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OCR for page 58
3
VAPOR CONTROL TECHNOLOGY
. ...
Attaining the emission reductions proposed in several state implemen-
tation plans to meet the ozone deadlines in the National Ambient Air
Quality Standards would require substantial investments by the operators
of marine vessels and terminals. Special vapor handling systems would
be needed at loading terminals and aboard vessels. Compartments on both
tankships and barges would need to be closed to the atmosphere during
loading (with appropriate automated gauges to prevent overfilling and
overpressuring). Vapors would need to be collected and piped to recov-
ery or disposal systems such as flares, incinerators, refrigeration
systems, carbon adsorption beds, or lean-oil absorption units.
The essential technologies for these measures are available commer-
cially. They are used routinely in tank farms and tank truck terminals,
although the scales of these systems are often smaller than those re-
quired to control vapor emissions during tank vessel loading or
ballasting.
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.
Operations throughout the industry will need to be raised to the level
of subchapter O cargo handling. Communications must be fail-safe,
procedures must be consistent and thorough, and equipment must be well
maintained.
The modest skills required of the barge-trained tankerman, especi-
ally in smaller operations/ports, should be taken into account in
devising technical solutions and management approaches. Coincidentally,
the Coast Guard is in the process of revising and upgrading tankerman
certification requirements for a number of safety oriented purposes.
Among the technical challenges is the gauging of closed tanks on
barges as they are loaded. In loading some tankships and most barges,
the practice generally is to gauge the height of the cargo by eye,
through open hatches. With vapor recovery systems, tanks will be loaded
with hatches and vents closed to the atmosphere, so that accurate gauges
will be needed. The closed gauging requirement is particularly impor-
tant since overfilling can result in spills, ruptured tanks, and
58
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59
damaging mechanical shocks to vapor handling equipment, with possible
subsequent fires and explosion. In addition, detonation arrestors
adequate for the sizes and flow rates of vapor pipelines will need to be
developed and tested.
MAXIMUM CONTROL OF EMI S S IONS
Loading and ballasting emissions from vessels carrying crude oil and
gasoline can be reduced by over 90 percent, using components that are
commercially available. However, since few complete systems of the
appropriate scale have been constructed and used, some engineering
challenges would have to be met to ensure a safe and cost-effective
solution to the regulatory requirement for such control. The following
sections describe the available options and comment on the technical
uncertainties.
Closed Loading of Tank Vessels
Controlling vapors from tank vessels, obviously, will require load-
ing with all hatches and ports closed. Closed loading departs from
barge practice, but it is routine on most tankships . I t presents cer-
tain problems not confronted when loading with open hatches, but the
practice does not present any unusual risk if the vessel is properly
outfitted and operated.
Liquefied gas carriers, specialty vessels carrying certain hazardous
chemicals, and most tankships have been closed loaded for many years
with very good safety records. The installation of inert gas (IG)
systems on the majority of tankships during this decade has resulted in
a great increase in closed loading experience, since closed loading is
necessary to maintain the legally required minimum inert gas pressure
above the cargo.
Equipment for closed loading falls into three categories: (~)
protection from tank overpressurization, (2) final (custody transfer)
gauging and sampling, and (3) level monitoring and alarms. With some
greater risk, closed loading could be done without specifically address
-
Two possibilities for eliminating volatile organic compound (VOC)
emissions from tank vessels were not deemed appropriate for further con-
sideration by the committee. The first was to construct the cargo tanks
of all tank vessels as pressure vessels, to retain the VOC in the tank-
ships. This was considered to be too expensive. The second possibility
was to equip vessels so that VOC loading emissions would be transferred
to the segregated ballast or clean ballast tanks and eventually dis-
charged at sea. This would require changing or abrogating U.S. and
inte**national regulations, which was judged impractical.
The term "closed loading" does not necessarily imply the capture
of vapors. Closed loading today is generally carried out with tank
vents open to the atmosphere.
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60
ing each of these, but each category should be given careful considera-
tion to determine the degree of risk an operator is willing to assume.
Tank overpressure protection should especially be considered for
barges, since the installed pressure/vacuum (PV) 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, rup-
ture disks, and full-flow relief valves in conjunction with proper
piping aeslgn.
Spill valves are high-volume, quick-acting relief valves that are
closed when gas is being exhausted and open when liquid is present.
