Automatic Pressure Correcting Phase II/ORVR Compatible Vapor Recovery System

This paper describes an innovative Phase II vapor recovery system that provides for efficient fuel vapor collection while automatically correcting for pressure differentials between the pressure at the vehicle fill-pipe and the pressure in the fueling facility’s UST, while automatically providing the appropriate Vapor to Liquid Ratios (V/Ls) to ensure efficient vapor collection.



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Author: Simpson W. Dwain , PhD

Abstract
This paper describes the SaberVac VR, a Phase II Vapor Recovery System that provides for efficient fuel vapor collection while automatically correcting for pressure differentials between the pressure at the vehicle fill-pipe and the pressure in the fueling facility's Underground Fuel Storage Tanks (USTs). The system combines the unique features of the SaberVac®, Model V01001 vapor recovery pump, coupled with a matched vapor recovery nozzle (Husky model 6051) that provides a fairly tight seal with the vehicle fill-pipe and the vapor recovery path. The combination of these two components eliminates the problems associated with earlier vacuum assist Phase II vapor recovery systems that work well with non-ORVR equipped vehicles, but have unique problems when used with the mandated Onboard Refueling Vapor Recovery (ORVR) systems. While refueling either ORVR or non-ORVR vehicles, the SaberVac VR system provides only the system pressure differential necessary to recover vapors efficiently without under- or over-pressurizing the USTs. The appropriate values of Vapor to Liquid Ratios (V/Ls) are automatically provided to ensure efficient vapor collection.

Background
There are two levels of systems mandated by US and other governments for the safe, efficient recovery of fuel vapors, especially those produced during the process of providing gasoline, and other aromatic automobile fuels such as reformulated gasoline (RFG), to the automotive vehicle market. These two levels are referred to as Phase I and Phase II vapor recovery systems.

Phase I Vapor Recovery is accomplished when a fuel transport vehicle discharges fuel into the UST, a return hose allows the vapors in the UST ullage to be forced back into the fuel transport tank by the displacement of the vapor volume with the newly dispensed fuel. Phase I is an important part of the overall vapor recovery requirements, but it is not addressed in this paper.

Phase II vapor recovery involves recycling the vapors displaced from vehicle fuel tanks during refueling of the vehicle. Phase II systems must efficiently return the vapors from the vehicle tank to the UST during the refueling process in order to prevent vapors escaping into the atmosphere. There are two main types of Phase II vapor recovery systems: the Balance System and the Vacuum Assist System.

The proposed "membrane" systems add complex electronic logic and vapor handling equipment. In addition to the additional expense, the complexity of these systems increases maintenance and support costs.

Each of these technologies have unique advantages and disadvantage which have been discussed in previous issues of PE&T Online.

There is an obvious need to supply a better vapor recovery system to the fuel dispensing industry - one that is compatible with:

• the current largely non-ORVR vehicle population,

• the expected population of mixed non-ORVR and ORVR equipped vehicles in the next 20 years, and

• the expected majority population of ORVR equipped vehicles after the year 2020.

This paper describes such a system and the technology upon which it is based.

Concept: Use of an Automatically Adjustable Low Pressure Vacuum Pump
A vapor recovery system that utilizes an automatically adjustable low-pressure vacuum pump in conjunction with coaxial or equivalent fuel/vapor delivery hoses, appropriate vapor recovery nozzles, vapor valves in the dispensing end of the hose (either in hose or nozzle), and properly specified pressure-vacuum (PV) valves in the vent pipes of the USTs can produce excellent vapor recovery efficiencies while providing the advantages of both the balance vapor recovery system and the vacuum-assist vapor recovery system without the inherent problems of either. It also eliminates the need for expensive and difficult-to-maintain "processor" or "membrane" systems described earlier.

A vapor recovery pump (SaberVacÒ) has been designed and implemented (US Patents 5,217,051 & 5,858,857, others pending) which has exactly the features required. When this pump is used in the system described below, combined with the appropriate nozzle, efficient and inexpensive vapor recovery is possible for all combinations of vehicles to be fueled, including any mix of ORVR or non-ORVR equipped vehicles.

A low-pressure pump is desirable because of the need to control the pressures in the USTs. It is desirable to keep the pressure in an UST as near atmospheric pressure as possible without either allowing vapors to escape into the atmosphere or allow too much air into the UST. The pressure must also be maintained such that the vapor recovery return lines to the UST from the fuel dispensing position are not pressurized enough to cause "fugitive emissions" at any leak points in the nozzle/hose/dispenser/plumbing system. Fugitive emissions are of particular concern if a fuel dispensing facility is not "tight" and vapors can escape into the atmosphere in several locations not easily monitored by vapor detection systems.

