September 6, 1995


MEMORANDUM


SUBJECT:	 Calculating Potential to Emit (PTE) for Emergency


Generators 


FROM:	 John S. Seitz, Director


Office of Air Quality Planning and Standards (MD-10)


TO: Director, Air, Pesticides and Toxics 


Management Division, Regions I and IV


Director, Air and Waste Management Division,


Region II


Director, Air, Radiation and Toxics Division,


Region III


Director, Air and Radiation Division,


Region V


Director, Air, Pesticides and Toxics Division,


Region VI


Director, Air and Toxics Division,


Regions VII, VIII, IX, and X


The purpose of this guidance is to address the determination


of PTE for emergency electrical generators. 


Background


In a memorandum dated January 25, 1995, the Environmental


Protection Agency (EPA) addressed a number of issues related to


the determination of a source's PTE under section 112 and title V


of the Clean Air Act (Act). One of the issues discussed in the


memorandum was the term "maximum capacity of a stationary source


to emit under its physical and operational design," which is part


of the definition of "potential to emit." The memorandum


clarified that inherent physical limitations, and operational


design features which restrict the potential emissions of


individual emission units, can be taken into account. This


clarification was intended to address facilities for which the


theoretical use of equipment is much higher than could ever


actually occur in practice. For such facilities, if their

physical limitations or operational design features are not taken 


into account, the potential emissions could be overestimated and


consequently the source owner could be subject to the Act


requirements affecting major sources. Although such source


owners could in most cases readily accept enforceable limitations


restricting the operation to its designed level, EPA believes


this administrative requirement for such sources to be


unnecessary and burdensome. 


On the topic of "physical and operational design," the


January 25 memorandum provided a general discussion. In


addition, EPA committed to providing technical assistance on the


type of inherent physical and operational design features that


may be considered acceptable in determining the potential to emit


for certain individual small source categories. The EPA is


currently conducting category-specific analyses in support of


this effort, and hopes as a result of these analyses to generate


more general guidance on this issue as well.


The purpose of this memorandum is to address the issue of


PTE as it relates specifically to emergency generators. There is


a significant level of interest in this source category because


there are many thousands of locations for which an emergency


generator is the only emitting source. Moreover, based on a


review of this source category, there exists a readily


identifiable constraint on the operational design of emergency


generators. Hence, the EPA believes it would be useful to


provide today's guidance before the entire effort is complete. 


The policies set forth in this memorandum are intended


solely as guidance, do not represent final Agency action, and


cannot be relied upon to create any rights enforceable by any


party.


Guidance for Emergency Generators


For purposes of today's guidance, an "emergency generator"


means a generator whose sole function is to provide back-up power


when electric power from the local utility is interrupted. The


emission source for such generators is typically a gasoline or


diesel-fired engine, but can in some cases include a small gas


turbine. Emissions consist primarily of carbon monoxide and


nitrogen oxides. Other criteria pollutants, and hazardous air


pollutants, are also emitted, but at much lower levels. 


Emissions occur only during emergency situations (i.e., where


electric power from the local utility is interrupted), and for a


very short time to perform maintenance checks and operator


training. 

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The EPA believes that generators devoted to emergency uses


are clearly constrained in their operation, in the sense that, by


definition and design, they are used only during periods where


electric power from public utilities is unavailable. Two factors


indicate that this constraint is in fact "inherent." First,


while the combined period for such power outages during any one


year will vary somewhat, an upper bound can be estimated which


would never be expected to be exceeded absent extraordinary


circumstances. Second, the duration of these outages are


entirely beyond the control of the source, and when they do occur


(except in the case of a major catastrophe) rarely last more than


a day.


For emergency generators, EPA has determined that a


reasonable and realistic "worst-case" estimate of the number of


hours that power would be expected to be unavailable from the


local utility may be considered in identifying the "maximum


capacity" of such generators for the purpose of estimating their


PTE. Consequently, EPA does not recommend the use of 8760 hours


per year (i.e., full-year operation) for calculating the PTE for


emergency generators. Instead, EPA recommends that the potential


to emit be determined based upon an estimate of the maximum


amount of hours the generator could operate, taking into account


(1) the number of hours power would be expected to be unavailable


and (2) the number of hours for maintenance activities.


The EPA believes that 500 hours is an appropriate default


assumption for estimating the number of hours that an emergency


generator could be expected to operate under worst-case


conditions. Alternative estimates can be made on a case-by-case


basis where justified by the source owner or permitting authority


(for example, if historical data on local power outages indicate


that a larger or smaller number would be appropriate). Using the


500 hour default assumption, EPA has performed a number of


calculations for some typically-sized emergency generators. 


These calculations indicate that these generators, in and of


themselves, rarely emit at major source levels. (Of course,


there may be unusual circumstances where these calculations would


not be representative, for example where many generators are


present that could operate simultaneously).


Cautions


Today's guidance is only meant to address emergency


generators as described. Specifically, the guidance does not


address: (1) peaking units at electric utilities; (2) generators


at industrial facilities that typically operate at low rates, but


are not confined to emergency purposes; and (3) any standby

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generator that is used during time periods when power is


available from the utility. This guidance is also not intended


to discourage permitting authorities from establishing


operational limitations in construction permits when such


limitations are deemed appropriate or necessary. Additionally,


this memorandum is not intended to be used as the basis to


rescind any such restrictions already in place.


Distribution/Further Information


The Regional Offices should send this memorandum to States


within their jurisdiction. Questions concerning specific issues


and cases should be directed to the appropriate Regional Office. 


Regional Office staff may contact Tim Smith of the Integrated


Implementation Group at 919-541-4718. The document is also


available on the technology transfer network (TTN) bulletin


board, under "Clean Air Act" - "Title V" - "Policy Guidance


Memos". (Readers unfamiliar with this bulletin board may obtain


access by calling the TTN help line at 919-541-5384).


cc: 	Air Branch Chief, Region I-X


Regional Air Counsels, Region I-X 


Adan Schwartz (2344)


Tim Smith (MD-12)