Memorandum

Date: January 28, 2010

To: Brenda Shine, EPA OAQPS

From: Matt Todd, API

cc: Jan Tierney, EPA

API Air Toxics Task Force

Subject: Cooling Tower Monitoring Issues

Identifying Leaking Heat Exchangers in Organic HAP Service

Under §63.654(d) of subpart CC, when a leak is indicated by the monthly monitoring, sources 

have 45 days to reduce the leak to below the leak definition. Since the Clean Air Act requires 

that part 63 rules address HAPs, §63.654(e) of subpart CC provides a procedure for 

demonstrating that the leak is not from an exchanger in organic HAP service and thus not subject 

to the repair requirements. It specifies:

(e) If the owner or operator detects a leak when monitoring a cooling tower return line 

under paragraph (c)(1) of this section, the owner or operator may conduct additional 

monitoring to identify leaks of total strippable VOC emissions using Modified El Paso 

Method from each heat exchanger or group of heat exchangers in organic HAP service 

associated with the heat exchange system for which the leak was detected. If the 

additional monitoring shows that the total strippable VOC concentration in the stripped 

air at the heat exchanger exit line for each heat exchanger in organic HAP service is less 

than 6.2 ppmv for existing sources or less than 3.1 ppmv for new sources, the heat 

exchange system is excluded from repair requirements in paragraph (d) of this section.

Thus, the only allowed procedure for demonstrating that a particular leak is not from an 

exchanger in organic HAP service is to perform modified El Paso monitoring at every individual 

or group of heat exchangers in organic HAP service associated with that heat exchange system, 

even including heat exchangers that are on different return lines than the one where the leak was 

found. This is not how leaking exchangers are identified today and thus does not represent the 

MACT floor and will result in a great many wasteful El Paso analyses and impose significant 

costs. Furthermore, this approach would appear to be required even if the source of the leak is 

identified by other means as not being from a heat exchanger in organic HAP service and would 

have to be repeated monthly until the non-HAP leak is repaired. 

Typically, when a leak is found sources perform a gas chromatographic analysis of the stripper 

offgas or of headspace gas collected from a return water sample. This analysis generally narrows 
the potential leak source to a few heat exchangers. While there is always some background level 

of chemicals, the fingerprint of the leaking material almost always stands out because those 

components are present at many times the background level. Using this information a source can 

usually identify the leak source with one or just a few confirming El Paso analyses. Since 

§63.655(i)(4)(i) and (ii) require that sources identify all heat exchangers in organic HAP service 

in each heat exchange system, it only takes identifying the leaking heat exchanger to determine if 

the leak is from an exchanger in organic HAP service or not. Thus, sources should be allowed to 

use whatever means they have to identify the leaking exchanger, confirm that conclusion with an 

El Paso analysis at the leaking exchanger, and then use their recorded process information to 

determine if the exchanger is in organic HAP service and thus subject to the repair requirements 

or not.

Monthly Monitoring

Issue: The new §63.654(c) of Subpart CC requires “The owner or operator must perform 

monthly monitoring to identify leaks of total strippable volatile organic compound (VOC) from 

each heat exchange system...” Does “monthly” mean any time within a calendar month or 

something else?

Discussion: Neither the preamble or the Response to Comments document addresses this issue. 

Nor is this issue addressed in the Part 63 General Provisions. However, there is considerable 

precedent dealing with this question.

Monthly requirements are prevalent throughout Subpart CC and have always been interpreted as 

referring to calendar months. For example, the requirement for “weekly” visual monitoring and 

“monthly” Method 21 monitoring of pump seals has always been interpreted to mean that pumps 

must be monitored once within each calendar week and month, respectively. 

The specific language in Part 63 Subparts F and H of the HON rule have generally been accepted 

as the basis for interpreting terms such as monthly under the refining and chemicals rules, unless 

a subpart specifically specifies a different interpretation. §63.100(k)(9) states:

(9) All terms in this subpart F or subpart G of this part that define a period of time for 

completion of required tasks (e.g., weekly, monthly, quarterly, annual), unless specified 

otherwise in the section or subsection that imposes the requirement, refer to the standard 

calendar periods. 