Their principal disadvantages are the very large size and high cost of
valves that handle high-loading rates (more than 5,000 bbl per hour).
Rupture disks are devices with carefully machined carbon or metallic
disks that rupture at a preset level below the design pressure of the
vessel tank structure. Their major disadvantage is that, when actuated,
they provide a free path from the atmosphere back to the cargo tanks,
with the associated fire hazard. The main purpose of either device is
to prevent rupturing the vesselts hull.
If practical, the spill valve or rupture disk should be piped to a
tank or enclosure to prevent oil from spilling into the water. Even if
this is not possible, the spill that might result from the operation of
one of these devices could be expected to be much smaller than that from
a hull rupture.
PV valves are available on the market in a variety of configura-
tions. These valves, however, are designed to vent gas rather than
liquid at full-loading rates. The limitation can be overcome with a
piping design, so that any liquid overflowing one tank and entering a
gas exhaust header can flow down into a tank that is not being
loaded. Even if this contaminates one cargo with a different one,
the cost of reprocessing the contaminated cargo should be considerably
lower than the costs of potential damages and cleanup of a major spill.
Final manual gauging and sampling of cargo is a routine practice in
ship and barge operations. Cargo quantity and quality are verified for
both the cargo owner and the transporter, and this practice can be
expected to continue as an accepted standard for some time. Manual
gauging and sampling on close-loaded vessels, however, cannot be carried
out in the same manner as on open-loaded vessels. Whether or not a
vessel is inerted with a pressurized inert gas, residual pressure in the
tank could present a hazard to the gauger and create inaccuracies in
measurement. Several methods are available to overcome these problems
and should be included in the design of the gauging and sampling system.
On noninerted vessels, when loading is stopped and there is no
pressure in the tank above the cargo, a restricted ullage cover in the
tank top may suffice. As an alternative, many operators use a standpipe
Such piping designs employ vapor headers equipped with valves to
permit selection of an empty versus a full tank for possible overflow.
The very presence of such valves carries with it the risk of shutdown
against a stream of vapors and therefore tank overpressuring, a major
risk of closed loading.
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61
extending from the deck to just above the bottom of the tank. Measure-
ments taken in the standpipe will always be higher than the actual
level, if there is pressure in the tank ullage and there are no
pressure-equalizing ports. However, if pressure-equalizing ports are
provided, gas can flow into the vicinity of the gauger. (Monitoring of
the unequalized standpipe level during the first 70-80 percent of
loading can be useful, in that the positive ullage pressure will provide
a conservative indication of the tank level.)
Samples and accurate level readings can be taken from pressurized
tanks using a vapor lock and valve. This device consists of a length of
pipe extending above the deck to a ball valve, with an additional length
of pipe above the valve terminating in a fitting that mates with the
gauging tape. The fitting height is set to a datum for which the tank
is calibrated. A special ullaging tape in a vapor-tight reel can be
mated to the fitting on the end of the pipe, the ball valve opened and
measurements taken without releasing tank pressure (Figure 3-1~. Sam-
ples, water interface measurements, and temperature readings can also be
taken with attachments to the tape. At least two companies make these
devices.
One consequence of closed loading is there may not be the opportu-
nity to observe the cargo level directly, as done with open loading.
Indirect determination of cargo levels has generated concern about know-
ing the "true" cargo level. 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.
Virtually all the cargo level measuring and indicating systems for
closed loading incorporate some redundancy. Simple systems may have two
independent passive devices. More sophisticated ones may have multiple
active and passive devices, independent alarms, and remote repeaters.
At the simple end of the spectrum is an unpowered tank barge with a tank
viewing port and one other unpowered device to provide warning of nearly
full tanks. Among the most complex systems are those of chemical car-
riers, which by regulation are required to have a full-range, level-
measuring instrument and two independent, high-level alarm instruments.
One large domestic operator, which loads all of its 20 operating
tankships closed, modified five ships of its fleet from 1980-1981 for
use with a vapor recovery system at an offshore facility in the Santa
Barbara Channel. Each of these tankships is equipped with a full-
length, float-and-tape gauge and a magnetic float-with-reed-switches
gauge for the top 10 ft of each tank. In addition, two independent
dual-float alarms and a vapor lock for manual ullaging are installed in
each tank.