Typically, USTs are maintained within pressure limits of only a few inches of water (" in H2O") pressure relative to atmospheric pressure (Standard atmospheric pressure is about 407 inches of water). PV valves may be set, for example, to a positive pressure of 2.0 in H2O and a negative pressure (vacuum) of -6 in H2O. Except in very rare cases of extremely sudden atmospheric pressure changes (low fronts due to hurricanes, etc.), the established ranges of pressures set for the PV valves will prevent vapors from escaping the USTs and prevent too much fresh air from entering the USTs.

A particular concern about Phase II vapor recovery systems as more and more vehicles become equipped with ORVR is the problem of "vapor growth" in the USTs. Vapor growth occurs when fresh air is ingested into the Ullage of the UST and mixed with the vapors already in the UST. Because of the diverse mix of hydrocarbons in the fuel (gasoline can contain hundreds of hydrocarbons in its formulation) various components of the fuel can vaporize and mix with the air/vapor mixture until a stable vapor concentration is achieved. The resulting stable UST pressure is usually at a higher pressure than before the excess air was ingested.

Since the extant Vacuum Assist systems are designed to deliver a constant ratio (within tight limits) of the vapor volume returned to the volume of liquid fuel dispensed (V/L ratio), a serious over-pressurization (due to vapor growth) can occur when some of the vehicles fueled are equipped with ORVR . The only protection for the UST is then to expel vapors into the atmosphere via the PV valve(s). The effective efficiency of these Vacuum Assist systems will then decrease and become essentially non-effective as the ORVR population increases over the next 15 to 20 years.

The "Processor" systems will be able to permit stabilization of the USTs by simply burning off the excess vapors - but this means that more and more fuel will be burned in a wasteful manner and even more greenhouse gases will be expelled into the atmosphere. A better system would prevent the burning of this fuel, which should be safely contained in the USTs.

"Membrane" systems have been proposed that would correct the problems associated with processor systems, recovering the majority of the vapors as condensed fuel while allowing only air or non-VOC gases to escape the UST via the vent lines. However, as with the processor systems, more electronic logic and vapor handling equipment must be added to a conventional Vacuum Assist system, making the system even more expensive and more unreliable as more complexity is added.

As discussed above, both the processor and membrane systems must be "tuned" carefully to avoid return of excess amounts of fuel from the vehicle fill-pipe during top-offs and splash-backs while refueling.

When the SaberVacÒ pump is incorporated into a vapor recovery system such that one pump is installed for each dispensing point (either external to the fuel dispenser or installed within the dispenser), then the vapor recovery is controlled independently for each fueling position. The pump is so designed that at pressure differential between the vehicle fill-pipe and the UST is near zero, it operates with a V/L ratio of very nearly 1.0. However, at pressure differentials greater than zero, the value of the V/L will decrease slightly so that pressurization of the tank will not occur. At pressure differentials less than zero, the pump will operate at a V/L of slightly greater than 1.0 so that the UST pressure is automatically adjusted upward. Since these very minor adjustments are made on every fueling episode, any variation in UST pressure due to atmospheric changes or vapor growth due to air ingested (from fueling ORVR vehicles, for example) into the UST will automatically be corrected. No vapors will be lost from the UST, and there is no requirement for additional vent vapor processors or electronic control of the vapor pump operation.

Overview of System
An overview of the SaberVac VR system is given in Fig. 1. The system is shown as a single fueling point with a single fuel and vapor return line and a single UST, but the extension to actual fueling facilities with multiple fueling points and multiple USTs is obvious.

In Fig. 1, the system is shown to consist of 1) an automotive fuel tank and fill-pipe, 2) a fuel dispensing nozzle with vapor recovery capabilities and with the capability of making a fairly tight seal between the fill-pipe and the vapor return path, 3) a hose system which provides for both fuel delivery and vapor recovery (i.e. coaxial vapor recovery hose), 4) the SaberVacÒ low-pressure, automatically adjustable vapor pump, 5) a dispensing unit which controls the fuel delivery, 6) fuel and vapor return lines to the UST, 7) a sealed UST which contains both fuel and fuel vapor space, 8) vent lines which permit vapors to flow from the UST to atmosphere or air to flow from the atmosphere to the UST, and 9) pressure/vacuum (PV) valves on the vent lines to maintain the UST vapor pressure within set limits. The Phase I ports for fuel delivery (with drop tubes) and vapor recovery are not shown, although they are required in all UST installations.

