4 replies to this topic

[asade abiodun]

There all,

I need solution to the problem below:


A purge gas is connected to a flare header line into a flare scrubber to ensure that there is no ingress of oxygen into the flare through the tip. The vessel is expected to be maintain at a certain pressure. Also, the purge gas pressure is required to maintain the pressure in the vessel when there's no relief load through the vessel. The pressure maintained in the vessel should be capable to push the liquids out through the flare scrubber liquid control valve into the sump tank.

A study was performed to show that at a certain pressure in the vessel, liquid can flow through the liquid line without the aid of a liquid pump. From the study, it was obtained that at 3 psig, liquid can flow from the flare scrubber into the sump tank. The flare scrubber is located on a upper deck and the sump tank is located on lower deck at the oil platform.

Problem: I need to validate the purge gas flow rate required for the flare header/ flare system. Also, I need to determine the amount of purge gas required to maintain the estimated pressure expected to be maintained in the vessel.

Is there a relationship between pressure and gas flow rate in a vessel?

Your response is highly appreciated.

[Dacs]

I can't digest the vessel part yet, but I believe there's some correlation that tells you how much purge you need to flow to avoid O2 ingress in your flare stack.

[flarenuf]

Dear Anonymous person,

Firstly you do not use purge gas to pressureise up a KO drum so you can force liquids out of the drain. Th e amount you would require on a continuous basis would be prohibitive due to waste of gas,
.
Each flaretip will have a recommended amount of gas required in order to prevent ingress of air into the system
This amount will either be specified by the vendor or can be calculated using various methods HUSA, API ( try googling it) or look at API 520

Also i do not understand if the KO drum is on the upper deck and the sump tank is on the lower deck whats wrong with Newton's dicovery of gravity to drain the KO drum

flarenuf

[jrtailor09]

As per API guidelines purge gas flow rate would need to have a velocity of 0.3 m/sec.

[kkala]

In addition to previous posts, following notes can be of interest.
1. For purge gas flow rate to eliminate possibility of air ingress, see http://www.cheresources.com/invision/topic/12530-flare and web references mentioned there. Also http://www.cheresources.com/invision/topic/13221-flares-pilots-and-purge-gas for general info.
2. For any purge gas flow, ΔP from atmosphere to the vessel could be estimated (including ΔPs related to exit, flare tip, riser, any seals, header, exit from vessel); and then find the purge gas flow to cause total ΔP=3psi (that is 3 psig in the vessel).
Resulting loss of purge gas is expected to render this option uneconomical (look at posts No 2 and 3).

Note: A sketch would also help clarify the picture.

Edited by kkala, 15 November 2012 - 10:53 AM.