Because the vessels are of the older two-houses design, there is no
cargo control room and all cargo operations must take place on deck. To
provide visual indication of high-level warning and alarms, mimic dis-
plays are mounted on the fronts and backs of the midships houses. This
arrangement has been very successful. None of the vessels has had a
OCR for page 62
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OCR for page 63
63
cargo spill during the 6 years since the system was installed. In fact,
this company has had no cargo overflow spills in its entire fleet since
converting to closed loading.
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. If anything, the overflow incidence of close-
loaded barges is lower than that on open-loaded barges' perhaps because
of the higher degree of operational control that is necessary in closed
loading.
Level-monitoring equipment can be conveniently categorized by actua-
tion method and display technique, as done by Southwest Research
Institute in a study (Johnson et al., 1981) for the Maritime Administra-
tion. Of more practical interest to the operator, however, is whether a
device is active (requires external power) or passive (can stand alone).
Active devices most commonly use electric or pneumatic power, and
have the ability to actuate visual and audible alarms and to provide
remote indication. The most common electric instruments measure the
liquid level by means of magnetic floats sequentially operating reed
switches, radar or sonar impulses bounced off the liquid surface, or
hydrostatic pressure transducers located in the tank. Alarm indication
may be centralized or distributed, and remote readouts may be single or
multiple. At least two manufacturers offer hand-held radio receivers
that allow an operator on deck to monitor levels and receive alarms from
all tanks regardless of location.
Since an external source of electricity is needed, this type of
monitoring instrument is suited mainly to ship installations. Several
manufacturers do offer instruments that are solar powered and could be
installed on unpowered barges. The solar-powered devices cannot,
however, provide an alarm, owing to the low power available from their
photovoltaic collectors.
Pneumatically operated level detection systems have found only
limited application aboard ships and barges. In most cases, the reasons
have been the limitations on supplies of clean, dry air and the poten-
tial for creating an explosive mixture by admitting oxygen into an
otherwise safe atmosphere in the tank.
The most common passive level monitors found on barges are simple
visual, mechanical/magnetic, or purely mechanical types that require
monitoring by the operator. The simplest of these monitors is a glass
viewing port that can be mounted either in the deck or in the expansion
trunk hatch cover. More elaborate versions of this device can have a
hand-operated wiper for clearing condensation from the underside of the
glass, a stepped and calibrated scale that can be viewed through the
glass, and a second port to allow a light to be directed into the tank.
Another simple but effective device for monitoring the top 4-6 ft of
the tank consists of a nonmagnetic tube that penetrates the tank with a
float and magnet outside the tube (Figure 3-2~. The float magnet inter-
locks with a magnet at the bottom of the lightweight stick inside the
tube. As the float rises, the coupling of the magnets causes the stick
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64
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Cargo Level
OCR for page 65
65
to rise with the liquid level, providing a reliable and reasonably
accurate indication of tank level. With green, yellow, and red bands on
the stick, the tanks can be monitored at reasonable distances if the
deck is not too cluttered.
A variation on this principle has the stick attached to the float
and rising in a tube that projects above the deck. A magnet at the top
-of the stick causes magnetic flags to flip over as the top magnet passes
them, changing the visible color of the column from white to red. The
range of indication on each of these types is limited by the weight of
the stick and the vertical clearance above the barge deck.
A more complicated but effective full-length gauging instrument uses
a wire-guided float, which unreels and retracts a calibrated tape from a
spring-loaded drum as the float falls or rises with the tank level.
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. Any alarm
system should have a means of checking the complete alarm operation
before loading and, if electrically powered, should have intrinsically
safe circuitry. The most common type of alarm in marine use has a mag-
netic float that holds a reed switch closed. When increasing cargo
level lifts the float above the switch, the switch opens, breaking the
circuit and sounding the alarm. A mechanical or magnetic link is normal-
ly included to lift the float to check for float freedom or proper
electrical function. In addition to the float-actuated type, several
manufacturers offer capacitance or optical alarm devices that might be
adapted to marine use. Functional testing of these types, however,
might be more difficult.
While each operator will have ideas about acceptable risk, the
possible consequences of a cargo overfill incident are severe enough to
require very careful consideration of the vessel's need for additional
equipment prior to converting to closed loading. As a minimum, a full-
depth level monitor with an alarm, an independent high-level alarm, and
a closed gauging and sampling connection should be fitted on ships with
IG systems. Unpowered barges should have at least a means of monitoring
the top few feet of the tank, a restricted standpipe, and, because of
their lower design pressure rating, a rupture disk, spill valve, or
high-capacity relief valve with intertank overflow capability.
