9 replies to this topic

[Roland2009]

Hi everybody!!!

This is my first time in this forum. I need some help about water seal drum calculation sizing procedure, or where can i get that info?. I´ve already have diameter of flare stack, so I have pressure, temperature, flow rate, composition, but i dont know how to SIZE a water seal drum, which it will be located at flare stack base.

Please please please.

Thank you very much.

[Art Montemayor]

Roland:

You can probably find an immediate answer to your problem in API 521, I believe.

If you are a student, you probably don't have access to API 521 and, in that case, I would employ the Brown-Souders relationship for vapor-liquid separator design in order to eliminate entrainment of water particles.

You can find a lot of threads and explanations on the Brown-Souders relationship in our Forums by using the SEARCH function. In effect, what the water seal serves to do is to prevent a back-flow or a propagation of a possible flame in the reverse direction by presenting a water barrier that the flowing flare gas has to overcome. In going through the water barrier, the gas tends to carry water particles and therefore must have a vapor disengagement space and size adequate to do so. The Brown-Souders relationship was developed for that very purpose.

[astro]

You've probably got a copy of Perry.

The 7th edition has a very useful section entitled "Process Safety", viz. chapter 26. There's a sub-section on safety devices starting at p.26-26 through to 26-49. I'd recommend that you read it and take particular note of the run down on flare/vent knock out drum design. There are even step by step design methods to take you through the points to consider.

Perry gives a good summary of many of the important points that you will find in API RP 521. Even though it's not a replacement for that source of knowledge, it should get you a fair way down the right path.

One question, why have you selected a vertical drum configuration? I would have thought that the selection of a vertical or horizontal drum would include a calculated assessment of the vapour / liquid load that the system needs to process. Are you putting the cart before the horse or are you being driven by some other constraint, e.g. plot space?

[JoeWong]

One question, why have you selected a vertical drum configuration? I would have thought that the selection of a vertical or horizontal drum would include a calculated assessment of the vapour / liquid load that the system needs to process. Are you putting the cart before the horse or are you being driven by some other constraint, e.g. plot space?

Water seal drum is

1) to prevent any flashback,
2) to prevent air ingress

It is based on water seal isolating flare header from flare stack. Static head is the main force to maintain the seal. Vertical drum arrangement is good in providing static head.

Flare KOD is normally provided upstream of seal drum to knock-out HC liquid. You don't aim to separate liquid from relief vapor in water seal drum. Thus, Horizontal arrangement is not preferred.

Unless you intended to combine KO and seal functions into single drum. Then the KOD/seal drum type would subject to studies. However, experiences still shows KO function is dominant.

Follow advice from Mr. Montemayor, check API STD 521 (2008 may Addendum). Section 7.3.2.2.

[astro]

One question, why have you selected a vertical drum configuration? I would have thought that the selection of a vertical or horizontal drum would include a calculated assessment of the vapour / liquid load that the system needs to process. Are you putting the cart before the horse or are you being driven by some other constraint, e.g. plot space?

Water seal drum is

1) to prevent any flashback,
2) to prevent air ingress

It is based on water seal isolating flare header from flare stack. Static head is the main force to maintain the seal. Vertical drum arrangement is good in providing static head.

Apologies for getting my wires crossed with KO Drum and Water Seal Drum. That will teach me to read the thread properly.

However, I consider that the validity of my question still stands.

Perusing API STD 521 (2008 May Addendum), Section 7.3.2.2., gives me this in the final paragraph:

Examples of liquid-seal drums are provided in Figures D.1 and D.2. They act as a final or secondary knockout drum and, as such, should be designed based on the same principles as a vertical settling drum.

However if the vapour load through the vessel is high, the design will tend to favour a horizontal vessel (as indicated by figure D.1).

This guidance is expanded in Chereminisoff's book:
Pressure Safety Design Practices for Refinery & Chemical Operations

Fortunately pages 268-271 can be accessed and a critical velocity equation is given on p.271. Apply the equation and work out the required disengagement area. A vertical or horizontal vessel will be implied as a result.

[JoeWong]

astro,

\However if the vapour load through the vessel is high, the design will tend to favour a horizontal vessel (as indicated by figure D.1)

For high vapor load, isn't vertical drum is preferred as liquid at the bottom of horizontal drum will occupy some room and tend to make the drum bigger compare to a vertical drum ?

[iplan]

hi,

I have worked with horizantal liquid seal drums having a vertical partition plate with oil skimming facility. At one side of partition the inlet pipe dips into the sealant and on the other side exits the outlet connection to flare. The oil is periodically skimmed from the vessel. The vessel is so constructed that if the oil is skimmed and the level is maintained, then the oil does not come to the outlet side of the sealdrum. I m not sure how this arrangement can be made in case of a vertical seal drum.


thanks
iplan

[sanuj]

Hi everybody!!!

This is my first time in this forum. I need some help about water seal drum calculation sizing procedure, or where can i get that info?. I´ve already have diameter of flare stack, so I have pressure, temperature, flow rate, composition, but i dont know how to SIZE a water seal drum, which it will be located at flare stack base.

Please please please.

Thank you very much.

you can refer API- RP page 67

[sachindhopade]

hope this helps u
http://www.processca...sel_Sizing.aspx

[PKS]

Sachin

have u ever visited the site processcalculator.com?
Are you any agent for this site?
have u ever visited vertical sizing option in processcalculator.com?

First study and then post such things.

Question is about water seal drum? not about calculating the volume of vessel with known dimensions.

first read the question carefully.

hope this helps u
http://www.processca...sel_Sizing.aspx