TANK ULLAGE PRESSURE CONTROL
The present invention relates to apparatus for controlling the pressure in a tank ullage to
prevent fugitive emissions of polluting Volatile Organic Chemical ("VOC") vapours.
Common sources of these hydrocarbon vapours include dry-cleaning solvents and liquid
fuels.
Vapour recovery fuel dispensers have been in use for some time. The development of
vapour recovery fuel dispensers began in the early '70's and included some dispensers
that had assist-type mechanisms for pumping the vapours to the underground storage tank
in service stations. These did not generally gain favour and, throughout the 1970' s and
1980' s, balance system vapour recovery fuel dispensers were more common. In the
balance system, a closed, sealed path is established between the fuel tank being filled and
the underground tank. The movement of the liquid from the underground tank to the
vehicle tank creates a higher pressure area in the vehicle tank and a lower pressure area
in the underground tank to induce the vapour to move from the vehicle tank to the
underground tank. However, these systems tend to be cumbersome and prone to failure.
More recently, vapour recovery fuel dispensers have been equipped with vapour pumps
to actively pump the vapour to the underground storage tank. These systems are
exemplified by numerous issued U.S. patents including 5,040,577 to Pope, 5, 195,564
to Spalding, and 5,333,655 to Bergamini.
A problem which may be encountered with vapour pumps is that the act of pumping of
the vapour can lead to pressurisation of the underground storage tanks and associated
piping. Also, other factors such as temperature changes can lead to pressurisation. The
underground storage tanks and piping have an area above the liquid known as the ullage,
in which air and fuel vapours reside. The pressurised air and fuel vapours will have a
tendency to leak out of any hole in the tank or piping of the system, thus allowing the
release of the polluting fuel vapour to the atmosphere, precisely the situation that the
vapour recovery fuel dispensers are intended to avoid.
It has also been found that the balance systems can be subject to fugitive emissions of this
sort. Various pressure changes can occur in the tank, regardless of whether there is
pumping going on, including diurnal temperature changes and the like, leading to an
overpressure in the underground tank. These overpressures are of concern, since the
result can be fugitive emissions of pollutants to the atmosphere.
The assignee of the present applicant addressed this problem in U.S. Patent No,
5,571,310 issued November 5, 1996, and in U.S. Patent No. 5,464,466 issued November
7, 1995. These patents disclose a fuel storage tank vent filter system in which vapours
from the underground tanks are directed to a chamber having a fractionating membrane.
The membrane permits transmission of hydrocarbons through it in preference
differentially to atmospheric vapours. That system calls for a pump to be arranged to
draw the pollutants through the membrane as permeate and redirect them to the
underground tank, permitting air as retentate to be released to the vent pipe of the service
station tank arrangement.
The present invention provides a system to remove volatile hydrocarbons from the ullage
that is less expensive and easier to maintain than the membrane-based system.
According to the present invention there is provided apparatus for controlling a tank
ullage pressure comprising: a) a vapour pump for withdrawing vapour from the ullage;
b) a pressure sensor adapted for detecting pressure in the tank ullage; c) a canister
containing a VOC-adsorbent material downstream of the vapour pump; d) an exhaust
downstream of the canister to release VOC-depleted vapours to the atmosphere; and e)
a controller adapted to receive input from the pressure sensor and to output control
signals to the vapour pump to activate the vapour pump when the ullage pressure exceeds
a pressure threshold to adsorb VOC in the VOC-adsorbent material and vent substantially
VOC-free air to the atmosphere.
Preferably, the apparatus further comprises: an inlet manifold in communication with the
vapour pump and having a first exit and a second exit; a first canister containing a VOC-
adsorbent material positioned between the inlet manifold first exit and an exhaust
manifold; a second canister containing a hydrocarbon-adsorbent material and positioned
between the inlet manifold second exit and the exhaust manifold; wherein the exhaust
manifold includes a first entrance in communication with the first canister, a second
entrance in communication with the second canister, a first exit in communication with
the tank ullage and a second exit connected to a recirculation line, the recirculation line
connecting the inlet manifold and the exhaust manifold; wherein the controller is adapted
to receive input from the pressure sensor and output control signals to the vapour pump
and valve means to activate the vapour pump and control the valve means when the ullage
pressure exceeds a pressure threshold to vent substantially VOC-free air to the
atmosphere and to return hydrocarbons trapped in the adsorbent material to the fuel tank
ullage.
The provision of a second cannister enables tank ullage vapours to be pumped from the
tank after the material in the first canister becomes saturated.
