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Gossman Consulting, Inc

GCI TECH NOTES©


Volume 7, Number 5           A Gossman Consulting, Inc. Publication        October 2002


 

 

Vapor Balance System Control Efficiency

by

Dave Constans, Gossman Consulting, Inc.


Logic might dictate that AP42, EPA’s bible for estimating emissions from various processes, would in section 7.1 (Organic Liquid Storage Tanks) discuss vapor balance systems and provide equations for calculating emissions from tank systems with such equipment.  Indeed in 7.1.2.1 vapor balancing is discussed as one of four methods of controlling emissions from the tank.  The four methods are: Floating roof with a 60 to 99% control efficiency, vapor balancing with a 90 to 98% control efficiency, vapor recovery with a 90 to 98% control efficiency and combustion with a 96 to 99% control efficiency.  When it comes to utilizing the “Emission Estimation Procedures” in section 7 of AP42 to calculate emissions from systems utilizing vapor balance systems the formulas are useless.  Indeed the EPA has provided a software program to calculate tank emissions. The TANKS4 (version 4.096), developed by the API.  However, in a FAQ accessed via the Internet, one of the FAQ’s listed is: “I have a gasoline service station with underground fixed roof tanks that are vapor balanced.  What equations should I use to estimate emissions?”  The response to this question is illuminating, not because it is useful in calculating the answer but because it points out how little thought has gone into providing assistance in calculating emissions for one of the four recognized methods of control listed in 7.1.2.1. 

The response to this FAQ is to utilize the equations in 7.1.3.3.  Section 7.1.3.3 deals with “Variable Vapor Space Tanks” – by definition these are tanks that have an “expandable vapor reservoir to accommodate vapor volume fluctuations”... “most common types” ... “are lifter roof tanks and flexible diaphragm tanks” (Section 7.1.1.5 AP42).  In addition the text of 7.1.3.3 states... “Special tank operating conditions may result in actual losses significantly different from the estimate provided by equation 3-1.  For example, if one or more tanks with interconnected vapor spaces are filled while others are emptied simultaneously, all or part of the expelled vapor will be transferred to the tank, or tanks, being emptied.  This is called balanced pumping.  Equation 3-1 does not account for balanced pumping, and will overestimate losses under this condition.”  The text does not provide a clue as to where, or even if, any equations exist in AP42 to address a known and listed emission control method.

Gasoline storage is a system that utilizes vapor balance systems.  Section 5.2 of AP42 covers “Transportation and Marketing of Petroleum Liquids”.  Section 5.2.5.2 the text acknowledges: “Emissions from underground tank filling operations as service stations can be reduced by the use of a vapor balance system... The control efficiency of the balance system ranges from 93% - 100% percent.”  The EPA thought highly enough of vapor balance systems that they mandated the use of them for service stations in certain urban areas where “tests on a few systems have indicated overall systems control efficiencies in the range of 88% - 92%” (Section 5.2.2.3 AP42). Again, AP42 does not provide equations for calculating the reduced emission rates.

What is the purpose in detailing AP42’s inadequacies for addressing emissions estimates for systems utilizing vapor balancing systems?  Clearly this is a valuable technology.  This technology is documented by the EPA to reduce VOC emissions by 88% - 100%, yet it does not use any energy or consumable resource, such as activated carbon, to do so.  Furthermore, AP42 is nearly useless in providing guidance on how to estimate emissions from tank systems utilizing this tank emissions reduction technology.  This puts the facility owner/operator who utilize such systems at a disadvantage.  Generally the mere use of AP42 equations and subsequent demonstrations of the required VOC capture efficiencies would be sufficient to demonstrate compliance to the regulations.  However, because of AP42’s inadequacies the documentation becomes more difficult.  In addition, the lack of an EPA recognized methodology impugns the owner/operator’s efforts in the eyes of many regulators.

Vapor Balance System

A widely used vapor balance system is described in AP42 section 7.1.3.3 (noted above) as a “Special tank operating condition”….”called balanced pumping”. 

The vacuum/pressure vent valve located next to the carbon canisters is designed to be the “first to open”, that is, this pressure vent will open before any other relief valve on the piping or tanks, and release VOC laden gases into the carbon canister.

