EPA-AA-SDSB-86-3
Technical Report
Evaluation of the Feasibility
of Liquid Fillneck Seals
December 1986
NOTICE
Technical reports do not necessarily represent final EPA
decisions or positions. They are intended to present technical
analysis of issues using data which are currently available.
The purpose in the release of such reports is to facilitate the
exchange of technical information and to inform the public of
technical developments which may form the basis for a final EPA
decision, position or regulatory action.
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Sources
Office of Air and Radiation
U.S. Environmental Protection Agency
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Table of Contents
Section Page
I. Introduction/Overview 2
II. Evaluation of Liquid Seal Concepts 5
A. Preliminary Analysis and Experimental Design 5
B. In-Tube Liquid Trap 13
C. J-Tube Liquid Seal 17
D. Submerged Fill 20
E. Techniques for Reducing Required Fill Height 23
F. Discussion/Conclusions 28
III. Bench Testing of a Liquid Seal Onboard System 35
A. Description of the Prototype System 35
B. Evaluation of System Efficiency 41
IV. Conclusions 49
References 53
Appendix 54
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I. Introduction/Overview
The only significant uncontrolled source of hydrocarbon
emissions from motor vehicles is the gasoline vapor which
escapes during vehicle refueling. In . an uncontrolled
refueling, the vapors that are displaced during the refueling
event pass through the fillneck and escape to the atmosphere.
Systems designed to control these emissions are called
refueling vapor control systems. When the controls are vehicle
based, they are generally referred to as onboard vapor recovery
systems. The essential ingredients of an onboard vapor
recovery system are: 1) a fillneck seal to prevent vapors
from escaping to the atmosphere via the fillneck, 2) a fuel
tank vent and vapor line from the fuel tank to allow for
displacement of refueling vapors, and 3) a method of removing
hydrocarbons from the vapors displaced from the fuel tank and
storing them for purging at a later time. Normally this is
accomplished through the use of a carbon canister.
This report is primarily concerned with fillneck seal
portion of the onboard vapor recovery system. Historically,
the fillneck seals most often discussed as part of onboard
systems are mechanical seals. In a mechanical seal system, a
temporary physical barrier is created between the seal and the
nozzle when the fuel nozzle is properly inserted into the
fillneck (see Figure 1). This barrier (usually referred to as
a seal) prevents the vapors from escaping out the fillneck when
the fuel is dispensed.
In an extensive development and testing program conducted
in 1978, API demonstrated that mechanical seals can effectively
prevent vapors from escaping through the fillneck over the
useful life of a light-duty vehicle when used under normal
conditions.[1] However, some concern has been expressed about
the safety, durability, and integrity of the mechanical seal
approach.
Concerns about the mechanical seal fall in three areas:
Fillneck seals may require a pressure relief valve
to deal with potential fuel tank overpressures which
could occur as a result of failure of the fuel
nozzle automatic shut-off mechanism or a blockage in
the vapor line between the fuel tank and the carbon
canister. Without a pressure relief valve, tank
overpressures could result in a fuel spitback with
potential fuel tank overpressures.
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Figure 1
FILL RPE MODIFICATIONS
ROTARY SEAL
ROTARY SEAL
TRAP DOOR
LEAD RESTRICTOR
FILL PIPE MODIFICATIONS
ROTARY SEAL
ROTARY SEAL
TRAP DOOR
SPOUT
LEAD RESTRICTOR
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Under some adverse in-use conditions, a mechanical
seal may require maintenance or replacement to
retain a high in-use efficiency. This depends in
large part on the material used to make the seal and
the degree to which the mechanical seal mounting
within the fillneck protects it from unusual wear or
abuse.
If not properly designed, the mechanical seal may be
susceptible to tampering by the consumer.
Instead of using a mechanical seal in the fillneck, one
alternative is the use of a liquid seal. The idea of using a
liquid seal in the fillneck was introduced early in the
development of onboard vapor recovery systems. A report
published in 1973 by the American Petroleum Institute
identified and briefly evaluated several different liquid seal
concepts.[2] A liquid seal system uses a fillneck design which
routes the incoming fuel in such a way that the fuel itself
prevents the vapors from escaping via the fillneck.
The use of a liquid seal design would help to eliminate
the potential problems associated with a mechanical seal.
Since a new seal is formed at each refueling, in use durability
and maintenance are not in question. Also, since the liquid
seal fillneck is no different in appearance than current
fillnecks and the fuel itself forms the seal, liquid seals are
essentially tamper-proof. The problem of overpressure during
refueling would be eliminated, because fuel would back up in
the fillneck as soon as any pressure built up. The fuel nozzle
would then either shut off automatically or could be shut off
manually, since the nozzle operator could see the overfill
condition. In addition, liquid seal systems may be somewhat
less costly than mechanical seals since a pressure relief
device may not be required.
The purpose of this report is to evaluate the liquid seal
approach to fillneck vapor containment as a practical
alternative to the more widely recognized mechanical seal.
After evaluating several different liquid seal approaches, the
performance of one of the liquid seal designs is then evaluated
in a simple prototype onboard control system.
The first portion of the report covers evaluation of the
liquid seal concepts. It starts with a conceptual analysis of
the liquid seal and provides an overview of the experimental
and analytical methodology used in the testing. Then it
examines each of three basic liquid seal designs individually:
the "in-tube" liquid trap, the "J-tube," and the submerged
fill. The testing done on each design is outlined and the
strengths and weaknesses of each are noted. Next, a problem
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common to each design, air entrainment and bubble proliferation
in the fillneck, is discussed and various methods of
controlling these problems are evaluated. The first portion of
the report closes with conclusions about the adaptability of
these liquid seal designs to current fuel tank fillneck
configurations.
The second portion of the report discusses the performance
of a crude, first generation liquid seal fillneck (J-tube) on a
simple prototype onboard system. It begins with a description
of the key components needed for an onboard control system and
then describes the components used in EPA's system and how
these met the performance criteria needed for the liquid seal.
The test procedure used to evaluate the performance of this
system in a series of bench tests is described and this is
followed by a discussion of the test results including system
efficiency and sources of emissions.
The report then closes with some conclusions about the
performance of liquid seals and compares them to results
achieved for mechanical seals.
II. Evaluation of Liquid Seal Concepts
A. Preliminary Analysis and Experimental Design
1. Introduction
The term "liquid seal" in the context of an onboard vapor
recovery system refers to any system of fuel tank and fillneck
in which a column of liquid separates in-tank vapor from
atmospheric air. One key parameter in evaluating the
feasibility and performance of liquid seals is the fill height
required to maintain a liquid seal during a refueling event.
If the fill height available is less than that needed, the fuel
rising in the neck will cause a premature shut-off of the
nozzle. One of the major goals in these evaluations was to
determine the minimum fill heights necessary to completely
refill the tank using liquid seals in the fillneck. This
section develops some models which help to predict the manner
in which these liquid seals would perform and what fill height
might be necessary to make use of a liquid seal as an
alternative to a mechanical seal. First, the static situation
is discussed and a mathematical model relating fillneck height
to tank backpressure in the static situation is presented.
Then the dynamic effects that are part of a typical refueling
event and that complicate the mathematical model are
introduced. Third, an experimental design for testing the
various fillneck configurations is outlined. Fourth, the
general test procedure for testing the liquid seal concepts is
developed.
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2. Static Situation
Any liquid trap system can be modeled by the simple 'U1
tube system shown in Figure 2. In the static situation, the
vertical fill height, hf, required to support any induced
tank backpressure, PB, can be found using the basic equation
of fluid statics. In this case:
PB = Po + pghr, where
PB = tank backpressure, gauge
P0 = reference gauge pressure at reference height, h = 0
p = density of fluid in tank
g = acceleration of gravity at test altitude
hf = required fill height above reference height
For example, to support a backpressure of 5 inches H20
under static conditions, a gasoline column (specific gravity =
0.72) would have to rise approximately 7 inches above the
reference height. So vertical fill height would have to be at
least 7 inches. During testing, a water manometer was used to
gauge pressure so that pressure could be read directly in
inches of H20. If fuels were dispensed very slowly, this
model could be used to analyze the system. It is not, however,
and the next section looks at the dynamic effects of a
refueling.
3. Dynamic Effects
A number of factors complicate this model for a typical
refueling event. These include:
Dynamics: At a typical gasoline pump fuel is
dispensed at a rate of 7-10 gallons/minute
(gal/min); thus the system is not static.
Injection effect: The kinetic energy of the
gasoline as it flows out of the dispensing nozzle
tends to aid the flow of gasoline into the tank and
to reduce the required.fill height.