Preferably the inlet manifold has valve means comprising first and second inlet valves
positioned between the vapour pump and respective inlet manifold exits and first and
second purge valves positioned between the recirculation line and respective inlet
manifold exits, although the inlet valves could be replaced by a two-way inlet valve and
the purge valves could be replaced by a two-way purge valve. The inlet valves and purge
valves direct the vapours in desired directions.
Preferably interlocks are provided to prevent both purge valves, and/or both return
valves, from opening simultaneously. This is an important safety feature.
Preferably the exhaust manifold has valve means comprising first and second return
valves positioned between the respective canisters and exhaust manifold exits, first and
second atmospheric vents connected to respective canisters and first and second
recirculation valves positioned between the respective canisters and the recirculation line.
Such valves are provided in order to direct most of the non-polluting components to the
atmosphere while recirculating a portion of the non-polluting components.
Preferably further interlocks are provided to prevent respective valves from opening
simultaneously as a further safety feature. The recirculation valves may be proportional
valves.
Preferably at least one VOC vapour level detector is provided to monitor the VOC
content of the flow exiting the canisters and to output a signal to the controller indicating
the VOC content. The provision of vapour level detectors permits the operation of the
two canisters to be alternated between adsorption and regeneration in dependence on the
amount of saturation of the VOC-adsorbing material in the canisters.
Further safety features which may be provided include an interlock to prevent the first
inlet valve and second inlet valve from opening simultaneously, a heat exchanger
positioned in the recirculation line, flame arrestors positioned either side of the vapour
pump and a heat exchanger between the exhaust manifold and the tank ullage. Further
canisters for the adsorption of VOC vapours may be provided.
Embodiments of the present invention will now be described, by way of example only,
with reference to the accompanying drawings, of which:
Figure 1 is a specific schematic view of a preferred embodiment of the invention; and
Figure 2 is the schematic view of Fig. 1 showing vapour flow reversed through the
invention.
Referring to Figure 1, there is shown a storage tank 8 which is typically underground,
but need not be an underground tank for purposes of this invention. The tank 8 holds a
liquid containing VOC's which could include, but is not limited to, a fuel 22 such as
gasoline. As discussed above, the present invention may be used with a variety of liquids
containing VOC's. Any reference herein to use of the present invention for hydrocarbons
is exemplary only. The scope of the present invention includes its application to any
VOC-containing liquid. Above the liquid 22 in the tank 8 is a volume known as the
ullage 11 holding vapour within the tank 8. The ullage 11 contains, as well, vapours
recovered from automobile fuel tanks via a vapour recovery system. The vapour
recovery system may be an assist system as described herein above or a balance system.
Other contents of the ullage 11 will be vapours of the liquid fuel 22 and, typically, air.
The air arrives in the tank 8 through numerous possible paths. Primarily, the air moves
through a vapour recovery system as discussed above. The vapour return from such a
system to the tank 8 is shown at 16. Air might also be ingested through pressure vacuum
valve 15 atop vent pipe 14. If the pressure in the ullage 11 falls to a low level, the
pressure vacuum valve 15 opens in conventional fashion to permit air to be ingested and
avoid potentially dangerous under pressure in the tank 8.
An electronic controller 20 is connected between various components of the apparatus and
pressure sensors 24 and 25. Pressure sensor 24 is in communication with the ullage 11
and outputs a pressure signal to electronic controller 20. Pressure sensor 24 may also
output a temperature signal indicative of the temperature in the ullage 11. Sensor 25
measures and signals to controller 20 the current atmospheric pressure as a reference for
comparison to the pressure in the tank ullage 11.
The controller 20 can be a suitable controller such as a programmable controller or other
microprocessor based control. Those of ordinary skill in the art will realize that the
controller may be made using various forms of analog or digital electrical circuits or
perhaps pneumatic, hydraulic, mechanical or fibre optics technology. The controller 20
has outputs to various items shown in Fig. 1 including a vapour pump 32, inlet valve
manifold 40, exhaust valve manifold 60, and, if installed, a heat exchanger 80. It also
receives temperature or pressure signals from sensors 24, 25. The dotted lines shown
extending to the various major components should be understood to represent and to
include individual operable connections to the sub-components thereof.
The apparatus also includes vapour pump 32 which is in communication with ullage 11
via conduit 30. The vapour pump is in communication via conduit 39 with the inlet valve
manifold shown in dotted line at 40. The inlet valve manifold 40 has a first exit 46 and
a second exit 47.
