8. Disposal System Design Criteria for Liquid Discharge
8.1 Liquid Discharge to Onsite Liquid Knockout Drum
8.1.1 Onsite liquid knockout drums shall comply with Section 9.3 and have an effective liquid reservoir that
is sized to retain the required liquid relief discharge rate for the greater of the expected duration of the
discharge or 15 minutes. A vapor discharge line to the flare and a vapor space shall be provided to handle
that portion of the liquid discharge that is expected to vaporize.
8.1.2 The onsite drum shall be provided with a vapor line to a closed pressure relief system. The vapor line
shall include a car sealed open block valve and a spectacle blind. The drum and piping shall be designed
to withstand the maximum backpressure of the closed pressure relief system under maximum emergency
relief conditions. The vapor space shall be designed to separate the liquid from the entrained vapor of the
material that is discharged to the drum.
8.1.3 Onsite liquid knockout drums shall be provided with a pump and a spare, each sized to empty a
half-full drum within two hours (see Section 9.3). The pump shall discharge to a suitable waste system or
recycled back into the process. If the liquid volume is small a pump may not be necessary as long as an
adequate drainage capacity is designed in the system.
8.1.4 Onsite knockout drums shall be provided with a high-level alarm to alert the operator when liquid
begins to accumulate in the drum so that remedial action can be taken. The pump(s) may be started
manually or automatically by the drum level controller. In addition, an independent high-high level alarm
shall be provided, set to alarm at the design liquid capacity of the drum.
8.2 Liquid Discharge to Lower Pressure Process System or Vessel
A lower pressure process system or vessel shall be capable of handling the required liquid relief discharge
rate. Flashing at the lower pressure shall be taken into consideration.
8.3 Liquid Discharge to an Oily-Water Sewer
Liquid discharge to an oily-water sewer shall be nonvolatile and nontoxic and shall be via a blowdown
drum. The maximum liquid relief rate shall be within the oil removal capability of the oily-water treating
system.
8.4 Liquid Discharge to a Pump Suction
Liquid relief shall discharge to the upstream liquid reservoir from which the pump takes suction.
8.5 Cold Liquid Disposal
8.5.1 Special precautions shall be taken to prevent freezing of relief system contents in pressure relief
systems which are required to dispose of both cold and wet streams. Wherever practical, separate systems
shall be provided for cold streams with segregation maintained until the streams become compatible.
Except for LNG systems, cold liquid knock-out drums shall be provided with vaporization facilities.
8.5.2 Special precautions shall be taken to prevent low-temperature embrittlement of piping and equipment
in pressure relief systems which are required to dispose of streams that enter the system at 20 F (-5 C) or
colder or where streams may autorefrigerate to a temperature below which carbon steel can become
brittle.
8.6 Thermal Relief
Thermal relief valves shall discharge to a lower pressure system whenever possible. When this is not
practical, they may discharge to an oily water sewer or to grade.