Turbulence and air entrainment: The interaction
between liquid and air in the fillneck is quite
turbulent. This interaction tends to greatly
increase the required fill height.
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Figure 2 - 'U1 Tube
Pb = tank backpressure, guage
Po = o = reference guage pressure
hf = required fillneck height
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Observations during informal testing of the 'U' tube set
up of Figure 2, support the intuitive hypothesis that required
fill height is directly related to tank backpressure for any
given fuel flow rate and fillneck configuration. In the static
situation, PB and hr are related by a constant conversion
factor, the liquid density. In the dynamic situation, the
relationship is much more complex, with fluid density,
viscosity, surface tension, and other factors all interacting
in a highly turbulent flow. In this dynamic, turbulent
situation, it was expected that PB and hf would vary
directly and that an increase in tank backpressure would
increase the fill height necessary to support that pressure. A
mathematical analysis of such a turbulent, dynamic flow was
outside the scope of this investigation, so a mathematical
model was not developed. Instead, the relationship between
tank backpressure and fillneck height was determined
experimentally for the three liquid seals evaluated.
4. Experimental Design
The ultimate goal of the tests conducted by EPA was to
compare the performance of a number of liquid seal fillneck
configurations under uniform test conditions and procedures.
Because the optimal experimental design was not intuitively
obvious when testing began, a series of developmental tests
were performed with water as the test fluid. These tests were
done to evaluate a number of experimental designs without the
safety concerns and expense involved with gasoline testing.
These preliminary tests were performed using standard tap
water, and fillneck systems were constructed using metal,
tygon, and plexiglass. These systems were then attached to a
standard automotive fuel tank having a nominal capacity of
about 18 gallons. All gasoline tests were performed with
indolene clear as the test fluid, and since plexiglass is
incompatible with gasoline, systems were built of metal and
tygon. The temperature during this testing was the prevailing
ambient temperature (nominally 80°F).
The developmental water tests were carried out in a 'U'
tube system such as the one described above and diagrammed in
Figure 2. Data was taken relating fill height to tank
backpressure for different fillneck configurations during these
developmental water tests. The numerical results of these
tests proved to be of little use in predicting the results of
the gasoline testing, however, because of the great differences
in the properties of water and gasoline (see Table 1).
However, the tests . were very helpful in isolating the
parameters that would prove critical in determining required
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Table 1
Properties of Water and Gasoline at 20°C
Property Water Gasoline (n-Qctane)
Density (Ib/gal) 8.30 5.83
Viscosity (Centipoise) 1.0 0.54
Surface Tension (Dynes/cm) 72.8 21.8
Source: CRC - Chemistry and Physics Handbook and Perry's
Chemical Engineering Handbook, 5th Ed.
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fillneck height. The qualitative results of this preliminary
testing which are important to the experimental design are
outlined below:
The system performed as predicted by the 'U1 tube
model - required fill height varied directly with
tank backpressure.
Increasing fuel inflow rate increased required fill
height. Bubbling caused by the mixing of fluid
entering the fillneck with air in the fillneck was
observed to increase as dispensing rate increased.
Changes in fillneck diameter (or volume per unit
length) affected required fill heights. Generally,
required fill heights increased with decreasing
fillneck diameters.
The orientation of the dispensing nozzle in the
fillneck affected fill height. If the nozzle was
oriented appropriately, the liquid tended to adhere
to the fillneck wall. If this was done, air
entrainment and bubbling were reduced leading to a
decrease in required fill height.
The results of these tests confirmed that these factors
would have to be controlled to ensure consistent test results.
Based on observations during the water testing, the U-tube
system appeared to create a very adequate seal. To further
evaluate the feasibility of this approach, a "u" was built into
a 2-inch plexiglass pipe and tested using water. As will be
discussed below, the testing showed that the smaller cross
sectional area for flow within the trap required too high a
fill height to be practical. However, this experiment revealed
useful information on flow and turbulence characteristics.
Based on the experience gained from this preliminary
testing and supplemented by information contained in the SAE
Recommended Practice regarding refueling emissions, standard
test conditions were chosen to test the liquid seal fuel tank
systems using gasoline under conditions likely to occur during
a typical refueling event.[3] These conditions were used in
the tests which evaluated the fill height requirements and
other performance aspects of the liquid seals system.
The backpressure was set at five inches H2O. It is
expected that carbon canisters and their accompanying tubing
and valves can be designed to accommodate normal refueling
rates with tank backpressures of 5 inches H2O or less. This
is supported by experimental evidence presented in an API
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report dated October, 1978.[1] The report quotes backpressures
as low as 2 inches HZO, at a refueling rate of eight
gal/min. All three of the fillneck seal approaches evaluated
by EPA were tested at 5 inches H20 backpressure and most were
tested at some lesser backpressures.
Fuel dispensing rate was also standardized. Typical fuel
dispensing rates average from 7 to 10 gal/min, but the fuel
cart available for use in this testing had a maximum fuel
dispensing rate of 7.5 gal/min. Therefore, this was the
designated fuel flow rate for gasoline testing.
As was mentioned above, fillneck cross sectional area also
affects required fillneck height and had to be standardized.
Because most stock fillnecks have an outer diameter of
approximately two inches, this was chosen as the base fillneck
diameter and tygon or metal tubing of this diameter was used.
Also mentioned above, fuel nozzle orientation can also
affect required fill height, but the optimal orientation is
different for each fillneck configuration. Also, nozzle
orientation is one of the key human factors in vehicle
refueling, and may vary significantly from operator to operator
and vehicle to vehicle. For these reasons, fuel nozzle
orientation was not standardized to minimize fill height.
Instead, for each fillneck configuration the nozzle was
oriented in the manner that would normally occur in-use if the
nozzle were to rest vertically in the fillneck perpendicular to
the surface.
The standard test conditions described above were then
incorporated into the standard test procedure developed for the
gasoline testing. The standard setup for testing the liquid
seal designs is shown in Figure 3. The gate valve mounted on
top of the tank was used to regulate the size of the orifice
through which any vapor leaving the tank must pass. The
backpressure induced in the fuel tank was controlled for any
fuel inflow rate by changing the position of the gate valve.
Prior to the start of each test, the manometer was zeroed,
the fuel tank was leveled, and the gate valve opened to about
one fourth of wide open. The fuel cart was turned on and fuel
dispensed at the highest available rate. The gate valve was
then closed until backpressure was stabilized at the desired
value (initially, 5 inches H20) . The height of the
fluid/bubble column in the fillneck was then measured when
backpressure stabilized. This value was recorded as the
required fill height. The tank was drained and the operation
repeated.
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Figure 3
Standard Experimental Setup
h = required fillneck height
Fill Nozzle
Box Represents
Liquid Seal
System
Gate Valve
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At this point, fuel RVP and dispensed and tank temperature
were not important. Neither the RVP of the fuel or the fuel
temperatures mentioned above were controlled.
The testing of each system generally adhered to the test
procedure outlined above, although some details were specific
to the individual systems being tested. The most significant
difference between individual tests was the measurement of fill
height. Fill height can be generally defined as follows:
The value of the vertical component of a vector from a
point on the plane of full tank fuel level to the point
where the tip of the dispensing nozzle rests in the
fillneck (the vertical distance from the tank full level
to the nozzle tip).
The peculiarities of the measurement of fill height for
each individual system will be discussed in the sections of
this report which deal with the specific designs. It should
also be noted here that fill height, as used in this report,
does not include the distance from the inserted nozzle tip to
the gas cap location. This distance (2-3 inches) would have to
be included in any fillneck design.
B. In-Tube Liquid Trap
1. Introduction
The first liquid seal approach tested was the "in-tube"
liquid trap. The in-tube trap was chosen for study because the
preliminary water testing showed that the system could provide
a positive seal and could be installed by modifying only the
fillneck, leaving the tank essentially unchanged. If this were
possible, and the fillneck height requirements permitted, the
design could be adapted to fit a number of vehicles. This
section looks at the in-tube trap and its performance under the
standard test conditions.
An in-tube trap in the fillneck operates by creating a
liquid barrier between the gasoline vapor in the tank and the
atmosphere. Its operation is similar to that of the home sink
drain pipe, but the 'U1 shaped trap is built into the fillpipe
rather than bending the fillpipe into a 'U' shape.