The apparatus further includes a first canister 50 and a second canister 52, each
containing a VOC-adsorbent material, such as activated charcoal. Each canister is in
communication with the inlet valve manifold 40. The first canister 50 is positioned
between the inlet valve manifold first exit 46 and an exhaust valve manifold 60 shown
in dotted line. The second canister 52 is positioned between the inlet valve manifold
second exit 47 and the exhaust valve manifold 60. The exhaust valve manifold 60
communicates with the inlet valve manifold 40 via recirculation line 53.
The inlet valve manifold 40 includes a first inlet valve 42 and a second inlet valve 43
each in communication with the vapour pump 32. The first inlet valve 42 is positioned
between the inlet valve manifold vapour entrance 41 and the valve manifold first exit 46.
The second inlet valve 43 is positioned between the inlet valve vapour manifold entrance
41 and the inlet valve manifold second exit 47. The inlet valve manifold also includes
a first purge valve 44 and a second purge valve 45 each in communication with
recirculation line 53. The first purge valve 44 is positioned between the recirculation line
53 and the inlet line manifold first exit 46. The second purge valve 45 is positioned
between the recirculation line 53 and the inlet valve manifold second exit 47.
Although the inlet valves 42,43 and the purge valves 44,45 are shown as separate valves,
the function of each pair of valves could be combined in a single unit. For example, inlet
valves 42,43 could be combined into a single two-way valve capable of shifting the
discharge of the vapour pump 32 to either the inlet valve manifold first exit 46 or inlet
valve manifold second exit 47 responsive to a control signal from controller 20. The use
of either a single valve or dual valves as is illustrated in Fig. 1 will vary depending upon
economic considerations and operating conditions. The selection of a valve configuration
of the inlet valves and the purge valves is within the capability of one of ordinary skill
in the art.
The exhaust valve manifold 60 communicates with the first canister 50, second canister
52, recirculation line 53 and the tank ullage 11 (through optional heat exchanger 80).
The exhaust valve manifold 60 includes first recirculation valve 62 and second
recirculation valve 63. The first recirculation valve 62 is positioned between first canister
50 and recirculation line 53. The second recirculation valve 63 is positioned between the
second canister 52 and the recirculation line 53. First atmospheric vent 72 and second
atmospheric vent 73 are provided for first canister 50 and second canister 52,
respectively. These vents permit the release to the atmosphere of the pollutant-free air exiting either the first canister or the second canister. The recirculation valves 62,63
return a portion of the stream exiting their respective canisters to the opposite canister
undergoing regeneration as is explained in more detail below. The exhaust manifold 60
also includes first return valve 64 and second return valve 65. The first return valve 64
is positioned between the first canister and the tank ullage 11. The second return valve
65 is positioned between the second canister and the tank ullage 11. Vapours are
returned to the tank ullage 11 via return conduit 82 (and, optionally, heat exchanger 80).
As was the case with the inlet valves 42,43 and the purge valves 44,45 in the inlet valve
manifold 40, the functions of the recirculation valves 62,63 and the return valves 64,65
may be combined into a single two-way valve for each pair of valves. Again, selection
of a valving arrangement is within the ordinary skill in the art.
The flow exiting the exhaust valve manifold 60 can follow two paths. First, non-
polluting components exiting either canister may exit via the recirculation line 53 as a
regeneration gas as described below. Alternatively, the flow may return to the tank
ullage 11 via the return valves 64,65. As will be explained in more detail herein below,
the flow follows only one of these paths at any given time.
Although the apparatus is illustrated with a single recirculation line 53, other
arrangements are possible. By way of non-limiting example, a separate recirculation line
may be provided for each canister. This arrangement would require separate purge
valves as opposed to the single two-way valve option described above. Also the purge
valves need not share the inlet valve manifold's first and second exits 46,47. The
discharge of each of those valves could be directed to the appropriate canister via a
separate line.
Optional components for the apparatus of the present invention include flame arrestors
33, 34 which may be positioned on the suction and/or discharge sides of vapour pump
32. The flame arrestor is a safety item that may be installed on an as-needed basis. A
second optional component is heat exchanger 80 positioned between the exhaust valve
manifold 60 and the tank ullage 11. During the operation of this apparatus, it may be
desirable to cool the flow returning to the tank ullage. The need for such cooling would
depend on a variety of factors to include ambient temperature, tank ullage temperature,
and the temperature of the flow returning to the tank ullage. The design and sizing of
this heat exchanger is well within the ability of one with ordinary skill in the art.
