3 replies to this topic

[Vis]

Has any one come across projects in which the closed drain drum was also used as the LP Flare KO Drum? Would the sizing criteria be different in terms of liquid hold up volume / residence time etc.. if the closed drain drum is used as the LP flare drum?

[paulhorth]

Vis,

This idea is often raised at the beginning of a project, but on the jobs that I remember, the two drums were always kept separate. There are good reasons for this.
I cannot back this up with personal experience, but I have been told by more than one oil company client that combining the drums is a bad idea. The reasoning runs roughly as follows.
if the drums are combined, then there is a greater possibility of liquid flow into the combined drum leading to a high level. This in a flare drum should cause a total plant shutdown, which is inconvenient if it happens due to a routine draining of an offline vessel. Sometimes the operators will override the shutdown to avoid this, which leads to a real risk of flooding a flare during an emergency. This apparently has actually happened - so there must be plants in service which do (or did) have combined flare and drain drums.
It was emphasised to me that it is vital that the operators see rising level in a flare drum as an emergency and that the causes can be clearly and quickly identified and acted on. if the source of liquids is from the drains system this is (1) not obvious and (2) cannot be shut off. Thus it is best to keep the two drums separate.

If you are going to combine the drums, you need to think very carefully about the basis for the holdup volume between "normal" level (which controls the pump - near the bottom) and high high level trip.There must be sufficient volume above this to accommodate all the liquid from an emergency blowdown or relief, and still leave the required cross section for gas flow.

In the case of a large onshore plant, there is another reason to have separate drums. The flare drum is usually a long distance from the centre of the plant, at the end of the flare header just at the flare sterile radius. In contrast, you want to locate the drains drum in the middle of the plot area to simplify the pipe routing.

Paul

Edited by paulhorth, 30 September 2011 - 05:57 AM.

[Vis]

Paul,

Thank you for you reply. As you have indicated, the critical aspect of combining the two drums is the volume of the knockout drum / and sizing it to handle the largest liquid dump in the closed drain + accommodate any liquids from relief devices.

I have heard from some of my colleagues that they have seen systems where the two LP flare drum was sitting right on top of the Closed drains.. like a double barrel. And the lower barrel was sized to handle all the liquids from drains + additional volumes for any reliefs.

Thank you.

Vis

[R NESAMANI]

Hi, Do you mean LPG flare KO drum?
Have you come across the KO drum in LPG flaring applications?