The vacuum vent, in turn, will be the first to open before any other vacuum vent on the piping or tanks, to allow air into the system to preclude the vacuum crush of the tanks. Typical vacuum/pressure settings are 0.035-psi vacuum/0.5 psi pressure.  In addition the fuel delivery tank trucks are connected to the system during unloading.  This directs the vapors displaced from a receiving tank back to the unloading truck, resulting in no loss of VOC during the transfer.  Similarly when the receiving tank has been blended and analyzed and is transferred to a burn tank the displaced vapor is routed back into the transferring tank.  However unlike virtually all systems described in AP42 the fuel in the burn tanks is being consumed at a nearly constant rate.  This reduction of fluids in these tanks creates a partial vacuum in the burn tank pulling vapor from the other tanks, frequently in excess of the vapor generation rate attributed to daily temperature changes.  Indeed the system is generally under a partial vacuum except during periods of time when the kiln is not utilizing the hazardous waste fuel.

To document the control efficiency of the vapor balance system it is first necessary to calculate the uncontrolled emissions from the various tanks using the equations in AP42 section 7.1 as these equations are utilized in the TANKS4 software.  This emission rate would then be compared to the emission rate for the tank system utilizing the vapor balance equipment, but excluding the additional reduction attributable to the carbon canisters.

To calculate the vapor emissions from the tank system equipped with a vapor balance system it is first assumed that the use of the vapor balance system precludes the emissions of VOC attributable to “working loses” and only those emissions attributable to breathing losses in excess of the volume reduction caused by the consumption of the fuel in the kiln or during those time periods when fuel is not being consumed.

Uncontrolled Emission Calculation:

Example hazardous waste storage tanks are described below:

Tanks 1 through 4 are identical, each is a:  Nominal 40,000 gallons, 15 ft. dia. x 30’ straight side, vertical tanks with cone roof.  Tank 5 and 6:  Nominal 150,000 gallons, 30’ dia. x 30’ straight side, vertical tank with  cone roof.

The organic vapor partial pressure is 0.285 atmosphere and the average molecular weight of the organic fuel is 66.84 (This is typical for a kiln burning commercially available waste fuel.)  Other assumptions: The facility processes 16,000,000 gallons per year.  Each of the 40,000-gallon blend tanks sees one-fourth of this throughput and each of the burn tanks sees one-half of this throughput annually.  The vacuum/pressure setting for the tanks is 0.035 psi and 0.5 psi.  The other factors are properly selected from the TANKS4 program menus.  The TANKS4 program is run and the uncontrolled vapor emissions coming from each of the tanks is as follows:

 

Tank

Working Losses, lbs/year

Breathing Losses, lbs/year

Total Emissions, lbs/year

1

12,311

1,591

13,902

2

12,311

1,591

13,902

3

12,311

1,591

13,902

4

12,311

1,591

13,902

5

41,031

6,364

47,395

6

41,031

6,364

47,395

Total

131,307

19,093

150,400

 

Emissions Controlled by Vapor Balance System

All of the working losses are reduced to zero by the vapor balance system.  The breathing losses are reduced due to the volume of fuel being consistently consumed by the kiln as fuel.  The 16,000,000-gallon throughput each year is consumed in about 7920 hours of operation (330 days).

The breathing losses of 19,093 lbs./year are approximately 285.6 pound-moles or 95,847 ft3 at the average annual temperature of 52°F or 262.6 ft3 daily.  There are 35 days each year when hazardous waste fuel is not consumed.  For these 35 days 9,191 ft3 of VOC’s are released from the tanks.

During the other 330 days the consumption of the fuel causes 48,484.8 gallons of fuel to be drawn from the burn tank or 6482 ft3 each day.  This volumetric flow rate out of the tanks is 270.1 ft3 per hour, more volume than the breathing losses from all tanks each day.  Even if the VOC’s were diluted with air at a ratio of 20 to 1 the volumetric removal rate from the tanks would exceed the VOC generation rate.  Therefore on the 330 days when fuel is consumed in the kiln there will be no breathing losses.

Consequently the only emissions from the system are those attributable to the 35 days each year when hazardous waste fuel is not consumed.  The 9191 ft3 of VOC’s is equal to 1,831 lbs./year of VOC, in comparison to the 150,400 lbs./year of uncontrolled emissions.  This demonstrates a 98.9% control efficiency for the vapor balance system alone.  In addition if these emissions are then routed through a series of 2 carbon canisters for an additional 95% + reduction, the overall control efficiency is in excess of 99.9%.