A second heat exchanger 84 may be installed in recirculation line 53 between the inlet
valve manifold 40 and the exhaust valve manifold 60. The purpose of this heat exchanger
84 is to heat the air flow through the recirculation line 53 to improve the regeneration of
the adsorbent material contained in canisters 50, 52. Typically, this heat exchanger 84
will be cycled on and off by controller 20 as needed for the proper operation of the
system. For instance, it is believed that during winter months having a low average daily
temperature, it may be desirable to heat the flow through the recirculation line.
Determining the need for and sizing this heat exchanger is within the capability of one
of ordinary skill in the art.
Though the apparatus of the present invention will function properly with the components
described above, the inclusion of certain safety items is desirable. These safety items
include temperature sensors (not shown) positioned at the discharge of the vapour pump
32 and at the outlet of the canisters 50, 52 to detect overheating and shut down operation
of the apparatus, if needed. The apparatus may also include hydrocarbon detectors 54,
55 positioned at the canister outlets to detect potentially explosive hydrocarbon levels and
again shut down the apparatus if need be. Additional hydrocarbon detectors may be
incorporated into atmospheric vents 72, 73 to shut down operation of the apparatus if
excessively high levels of hydrocarbons are being vented to the atmosphere.
An additional safety device may be incorporated into the inlet valves 42, 43, purge valves
44, 45, return valves 64, 65 and the recirculation valves 62, 63. As will be explained
in more detail below, during the operation of the apparatus, only one of each of these
pairs of valves should be open at any given time. Accordingly, it is desirable to include
a valve interlock to ensure that if one valve of a pair is open, then the second will be
closed. The interlock could take either a mechanical or electronic form. In a mechanical
form, the interlock could include a member attached at each end to the valves and
physically situated so that when one valve is open the other is forced to close.
Alternatively, the interlock could be incorporated into the control logic of controller 20
to permit only one valve of the pair to be open at any given time.
The canisters 50, 52 are filled with a VOC-adsorbent material. The practice of the
present invention includes the selection and use of a suitable material for the VOC being
stored. However, activated carbon is believed to be most desirable for adsorbing
hydrocarbon from the vapours drawn from the tank ullage. Moreover, activated carbon
is believed to be desirable for removing VOC's of other types as well.
The operation of the apparatus will be described with reference to Figures 1 and 2. The
electronic controller 20 monitors the pressure in tank ullage 11 through the pressure
sensor 24. When sensor 24 detects a pressure in tank ullage 11 above a threshold level,
a signal is output from controller 20 to vacuum pump 32 to initiate withdrawal of vapours
from the tank ullage. The valve arrangement at this point is shown in Fig. 1 and is
detailed in Table 1 below:
Table 1
Valve Position
First inlet valve (42) closed
First purge valve (44) closed
Second inlet valve (43) open
Second purge valve (45) closed
First atmospheric vent (72) closed
Fist return valve(64) closed
First recirculation valve (62) closed
Second atmospheric vent (73) open
Second return valve (65) closed
Second recirculation valve (63) closed
Thus, the vapours are directed through the second inlet valve 43 to the second canister
52. In the canister the hydrocarbon component of the tank ullage is adsorbed by the
material in canister 52. The flow exiting canister 52 consists of the substantially
hydrocarbon-free air which is vented to atmosphere by atmospheric vent 73. This process
will continue until the material in the second canister 52 is saturated with hydrocarbon
or until the pressure in the tank ullage 11 is reduced to below a threshold level. It should
be noted that the operation of the apparatus at this juncture may be continuous or
intermittent, depending on the amount of running time required to bring the tank ullage
pressure within limits. In its simplest sense, the method of the present invention includes
monitoring the pressure in the fuel tank ullage; withdrawing vapour from the fuel tank
ullage when the ullage pressure exceeds a threshold value, feeding the vapour through a
canister containing a hydrocarbon vapour adsorbing material; and exhausting the
hydrocarbon-free air exiting the canister to the atmosphere.
The hydrocarbon removal taking place in second canister 52 is monitored by a
hydrocarbon detector 54 positioned at the canister outlet. The hydrocarbon detector
evaluates the proportion of hydrocarbons in the canister exhaust and applies a signal
indicative of same to the electronic controller 20. When the hydrocarbon content
approaches limits indicating the hydrocarbon-adsorbent material in canister 52 is nearly
saturated, vapour flow is diverted by the inlet valve manifold to the first canister 50.