(i) Notwithstanding time periods specified in this subpart F or subpart G of this part for 

completion of required tasks, such time periods may be changed by mutual agreement 

between the owner or operator and the Administrator, as specified in subpart A of this 

part (e.g., a period could begin on the compliance date or another date, rather than on 

the first day of the standard calendar period). For each time period that is changed by 

agreement, the revised period shall remain in effect until it is changed. A new request is 

not necessary for each recurring period. 

(ii) Where the period specified for compliance is a standard calendar period, if the 

initial compliance date occurs after the beginning of the period, compliance shall be 
required according to the schedule specified in paragraphs (k)(9)(ii)(A) or (k)(9)(ii)(B) 

of this section, as appropriate. 

(A) Compliance shall be required before the end of the standard calendar period within 

which the compliance deadline occurs, if there remain at least 3 days for tasks that must 

be performed weekly, at least 2 weeks for tasks that must be performed monthly, at 

least 1 month for tasks that must be performed each quarter, or at least 3 months for 

tasks that must be performed annually; or 

(B) In all other cases, compliance shall be required before the end of the first full 

standard calendar period after the period within which the initial compliance deadline 

occurs. 

(iii) In all instances where a provision of this subpart F or subpart G of this part requires 

completion of a task during each of multiple successive periods, an owner or operator 

may perform the required task at any time during the specified period, provided the task 

is conducted at a reasonable interval after completion of the task during the previous 

period.

EPA explained the basis for this provision at 61 FR 43702 (August 20, 1996) as follows:

3. Clarification of Compliance Periods

The proposed revisions to subpart F also would add a new paragraph (k)(9) to § 63.100, 

and a new paragraph (g) to § 63.162 to clarify that when the rule specifies a period of 

time for completion of required tasks (e.g., weekly, monthly, quarterly, annual), this 

refers to standard calendar periods unless it is specified otherwise in the section or

paragraph that imposes the requirement. The current rule does not specify this, and this 

text is being added to the rule to remove any potential for ambiguity. The new 

§63.100(k)(9) and § 63.162(g) also provide that time periods may be changed by mutual 

agreement between the owner or operator and the Administrator, as provided in subpart 

A of this part. Finally, this new set of provisions also provides that if the rule requires 

completion of a task during each of multiple successive periods, an owner or operator 

may perform the required task at any time during the specified period, provided the task 

is conducted at a reasonable interval after completion of the previous task. When the 

rule was originally drafted it was assumed that this could be done, but an oversight in 

drafting language specifying this was omitted from the rule.

It should also be noted that Subpart H of the HON includes essentially the same language at 

§63.162(g) and that Subpart H is a specified compliance option for equipment leaks under 

Subpart CC. Therefore, this language already specifically applies to a portion of Subpart CC.

Any other interpretation of the term “monthly” causes major problems because 1) it would be 

inconsistent with the MACT floor determination, which was based on monitoring the Heat 

Exchanger System twelve times per year, as done under Texas rules, not monitoring 13 or more 

times as would result from other interpretations, 2) the compliance date for the next monitoring 

would change after each monitoring, making it difficult for sources and inspectors to determine 

if compliance was timely, and 3) would interfere with efficient scheduling of resources to do the 

monitoring, because the monitoring would be required at different times each month.
Reading the term “monthly” to mean anything other than calendar month in this case would also 

raise serious questions about due process, since the rulemaking did not provide for notice and 

comment on any other interpretation than the generally accepted one, and would raise questions 

about compliance with all the other provisions in Subpart CC where this term is used (e.g. 

equipment leaks, wastewater) and where similar terms, such as daily1, quarterly and annually are 

used. 

 

1

§63.1(a)(10) specifies “For the purposes of this part, time periods specified in days shall be measured in calendar days, even if 

the word “calendar” is absent, unless otherwise specified in an applicable requirement.”