RATA Load Analysis Errors and How to Avoid Them

Load Analysis Errors are usually discovered after a RATA test has been conducted and can lead to hundreds of hours of invalid data. To avoid this error at your facility, learn about some of our best practices to consider when planning your next RATA test.
RATA Load Analysis

Relative Accuracy Test Audit (RATA) Refresher

Relative Accuracy Test Audit, or “RATA,” is a test method required by Title 40 of the Code of Federal Regulations (CFR). A RATA test must be conducted once every four calendar quarters to determine the accuracy of an installed CEMS and/or Flow Monitoring system. Read more about the Anatomy of a RATA on Monitoring Solutions blog post.


What is Load Analysis?

The load analysis defines the percentage of operating time in each of the three load ranges – low, mid, and high. The load range with the highest percentage of operating hours is considered the “normal” load. “Normal” is the load range that the unit must operate in when the gas RATA (Relative Accuracy Test Audit) is performed.

According to 40 CFR Part 75 Appendix A, for a common stack/duct, the three different exhaust gas velocities may be obtained from frequently used unit/load or operating level combinations for the units exhausting the common stack.


Why Does Load Analysis Matter?

Suppose the unit cannot operate in the “normal” load range on the day of the actual RATA for economic reasons. In that case, the unit can legally operate in the load range with the second-highest percentage of operating hours. The second operation range is known as the “next normal” load, and the gas RATA can legally be performed at any operating level within the “next normal” range.


The ESC Spectrum Reporting Team has recently encountered higher-than-normal situations where customers did not perform RATAs at the appropriate operating levels. Unfortunately, this type of error is usually discovered after the RATA test has been conducted and can lead to hundreds of hours of invalid data.


How to Avoid Load Analysis Errors

To avoid this error at your facility, here are some best practices to consider when planning your next RATAs:

  • Operating Level for Part 60: For Part 60 systems, it can vary somewhat from state to state, but in general, RATAs are conducted at 90% or greater of the full operating level. Some exceptions are allowed, but generally, the reason for not testing at 90% or greater must be justified and documented.
  • Operating Level for Part 75: Under Part 75, the normal operating range for an operating source is divided into three separate ranges. The bottom of the operating range is defined as the lowest, safe, and stable load that a unit can operate. The top of the operating range is the maximum sustainable operating range. The low range is defined as the first thirty (30) percent of the operating range. The mid-range is also defined as the next thirty (30) percent, or from 31% to 60% of the operating range. Finally, the high range is the remaining forty (40) percent, or from 61% to 100% of the operating range.
  • Over the past 5 to 10 years, the way electrical generating units are dispatched has changed. Generating units that previously were operating at the high load range are now being operated more frequently at the mod or even the low load ranges. Due to these changing load dispatch profiles, a load range screening needs to be performed on operating data since the last RATA to define the acceptable load ranges for the next round of RATAs.
  • RATAs for gas monitoring systems, such as NOx or SO2 ppm, O2/CO2 percent and NOx emission rate (lb.MMBtu) systems are typically conducted at their “normal load,” which is the load range (low, mid, high) that has the highest percentage of operating hours. This normal load selection must be defined in the “load” record as part of the Monitoring Plan records. A “second normal load” can also be defined in the “load” record, and if the second normal operating range is defined, the gas RATA can actually be performed at either of the two “normal loads.”

Final Thoughts on RATA Load Analysis

Operating Level Errors for Part 60 and Part 75 RATA tests are the focus of this blog post. However, we provide many other resources for all things RATA Testing. To understand a Relative Accuracy Test Audit (RATA) and how to prepare for one, read How to Plan for a RATA test. To learn more about the activities required for 40 CFR Part 60/63 and 40 CFR Part 75 reporting, read Air Quality Compliance Reporting 101.  


Part 75 Policy Manual Question 8.30 (Load Analysis) references Section 6.5.2 of Appendix A (Flow Monitor RATAs – Special Considerations) and describes performing the load analysis in greater detail than what is provided above. Use these resources in planning and conducting your load analysis.


How ESC Spectrum Can Help with RATA Testing

ESC Spectrum has performed air emissions Stack Testing since 1991. We understand that making informed decisions when it comes to emissions is crucial to operations. Our team of experts is well-versed in Environmental Protection Agency test methodologies, Relative Accuracy Test Audit (RATA) testing, and CEMS operations. Our stack testing team uses RATAView, a stack testing software they developed to create workflows and automate testing runs.

Discover Upcoming Events

Upcoming Events at ESC Spectrum
Skip to content