A diagram of the in-tube liquid trap and illustrations of
the parameters describing its construction are shown in Figure
4. The trap was designed to divide the tube into three
channels of equal cross-sectional area. Liquid is trapped in
the upward opening 'U', and if tank backpressure increases
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Side View
Figure 4
In-Tube Trap
Bottcm
View
e-b
T
At
11
Dimensions
Plexiglass
a = c = 0.73"
b = 0.54"
D = 2.0"
Metal
a = c = 1.1"
b = 0.8"
D = 3.0"
d = Trap Depth
A = B = C
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during refueling, a liquid column will rise above that 'U'
until the pressure is offset. "Trap security" describes the
susceptibility of the trap system to allowing vapor to pass
through it during the refueling event. Trap security can be
increased by increasing the depth of the liquid seal, but
increasing this depth might also increase the required fillneck
height. Normally, with an in-tube liquid trap, the depth would
just need to be adequate to accommodate refueling on sloped
terrain. So, to refuel on a 30° incline, a trap depth of
one-half inch would be sufficient (for the large trap shown in
Figure 4).
Two traps were built for testing. The smaller trap was
set in a two inch diameter plexiglass tube. This trap was
built so that flow through the trap could be visually monitored
during the preliminary water testing. A larger 3 inch diameter
trap was built entirely of metal so that gasoline could be used
as the primary test fluid.
2. Test Procedures
Figure 5 is a schematic of the experimental setup used to
test the in-tube liquid traps. It is important to note the
location of the trap in relation to the tank full level. If
spatial efficiency were optimized in a production
configuration, the trap would actually be at the roof of fuel
tank inside the vapor space and, the upward opening 'U' would
open at the tank full level. However, for the sake of
convenience, these traps were built on top of the tank, and
hence required fill height was measured from the top of the
upward opening 'U' to the nozzle tip.
3. Results/Discussion
As was mentioned previously, some of the developmental
testing discussed earlier in this report was done on an in-tube
liquid trap. The dimensions of that trap are those listed
under "Plexiglass" in Figure 4. The developmental testing
showed that a trap built inside a 2 inch diameter tube would
not be large enough to support a backpressure of 5 inches
H2O. The channels through which liquid was supposed to flow
(approximately 1 square inch) simply were not large enough to
allow liquid to pass through quickly.
In order to accommodate gasoline and higher flow rates and
to reduce required fillneck height, a larger in-tube trap was
set in a 3 inch diameter tube. The dimensions of the trap are
shown in Figure 4 under "Metal". Using the standard test
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Figure 5
Experimental Setup For In-Tube Trap
Fill Nozzle
Required
Fillneck
Height
Full Tank
Level
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procedure, the tank was filled with a required fill height of
only 11 inches. This is a very good result, considering that
in the static situation a neck height of 7 inches would be
needed to support a 5 inch H20 backpressure (5 inches/0.72 =
6.94 inches). The implications of this result are discussed
later in the report.
4. Conclusions
A fill height of approximately 11 inches for the in-tube
liquid trap is quite acceptable for many vehicle applications.
However, the in-tube liquid trap approach has some drawbacks.
First, the in-tube trap could be relatively complex to
construct on a production basis. Also, since the trap required
a 3 inch diameter fillneck to perform adequately under the
standard test conditions, current fillpipe designs might have
to be modified to accommodate its use.* A 3 inch diameter
expansion in some fillnecks may create vehicle packaging
problems. However no fuel tank changes would be necessary and
the in-tube liquid trap has all the advantages of liquid seal
systems that were described above. Packaging problems could be
eliminated if the in-tube trap were built into the inside top
of the fuel tank. The advantages and disadvantages of the
in-tube trap are discussed further following the examination of
the other liquid seal approaches.
C. "J" Tube Liquid Seal
1. Introduction
The second liquid seal approach to refueling vapor
containment examined was the 'J' tube liquid seal. The 'J'
tube was chosen for study because it was a logical extension
from the in-tube trap approach and would take only minor
changes in fuel tank and fillneck to install and has all the
advantages of liquid seals. This section looks at the fil'l
height necessary for the 'J' tube system when evaluated under
the standard test conditions.
The 'J' tube system is shown in Figure 6. The 'J' tube is
simply a modified 'U' tube (like that shown in Figure 2). The
difference is that in the 'J' system the fuel tank itself
replaces the downstream, vertical portion of the drainpipe.
The ultimate size of the trap and fillpipe diameter needed
depend in part on the system backpressure. System
backpressure less than the 5-inch H2O used here are
achievable, and would reduce the diameter of the in-tube
trap.
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Figure 6 - 'J1 Tube
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Th e dimensions of the 'J1 system along with illustrations of
some of the descriptive terms used in the discussion are given
in Figure 6. it should be noted that the liquid trap is
located inside the fuel tank, and the top of the trap is
located on or near the full tank plane. This promotes spatial
efficiency and makes maximum use of available fillneck height.
- 2. Test Procedure
Because the 'J'-tube trap is located inside of the fuel
tank and fill height is measured from full tank level to the
nozzle insertion point, the standard test method was followed
exactly in this case.
3. Results/Discussion
The 'J' tube test results are listed below:
Fuel dispensing rate = 7.5 gal/min.
Backpressure Required
(inches H20)
5.0 16.0
4.0 13.5
3.0 11.0
The 16 inches of fill height required to fill the tank at
five inches H20 backpressure was more than expected based on
extrapolation of the results of the developmental water tests
discussed earlier. Although bubbling, due to the mixing of
water with air in the fillneck, was observed during
developmental tests, the violence of the bubbling during
gasoline testing was not fully expected. The extensive
bubbling in the fillneck was the cause of the large required
fill heights. It was clear from this testing that if bubbling
could be controlled, required fillneck heights could be
reduced. This cou-ld be managed by reducing the backpressure or
controlling the proliferation of bubbles in the fillneck during
refueling.
4. Conclusion
The 'J1 tube, a simple modification of the 'U1 tube, was
tested using the standard test method. Excessive turbulence
and bubbling in the fillneck made a fill height of 16 inches
necessary for refueling under the standard test conditions.
The control of the bubble problem, and some methods of fillneck
height reduction are discussed in section E.
It is important to note that the 'J1 tube trap appeared to
be doing its job of controlling emissions. A rough measurement
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of the hydrocarbon concentration at the fillneck mouth was
taken with a lower explosive limit (LEL) meter. The meter
showed no difference between the readings taken at the fillneck
mouth and readings taken around the test chamber.
D. Submerged Fill
1. Introduction
The third and final liquid seal system investigated was
the submerged fill. In a submerged fill system, the fillneck
opens into the fuel tank below the liquid surface. By filling
the tank from below the surface, a liquid seal is naturally
formed between in-tank vapor and atmospheric air.
A drawing of a submerged fill system is shown in
Figure 7. The significant features of the submerged fill
system are listed below:
The security of the liquid trap is good and it
improves as the tank fills.
A pressure relief valve would have to be
incorporated into any submerged fill system. If the
lines from fuel tank to carbon canister became
blocked, tank pressurization during refueling could
become a problem. If the backpressure in the tank
rose between refuelings, when the gas cap was
removed from the fillneck, the overpressure could
cause gasoline to spit out of the neck. The
pressure relief valve is needed to eliminate this
problem.
The fillneck height is not critical during the early
stages of refueling. Only in the last 10 percent of
the fill is the full fill height used, as the back
pressure caused by the fuel level in the tank
increases.
2. Test Procedure
In a submerged fill system, the location of the top of the
liquid trap rises as the tank fills. One major change was made
in the standard test procedure to accommodate this unique
feature of the submerged fill. Since the top of the liquid
trap rises during refueling, the entire fillneck height is only
needed during the final stage of refueling; when the top of the
trap approaches the full tank liquid level. Therefore required
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Figure 7 - Submerged Fill
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fill height must be measured just prior to the end of the
refueling event. For these tests, fill height was measured
when about 18 gallons of fuel had been dispensed (into a
nominal 18 gallon tank).
3 . Results/Discussion
The results of the testing of the submerged fill system
are presented below:
Flow rate: 7.5 gal/min
Backpressure Required Fill
(Inches H20) Height (inches)
5.0 16
4.5 13
These results are similar to the results for the 'J' tube,
and the bubble problem was evident once again.
There has been some previous work done to evaluate the
effectiveness of the submerged fill seal in controlling
refueling vapors. Preliminary lab work done by EPA in 1979
found the submerged fill to be an effective method of refueling
vapor control.[4] Crude sampling done with an LEL meter
supports this finding. Samples taken during refueling from
near the mouth of the fillneck showed no higher hydrocarbon
concentrations than did background samples.
4. Conclusions
The submerged fill approach was evaluated using the
standard test procedure, and required fill height was measured
just prior to completion of the refueling. At a backpressure
of 5 inches H20 a fill height of about 16 inches was needed
to accommodate the large liquid/bubble column in the fillneck
caused by excessive turbulence and air entrainment during
refueling.