The logical next step for barges is to provide a system that reads
level warning and alarm signals aboard the barge and actuates alarm and
control devices at the terminal. A practical method of doing this can
be realized by installing currently available sensors aboard the barge,
explosion-proof alarm and control enclosures routinely fabricated for
refineries, and intrinsically safe circuitry, also currently available,
between dock and barge. The configuration of the connection between the
barge and the dock cable, presumably a plug and socket arrangement,
would need to be accepted as an industrywide standard to ensure that any
barge can connect to the alarm system at any marine terminal.
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66
Hydrocarbon Vapor Recovery and Disposal Systems
Several established processes can be used to reduce the hydrocarbon
vapor emissions from crude oil and gasoline loading. The control pro-
cesses fall into two broad categories: combustion and recovery. Combus-
tion processes include flares and incinerators. Recovery processes
include lean oil absorbers, refrigeration systems, and carbon-bed
absorbers.
The optimal process for one vapor control application may not be
optimal for another. In selecting a process for a given situation, the
most important decision is whether or not to recover the hydrocarbon.
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.
This is especially important for the line between the cargo compartments
and the combustion or recovery equipment.
When Combustion Is Preferable to Recovery
Compared to hydrocarbon recovery systems, flares and incinerators
are inexpensive to install and easy to operate. They will probably be
more economic at low-volume terminals that are located far from existing
utility hookups. This is especially true if the vapor vented from tank-
ships and tank barges is lean, and the potential value of the recovered
hydrocarbon is low.
When Recovery Is Preferable to Combustion
It may be economic to recover hydrocarbon from large, relatively
rich streams at high-volume terminals that have adequate space and
easily accessible utilities. Recovery equipment costs more to install
and operate, but the value of the recovered hydrocarbon makes recovery
cost-effective, especially at terminals with adequate space and easily
accessible utilities.
Recovery Followed by Combustion
Most recovery processes can recover 80-95 percent of the hydrocarbon
with moderate installation and operating costs. However, it becomes
prohibitively expensive to remove much more because the operating condi-
tions become too severe (e.g., temperatures below -200°F, pressures
above 250 psia).
OCR for page 67
6_
7
If further reduction is needed, a small flare or incinerator should
follow the recovery unit to polish the outlet stream. A polishing com-
bustor can be designed small, since it will see a lean, steady feed.
Combustion Processes
Flares and incinerators combust hydrocarbon-containing vapors as
they arrive from the vessel or from intermediate vapor recovery equip-
ment. The combustion products are mainly CO2 and H2O; small amounts
of NOx and CO are also produced.
Both flares and incinerators are more than 98 percent efficient if
operated properly. They can perform reliably as the sole hydrocarbon
control process; and even more reliably as polishing units.
The primary drawback is that they do not recover the hydrocarbon.
The value of unrecovered hydrocarbon can be significant when crude or
gasoline is being shipped.
Another potential drawback is that combustion devices can be rela-
tively unsafe, simply because they are potential ignition sources. This
concern is especially important if the displaced vapors are not inerted.
Vapors from vessels with inert gas systems will have oxygen contents
below 11 percent--too low to support combustion. The lack of oxygen
will greatly reduce the risk of explosion. It will also require the
combustion system to draw in additional air (to raise oxygen levels to
the point where the mixture will burn). Diluting the vapors will
increase the size of the combustor and the amount of supplemental fuel
needed to maintain minimum combustion temperatures.
Open Flares
Open flares have been used by refineries and chemical plants for
decades. Almost all were installed as plant protection and safety
devices. However, during the past 5 to 10 years, an increasing number
have been installed specifically to reduce hydrocarbon emissions.
The vapors ignite as they pass through one or more burners. 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. To maintain a
flame at all times, every flare needs a pilot burner in case the main
flame goes out. The pilot burner is much smaller than the primary
burners.
Advantages Open flares are the least expensive control option. They
require little operator attention and will sustain burning on their own
as long as the incoming vapors contain enough hydrocarbon. As long as
the combustion zone stays properly lighted, they are usually more than
98 percent efficient.
