Controlling Hydrocarbon Emissions from Tank Vessel Loading (1987) / Chapter Skim
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3 Vapor Control Technology
3 Vapor Control Technology
Pages 58-92
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From page 58... ...
Vapor control technology is used at marine terminals mainly for handling highly toxic or noxious cargoes with volatile vapors, such as ammonia, chlorine, acrylonitrile, and epichlorohydrin. Applying these technologies more widely, particularly to the high volumes and loading rates typical of gasoline and crude oil, will challenge the ability of vessel and terminal operators to maintain safe operating practices.
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From page 59... ...
The first was to construct the cargo tanks of all tank vessels as pressure vessels, to retain the VOC in the tankships. This was considered to be too expensive.
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From page 60... ...
relief valves are not normally designed to pass the full volume flow rate of liquid during loading. Three types of protection are available: spill valves, rupture disks, and full-flow relief valves in conjunction with proper piping aeslgn.
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From page 61... ...
Before the transition to closed loading aboard inerted ships, many operators feared that cargo levels would not be reliably known and that overfilling of tanks and spill incidents would increase significantly. While there has been very little reported on recent closed loading experience, the absence of casualty reports suggests not only that there has been no serious increase in overfill incidents, but that the incident rate has actually decreased.
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From page 63... ...
In contrast with these complex systems, some operators of barges carrying benzene and other hazardous chemicals regularly load closed with nothing more than a glass viewing port for observing the cargo and ma restricted standpipe for final gauging. Even with this arrangement, there was no evidence of cargo overflow incidence rates in excess of open-loading experience.
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From page 64... ...
J ~' 1 Stainless Steel Float Follower Magnet r Float Stop Cargo Level
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From page 65... ...
This instrument may be supplied either unpowered or powered to drive a remote gauge and alarms. Independent, power-operated, high-level alarms should be considered for tank vessels that have neither redundant cargo level monitoring systems nor level monitors with built-in alarm capability.
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From page 66... ...
This decision depends primarily on · the nature of the vapor stream, specifically, its expected variability in flow rate and hydrocarbon content; and · locational factors, such as the availability of utilities and the distance from the tankship or barge to the vapor control facility. To prevent flame flashbacks, each hydrocarbon-containing line that feeds the flare needs to pass through at least one detonation arrestor.
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From page 67... ...
Several different burner head designs are available to maximize combustion. They vary in size and shape depending on the design flow rate, the design hydrocarbon content, and turndown requirements.
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From page 68... ...
. In shipping applications, supplemental fuel will probably be needed until the end of the loading cycle, unless the cargo is exceptionally volatile.
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From page 69... ...
Incinerators may be slightly more efficient than flares. If operated properly they can achieve more than 98 percent hydrocarbon destruction over larger ranges of flow rates and hydrocarbon contents.
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From page 70... ...
70 Supplemental Fuel Lean Vapor to Atmosphere Split Range 1 HO ~ TRC 'l ~ b PCV Flame Arrestor 1 O2}~__ Rich Vapor from the Vessel . ~ ,, Incinerator/ Thermal Oxidizer Pilot Fuel Combustion Air mu.
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From page 71... ...
The bladder will help dampen variations in vapor flow rate and hydrocarbon content and thus allow a smaller absorber to run for longer, lined-out periods, yielding vapor with a low, predictable hydrocarbon content. Any hydrocarbon liquid with sufficiently low vapor pressure can be used as the lean oil; the decision is an economic one.
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From page 73... ...
This performance limitation could limit the usefulness of lean oil absorbers in shipping applications, since vented vapors are likely to be lean during most of the loading cycle. (If the incoming vapors contain less than the absorber's equilibrium hydrocarbon content, say 2 percent, then the absorber will actually enrich them.)
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From page 75... ...
Carbon Bed Adsorption Carbon bed adsorbers use activated carbon or a similar adsorptive medium to adsorb hydrocarbon selectively. Air and very light hydrocarbons pass through the medium, while heavier hydrocarbons are adsorbed to the medium's surface (Figure 3-6~.
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From page 76... ...
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From page 77... ...
Advantages Carbon bed adsorbers can be more than 99 percent effective at removing hydrocarbon. Disadvantages Carbon beds do a poor job of recovering light ends, such as ethane and propane.
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From page 78... ...
Ideally, the feed vapors contain between 0.05 and 0.75 percent hydrocarbon, corresponding to hydrocarbon concentrations between 5 and 75 percent of the lower explosive limit (typically around 1 percent hydrocarbon in air)
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From page 79... ...
For the foams to achieve a high degree of vapor control, vessel operators would need to generate them on board, then spread and maintain a 2- to 6-in. layer on the liquid hydrocarbon's surface throughout the offloading and loading cycle.
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From page 80... ...
VAPOR BALANCING AS AN ADJUNCT TO VAPOR CONTROL The technique known as vapor balancing can be used as an adjunct to vapor control to reduce instantaneous processing rates, or for other reasons. For example, at Exxon's offshore Hondo Field in California, loading emissions are pumped into a large tank vessel where they are retained for subsequent burning.
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From page 81... ...
Coast Guard regulations now require that "each tank vessel having a COW system -- without sufficient SBT or CBT -- must have a means to discharge hydrocarbon vapors from each cargo tank that is ballasted to a cargo tank that is discharging crude oil." This transfer of vapors is accomplished using IG vent lines. Using this arrangement, VOC emissions from ballasting are eliminated.
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From page 82... ...
TABLE 3-1 Atmospheric Emissions Loading 250,000-dwt Crude Carrier (all tanks COW) Vapor Emissions Vapor in empty tanks before loading Evaporative loss during loading and gauging Subtotal In ullage space after loading and gauginga Atmospheric emissions during loading and gauging Initial gauging Emissions during loading and final gauging Tons 35 58 93 86 3 83 aThe ullage space after loading eventually reached equilibrium and registered 50 percent hydrocarbon equal to 15 tons on arrival at the discharge port.
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From page 83... ...
· Tank vessels loading at docks that serve only tankships and large inerted tank barges will have onboard systems for closed loading with redundant gauging and alarm capability and an inert gas system designed for less than 7 percent O2. · All tank vessels will be outfitted with vapor collection headers sufficient to accept vapors at full-loading rates.
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From page 84... ...
Vapor Collection Headers For most tankships fitted with IG systems, the installed inert gas headers can, with a few modifications, be used as vapor collection headers. Noninerted tankships and tank barges will require the installation of one or more deck headers to collect vapors from the tanks and carry them to the vapor hose connections located in way of the cargoloading manifold.
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From page 85... ...
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From page 86... ...
. Special Considerations for Tank Vessel Inert Gas Systems Either flue gas or independent IG systems are acceptable if the vapor mixture leaving the tank vessel has an oxygen content of less than 8 percent at all times.
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From page 87... ...
Figure 3-9 shows the dockside enclosure panel for the system. Vapor-Handling System for Terminals At terminals loading large, inerted tank vessels, the incinerator or other vapor control process and the vapor transfer piping system are sized to receive the maximum loading rate expected for gasoline and crude oil, with a suitable safety margin.
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From page 89... ...
(3) Wiring to the barge is intrinsically safe.
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From page 90... ...
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From page 91... ...
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From page 92... ...
At a minimum, detonation arrestors and rupture disks should be located at the terminal flangefs) and at the inlet to the vapor control process.
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