There are fuel tank fillneck configurations currently in
use that could not be easily adapted to accommodate fillneck
heights of 16 inches. Since bubbling in the fillneck is the
reason for the larger fill heights, a reduction in the bubbling
should lead to a lower required fill height. The next section
of the report addresses the bubbling problem and possible
control techniques.
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E. Techniques For Reducing Required Fill Height
1. Introduction
All the liquid seal systems built with 2 inch diameter
tubing needed extended fillnecks to accommodate the large
gasoline/bubble column that rose above the liquid/air interface
in the fillneck during refueling. This section of the report
examines bubbling and air entrainment during refueling and
evaluates some possible methods of fill height reduction.
Bubbling is caused by: 1) the violent mixing of the gasoline
leaving the dispensing nozzle with that already in the
fillneck, and 2) air entrained in the fluid flow by the venturi
system in the dispensing nozzle.
There are three basic means of controlling bubbling in the
fillneck through modification of the fillneck. The first is to
reduce the violence of the mixing of dispensed fuel with that
already in the fillneck. Another is to physically obstruct or
direct the backup of bubbles in the neck. The third is to
create a system to dissipate the kinetic energy of the
dispensed fuel.
Also, part of the bubble problem could be eliminated by
reducing the amount of air being entrained in the fuel during
refueling. This air entrainment is caused by two phenomena.
First, the automatic shutoff venturi system in standard fuel
dispensing equipment adds air to the fuel being dispensed.
Second, air is entrained in the fuel as a by-product of the
turbulence in the fillneck. The second of these two phenomena
can be affected by fillneck modifications while the first could
only be changed by modifying the nozzle. This section looks at
the problems of air entrainment and bubbling, and examines some
fillneck modifications designed to control bubbling and reduce
fill heights.
2 . Air Entrainment
The entrainment of air in the fluid flow affects fill
height in two ways. First, as fuel is dispensed and mixes with
fuel in the fillneck, air is drawn into the mixture and bubbles
are formed increasing required fill height. Second, if air
makes its way into the fuel tank with the fuel, the amount of
vapor forced out of the tank during refueling and the tank
backpressure (for a given flow rate) are increased. The
bubbling problem is discussed in the next section of the
report. This section looks at the entrained air that makes its
way into the fuel tank.
-------
A test was performed on the 'J' tube system to determine
if air was entering the tank along with gasoline. The tank was
filled with 20 gallons of gasoline and the vapors displaced by
the gasoline were collected and the volume of those vapors
measured by pulling the vapors through a dry gas meter.* Both
the dispensed and tank fuel were at the prevailing temperature,
so vapor expansion or shrinkage should have been near zero.
The results of the tests are shown below:
Gasoline Dispensed Vapor Collected
Trial No. (gallons) (gallons)
1 20 22.2
2 20 22.4
3 20 22.8
No attempts were made to identify the sources of the
entrained air, but it was fairly evident that it was caused by
air bubbles being trapped in the fuel entering the tank. This
in turn was caused by the turbulent mixing in the fillneck.
Although this testing was conducted on the J-tube system, it is
also likely to occur to similar degrees in both the submerged
fill and in-tube trap.
The phenomenon of entrained air entering the fuel tank is
significant. When the carbon canister and tubing are designed
for a given liquid seal configuration, the amount of air
entrained must be determined and figured in the design.
Systems that allow more air to enter the fuel tank will need to
transport more vapor at a given fuel inflow rate. This may
also increase the hydrocarbon load to be captured by the
canister, since the entrained air may enhance evaporation of
the dispensed fuel. Some of the entrainment may be avoided
through the control of the mixing of gasoline and air in the
fillneck. This is examined next.
3. Bubbling and Turbulence
The most obvious source of bubbling in the fillneck is the
violent mixing, of the gasoline being dispensed with the
gasoline already in the fillneck. To examine bubble formation,
a tygon fillneck was mounted on the 'J' tube system, and the
pattern of bubble flow in the neck was observed. Although the
pattern of flow depends on the orientation of the fuel
It was possible to put 20 gallons of liquid in a tank
having a nominal capacity of 18 gallons because the tank
was modified with a liquid seal and vent, but did not have
a fill limiter. Thus the vapor space of the tank also
contained fuel, which would not normally be the case.
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-25-
dispensing nozzle in the fillneck and on the configuration and
cross-sectional shape and area of the fillneck, a typical
bubble pattern is shown in Figure 8. The pattern shown was
generated in a two inch diameter tube with a round
cross-section. The sketch is helpful in that it highlights the
point of bubble generation and shows the route that the bubbles
take in climbing toward the automatic shutoff port on the
nozzle. Since gasoline bubbles in the fillneck could
contribute to premature nozzle shut-offs, several different
techniques to reduce bubble generation were examined. These
included approaches to eliminating bubbles before they reached
the fuel nozzle and approaches to reducing bubble formation by
decreasing the amount of turbulent mixing in the fillneck.
While the approaches examined here were by no means
comprehensive, (i.e., many other options exist) one approach
identified was quite successful.
This approach involved replacing a section of the standard
2 inch diameter tubing with a section of 4 inch diameter
tubing. The resulting 'reservoir1 added volume to the fillneck
and served as a settling chamber in which the bubbles formed
and burst without rising in the fillneck. The use of such a
reservoir, the dimensions of which were chosen rather
arbitrarily, had a significant impact on required fillneck
height. -
The reservoir idea was tested as an addition to the j-tube
setup discussed in Section C. The reservoir is diagrammed in
Figure 9. The procedure used to gauge required fillneck height
was the same as that used for the J-tube without the
reservoir. The results of testing this system are shown below:
Fuel inflow rate = 7.5 gal/min
Backpressure Required Fillneck
(inches f^O) Height (inches)
5 11
4 9.5
3 6.5
Similar results were obtained when the in-tube trap and
submerged fill systems were tested with reservoir type
fillnecks. The in-tube trap set in 3 inch diameter tubing has
a "built in" reservoir - the tubing is larger than the stock
system already. This seal system needed a fill height of only
11 inches. The submerged fill system, with the reservoir used
for the 'J1 tube attached, achieved similar results (11 inches
required fill height). The results of this test show that a
properly designed reservoir system can greatly reduce required
fillneck height.
-------
Figure 8
Bubble Flow
Pattern
Static Case
Liquid Level
-------
Figure 9
Resevoir
Top View
Side View
To Nozzle
2. in.-
« 4 in.
Stock Fillneck
Reservior
To Tank
-------
-28-
Another approach to bubble control involves the use of a
simple baffle to control flow in the fillneck as is now used in
some vehicle designs. More specifically the inflow of fuel and
the backflow of bubbles can be separated with the use of a
baffling system. The simple baffling system shown in Figure 10
directs and smooths the inflow of fuel and also directs and
restrains the backflow of bubbles. The baffle restricts inflow
to one portion of the fillneck and encourages much of the
bubble backflow to rise in the other. The vents near the top
of the baffle allow circulating air and vapor to pass through
the baffle, but any remaining bubbles are popped by the edges
of the vents before they can reach the nozzle venturi tube.
Tests were conducted to determine the fillneck height
requirement for the submerged fill tank with a baffled
fillneck. The procedure described in Section D.3. for the
submerged fill without baffling was followed. At a fuel inflow
rate of 7.5 gal/min and tank backpressure of 5 inches H20,
required fillneck height was reduced to 11.5 inches, a decrease
of 5.5 inches.
The baffle approach was not evaluated with the in-tube
trap or J-tube, but could be used if the baffle extended for
enough into the liquid to separate the liquid/bubble flow.
4. Discussion
The reservoir and baffle approaches discussed above were
successful in reducing fillneck height requirements by about 30
percent. Of the portion of the fillneck height that can be
attributed to the dynamic affects of refueling, the reduction
is approximately 55 percent. This reduction is quite
significant given the limited time and resources committed to
this effort, and suggests further improvement is possible.
F. Discussion/Conclusions
1. Summary
One of the key components of an onboard refueling vapor
recovery system is a fillneck seal to prevent the escape of
vapors during refueling. While mechanical seal approaches have
been demonstrated successfully, there are concerns about the
safety, durability, and integrity of the mechanical seal
approach. As an alternative to a mechanical seal, EPA has
evaluated the practicality of implementing a liquid seal
approach.
