5 replies to this topic

[Rodscott]

Hello forum members!

Just a brief introduction: we are doing a project of a new facility in our plant. There are two existing spheres that we use to store 1,3 Butadiene. We are changing these spheres to store n-Hexane for the new process. Since we have some available nozzles at the top of these vessels, we are using them to connect some liquid streams. To do that we included dipped pipe for each nozzles we are going to use.
I am going through the HazOp of this new facility (still in design phase) and i have found the following statement, regarding the dipped pipes analysis:

"Dipped pipe has to be covered at minimum 400 mm with hexane."

This statement is for all dipped pipe that we are foreseen! Each dipped pipe has a different minimum coverage.

So, my query is: Is there some standard or best practice that consider this evaluation, regarding the minimum pipe coverage? How can i find the minimum pipe coverage?

Please, can some forum members help me?

I will really appeciate any help.

Regards,

Rodscott

Edited by Rodscott, 24 January 2013 - 11:00 AM.

[Pilesar]

The idea of the recommendation is to avoid splashing which might result in a static electric ignition source. The time to complain about HAZOP findings is before the report is written. It is difficult to argue with a HAZOP report, especially when you were not in the HAZOP for the discussion. Whether best practice or not, you are almost forced to follow the HAZOP recommendation if it is feasible. Unless you have information that shows the recommended practice to be unsafe, pick another battle to fight. Just set your tank low-low level to be above the bottom of the dip pipe by at least the minimum amount. Interlock the discharge pump so it will not operate below that limit.

The HAZOP members should be listed in the report. One of them knows the details of why the recommendation was specified as it was and is likely very proud of it and will be excited to explain it to anyone who asks.

Edited by Pilesar, 24 January 2013 - 12:39 PM.

[Rodscott]

Pilesar,

thanks for your reply.

I know what was the reason to cover the pipe end. That was due to static electricity development. But the only thing i do not know is how they specify the height these pipes must be covered and why it is different for each pipe.

I tried to find on technical documents like books, articles and googling it but i did not find anything about this subject.

Was this height specified taking some criteria as a basis?

Rodscott

[fallah]

I am going through the HazOp of this new facility (still in design phase) and i have found the following statement, regarding the dipped pipes analysis:

"Dipped pipe has to be covered at minimum 400 mm with hexane."

Rodscott,

The above mentioned statement is a HAZOP recomendation/action. What was relevant cause and consequence(s)?

Edited by fallah, 24 January 2013 - 12:49 PM.

[Art Montemayor]

Rodscott:

To my knowledge, there is no “standard” or best practice recommendation(s) that consider the detailed design of an inlet nozzle dip pipe installation and its minimum pipe coverage? The principle issue here is NOT the minimum pipe coverage.

As Pilesar has stated the issue with dip pipes is the prevention of free fall for hydrocarbon liquids inside of a storage tank. The free fall of these liquids has been reported to generate static electricity and, consequently, constitute a recognized potential hazard. Ergo, the Hazop report and the recommendations on the “proper” design details. If you have hazoped the proposed installation prior to final engineering detailed design, this is great and well-organized. As Pilesar also recommends, you are mandated to follow the Hazop results – period. The objective is to avoid liquid free-fall inside the sphere(s) and this should be obeyed. The immersion of the dip pipe is simply someone’s idea of being “safe”. How the 400 mm was arrived at should be available to you, but in the end some dimension has to be given.

A dip pipe installation generates many side issues:

• If the objective is to eliminate free-fall, how is this accomplished at the onset when the tank is initially being filled? Certainly there will be free-fall during this period, however brief.
• How high off the tank bottom should the dip pipe be?
• What ensures that the dip pipe is 100% liquid-filled during the pump-in?
• What is the recommended liquid velocity in the dip pipe?
• What about the required vent hole at the top of the dip pipe, slightly under the tank’s roof? What diameter should this be to avoid any syphon effect?

What about the minimum pipe coverage during the initial tank fill? Certainly, it will be: ZERO coverage. You will be in violation of the Hazop directive during this time until you achieve a 400 mm Hexane coverage of the dip pipe. This reality should be addressed in the tank’s Operating Procedures and specifically explained there.

[kkala]

1. Considering present thread and http://www.cheresources.com/invision/topic/16948-is-diked-area-needed/page__hl__hexane#entry68074, I understand that these spheres will operate as atmospheric tanks with nitrogen blanketing. And this blanketing is not enough to assure no air ingression through vents all the time, thus possibility of splashing has to be eliminated, so that an electric spark may not cause internal explosion. Can you please confirm or clarify?
2. Some spheres have a manhole at bottom with a lot of nozzles on its cover. If this is the case, an option is to replace this cover with another, having the new nozzles in addition (it may be difficult to weld them on the existing cover). In this way the new pipes will be vertical entering from bottom. You can so avoid vertical pipes inside the spheres (they may also need support), as well as the disadvantages mentioned in post no 5 (by Art Montemayor). And bottom nozzles (plus a length of upstream pipe) can have sections broad enough for velocities < 1 m/s at max flow, to avoid static electricity events.

Edited by kkala, 25 January 2013 - 05:10 PM.