A knockout drum is a vapor-liquid separator installed in a collection system to limit the quantity of liquids carried to downstream sections of the system. A knockout drum is essential in this location because a flare stack is not capable of handling liquids, and because liquid droplets can pose a serious hazard to equipment and personnel when ignited as they exit a flare tip. In some cases, multiple knockout drums are installed in series in a given relief header. This may be necessary if an individual vessel or unit has the potential to discharge a large volume of liquid to the collection and disposal system.
The Company prefers to locate knockout drums near the plot limit of each process plant where the plant relief headers enter the main interconnecting pipeway. Occasionally it is economical to provide a single knockout drum for a group of plants.
Knockout drums should be designed or evaluated for each global relief contingency identified, using the methods described in API Recommended Practice 521, to meet the following criteria.
Liquid Droplet Separation
The final knockout drum in a collection system that discharges either to a flare or to atmosphere should separate out entrained liquid droplets larger than some specified size. To avoid “burning rain” from a flare, API Recommended Practice 521 recommends this size be in the range of 300 to 600 micrometers diameter; flares can normally handle droplets smaller than 300 micrometers. Methods for evaluating the separation performance of a knockout drum are described in RP 521.
Liquid Hold-up Capacity
The hold-up volume of a knockout drum is the minimum volume available for temporary containment of liquids without interfering with the drum’s ability to achieve the specified vapor/liquid separation. The combined hold-up volume of
the knockout drums in a pressure relief collection system should be sufficient to contain the greatest amount of liquid predicted to be discharged to the collection system for some specified length of time by any single contingency. Knockout drums should be sized so that a 10-minute blow at maximum rate will fill the drum half-full. this conforms with API RP 521 recommendations.
When evaluating the sufficiency of a knockout drum’s hold-up volume, credit is often taken for the flow capacity of knockout drum pumps. Maximum liquid volume also depends on the operator’s reaction time to the relief event. Operating management should concur on the selection of the knockout drum size. Figure 1200-36 shows a typical knockout drum installation and illustrates the relationship between liquid hold-up volume and droplet separation.
Producing facilities often operate knockout drums at 20 to 30 psig, and liquids are forced by that pressure to temporary storage tanks or sumps from which they can be pumped whenever it is convenient. Operating the scrubber under pressure allows both the knockout drum and the associated pumping equipment to be smaller, since the knockout drum should reasonably never reach one-half capacity.
Level Control Requirements
Instrumentation should normally be installed on a knockout drum for automatic control of the liquid level in the drum. A high level alarm signal should be delivered to the appropriate control room to warn of possible liquid carryover to the flare or atmospheric discharge. Due to the low pressure normally in the drum, a pump is usually required to remove liquids from the knockout. Careful analysis needs to be used in specifying the power source for this pump.
Because of the low pressure normally seen by knockout drum, designers are often tempted to specify a very low design pressure for this vessel. Operating experience has proven this a false and short-sighted economy. Most operating companies currently require a minimum MAWP of at least 50 psig for flare knockout drums.