The practicality aspect of the liquid seal approach
involves three factors: fill height requirements, safety, and
system efficiency. System efficiency is discussed in the next
-------
Figure 10
Baffled Fillneck
Nozzle
Side View
Front View
-------
-30-
portion of this report. Fill height requirements are important
because of concerns regarding premature nozzle shut-off. In
order to evaluate the potential suitability of various liquid
seals with respect to fill height requirements, systems were
tested under standard test conditions: a fuel dispensing rate
of 7.5 gal/min and a tank backpressure of 5 inches H20. The
vertical fillneck height required for normal refueling on each
at the three liquid seal approaches was measured and recorded
for each system.
Due to the excessive amount of bubbling generated during
refueling, fillneck heights of at least 16 inches were needed
to refuel the tank for the J-tube and submerged fill
approaches. Because of fillneck height limitations imposed by
some current vehicle models, efforts were made to reduce the
fill height requirements of the liquid seal systems. This
involved fillneck modifications aimed at controlling air
entrainment and bubbling in the fillneck. Through the use of
reservoirs and baffles, fill heights were reduced to 11
inches. The reduction of 5 inches is significant, considering
that seven inches of fillneck height would be necessary to
support a backpressure of 5 inches H20 under static
conditions. The effect of dynamics was reduced to 4 inches.
The results of the fill height evaluations are summarized in
Table 2.
The safety of liquid seal approaches must also be
considered. Most notably, liquid seal systems cannot
pressurize during refueling such as could occur for mechanical
seal systems. Any increase in pressure during refueling would
cause fuel to rise in the fillneck and automatic shutoff to
occur. The one safety problem that must be addressed for the
liquid seal system is the possible spitback of fuel when the
gas cap is removed prior to refueling.
Spit back could occur if the lines to the carbon canister
became blocked and pressure built in the tank prior to
refueling. When the gas cap was removed, some of the fuel
forming the liquid seal could be forced out of the fillneck as
pressure equalized. Spit back can be avoided by either 1)
removing the liquid seal after refueling or 2) including a
pressure relief system to vent tank pressure before spit back
can occur. The in-tube trap and 'J' tube use the first
alternative. By drilling a small hole at the bottom of the
upward opening 'U' in these designs, the liquid trap can
operate during refueling, and then drain soon after the event
has been completed.
-------
-31-
Table 2
Bench Test Fill Height Requirement
System Static
In-Tube Trap 7"
J-Tube 7"
Submerged Fill 7"
Base
System
16"
16"
Modified
System
11" (3" pipe diam.)
11" (reservoir)
11" (baffle)
5" H2O backpressure, 7.5 gpm dispensing rate, 2" fillpipe
diameter.
-------
-32-
The submerged fill system, on the other hand, must include
a more complex pressure relief system. There are a number of
ways to incorporate the pressure relief system into the
submerged fill. One of the simpler pressure relief systems is
shown in Figure 11. The small tube enters the tank above the
full tank liquid level, so it is never submerged. As refueling
takes place, the 'U1 portion of the small tube is filled with
fluid. When the gas cap is removed prior to refueling, the
pressure is vented through the small tube before spitback can
occur. The amount of pressure needed to vent the system can be
changed by changing the depth of the 'U' portion of the small
tube. Although the spit back problem can be easily solved for
any of the systems, the alteration of the submerged fill is
clearly more complex than that required for the in-tube trap or
'J' tube liquid seals.
2. Conclusions
There are two main questions to be addressed in concluding
the first portion of this report. First, is the liquid seal
approach to fillneck vapor containment a practical alternative
to the mechanical seal as part of an onboard refueling vapor
recovery system? Second, which systems tested in the study are
the most promising for further development?
The answer to the first of these questions is generally
yes. The liquid seal could be adapted to fit most lightduty
vehicles and trucks as currently produced, but there are some
vehicles that do not provide adequate fill height to employ the
liquid seal. There are two major classes of fillneck
configurations, the side fill and the rear fill. The side fill
configurations will be discussed first.
The liquid seal design is adaptable to most side fill
vehicles. In fact, most vehicles could probably be equipped
with an unmodified fillneck (no baffle or reservoir). For
other vehicle models, the length of the fillneck is sufficient
to permit the use of a liquid seal with a modified, bubble
reducing fillneck. Further, it appears that changes could be
made in most side fill vehicles to increase the fillneck height
if needed, by .changing the location of the gas cap on the
vehicle body.
Rear fill configurations are much less receptive to the
liquid seal approach than are side fills. The fillneck height
of some rear fill vehicles is as little as 5 inches. A fill
height of 5 inches could accommodate a backpressure of only
3.15 inches H20 under static conditions. The limited testing
conducted by EPA suggests that with a carefully designed
-------
Figure 11
Submerged Fill with Pressure Relief
,Tank Overpressure
Relief Valve
Full Tank Level
-------
-34-
reservoir, a fillneck height could probably be reduced to
approximately 6.0 inches (3.0 inch H20 backpressure and a 7.5
gal/min fill rate). If a manufacturer were convinced of the
superiority of the rear fill configuration, and wanted to use a
liquid seal system, changes in fuel tank and gas cap locations
would have to be made. However, these changes might be
relatively involved. Another approach would be to create a
horizontal reservoir by extending the fillpipe horizontally
across the top of the fuel tank, (either outside or inside).
The fuel would then enter the tank at the opposite end.
Another option open to manufacturers of rear fill vehicles
would be the use of a mechanical seal. The accompanying safety
concerns would have to be addressed. Nevertheless, it is
important to note that most current rear fill vehicle models
are being phased out and no new models of this design have been
introduced recently. In the post 1990 time frame few, if any,
new models with rear fill tanks are expected in the new
car/light truck fleet.
The second question to be addressed is which system
examined is promising for further development. There are four
characteristics that are important in comparing these liquid
seals. The first is the spatial efficiency of the system as
measured by required fill height. The test results suggest
that all of the systems can be modified to get nearly equal
results. Each system evaluated needed a fillneck height of
approximately 11 inches to support a backpressure of 5 inches
H2O at a fuel dispensing rate of 7.5 gal/min. The submerged
fill system does have an advantage in this area, however, since
the full fill height is needed only as the tank approaches the
full level.
Refueling safety is the second major characteristic for
comparison of the liquid seal systems. Both the in-tube trap
and 'J' tube need only one minor modification to avoid any
spitback problem. However, the submerged fill system needs a
pressure relief system like that described earlier. The
submerged fill system is at a disadvantage in this respect.
The third characteristic of comparison for these systems
is the complexity of the liquid seal itself. The in-tube trap
is the most complex of the three. The submerged fill and 'J'
tube designs would be much less difficult to build and are
probably more practical for this reason.
The final basis of comparison is the security of the
liquid seal defined in terms of the probability of vapor
escaping to the atmosphere during refueling. The biggest
concern about trap security arises when a vehicle sits on
sloped terrain during refueling. The in-tube trap and 'J' tube
-------
-35-
traps can be designed to ensure security up to any reasonable
angle chosen. The submerged fill system works differently than
these systems, however. For the submerged fill to function
properly, the fuel must enter the tank below the liquid
surface, but when the tank is nearly empty,, the mouth of the
fillneck will be only slightly below the liquid level. If the
vehicle were on an incline, the fillneck may not be submerged
at all or may be only partially submerged, and some vapor could
escape during the early stages of refueling.
It is clear from the discussion that each system has both
strong and weak points, but most of the problems with any of
the systems can be resolved with a moderate amount of effort.
The relative complexity of the in-tube trap suggests that this
design may be the most costly to produce, but if installed in
the inside top of the tank it could be implemented with no
other fillpipe modifications. The other two systems are also
clearly worth further investigation if the liquid seal concept
is being developed. Based on the safety advantage, the J-tube
is somewhat preferable to the submerged fill, and EPA has
selected that approach for inclusion in the prototype liquid
seal onboard system developed for bench testing. The J-tube
was also selected because of EPA's previous experience with the
submerged fill which showed very high efficiencies in a bench
test.[4]
The next portion of this report covers EPA's program to
evaluate the efficiency of a J-tube liquid seal. The
discussions cover the remainder of the components needed for a
bench evaluation of the system and the efficiency testing
conducted by EPA.
Ill. Bench Testing of a Liquid Seal Onboard System
A. Description of the Prototype System
1. Introduction
Once the evaluation of the three liquid seal concepts was
complete, EPA desired to evaluate the efficiency of a liquid
seal in a simple prototype onboard refueling control system.
As will be described below, the prototype system constructed
for this evaluation was relatively crude, and incorporated only
the essential components. No attempts were made to add
components or incorporate modifications to reduce the refueling
emission rate or optimize system efficiency. This section of
the report briefly examines the three components added
downstream of the fuel tank to complete the prototype system.
-------
-36-
To complete the prototype system three additional
components were needed. These include a fill limiter, a
properly sized vapor line, and an activated carbon canister.