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68
Disadvantages If the hydrocarbon content drops too low, supplemental
fuel will be needed to prevent significant drops in efficiency (poten-
tially to zero if the flame goes out). In shipping applications,
supplemental fuel will probably be needed until the end of the loading
cycle, unless the cargo is exceptionally volatile.
Although flares are effective hydrocarbon removal devices, it is
difficult to demonstrate whether or not they achieve the commonly be-
lieved 98 percent efficiency level. Industry and the EPA have conducted
numerous tests and have agreed that efficiency of more than 98 percent
is typical. Nonetheless, lack of demonstrability may limit applications
in areas where state and local regulators want proof in the form of
rigorous field tests.
The radiative heat given off by flares is a concern, but not a major
one. Flares, especially open ones, need to be located away from people
and equipment. By comparison, location of an incinerator is somewhat
less of a concern, since its combustion zone is enclosed.
Noise and visual impact are other minor disadvantages of flares,
particularly open flares. These factors do not affect performance or
safety, but may affect an operator's chances of getting permits for
equipment.
Enclosed Flares
An enclosed flare is essentially an open flare with a protective
cylindrical shroud around the burners. The shroud helps increase
natural draft and aerate the combustion zone. The shroud also helps
minimize the impact of wind and other disturbances.
Enclosed flares are open to the atmosphere on top. On the bottom
they have louvers to help control the inflow of combustion air. The
louvers increase the efficiency somewhat by reducing the excess air.
However, louver adjustment is usually performed manually and is not very
accurate. On some enclosed flares the louvers are not adjustable.
Advantages Enclosed flares are somewhat easier to test for compliance
than open flares. Flue gas samples can be drawn from within the stack.
Thus, even though it is difficult to determine how much air enters
through the louvers, measurements are more likely to be accurate than
those around open flares. Enclosed flares also radiate less heat and
are less noisy than open flares, especially when designed large enough
to contain the combustion zone within the stack.
Disadvantages Enclosed flares are more expensive than open flares.
They are also subject to capacity limitations.
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82
Explosive Limit (1 percent HC). As loading proceeds,
the hydrocarbon content of the vented gas rises.
. . . iT]he gas layer depth will be taken as the dis-
tance from the liquid surface to the level above it
where the gas concentration is 50 percent by volume.
. . . tG]as will be detectable at heights above the
liquid surface several times the layer depth defined in
this way.
Most high vapor pressure cargoes give rise to a gas
layer with a depth in these terms of less than 1 meter.
The 83 tons of emissions that result, on average, from loading a 100
percent crude-oil-washed VLCC have been analyzed as shown in Table 3-1
(Uhlin, 19849.
Atmospheric emissions while loading cargo are minimized by filling
each compartment as rapidly as possible, to reduce the amount of evapora-
tion into the ullage space (an exception to this is at the start of load-
ing when rapid rates may cause splashing, which increases evaporation).
Loading into Gas-Free Cargo Tanks
Table 3-1 shows that gas-freeing of cargo tanks on the ballast pas-
sage combined with loading into the gas-freed tanks would reduce VOC
vapor emissions by about one-third.
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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83
Gas-freeing all the cargo tanks on the ballast passage would in-
crease bunker fuel costs. In addition, it might delay crude oil washed
tankships at the discharge port.
Loading to 70 Percent of Capacity
Loading each gas-freed cargo tank only 70 percent would retain most
of the vapors in the ullage space. To minimize sloshing at sea, the
cargo could later be transferred to fill most of the tanks to capacity.
This technique could reduce carrying capacity by 30 percent; thus its
economic acceptability would need to be evaluated.
HYDROCARBON VAPOR CONTROL SYSTEMS:
ASSUMPTIONS FOR PURPOSES OF ASSESSMENT
Since vapor control systems are not widely used in maritime applica-
tions, the committee found it necessary to set some ground rules for
analyzing cost estimates. The resulting hypothetical system is intended
to meet all likely regulatory requirements and incorporate all safety
features. It uses available technology and would be capable of reducing
loading and ballasting emissions at terminals by more than 99 percent.
The assumptions that underlie the system are:
· Vapors will be inert or overrich prior to being treated or trans-
ferred any significant distance.
· Incineration is the control process in all estimates. It has low
capital cost and universal application.