Figure 1:
SaberVac® VR System

System Operation
When fuel is dispensed via nozzle (2) into the vehicle fuel fill pipe (1), a vapor/air mixture is drawn back from the fill-pipe area via the vapor recovery hose (3) by the operation of the vapor pump (4). In standard vehicles without ORVR, the semi-tight seal with the fill-pipe, with the assistance of the SaberVac, essentially provides a "balance" type of recovery, collecting only the vapors forced out of the vehicle fuel tank by the fuel during refueling. In vehicles equipped with ORVR, very little vapor is available to be returned from the vehicle fill-pipe, and with the lower pressure at the nozzle/fill-pipe interface, very little air/vapor mixture is returned to the UST. For all refueling events, the collected vapors or vapor/air mixtures are returned to the UST via the vapor return lines (6). For non-ORVR vehicles, the V/L ratio is near 1.0 ("balanced"), the vapor pressure in the ullage of the UST will not change significantly, and no vapors will escape via the vent lines (8) through the PV valves (9). For ORVR vehicles, the V/L normally will be much lower than 1.0, typically 0.5 or less. Even if the vapor/air mixture returned from ORVR vehicles is mostly air, the lower volume returned assures that vapor growth in the ullage of the UST will not be excessive, and the UST pressure will tend to be reduced. If there are changes in the UST vapor pressure due to other effects (atmospheric pressure changes, large fuel drops, etc.), then the system operation will change automatically as described below, so that the UST pressure is always adjusted toward a stable pressure near or below atmospheric. Since these minor changes are made dynamically, responding to entire system parameters during each vehicle fueling episode, the efficiency of the vapor recovery system stays at optimum levels.

The active pressure of the SaberVac low pressure vapor pump is such that very little, if any, fuel can be drawn back via the nozzle vapor path into the vapor path of the coaxial hose. However, some condensation can occur and liquid fuel can accumulate in the vapor path over time. Normally the flow of the vapor/air mixture in the vapor path during refueling is enough to evaporate any condensed fuel but, in extreme temperature conditions, a small amount can remain in the hose. With proper hose configurations, this fuel can drain back into the vehicle fill-pipe on the same or next refueling episode. For other hose configurations, it may also be removed by standard venturi techniques included in coaxial hoses. In either case, the liquid fuel is thus returned to the customer rather than to the UST.

SaberVac® Vapor Pump Operation
A cut-away view of the SaberVac® vapor pump is shown in Fig. 2. The vapor pump is a fuel driven vapor pump, basically consisting of a fluid driven motor consisting of a rotor placed in the fuel flow path and constructed such that the fuel flow causes the rotor to rotate. The fuel rotor has inserted in the outer ring of the rotor a number of magnets arranged in such a way as to provide for optimum magnetic coupling with a similar set of magnets arranged on the inner ring of a vapor impeller. The fuel rotor is supported by bushings and/or bearings that permit the rotor to rotate freely under the influence of the fuel flow and at the rotational speeds required for driving the vapor impeller. The fuel rotor and the fuel flow is contained within a non-magnetic stainless steel tube which permits magnetic field lines to pass through the tube and couple with the magnetic fields of the vapor impeller. The vapor impeller is so designed as to permit it to spin around the fuel tube at the same rotational speed as the fuel rotor, since the two are magnetically coupled and act as a single rigid body within the limits of the magnetic coupling force. The vapor impeller is supported by a precision bearing system which permits it to operate at the high rotational speeds required to pump the vapors from the vehicle fuel fill-pipe all the way back to the UST at the operational pressures of the system. The flow directors within the pump are so designed to provide for optimum vapor flow through the pump at the operational speeds and pressure.

Figure 2:
SaberVac®Model V01001 Vapor Recovery Pump

Unlike pumps used in other Vacuum Assist systems, the SaberVac Model V01001 pump is not a positive displacement (PD) pump, making it possible to vary the V/L values of the SaberVac VR vapor recovery system in response to the presence of an ORVR vehicle. With the semi-tight seal of the vapor recovery nozzle, the normally large negative pressure differential between the vehicle fill-pipe and the UST will cause the SaberVac VR system to return a volume of vapor/air mixture to the UST that will not cause a pressure increase, even allowing for vapor growth if the mixture is mostly air.