Each of these components is discussed below, in the context of
the flow rate and backpressure design characteristics mentioned
above and the overall efficiency of the prototype system.
2. Fill Limiter
While it does not directly affect system backpressure or
efficiency, a fill limiter is needed in onboard system designs
where the vapor vent outlet is located in the fuel tank vapor
drive. On present fuel tanks, the automatic shut-off feature
.of the fuel nozzle is activated when tank back pressure
increases at the end of the refueling event and causes the fuel
in the tank to back up into the fillneck and cover the nozzle
spout with liquid. However, with a properly designed onboard
system fuel tank backpressures are minimal since vapor is
vented during refueling. The small amount of system
backpressure which does exist is not adequate to back the fuel
up into the fillneck and allow operation of the automatic
shut-off feature of the fuel nozzle. Therefore, a fill limiter
is needed to close the vapor vent when the tank is full and
thus increase the backpressure. Without a fill limiter it is
quite possible that the tank could be filled well beyond its
nominal capacity. For onboard equipped vehicles using side
fill tanks with external vent lines (as in Figure 12), the
onboard system has little effect on the refueling process. The
refueling event terminates the way it does on present vehicles,
and thus a fill limiter is not needed.
The EPA prototype onboard system used a rear fill fuel
tank without an external vent line, so a simple float valve was
needed as a fill limiter. The float was constructed of the
same material used for the tank's fuel level indicator. It was
suspended from the roof of the tank on a simple wire in such a
way that it would float on the fuel as the tank approached
full, and would nest into the vapor vent when the tank was
full. This would stop the venting of vapor which would
immediately increase system backpressure and thus would quickly
lead to activation of the nozzle automatic shut-off.
Before discussing the vapor line, two points about the
fill limiter and its operation should be noted. First, EPA's
simple prototype did not include a liquid/vapor separator,
rollover valve or a vent closure valve. These were not
necessary for a simple bench set-up intended only for
evaluation of refueling emissions control. Second, EPA did not
experience significant problems with fuel spillage due to
operation of the fill limiter, as has been suggested by
-------
Figure 12
Typical Current Evaporative System
PRESSURE/VACUUM
RELIEF CAP
EXTERNAL VENT LINE
FLOAT/ROLLOVER
VALVE
3/8" DIA.
8' LONG
.05" DIA. LIMITING
ORIFICE
14 GALLON FUEL TANK
PURGE VALVE
1 LITER
CARBON
CANISTER
TO PURGE
INDUCTION
POINT
-------
-38-
some.[5] This perhaps was due to the fact that the system
evaluated by EPA included a reservoir in the fillneck.
3. Vapor Line
The purpose of the vapor line is to convey gasoline vapor
from the fuel tank to the carbon canister. It must be sized
and constructed of material such that it does not unnecessarily
increase the system backpressure (which affects the fill height
requirement), and must be impermeable to gasoline vapors. To
meet these requirements, three characteristics of the vapor
line and its use must be considered and evaluated
concurrently. These include the vapor line diameter, the vapor
line configuration or layout, and the vapor line material. The
affect of these three characteristics on system backpressure
and permeability of vapors is discussed below.
As gasoline vapors flow through the lines, both major and
minor pressure losses occur which inhibit vapor flow and
directly affect system backpressure. The vapor line diameter
makes a major contribution to the pressure losses, since it
represents a major contraction in the flow. The other major
pressure losses are caused by functional resistance to the
vapor flow. This is a function of the vapor line diameter,
length of vapor line used, and to some degree the vapor line
material.
Minor pressure losses are mainly caused by the vapor line
layout or configuration. This includes any bends, elbows,
expansions or contractions in the line, as well as any other
deviations from the straight flow in a vapor line of a constant
cross sectional area.
As was discussed earlier in relation to fillneck seals,
the control system was designed so that the vapor line and
canister result in a fuel tank backpressure of no more than 5
inches of H2O. A series of tests was designed to empirically
determine what size vapor line would be appropriate for a
typical light duty vehicle application. These tests were
performed on complete control systems (J-tube equipped fuel
tank, vapor line, carbon canister) subjected to actual
refuelings. The test conditions were constant except for the
cross-sectional area of the tubing which was varied from test
to test. Although only one tubing material was tested, the
tests (summarized in Table 3) showed that vapor line with an
inner diameter of at least 1/2 inch would have to be used to
keep back pressure below 5 inches H20 for the entire system.
To be safe, vapor line of 5/8 inch inner diameter was used for
-------
-39-
Table 3
Vapor Line Pressure Drop Contributions
P, Inches H2O
Tube ID, Inch Tube Configuration Bench Test
3/8
1/2
5/8
1/2
5/8
15
15
15
8
12
ft.,
ft. ,
ft. ,
ft. ,
ft. ,
6
6
6
4
4
bends
bends
bends
bends
bends
5
2
1
1
.2
.2
.5
.9
-------
-40-
control system testing, although 1/2 inch vapor would clearly
be reasonable for many vehicle applications.*
The tubing material is also very important to the
functioning of the control system. The most important
characteristic of appropriate tubing materials is
impermeability to gasoline and gasoline vapors. If a vapor
line which was permeable to gasoline vapors was used as part of
the control system, hydrocarbons could escape from the tubing,
defeating the purpose of the control system. There are a
number of tubing materials which would be appropriate for this
application. Flexibility, density and cost characteristics are
all factors to be weighed in deciding between hard rubber,
softer, more flexible rubber and steel materials. A double
walled, flexible rubber fuel line was used in EPA1a prototype
onboard control system since 5/8 inch impermeable vapor line
was not commercially available.
4. Carbon Canister
Activated carbon for capture and storage of gasoline
vapors has been used successfully in automotive applications
for more than ten years. To apply this technology to control
of refueling emissions in EPA's bench evaluation, the canister
must not increase system backpressure more than is necessary,
and it must be sized and loaded with enough activated carbon to
efficiently capture essentially all refueling emissions. These
two factors are closely linked since canister size and design
both impact backpressure.
Rather than designing an optimum canister, it was EPA's
goal to construct or obtain a canister which was adequately
sized to capture all refueling emissions from an 18 gallon fuel
tank and yet would not increase total system backpressure to
more than 5 inches H2O.
First, to assist in meeting the backpressure requirement,
EPA held discussions with several manufacturers of activated
carbon. Based on these discussions, EPA selected Westvaco
extruded wood base activated carbon for use in the canister.
Information presented by the vendor indicated that the extruded
activated carbon might have a higher working butane working
capacity and better pressure drop characteristics than the more
conventional activated carbons now used in evaporative emission
applications. However, it appears that any of the activated
carbon types sold by the different companies would also work
successfully in capturing refueling vapors, but these would
This discussion briefly addresses how vapor line diameter
affects backpressure. Any system design would have to
consider vapor line length, configuration, canister
effects, etc., in addition to vapor line diameter.
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-41-
have different performance characteristics and would require a
different amount of activated carbon.
In sizing the canister, the next step would be to estimate
the total amount of activated carbon needed, based on gasoline
vs. butane working capacity, apparent density, carbon aging and
other factors. As is shown on Table 4, EPA estimates that a
carbon bed -of 1050 grams would be required to capture all
refueling emissions from an 18 gallon tank. At an apparent
density of 30 gm/100 ml, a canister of about 3500 milliliters
would be needed.
Alternatively, however, EPA turned to work previously
conducted by Mobil Research and Development in their 1978
onboard demonstration program. In that program Mobil used a
4350 ml canister to control refueling and evaporative emissions
from a carburetted 1978 Pontiac Sunbird which had an 18.5
gallon fuel tank.[l] An inquiry to Mobil revealed that the
canister shell which is shown in Figure 13 was still on hand
and available for loan, so this canister shell was used by EPA
in the prototype system. This canister was loaded with
approximately 1400 grams of activated carbon, somewhat more
than needed based on Table 4.
5. Conclusion
We have now described all portions of the prototype
onboard system used to evaluate the efficiency of the J-tube
liquid seal. The prototype onboard system included the
following components:
J-tube liquid fillneck seal with reservoir
18 gallon fuel tank
fill limiter
5/8 inch vapor line
4350 ml activated carbon canister
The results of EPA's evaluation testing on this system are
described in the next portion of this report.
B. Evaluation of System Efficiency
1. Introduction
Once the prototype onboard system had been constructed in
a bench configuration, a series of refueling emissions tests
were conducted to evaluate the efficiency of the system and to
-------
-42-
Table 4
Total Required Carbon Bed
Refueling Emissions 144 g
(at 8 g/gal for 18 gallons)
Activated Carbon Needed: 700 g
50 Percent Carbon Aging Allowance and
Safety Factor 350
Total 1050 g
Based on a gasoline working capacity of 6.3 gm gasoline
vapor/100 ml carbon, and an apparent density of 30 gm
carbon/100 ml carbon.
