Relief scrubbers

A vessel can only be overpressured if the upstream vessel has a higher pressure than the vessel in question. A compressor scrubber with a MAWP of 285 that gets flow from a 285 MAWP separator does not need to have a relief valve sized for blocked discharge. The upstream relief valve will keep the upstream separator pressure from going higher than 285, so there is no way it can oveipressure the downstream scrubber. The scrubber PSV only needs to be sized for fire. 

In large facilities and offshore platforms where the escaping gases and liquids could present a source of pollution or ignition, it is common to route the relief valve discharges into a common header that discharges at a remote safe location. Often a vent scrubber is installed in this header to separate the bulk of the liquids and to minimize the possibility of liquid discharges to atmosphere. 

Figure 13-11. Schematic of a relief system showing pressure vessels, relief voives, relief header, vent scrubber, and vent boom. Process piping has been omitted for clority.

When the relieving scenarios are defined, assume line sizes, and calculate pressure drop from the vent tip back to each relief valve to assure that the back-pressure is less than or equal to allowable for each scenario. The velocities in the relief piping should be limited to 500 ft/sec, on the high pressure system and 200 ft/sec on the low pressure system. Avoid sonic flow in the relief header because small calculation errors can lead to large pressure drop errors. Velocity at the vent or flare outlet should be between 500 ft/sec and MACH 1 to ensure good dispersion. Sonic velocity is acceptable at the vent tip and may be chosen to impose back-pressure on (he vent scrubber. 

Inflow exceeding outflow is sensed by a high-level sensor (LSH). Back-up protection is furnished by the PSH (to keep the relief valve from operating) or an LSH in a downstream vent scrubber if the vessel gas outlet goes to atmosphere. That is, a vent scrubber must be installed downstream of any vessel that discharges directly to atmosphere. 

Vessel is final scrubber in a flare, relief, or vent system and is designed to withstand maximum built-up back-pressure. 

Vessel is scrubber or small trap, is not a process component, and adequate protection is provided by downstream PSL or design function (e.g., vessel is gas scrubber for pneumatic. safety system or final scrubber for flare, relief, or vent system). 

Vessel is final scrubber in a flare, relief, or vent system, is designed to withstand maximum built-up back-pressure, and has no internal or external obstructions, such as mist extractors, back-pressure valves, or flame arrestors. 

Six-tenths factor, 47 Yearly cost indices, 47 Critical flow, safety-relief, 438 Back pressure, 440 Sonic flow, 438 Critical flow, see Sonic Cyclone separators, 259-269 Design, 260-265 Efficiency chart, 263 Hydroclones, 265-267 Pressure drop, 263, 264 Scrubber, 269 Webre design, 265 Deflagration venting nomographs,... 

Sizing, 451, 453, 455, 459, 462 Sonic flow, 461 Types, illustrations, 411-421 Rupture disk, liquids, 462, 466 Rupture disk/pressure-relief valves combination, 463 Safely relief valve, 400 See Relief valve Safety valve, 400, 434 Safety, vacuum, 343 Scale-up, mixing, 312, 314—316 Design procedure, 316-318 Schedules/summaries Equipment, 30, 31 Lines, 23, 24 Screen particle size, 225 Scrubber, spray, 269, 270 Impingement, 269, 272 Separator applications, liquid particles, 235 Liquid particles, 235 Separator selection, 224, 225 Comparison chart, 230 Efficiency, 231... 

Disposal provision of effective vent stacks to disperse material vented from pressure relief devices or use vent scrubbers. 

Relief system The network of components around a relief device, including the pipe to the relief, the relief device, discharge pipelines, knockout drum, scrubber, flare, or other types of equipment that assist in the safe relief process. 

Specific types of relief devices are chosen for specific applications, such as for liquids, gases, liquids and gases, solids, and corrosive materials they may be vented to the atmosphere or vented to containment systems (scrubber, flare, condenser, incinerator, and the like). In engineering... 

If a scrubber is installed after PSV-lb and it has a pressure drop of 30 psig, how would this affect the size (qualitatively) of this relief system ... 

Equations 9-2 and 9-3 apply to rupture discs discharging directly to the atmosphere. For rupture discs discharging into a relief system (which might include knockout drums, scrubbers, or flares), the rupture disc is considered a flow restriction, and the flow through the entire pipe system must be considered. The calculation is performed identically to regular pipe flow (see chapter 4). The calculation to determine the rupture disc area is iterative for this case. Isaacs5 recommended assuming that the rupture disc is equivalent to 50 pipe diameters in the calculation. 

Depending on the relief stream components (with or without noncondensable gases) and quenching efficiency, this arrangement often obviates the need for a subsequent scrubber and/or flare stack. The design of the quencher arm is critical to efficient condensation and avoidance of water hammer. Figure 23-55 is the more conventional passive type quench pool used in the chemical and nuclear industry. 

Emergency Scrubbers (Absorbers) Emergency relief effluent streams are often sent to scrubbers (also called absorbers, columns, or towers) for final disposal by absorption of the gas into a solvent. Some gases or vapors can be removed by physical absorption. Other gases or vapors can be removed by chemical absorption (reaction of the vent gas/vapor into a liquid that reacts with it). 

Equipment Selection Criteria and Guidelines A number of factors should be considered to determine when to select which type ofvapor/gas/solid-liquid separator (blowdown drum or cyclone separator) to handle a multiphase stream from a relief device, and which final control or destruction equipment (emergency flare, emergency scrubber, or quench pool/catch tank) should be selected. These factors include the plot plan space available, the operating limitations of each type, and the physicochemical properties of the stream. 

Emergency Scrubbers (Absorbers) Emergency relief discharges are often passed through scrubbers (also called absorbers, or absorption columns or towers) for removal of flammable, corrosive, or toxic chemicals. The removal mechanism in some scrubbers involves physical absorption in a solvent, whereas in others chemical absorption (reactive scrubbing) is required. 

A discussion of emergency scrubber design and operating criteria and practices is given in Guidelines for Pressure Relief and Effluent Handling Systems (AIChE-CCPS, 1998). 

A scrubber has a closed-loop recycle of caustic. The make-up caustic solution is added continuously on an ORP (oxidation-reduction potential) controller. The caustic concentration will typically be 20 wt% and thus the blow-down stream will contain approximately 15 wt% hypochlorite. Such a system will typically be provided with a large reservoir of caustic that is released on a once-through basis for containment of emergency relief streams or operated with a permanent excess of caustic, resulting in a hypochlorite concentration in the blow-down stream in the range of 6-12 wt%. 

A scrubber fed with a continuous stream of moderate concentration caustic (1-5 wt%) without caustic recycle. The continuous stream is sufficient to abate pressure relief and emergency shutdown streams in addition to continuous treatment of normal tail gas... 

Methyl isocyanate (MIC) is a colorless liquid that must be stored in a cooled enclosure before it is subsequently used in the manufacture of carbamate, a common insecticide. MIC liquid is highly reactive in the presence of water and iron oxide, and it generates heat. In sufficient quantities, this heat may generate vapor, which, as explained previously, is highly toxic. Three adjacent bunkers were used for MIC storage. These were mounted in a berm and a refrigeration coil was used to ensure that the temperature did not exceed 5°C. A vent gas scrubber was used to prevent vapor escape, and despite a low operating pressure, a closed relief... 

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