The Knock out drum is a vessel in the flare header system designed to remove & accumulate condensed & entrained liquids from the relief gases.

Both the horizontal & vertical design is a common consideration for the Knock out drum, which is determined based on the operating parameters as well as other plant conditions. If a large liquid storage capacity is desired and the vapor flow is high, a horizontal drum is often more economical. Also, the pressure drop across horizontal drums is generally the lowest of all the designs. Vertical knockout drums are typically used if the liquid load is low or limited plot space is
available. They are well suited for incorporating into the base of the flare stack.

DESIGN FEATURES
Although horizontal and vertical knockout drums are available in many configurations, the differences are mainly in how the path of the vapor is directed. The various configurations include the following:
a) Horizontal drum with the vapor entering one end of the vessel and exiting at the top of the opposite end (no internal baffling);

b) Vertical drum with the vapor inlet nozzle entering the vessel radially and the outlet nozzle at the top of the vessel’s vertical axis. The inlet stream should be baffled to direct the flow downward;

c) Vertical vessel with a tangential nozzle. Vertical centrifugal separators differ from vertical settling drums in that the flow enters tangentially and spins around a centre tube, which extends below the liquid inlet nozzle. The gas and liquid flow radially downward through the annulus causing liquid droplets to coalesce along the walls and collect in the bottom of the drum. The vapour changes direction once below the centre tube and flows upward to the outlet nozzle. To avoid liquid re-entrainment, vapour velocity has to be kept low in the turnaround section of the drum. An additional measure to prevent liquid re-entrainment is a baffle plate below the turnaround section of the drum. The maximum liquid level is the same as vertical settling drums;

d) Horizontal drum with the vapor entering at each end on the horizontal axis and a centre outlet;
e) Horizontal drum with the vapor entering in the centre and exiting at each end on the horizontal axis
f) Combination of a vertical drum in the base of the flare stack and a horizontal drum upstream to remove the bulk of the liquid entrained in the vapor. This combination permits the use of larger values for the numerical constant in the velocity equation.

A question concerning requirement of overpressure protection device on “Final Vessel” was raised.

“API 14C SAC A.4.c.5 seems to indicate that a PSV is required on a pressure vessel (max pressure >5 psig) unless it is the final vessel on a flare header (i.e. Flare KO Drum). However, this implies that any other vessel venting into the flare header (e.g. Closed Drain Sump) needs a PSV to protect against blockage of the final vessel.”

The following examples show an overpressure protection device is required even though the pressure vessel is “final vessel”

a) Degassing drum with vent line and flame arrester discharge to atmosphere
This drum is a “final vessel” in the relieving path. However there is a potential of flame arrester blockage and may lead to overpressure of this drum. Thus, an overpressure protection device is required. Normally a Pressure relief valve or rupture disk is installed across the flame arrester. In this case, it is inline with the API14C SAC A.4.C.1.

b) Flare KO Drum with Flare Recovery system
In certain environment sensitive area, “zero emission” and As-Low-As-Reasonable-Practical (ALARP) principle are adopted. Those in most cases, a Flare Recovery System (FRS) will be implemented in Flare system. It could be a recovery compressor, ejector, etc. In most cases, the Flare KO Drum downstream flare line which connected to flare stack may have an on-off valve to facilitate plant maintenance. Under this case, even though the Flare KO Drum is the “final vessel” but an overpressure protection device (i.e. rupture disk) would be installed across the on-off valve to ensure a clear relief path. In this case, it is inline with the API14C SAC A.4.C.1.

The following examples show an overpressure protection device may NOT required even though the pressure vessel is NOT “final vessel”

a) Two drums connected together with no internal in the interconnecting pipe
As there is NO possibility existent of potential blockage on the interconnecting line and the overpressure protection device located at downstream has been designed for worst case (of two drums), an overpressure protection device is NOT required for the upstream drum. In this case, it is inline with the API14C SAC A.4.C.4.

b) Flare KO Drum with Water Seal Drum
As large relief will destroyed the water seal in the seal drum, thus there is NO possibility existent of potential blockage results overpressure scenario. Overpressure protection device is NOT required for the Flare KO drum. In this case, it is inline with the API14C SAC A.4.C.5.