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          Texas Natural Resource Conservation Commission
                      Interoffice Memorandum

To: Permit Engineers
Date: February 16, 1994
From: James E. Crocker, P.E., Manager, Combustion Section
Subject: Determining Maximum Rated Engine Horsepower for Standard Exemption No. 6

Standard Exemption No. 6 (SE 6) sets emission limits for engines "rated" 500 horsepower (hp)
or greater in paragraph (b)(1).  Applicants requesting registration under SE 6 have represented
several methods of derating engines to below 500 hp to avoid meeting the emission limits of
(b)(1).  The acceptable method of determining (b)(1) applicability is detailed in this memo.  

There are two parameters acceptable for use in determining the maximum "rated" hp of an
engine registered under SE 6:  the engine's maximum published continuous speed and the driven
equipment's maximum published continuous speed.  The engine manufacturer's maximum
continuous load (horsepower) rating at the lesser of these two speeds is the maximum "rated" hp
for the purposes of SE 6.  This value of hp shall dictate the applicability of the emission limits
stated in (b)(1) of SE 6.

Manufacturer's data may be obtained to demonstrate a loss in hp due to elevation.  For example,
an engine that achieves 525 hp at maximum speed when operated at sea level may only achieve
475 hp at the installed elevation.  The manufacturer may have hp ratings at various speeds for
various elevations.  This data may be used to derate the engine to below 500 hp due to elevation. 
However, if this data cannot be obtained, "rated" hp must be determined based on the data
available.  In addition, if the maximum speed of the driven equipment is less than that of the
engine and data is not available on hp at reduced speed, "rated" hp must be determined based on
the engine's nameplate hp.

Representations by an applicant to limit engine hp to below 500, by whatever means, do not
exempt the unit from the emission limits of SE 6 (b)(1).  The applicability of SE 6 (b)(1) can
only be determined using the maximum "rated" hp as defined above.  If the maximum "rated" hp
is 500 or greater, the emission limits of SE 6 (b)(1) apply.