other means.

I have seen such big relief devices being used in the Hydroprocessing units, LPG treating units where fast depressrisation is required for fire case to reduce inventory and also to avert a run away exotherm.

The unit I am referring is a kerosene merox unit where such emergency relief device has not been provided . Such a release form the unit will be a huge liquid release and it would required to be routed to some storage vessel.

Many thanks regarding the ASME clause that is being referred. I will check the requirements also.

But , I gather that the fire case pressure safety valve relieving rate does not depend on the thruput but on contents and vessel data. Can you kindly help to know that this understanding holds good or not.

thanks in advance
Iplan

[fallah]

The unit I am referring is a kerosene merox unit where such emergency relief device has not been provided .

iplan,

IMO, the vessel(s) in your above mentioned unit haven't equipped with emergency depressurization (BDVs) because:

1-They are full of liquid vessel(s)
2-They can be isolated and are exposed to fire

Fallah

[sheiko]

Since all the vessels ( there are no columns in the unit & vessels handle only liquid) are designed to the take care of the shutoff head of the feed pump, all the relief devices are for external fire case.

I quite disagree with this assumption.
You'd better analyze all possible scenarii and compare the corresponding required orifice area as recommended per API Standard 521.

Edited by sheiko, 11 December 2011 - 10:04 AM.

[kkala]

In a local refinery unit to desulfurize naphtha, two depressurization valves were installed on the two rather critical columns. Their intented function was as described by fallah.
There were other columns and vessels in the unit, without depressurizing valves. Of course all of them had PSVs.
Is depressurization mandatory on any vessel per ASME Sec VIII Div 1? Or it is mandatory under conditions, e.g. holdup bigger than that, diameter bigger than that, etc? The latter would give an explanation.
It is noted that the unit was designed in 2001 by a reputable licensor and everyone of the mentioned vessels had its own PSV. We executed detail engineering.
A clarification would be welcomed.

[ankur2061]

Fallah,

Quite some time back I had prepared a standard on "Vent & Depressurization" for a middle east O&G company. Unfortunately I had lost the file on my hard disk. Today I retrieved it from my external hard disk. Here I would like to quote what I had mentioned about emergency depressurization in that standard:

Equipment that cannot be isolated or that cannot be exposed to fire shall not be connected to the EDP system.

EDP capability shall be provided only for large equipment that can both be isolated and exposed to fire simultaneously and only if the equipment is operating at or above 250 psig. Reference is made to Section 5.20.1 of API STD 521 and Section 4.6 of BP RP 44-1.

This does not match with your observation in your last post.

Regards,
Ankur.

Edited by ankur2061, 12 December 2011 - 10:28 AM.

[fallah]

Fallah,

Quite some time back I had prepared a standard on "Vent & Depressurization" for a middle east O&G company. Unfortunately I had lost the file on my hard disk. Today I retrieved it from my external hard disk. Here I would like to quote what I had mentioned about emergency depressurization in that standard:

Equipment that cannot be isolated or that cannot be exposed to fire shall not be connected to the EDP system.

EDP capability shall be provided only for large equipment that can both be isolated and exposed to fire simultaneously and only if the equipment is operating at or above 250 psig. Reference is made to Section 5.20.1 of API STD 521 and Section 4.6 of BP RP 44-1.

This does not match with your observation in your last post.

Regards,
Ankur.

Ankur,

You missed to refer to most important part of my last post: Being in "Full of Liquid Condition".

Anyway the statement from your standard, which is prepared based on API 521 and BP practices is generally right, but i meant a vessel with having all conditions in your statement (large equipment that can both be isolated and exposed to fire simultaneously and also is operating at or above 250 psig...) wouldn't need to have EDP capability due to (and just due to) being in "Full of Liquid Condition".

The reason is clear; by absorbing heat of the fire the installed PSV after a short time (due to fast overpressure following the liquid thermal expansion) would be opened to relief at its set pressure certainly before the vessel (full wetted area) to be failed and the loss of the inventory would be minimized even if the PSV opens several times to relief sequential overpressures.
On the other hand, considering BDV for such vessel causes all inventory to be lost and the vessel gradually would lost its wetted area and to be failed faster than before.

Hope above helps you out to get my point.

Fallah

[ankur2061]

Fallah,

Your explanation is well taken. What I meant was your point no: 2 in context to why no emergency depressurization is not provided for the vessel in reference

They can be isolated and are exposed to fire

IMO, it should have been:

They cannot be isolated and are not exposed to fire

This is what I have mentioned in my post also.

Regards,
Ankur.

[fallah]

Ankur,

Unfortunately you didn't get my point yet. Actually my two points should be considered besides each other (not separately) for providing the case which doesn't need EDP capability, other wise it is obvious that a vessel cannot be isolated and are not exposed to fire (regardless of it is full of liquid or not) doesn't need to EDP capability.

Fallah

Edited by fallah, 13 December 2011 - 05:37 AM.

[iplan]

Since all the vessels ( there are no columns in the unit & vessels handle only liquid) are designed to the take care of the shutoff head of the feed pump, all the relief devices are for external fire case.

I quite disagree with this assumption.
You'd better analyze all possible scenarii and compare the corresponding required orifice area as recommended per API Standard 521.

Sheiko,

Many thanks. I completely agree that I need to look at all cases that can lead to overpressurization and need to select the highest orifice size amiong them.

Only I wanted to know whether it is correct to consider tha a fire case relief rate for any vessel would not vary with its throughput provided the vessel dimensions are same. I understand that increasing thruput may lead to other overpressurisation scenario that needs to be looked into.

Thanks
iplan

I

[fallah]

iplan,

Please before you terminate this thread, at least specify if the vessels in your plant are full of liquid or not.

Fallah

[iplan]

Fallah,

The vessels are full of liquid in the unit.


many thanks
iplan

[fallah]

Fallah,

The vessels are full of liquid in the unit.


many thanks
iplan

iplan,

Thanks for your response.

Actually it almost confirms my previous guess (in post No. 8 of this thread) about the reason of lack of EDP capability on the vessels in the unit.

Fallah

Edited by fallah, 14 December 2011 - 04:14 AM.

[kkala]

In a local refinery unit to desulfurize naphtha, two depressurization valves were installed on the two rather critical columns. Their intented function was as described by fallah. There were other columns and vessels in the unit, without depressurizing valves. Of course all of them had PSVs.
Is depressurization mandatory on any vessel per ASME Sec VIII Div 1? Or it is mandatory under conditions, e.g. holdup bigger than that, diameter bigger than that, etc? The latter would give an explanation.

Thinking of the posts in present thread can lead to following conclusions on the above matter:
1. Depressurization for any vessel is not mandatory. Typically the vessel is subject to depressurization only in case that
its capacity is big (but how much?)
and it can be isolated
and it can be exposed to fire
and its operating pressure is higher than 17 barg
and it is not full of liquid, but it contains both liquid and gas, or only gas
2. Depressurization valve is located at the top of the vessel to ensure gaseous exit.
3. Depressurization is not recommended for vessels that contain only liquid (without gas), even though they can be isolated and exposed to fire.
In such a case liquid discharge in fire case should be implemented through PSVs only.

Comment (if any) on these conclusions welcomed.