-------
FIGURE 13
REFUELING SYSTEM CARBON CANISTER
Carbon
/////// ////i/ / // / 77
W/////////77/777//////
Ill I|IMH ITTTIPPf
Foam
Foam
Wire Mesh
* Fiberglass Air Filter
Sunbird Carburetor
Bowl Fitting
Chevrolet Carburetor
Bowl Valves
0.047" Bleed to Fuel Tank
J_L
Side View of Top Disc
i! i- 1/8" Plexiglass
ILL
1" Dia. xl 3/4" Long
Plexiglass Tube
Wire Mesh Glued to Bottom of Tube
of Tube
Chevrolet Purge Valve,
Drilled to 0.180"
Canister Dimensions
6" High
8" Diameter
Carbon: BPL-F3
1550 Grams
4350 ML
-------
-44-
draw some conclusions as to whether liquid seal systems could
have as high an efficiency as mechanical seals. This portion
of the report describes how these tests were done, the results
of the testing, and draws some conclusions about the efficiency
of liquid seal systems.
2. Description of Test Procedure
The refueling tests were conducted in a standard
evaporative emissions SHED which had been modified to allow the
entry of a fuel hose fitting into the side wall. Fuel was
dispensed from a standard fuel delivery cart.
The general test procedure followed is outlined in
Table 5, and is discussed below. The refueling control system
bench system was placed in the SHED and filled to 10 percent of
nominal tank capacity. The fuel tank was then heated to the
desired temperature inside the open SHED with the fuel cap
on.* The purge fan was operating in the SHED during this
time. The dispensed fuel had been heated to its desired
temperature previously.
At the end of the heating, the heating blankets were
unplugged, the fuel nozzle was inserted into the fillneck, the
mixing fans were restarted, and the SHED was sealed. A
background reading was taken in the sealed SHED before
refueling began. The refueling was then performed by turning
on the fuel cart from the outside the SHED. The refueling was
terminated when automatic shut off occurred. A final FID
reading was then taken in the SHED, and the SHED floor and
fillneck area were checked for fuel spills. The refueling
emissions to the SHED were evaluated as the difference between
the initial and final FID readings, with appropriate
adjustments for the small portion of the SHED volume occupied
by the bench apparatus.
Following the termination of the testing, the carbon
canister was weighed and then purged with a total of 300 cfm of
ambient air to remove refueling vapors from the canister.
Based on reduction in canister weights, this amount of air was
more than sufficient. Following completion of the purge, the
canister was weighed again.
Finally, before describing the results of the testing, it
is important to discuss the temperature and RVP characteristics
As will be discussed later, a couple of tests were
conducted with the fuel cap loose during the heat build to
estimate the effects of heat build emissions.
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-45-
Table 5
Test Sequence
1. Drain and refuel tank to 10 percent of nominal capacity.
2. Connect heat blankets and thermocouples, fuel cap on.
3. Heat tank to desired temperature and stabilize.
4. Insert fuel nozzle, set latch at high level.
5. Close SHED and start mixing fans.
6. Take initial sample reading (using FID).
7. Remove cap, refuel tank to automatic shut-off.
8. Check for spills and nozzle shut-off.
9. Take final sample reading (using FID).
10. Disconnect heat blankets and thermocouples.
-------
-46-
of the test fuels used. The dispensed and fuel tank
temperatures were set at 92°F +_ 2°F to be consistent with
previous testing conducted by EPA and to be representative of
the temperature proposed by EPA in its recommended practice for
testing of refueling emissions levels. [6, 7] Fuels of 11.8 and
9.0 psi RVP were used in the program. However most tests were
conducted using the 11.8 RVP fuel since it would be expected to
create higher refueling emissions and thus would be a more
stringent evaluation of system efficiency.
3 . Refueling Emission Test Results
The results of the refueling emission tests are summarized
in Table 6; more detailed information on each test is provided
in Appendix A. The emission results shown include 7 tests at
11.8 RVP and 3 tests at 9.0 RVP.
The efficiency of the control system can be evaluated by
comparing the refueling losses to the SHED to the total
refueling emissions based on the sum of refueling losses to the
SHED and canister weight gain. Under this approach the
efficiency can be calculated as shown below:
EFF = 1 - losses to SHED _ x 100%
caniscer win gain T losses C^
Using this approach, the average efficiency is 98 percent
for the 11.8 psi fuel and 96.6 percent for the 9.0 psi fuel.
Values ranged from 95.8 to 99.4 percent with an average of
about 97 percent. However, these results clearly indicate that
theoretical efficiencies of 98 percent or more are available
for liquid seal systems. This compares to efficiencies of
96-99 percent for a mechanical seal system. [1,8]
As was noted previously, two tests were conducted with the
fuel cap loose during the heat build to assess how much heat
build emissions were affecting the total canister vapor load.
Referring to Table 6, tests 1-5 were conducted with the fuel
cap on and fully sealed during the heat build, while tests 6
and 7 were conducted with the cap only loosely in place. A
comparison of the tests shows that the canister weight gain and
the apparent refueling emission rate were greater when the cap
was on and sealed. To some degree the canister was accepting
vapor from the tank during the heat build similar to what now
occurs in a diurnal evaporative emissions test. However, as
can be seen from Table 6, this has little effect on the system
efficiency evaluation for the program. All values fall within
the 96-99 percent efficiency range.
-------
RVP
11.8 psi
-47-
Table 6
Liquid Seal Control System Efficiencies
Conditions
Test
1
2
3
4
5
6
7
TT
91.0
90.8
92.0
91.0
91.9
92.0
91.0
ID
92.0
90.3
90.5
91.7
91.9
92.0
92.5
Gal
12.5
15.5
15.0
15.1
14.0
15.5
12.5
Loss to
SHED(g)
2.3
3.6
6.1
4.9
2.5
2.0
1.5
Canister
Wt Gain(g)
153.3
168.1
164.1
129.3
123.9
170.5
163.0
Percent
Efficiency
99.4
97.9
96.4
96.3
98.0
98.8
99.1
Avg. EFF 98.0
9.0 psi 08 92.0 93.0 18.5 6.0 135.5 95.8
09 93.0 92.2 15.0 3.6 121.1 97.1
10 92.0 92.5 15.0 4.0 120.8 96.8
Avg. EFF 96.6
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-48-
The turbulent mixing in the fillpipe discussed previously
and the subsequent entrainment of bubbles in the liquid
entering the tank suggests that the liquid seal may be
generating additional refueling emissions relative to the
uncontrolled fuel tank. Uncontrolled refueling emissions were
not evaluated on the bench system, but a similar fuel tank was
evaluated on the 1983 Oldsmobile Cutlass evaluated in a
previous EPA test program.[7] The results of that testing
indicate that uncontrolled refueling emissions for this vehicle
can be predicted using the equation
Losses (g/gal) = -5.584 - 0.114[AT(°F)] + 0.0857 [TD(°F>] +
0.520 [RVP(psi)], where AT = TT - TD.
Substituting the temperature and RVP conditions of Table 6
into this equation (TD = 92°F, AT = OF0, RVP = 11.8 psi)
yields an emission rate of 8.4 g/gal.
For comparison, two tests were conducted on the prototype
onboard system without the canister in place. The refueling
emission rate averaged 9.9 g/gal for the two tests (Tests
85-1363, 1364 in Appendix A).
The comparison of the actually occurring and predicted
emissions discussed above, leads to the conclusion that in this
case the liquid seal increased the emission rate. However,
using this data it is not possible to precisely quantify the
effect; the use of an average could be misleading due to the
relatively small number of tests and the range in values seen.
Ideally, comparisons should be made between identical tanks
with and without a liquid seal with as many other variables as
possible controlled.
A close review of the detailed results in Appendix A
reveals that 14-15.5 gallons was dispensed in most tests, but
over 18 gallons was dispensed in one test. This later test was
the first conducted in the series and led to the conclusion
that the fill limiter installed in the tank did not work
properly and an overfill occurred. However, when the fill
limiter was modified and reinstalled it dropped too low into
the tank and about a gallon of nominal tank capacity was lost.
This accounts for the difference in the dispensed gallons among
the tests, and explains why the amount of fuel dispensed in
most tests was somewhat less than 90 percent of nominal tank
capacity.