· Detonation arrestors will be placed in vapor pipelines near the
treatment system, at the dock manifold, and at the tank vessel's mani-
fold. Flame arrestors are not considered to be an acceptable substi-
tute.
· Redundant tank gauging and alarm systems will be used for closed
loadings.
· Shoreside loading facilities will have provisions for automatic
shutdown (using contact signals from the alarm systems).
· Loading at terminals will remain at current loading rates.
· 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.
· All cargo tanks that are inerted will be fitted with vapor locks
for use with sonic gauging tapes. If not inerted (as with most tank
barges), cargo tanks will be fitted with restricted standpipes extending
to just above the tank bottoms.
· Docks at terminals serving only tankships and large inerted tank
barges will be designed to accept inerted vapors coming from the
vessels.
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84
· Except as noted above docks at terminals serving smaller, non
inerted tank vessels will provide a means for inerting vapors coming
from the vessels as they enter the vapor transfer system.
· For smaller tank vessels without onboard inerting equipment, the
vapor stream may be inerted during loading.
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 installa-
tion 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 cargo-
loading manifold. Figure 3-7 shows a typical vapor connection header
for a tank barge.
The vapor collection header will be of steel construction. Internal
coating must be compatible with the products to be carried.
Tank PV valves should be set for the highest pressure consistent
with tank design. If tanks are fitted with individual PV valves, one
additional PV valve should be installed on the vapor collection header.
This valve and its header should be sized for the maximum loading rate
to all tanks served by the header. A rupture disk or spill valve should
be installed in the vapor header to limit tank pressure to the hydro-
static test pressure of the tanks in case of overfill. Such a device
should relieve liquid to a cargo tank or another enclosure. The vapor
collection header should be designed to allow for 1.0 psi (0.5 psi for
harges) back pressure at the vapor hose flange during maximum-rate load-
ing with tank pressures below the PV valve setpoint. If multiproduct
loading of cargoes susceptible to cross-contamination is expected, tanks
should have individual PV valves. Line blinds or valves should be pro-
vided at the vapor connections to the tanks.
A detonation arrestor should be located as near as possible to each
vapor hose connection and installed to be easily removed for cleaning
and maintenance. A shipboard pressure control system should be consid-
ered to allow the ship to control the cargo tank pressure independent of
the shore facility.
A drip pan, wide enough to accept a reducer, should be located under
each vapor hose connection to catch any condensate during hose removal.
The vapor hose connection may be of either bolted or cam lock type
and should accept both standard 125 pound or 150 pound flanges from the
hose.
*
An alternative method of rendering the vapors nonexplosive would
be to enrich them. Enriching would minimize the quantity of vapors to
be processed.
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86
Tank Gauging and Alarms for Inerted Tank Vessels
Inerted tank vessels are assumed to have two independent tank gaug-
ing and alarm systems, one to measure the full tank depth, the other to
measure the top 6-10 ft below deck. A single gauging system may be used
if it is inherently redundant by design. In this case, however, a sepa-
rate and redundant alarm system should be installed.
The gauging and alarm systems are assumed to have an accuracy of bet-
ter than 0.5 in., and two alarm setpoints, each redundant. The system
includes a high-level warning at about 12 in. below the top of the tank
and a high-level alarm at 4-6 in. below the top of the tank. If loading
is conducted from a cargo control room, level indication and alarms are
displayed in the room with audible and visual alarm indications on
deck. If cargo operations are carried out on deck, level indicators are
located at the tanks, with audible and visual warning and alarm indica-
tions placed where they will be heard and seen from anywhere on deck.
The alarm system provides a means for supplying pump shutdown contacts
at the tankship's rail for use by the terminal's emergency shutdown
system (where available).
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. The system should therefore be designed to pro-
duce inert gas with as low an oxygen content as possible, but no greater
than 7 percent. The tank vessel's IG system must have sufficient instru-
mentation and a recorder to allow the terminal to verify the proper
inerting of the tanks during the prior discharge.
Dockside Tank Level and Alarm System for Tank Barge Loading
Each cargo tank is assumed to be fitted with a reliable high-level
alarm and shutdown sensor. Each has a fail-safe method for checking the
instrument and circuit prior to each loading. Each instrument provides
two separate, normally closed contacts to initiate the high-level warn-
ing and high-level shutdown independently.