The operation of the pump itself is such that at a differential pressure across the pump itself is equal to zero, the pump will pump at a V/L of essentially 1.0. At differential pressures less than zero, the pump operates at V/L values greater than 1.0, and at differential pressures greater than zero at V/L levels less than1.0. Using compliance tests as specified in the CARB Executive Order approving the SaberVac VR system (EO G-70-196), A/L values will remain in the limits 0.85 to 1.05 when tested.

However, as noted before, the conditions at the nozzle/fill-pipe are significantly different for the system when a fairly tight seal is made there. For non-ORVR vehicles, the SaberVac VR system operates as a "Balance" system, with the action of the SaberVac pump assisting when the seal is not perfect. For most ORVR cars, the lower pressure in the fill-pipe causes the SaberVac pump to pump at significantly lower V/L values (below 0.60), insuring that the UST pressures can not increase and can be maintained at near atmospheric pressure or at a small negative pressure. In either case, the effect of fugitive emissions will be negligible, and the V/L values permit efficient vapor recovery while maintaining the UST pressure at acceptable levels. Within manufacturing tolerances, the SaberVac pump can maintain the DP control within a few tenths of an inch of water pressure, so that no field adjustments to V/L values are ever required.

Performance

The SaberVac VR system has been approved by CARB (EO G-70-196) to be over 95% efficient in recovery of vapors generated during vehicle refueling. The results for the 100-car matrix tested are shown in Figure 3. The actual test efficiency average was over 99%. The average measured V/L for the test, which included 92% non-ORVR cars and 8% ORVR cars, was 0.94, and the UST pressure remained slightly less than atmospheric (-0.5" H2O) during the 100-car test, including the effects of all other activity in the test station.

Figure 3:
Synopsis if SaberVac VR 100-Vehicle Test
(CARB TP-201)

During the entire 90-day durability test period before and after the actual 100-vehicle test, the average UST pressure never exceeded +0.2" H2O, as shown in the rolling averages in Figure 4. This 90-day durability test period included the period when winter fuels were dispensed, with Reid Vapor Pressures greater that 11 psi.

Figure 4:
Average UST Pressures of SaberVac VR System

Installation and Maintenance
The SaberVac VR system is probably the easiest VR system to install and maintain. Installing the SaberVac pump itself is no more difficult than installing a hose into the dispenser outlet casting. Once the SaberVac pump is installed, the hose and nozzle components are simply installed onto the pump and installation is complete. With balance-plumbed dispensers, it is not necessary to make any changes in the dispenser or vapor recovery return lines.

Since all components of the system are external to the dispenser itself, replacement of damaged or malfunctioning components is again as simple as replacing a hose or nozzle.

A view of a SaberVac pump installed directly into the dispenser outlet castings is shown in Figure 5. As is seen, not only is the SaberVac easy to install, it essentially blends in with the hose-dispenser interface.

Figure 5:
SaberVac Pumps Installed in Dispenser Outlet Castings

 

A view of the Husky 6051 nozzle (approved with the SaberVac VR system) is shown in Figure 6. The unique features of this nozzle (solid spout, flexible vapor collection bellows, semi-tight seal) make it ideal to use with the SaberVac pump to form a very efficient SaberVac VR vapor recovery system.

Figure 6:
Husky 6051 Nozzle
With Flexible Semi-Tight Seal

 

EVR and ISD
The California Air Resources Board (CARB) has proposed new regulations termed "Enhanced Vapor Recovery (EVR) for control and monitoring of Phase I and Phase II vapor recovery systems. The first phase of these new regulations essentially includes three functions; 1) compatibility with ORVR vehicles, 2) the control of the UST pressures very near atmospheric to eliminate fugitive emissions, and 3) the monitoring of the UST pressures to assure compliance with these levels. The second phase is to expand EVR to include the automatically performing of diagnostics of the system and to report variations and alarm conditions outside acceptable limits. This phase is termed "In Station Diagnostics" (ISD).

The SaberVac VR system has already addressed the "control" functions of EVR with the capability of controlling the UST pressures within the desired limits. As the ORVR population increases, average UST pressures will continue to decrease, insuring, when using SaberVac, that fugitive emissions will be essentially eliminated. The UST monitoring functions, if not already in place, can be added without any modification to the SaberVac VR system itself.

Designs for logic signals appropriate for inclusion in ISD systems are being finalized, and will be incorporated into SaberVac VR systems once approved.

W. Dwain Simpson, PhD, is President of Synergetic Technologies, Inc. (Wilton, CT) and Saber Technologies, LLC (Fairfield, CT). Dr. Simpson has been active in the development of fuel dispensing and fuel vapor recovery systems since 1989.

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