4 . Source of Emissions
In addition to assessing the control efficiency of the
system, tests were also conducted to locate the source of
refueling emissions from the control system. The first step in
this process involved checking the fuel tank and vapor line for
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-49-
leaks. This was done by removing the canister from the system,
plugging the vapor line (at the canister end) and pressurizing
the tank. A liquid was then applied to the tank and vapor line
so that leaks could be seen. The system was free from leaks so
the only, possible sources of emissions were the fillneck and
the canister (if breakthrough were occurring). In order to
isolate the emissions from the fillneck, the canister was left
off of the system and was replaced with a plastic vapor bag.
Refueling operations were performed with the vapor bag in place
and the refueling loss was measured. In this way, only the
emissions from the fillneck were measured. The results from
this set of tests are shown in Table 7.
The refueling losses from the control system with the
vapor bag are very similar to those emitted from the control
system with the canister in place. The projected efficiencies
shown in Table 7 were developed by dividing the refueling
losses by the predicted emission level found using the equation
cited previously.
Since the refueling loss and efficiency numbers of Table 7
are very similar to those in Table 6, it appears that most of
the refueling loss comes from the fillneck.
IV. Conclusions
The purpose of the development and test programs conducted
was to evaluate the practicality and feasibility of liquid
fillneck seals as an alternative to the more widely
demonstrated mechanical seals. The EPA program covered a
laboratory evaluation of three different liquid seal concepts
and bench testing of a simple prototype onboard system using a
liquid seal.
The evaluation of the liquid seal concept led to several
conclusions.
The available fill height on most side fill
passenger cars and light trucks is sufficient to
permit the use of liquid seals without premature
nozzle shut-off.
For some side fill vehicles, reductions in fill
height would need to be achieved through internal
fillpipe modifications to control the bubbles caused
by the strong turbulent mixing in the fillpipe.
Rear fill vehicles would require more substantial
fillpipe, tank, or other modifications to use liquid
seals and thus may be better suited to mechanical
seals.
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-50-
Table 7
Sources of Refueling Emissions on a Controlled System*
Test
11
12
TT
91.0
91.0
Condi
TD
92.5
92.5
tions
Gal.
13.3
13.5
Loss to
SHED(g)
2.4
2.8
Predicted
Emissions (g)
115.0
116.8
Percent
Efficiency
97.9
97.6
Vapor bag used to replace canister; RVP = 11.8.
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-51-
Liquid seal systems (especially the J-tube and
in-tube trap) appear .to have safety advantages over
the mechanical seal and submerged fill.
The strong turbulent mixing in the fillpipe entrains
air into the fuel entering the tank, and increases
the volume of vapor which is displaced from the tank
as it is refilled.
Several key conclusions can also be drawn from the bench
testing of a prototype onboard system using a liquid seal.
The efficiency of the liquid seal systems is
comparable to that achieved by mechanical seals.
The mechanical seal systems evaluated by API showed
efficiencies of 96 to 99 percent. The liquid seal
systems evaluated in this project showed
efficiencies in that range.
With improvements in the control system,
consistently high control efficiencies (98 percent
and greater) are easily within reach for liquid seal
systems.
For a liquid seal system with an adequately sized
carbon canister, most refueling losses appear to
arise from the fillneck.
The air entrainment caused by the turbulent mixing
in the fillneck appears to increase the total
refueling emissions load to the carbon canister.
Data generated in this test program is insufficient
to precisely quantify this effect.
The primary purpose of this program was to demonstrate the
feasibility of liquid fillneck seal concepts. The bench
prototype developed here was clearly a first generation system;
no attempts were made to optimize the design to reduce the
effects of entrainment or to maximize control efficiencies.
Improvements in both areas are possible, as demonstrated in
later work conducted by API. [8] This work shows that onboard
systems using liquid fillneck seals can consistently control
refueling emissions with efficiencies of 98 to 99 percent.
The development and test programs conducted by EPA
demonstrate that liquid seal systems are indeed practical
alternatives to mechanical seal systems. Analysis suggests
that liquid seals are superior to mechanical seals in the areas
of durability, tampering, and safety, and have equal
efficiencies for controlling refueling emissions. However, the
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-52-
work conducted by EPA indicates that liquid seals may be more
attractive for side fill than rear fill vehicles, and the air
entrainment caused by turbulent mixing in the fillpipe may
actually increase refueling emissions and create the need for a
larger canister than that required for mechanical seals. Thus
some trade-offs may exist between mechanical and liquid
fillneck seals.
Either liquid or mechanical fillneck seals could be used
as part of an onboard system. Each approach has its strengths
a-nd weaknesses, but both have been demonstrated to have high
efficiencies on prototype systems. Whether a liquid or
mechanical seal system is preferable would have to be evaluated
separately for each vehicle model.
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-53-
References
1. "On-Board Control of Refueling Emissions,
Demonstration of Feasibility," Mobile Research and Development
Corp., for American Petroleum Institute, Washington, D.C.,
October 2 1978.
2. "Control of Refueling Emissions with an Activated
Carbon Canister on the Vehicle - Performance and Cost
Effectiveness Analysis," Olson Laboratories, Inc., for American
Petroleum Institute, Washington, D.C., October, 1973.
3. "SAE Recommended Practice (J 1045) - Instrumentation
and Techniques for Vehicle Refueling Emissions Measurement."
4. "Status of In-House Refueling Loss Measurements,"
Memorandum from Martin Reinemann to Robert Maxwell, U.S. EPA,
SDSB, March 6, 1979.
5. "Ford Motor Company Comments on the Evaluation of
Air Pollution Regulatory Strategies for the Gasoline Marketing
Industry," Nov. 1984.
6. "Draft Recommended Test Procedure for the
Measurement of Refueling Emissions," Lisa Snapp, EPA Technical
Report, EPA-AA-SDSB-85-5, July, 1985.
7. "Refueling Emissions from Uncontrolled Vehicles," D.
Rothman and R. Johnson, EPA Report EPA-AA-SDSB-85-6, 1985.
8. "Vehicle Onboard Refueling Control," API Publication
4424, March 1986.
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-54-
APPENDIX A
Detailed Test Results
-------
Fuel Tank Bench Tests
FUEL-COMMERCIAL UNLEADED, SUMMER, 92°F DISPENSED*
Temperatures °F
Dis- Refuel-
pensed ing
Test No.
85-0694
85-0120
85-0121
85-0123
85-0698
85-0699
85-1358
85-1362
85-1368
84-5633
84-5634
84-5635
85-1363
85-1364
Date
10-31-84
11-05-84
11-06-84
11-07-84
11-08-84
11-09-84
12-19-84
01-04-85
01-11-85
08-23-84
08-24-84
08-24-84
01-04-85
01-03-85
Ini-
tial
66.8
68.2
68.3
69.5
67.55
67.0
61.0
67.7
60.5
69.0
67.0
67.9
70.7
67.7
Final
91.0
91.0
90.8
91.0
92.0
91.0
92.0
91.0
91.9
92.0
93.0
92.0
92.0
92.5
Final
Vap
89.0
88.8
89.0
89.0
90.0
89.5
88.5
89.5
91.0
90.2
89.0
90.3
89.0
90.8
Max
T
3.0
3.0
3.0
3.5
3.0
3.0
3.5
3.0
5.3
2.0
2.0
2.0
4.0
3.0
Dis-
pensed
92.0
92.5
90.3
91.0
92.0
92.5
90.5
91.7
91.9
93.0
92.2
92.5
89.8
90.0
Gal-
lons
12.5
13.3
15.5
13.5
15.5
12.5
15.0
15.1
14.0
18.5
15.0
15.0
14.3
14.5
Loss,
gms.
2.3
2.4
3.6
2.8
2.0
1.5
6.1
4.9
2.5
6.0
3.6
4.0
134.8
151.5
Canister
Wt Gain
153.3
NA
168.1
NA
170.5
163.0
164.1
129.3
123.9
135.5
121.1
120.8
NA
NA
Dis-
pensing
Time
Mi n, Sec
1'46"
2'04"
2'11"
1'57"
2'19"
1'49"
2'16"
2'08"
2'50"
3'41"
2'56"
2'55"
2'00"
2'09"
Heat
Time
Hr,Min
57"
52"
56"
45"
51"
55"
58"
40"
45"
46"
48"
48"
56"
42"
Spi llage/Coimients
None
None, vapor bag
None, manual shutoff
None, vapor bag
None, can in box
multi-fueling
None, multi fueling
None, Benzene 13A
Controlled
None, canister only
during refueling
None
None, manual shutoff
None, manual shutoff
Controlled w/o canister
Controlled w/o canister
All tests were at 11.8 RVP except 84-5633, 5634, and 5635.
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