Each instrument has two setpoints: a high-level warning at 12 in.
below the top of tank and a high-level alarm at 4-6 in. below the top of
tank. Warning and alarm/shutdown signals are both audible and visual
and easily detected from both the loading manifolds and the barge dock.
Instruments are connected through intrinsically safe cable to weath-
ertight nine-pin connectors near the loading manifolds. Each connection
serves the instruments of four tanks. All instruments and cables out-
side the dockside enclosure are intrinsically safe. The dockside enclo-
sure may be either explosion-proof or intrinsically safe. The main
connecting cables from the dock to the tank barge allow connections for
the maximum number of tanks expected in barges that utilize the dock.
87
Two lights for each tank are located on the dockside enclosure.
Large (4-6 in.) lenses with dimmer controls ? arranged to represent the
layout of the tank barge deck and visible from the barge manifold, are
preferred.
The system sounds alarms of distinctly different pitch for the high-
level warning and high-level alarm/shutdown. Contacts will be provided
in the dockside enclosure to activate the dock's emergency shutdown on a
high-level alarm. The system has the capability to perform continuity
and function checks prior to the start of each loading.
To facilitate accurate loading, the barge operator may also wish to
provide an unpowered or intrinsically safe solar- or battery-powered
gauging system that does not require an external electric source. If no
gauging system is provided, a second set of high-level warning/alarm
devices must be provided with their contacts wired in series with the
other warning/alarm devices such that the signals will be activated if
either contact opens.
Figure 3-8 is a schematic illustration of a dockside gauging and
alarm system for tank barge loading. 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. At tank barge terminals, the
systems are sized for the maximum loading rate plus sufficient addi-
tional inerting gas to lower the oxygen content of the vapor stream to
less than 8 percent. This may take four or more volumes of inert gas
for each volume of barge-emitted vapor, depending on the oxygen content
of the inerting gas. As tankship loading rates are frequently five or
more times the tank barge loading rates, terminals serving both may find
that the required size of the system will be nearly the same, owing to
the large volume of inert gas added to the barge vapor stream.
Figure 3-10 is a schematic drawing of a simple vapor control system
for a tank barge and tankship terminal. Figure 3-11 shows an incinera-
tor system for a barge dock.
Since the oxygen content of the incinerator exhaust gas can be con-
trolled to less than 5 percent with some incinerator designs, operators
may consider using this gas as inert gas for tank barge loadings. Other
sources of inert gas include fuel-fired inert gas generators, nitrogen,
natural gas, and refinery flue gas.
To prevent oxygen being drawn into the system, all piping carrying
inerted vapors should be under a positive pressure, but not present more
than 0.5 psi back pressure at a tank barge flange or 1.0 psi at a tank-
ship flange. Underwater pipelines may be at negative pressure only if
any extension above water is of all-welded construction.
The inlet for inerting gas at barge docks should be as close as prac-
tical to the terminal flange. An oxygen analyzer (explosimeter for
enriched systems) should be located as close as possible to the terminal
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WARNING SHUTDOWN
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FIGURE 3-9 Dockside warning and alarm panel. The following applies:
(1) Warning lights are amber and alarm lights are red. (2) The enclo
sure is explosion-proof. (3) Wiring to the barge is intrinsically
safe. (4) Warning/alarm circuits are open to actuate. (5) High-level
alarm actuates siren and shutdown. (6) High-level warning actuates
3-second horn. (7) Warning lights flash for 3 seconds then are on
steady. (8) Alarm lights and siren must be acknowledged. (9) Each
nine-conductor cable serves four tanks.
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92
flange, but downstream. of any inert gas inlet. A trip valve at the
terminal flange should be designed to close if an oxygen concentration
of 8 percent or more is detected. If a booster is required because of a
pressure drop through vapor transfer piping, a recirculation loop with a
cooler or other type of capacity control should be used to maintain a
positive pressure at the booster suction.
Care must be taken in the design and operation of the vapor transfer
system to eliminate any ignition sources. Temperatures in piping and
other components of the vapor transfer system should be kept well below
the vapor ignition temperature, whether vapors are inerted or not. At a
minimum, detonation arrestors and rupture disks should be located at the
terminal flangefs) and at the inlet to the vapor control process. A
final oxygen analyzer (explosimeter) should be located near the vapor
control process, but far enough upstream to ensure closure of the trip
valve before the potentially explosive vapors reach that point.