Referring now to Fig. 2, this diversion is accomplished by output signals from controller
20 which close second inlet valve 43, second atmospheric vent 73 and opens first inlet
valve 42 and first atmospheric vent 72. The remaining valves are positioned by the
controller as shown in the Table 2 below:
Table 2
Valve Position
First inlet valve (42) open
First purge valve (44) closed
Second inlet valve (43) closed
Second purge valve (45) open
First atmospheric vent (72) open
Fist return valve (64) closed
First recirculation valve (62) open
Second atmospheric vent (73) closed
Second return valve (65) open
Second recirculation valve (63) closed
Vapour flow now enters the first canister 50, where hydrocarbon adsorption takes place.
A portion of the hydrocarbon-free air exiting the first canister 50 flows through first
recirculation valve 62 to recirculation line 53. The recirculation line 53 directs this flow
back to the inlet valve manifold 40 through second purge valve 45 and into second
canister 52.
Thus a portion of the flow exiting the first canister regenerates the saturated hydrocarbon-
adsorbent material in the second canister 52. It is believed that up to about 30 percent
of the first canister's flow is required for regeneration. Desirably, the recirculation
valves 62,63 are proportional valves so that the amount of regenerative flow can be
varied as needed.
The vapours exiting the second canister 52 during regeneration will have a high
hydrocarbon content and cannot be vented to atmosphere. These vapours are directed
back to the tank ullage 11 via second return valve 65 and return conduit 82. The
apparatus will continue to operate in this manner until the pressure in the tank ullage 11
has been reduced to a specified threshold level, at which time the vacuum pump 32 will
be stopped by controller 20. As will be apparent to one of ordinary skill in the art, the
pressure in the tank ullage 11 decreases because more volume is removed by vacuum
pump 32 than is returned from the canister undergoing regeneration.
The hydrocarbon sensor 54 at the second canister outlet evaluates the proportion of
hydrocarbons in the flow therefrom and supplies a signal indicative of same to the
electronic controller 20. If the hydrocarbon content of the flow decreases sufficiently to
indicate that regeneration is complete, then output signals from the controller 20 close the
first recirculation valve 62, second purge valve 45, and second return valve 65, isolating
the second canister 52. That canister is then ready for service when first canister 50
becomes saturated as detected by a hydrocarbon detector 55 positioned at the first canister
discharge.
The hydrocarbon sensor 55 at the first canister outlet evaluates the proportion of
hydrocarbons in the flow therefrom and supplies a signal indicative of same to the
electronic controller 20. When the hydrocarbon content approaches limits indicating that
the hydrocarbon-adsorbent material is nearly saturated, flow through the apparatus is then
reversed from the arrangement shown in Fig. 2 so that the second canister 52 begins to
adsorb hydrocarbon and the first canister 50 is regenerated. Valve positioning at this
point is illustrated in Table 3 below:
Table 3
Valve Position
First inlet valve (42) closed
First purge valve (44) open
Second inlet valve (43) open
Second purge valve (45) closed
First atmospheric vent (72) closed
Fist return valve (64) open
First recirculation valve (62) closed
Second atmospheric vent (73) open
Second return valve (65) closed
Second recirculation valve (63) open
The operation is the same as described above with hydrocarbon being adsorbed in the
second canister 52 and the material in first canister 50 being regenerated. Hydrocarbon
sensor 55 will monitor the regeneration of the hydrocarbon-adsorbent material in the first
canister 50 until that process is complete. Similar to the procedure described above, flow
through that canister will then be stopped. This cycle continues as directed by controller
20 to maintain the tank ullage pressure within set limits.
It should be recognized that a situation could arise where the canister providing the
regenerative flow could become saturated with hydrocarbon before regeneration of the
other canister is complete. In this situation, controller 20 may stop vapour pump 32 and
close all valves in the apparatus. Simultaneously, the controller 20 will generate an alarm
to alert the operator of the apparatus of this condition.
After a number of adsorbtion/regeneration cycles, the hydrocarbon-adsorbent material
will require replacement. The life of that material will vary depending on several factors
including the volume of tank ullage being controlled, size of the canisters, ambient
weather conditions and type of hydrocarbon-adsorbent material used.
The invention has been described with reference to a single tank. However, the scope
of the present invention includes using the apparatus for multiple tank installations,
adjusting the size of the canisters and other components as needed to handle the combined
ullage of the all the tanks. The practice of this invention also includes increasing the
number of canisters beyond two as the demands of the particular application grow. For
example, a third or a fourth canister may be added if the vapour production of the tank
installation so requires. A person of ordinary skill may adjust the size and number of
canisters without undue effort.