11 replies to this topic

[Fr3dd]

Hello everyone!,

 

Currently, I'm involved in the design of a closed drain system; it consists on, as usual, of drain connections from several equipment discharging to a header which collects all the fluids and directs them to a Closed Drain drum. This drum is equipped with pumps to discharge the liquids once a certain level within the vessel is reached and it is connected to the LP flare through an open vent. The LP flare system was designed based on the scenario of a fire occuring on this vessel (fire case). Our client is asking for verification of gas blowby scenarios to check if the vent of the vessel is correctly sized; this is understandable, however, I have the following concerns:

 

1) Not a single piece of equipment (mainly pig launchers, headers and manifolds) is connected to the closed drain system has an automatic level control; all drain connections are manually valved (or equipped with motorized valves) that will be opened once each system is depressurized. Is gas blowby scenario realistic to consider in this case?.

I ask this because I've always thinked about gas blowby when considering malfunction in the level control of a vessel or failure of a control valve (including bypasses), but: is it reasonable to consider it when the connections will be opened manually as well?.

 

2) Several equipment (pig launchers) are provided with mechanical interlocks to ensure the proper operation, avoiding opening the pigtrap doors when the equipment is still pressurized, etc. Can I take credit the mechanical interlock to rule out gas blowby scenario on these connections?.

 

3) Considering the fact that this is a vessel designed to collect the largest liquid inventory from a single source (one equipment, one header, or one manifold at a time) and that fire case in this vessel was considered for the design of the LP flare system; isn't it reasonable to think that the vent will be able to handle any peak of gas coming in case of a gas blowby scenario?.

 

I would love if you share your opinions and insights on this, as well as any other concern that should be considered and is not mentioned in my explanation. Thanks in advance!.

 

Cheers,

 

Fr3dd

 

 

[fallah]

Fr3dd,

 

Please provide a simple sketch of the system you described...then you will certainly get better response...

[shan]

You should size the LP Flare Scrubber to handle the blowby case, if it is the governing scenario.  All the fluids from the blowby will flow through the Closed Drain Drum, which just serves as large diameter piping in this case, to the LP Fare Scrubber.   

[Fr3dd]

@Fallah

 

Herewith a simplified sketch to support my explanation.

Attached Files

•  Closed Drain System.jpg   49.3KB   6 downloads

[Fr3dd]

@Shan

 

I don't agree with that approach because if the vent of the vessel is not large enough, you will be taking in more gas than you can leave out, and that may lead to a pressure buildup. I think that's the whole issue with the gas blowby verification (but that's my point of view, I could be wrong).

[Bobby Strain]

How does one verify a non-event? Your client won't be satisfied until you present an evaluation of venting gas through one wide-open drain valve; the one that gives the greatest venting requirement at the collection drum. The question seems to come from a member of your client's team. Maybe one with no experience.

 

Bobby

[shan]

The possible pressure buildup in the closed drain vessel has nothing to do with the size of the closed drain vessel.  It is a issue of design pressure of the closed drain vessel, which has open access to the atmosphere through LP flare scrubber and flare stack/tip. 

[Fr3dd]

@Shan

 

That's why I mentioned that the element in question was the vent size, and not the drum itself.

 

@BobbyStrain

 

Well, IMO it is pretty obvious that fire case will overrule any gas blowby case; but probably at the end I will need to present a calculation supporting this. Any insights regarding Point (2)?

[Bobby Strain]

Regarding point 2. You have no reason to take credit for mechanical interlocks. If your client wants to do this, then let her inform your leader in writing.

 

Bobby

[latexman]

Let's say section "E" is prepared to be drained near the end of day shift.  Day shift documents this in the log and communicates to night shift what was done and what needs to be done next.  The night lead operator tells a night operator what to do.  The night operator inadvertently opens the manual drain valve on section "F" which is pressurized.  One human error; this is a credible scenario that can happen on every section.  The mechanical interlocks in point 2 have no effect on it, do they?

[Fr3dd]

@Latexman

 

First of all, thanks a lot for taking some time to respond.

 

I understand your point, but the mechanical interlocks is not supposed to ensure that the valves are opened in a logical sequence?. In this case, mechanical interlocks are provided in pig launchers and receivers and it's my understanding that the won't allow drain lines to be opened without opening depressurization connections first. So, doesn't this allow me to take credit from them?.

 

Fr3dd

[breizh]

Fr3dd,

You may refer to interlock similar to those described in the document attached !

Hope this helps.

 

Breizh