Flare KO Drum Design Criteria

# Flare KODs

A KO drum is provided to separate and collect the liquid, before the vapors pass to the flare package, and to prevent large liquid droplets in the outlet vapor stream, which may cause burning rain (lit flare) or liquid rain (unlit flare) and to provide sufficient hold up time for the emergency liquid relief following an emergency relief scenario. The KO drum shall be design based on the criteria stated below:

1. For economic reasons, a horizontal KO drum should be considered for large vapor flow and large liquid releases;
2. For vertical KO drums, API STD 521 Annex C method usually leads to larger vessels than the gas load factor method. For horizontal KO drums API STD 521 Annex C method may lead to smaller vessels and it is recommended to use the gas load factor method. A gas load factor of 0.1 m/s shall be used;
3. If a KO drum is required to be combined with the maintenance closed drain drum, the KO drum must be designed for possible interactions of the fluids relieving to the flare. This scenario may be applicable only for the facilities that have minimum equipment to drain during maintenance or may have layout/space constraints, for example remote degassing stations, wellhead towers. The KO drum should be sized to include the volume expected from normal maintenance activities and maximum emergency relief;
4. For large process complexes such as refineries and petrochemicals, the requirement for providing flare KO drums within individual unit battery limits or an intermediate KO for more than one unit against one common KO drum for each flare system, should be evaluated on a case by case basis based on the cost benefit analysis;
5. A split-entry or split-exit configuration can be used to reduce the drum diameter (but increase the length) for large flow rates and should be considered if the vessel diameter exceeds 3.66 m (12 ft); Where a horizontal KO drum is provided with two inlets the vessel should have a length L > 5 D;
6. The distance between the bottom of the feed inlet device bottom and highest liquid level trip shall be a minimum of 150 mm for horizontal vessels and a minimum of 300 mm for vertical vessels;
7. No deflector plate or demisting gas outlet device is permitted in the KO drum gas outlet. The design shall recognize that the maximum gas release case and the maximum liquid release case are not necessarily coincident. If no valid liquid case exists and the vapor is either condensable or has a condensable component, then the design liquid case should be a minimum of 2 wt% of the maximum gas rate to the flare KO drum;
8. The height above the LZHH is to provide sufficient time for safe shutdown of the total facility, to provide sufficient hold up for the emergency relief incoming to the KO drum and is based on the closing time of the SIF, delay in operator's response and failure of starting the KO drum pump at LAH;
9. Where tripping of facilities is practical and the risks associated with liquid carryover are high, e.g. offshore facilities, a high level trip as recommended by API RP 14C shall be installed;
10. The flare KO drum must have sufficient hold-up capacity to prevent any surges of liquid filling the flare KO drum and entering the flare stack, leading to incompletely combusted discharges, flame out, soot, smoke and potential burning rain. To determine the liquid hold-up requirements, all potential emergencies and sources of liquid flow should be considered. The hold-up capacity of the drum is based on containing the largest in-flow of liquid for sufficient time to allow shutdown of the source. This liquid hold-up time is usually 30 minutes (API STD 521 Annex C ). It is important to consider liquid that may already be accumulated in the drum. A conservative estimate is to assume that the drum is initially at its high high alarm level;
11. The liquid levels and residence times shall be as per Table below.

Flare KO Drum Level Instrumentation

It is important that the level indication in the flare drum is reliable as a rising level can be a warning of a potentially serious upset condition. The level indication must span the entire drum, not just the operating range. The level instruments should be robust with a high reliability and with voting level specified by SIL requirements; it is preferable that two technology types are used. Level instruments should be designed to handle the expected range of liquids and densities that could be discharged to the drum.

In general, high high level in the flare KO drum shall initiate an automatic overall total plant shutdown, however this depends on project philosophy. Where the high liquid level trip causes a total plant shutdown, it is good practice to use a high integrity 2 out of 3 voting system to prevent spurious shutdowns.

Droplet Size Criteria for Flare Drums

Large liquid droplets and liquid loading can cause smoke, release of liquid droplets from the flare or mechanical damage. It is essential that an effective liquid separation KO drum is provided to prevent carryover of liquid droplets to the flare tip which could result in burning rain/liquid rain. It is recommended that the flare KO drum size is based on the maximum of the following:

• A maximum droplet diameter between 300 and 600 μm. This should be confirmed by the flare tip vendor to prevent carry-over of heavier hydrocarbons. Larger droplet sizes can be used for sonic tips such as high pressure sonic flare tips. However, vendor confirmation is required for the maximum droplet size which will not limit the flare tip’s performance;
• The gas cap area above the maximum emergency liquid level shall be designed with a maximum gas load factor of 0.1 m/s. If a Scheopentoter (or equivalent) inlet device is used, a maximum gas load factor of 0.15 m/s can be considered;
• The gas/liquid separation efficiency of a KO drum shall be determined by its gas load factor, λ (sometimes referred as K factor which is related to Souders-Brown velocity) and is calculated by the following equation

For essentially dry gas loads or for ISBL KO drum or intermediate KO drums, a higher gas load factor of 0.25 m/s can be considered. The higher loading factor will lead to lower area available for gas (Ag).

The momentum criterion for Flare nozzles shall be as follows:

• Inlet nozzles which are not equipped with any internals shall be sized for a dynamic pressure of 1,500 N/m2.
• Nozzles provided with a half open pipe shall not exceed a dynamic pressure of 5,000 N/m2;
• Nozzles provided with a Schoepentoeter inlet device shall not exceed a dynamic pressure criteria of 10,000 N/m2. (Schoepentoeter shall be of a sturdy design to be able to cope with high loads);
• Gas outlet nozzle shall not exceed 6000 N/m2.

Flare KO Drum Pumps

Relief liquid collected in the flare KO drum is typically pumped to tankage or back into the process. The flare KO drum pumps should be designed to discharge the emergency liquid hold-up volume within a reasonable time period for restarting the process, and not on the liquid load feeding the KO drum. The time to pump out the liquids is specified on a case by case basis, typically 2 - 8 hours.

The pumps are typically electric driven 2 x 100% (1 duty; 1 standby) with emergency power supply. The first pump will start at LAH and the second pump will start if the level continues to increase to LAHH. Both pumps will stop on low liquid level.

Flare KO Drum Heater

Heaters are required in flare KO drums to prevent water freezing or wax and hydrate solids forming. The heater shall be sized to heat low temperature releases due to depressurization and winterization. If the liquid is volatile (e.g. liquid propane) heating shall be provided to vaporize the liquid.

The heater will be submerged in the liquid below LLLL. The KO drum pump starts at HLL/HHLL and stops at LLL. Since the inventory below LLL will always be kept warm, the heater only needs to be sized for additional duty to heat the liquid above LLL. Therefore, only the liquid inventory between LLL and HLL should be considered for heater sizing. An extended period of time can be assumed for heating the cold liquids (e.g. 2- 8 hours).

The minimum temperature to be considered for liquid inventory prior to heating should be either minimum ambient temperature or minimum temperature at relief/blowdown discharge.

The heating may be provided by an electric heater or steam coil.