Apr 4

Generator and Engine Exhaust Stack Requirements

Updated: Apr 15

The emissions discharge requirements for Emergency Generators and Emergency Turbines in Massachusetts are commonly misunderstood. The requirements for Generator Stacks are listed in several codes, NFPA 37,4.1.3.1 and 4.1.4, International Mechanical Code 2015 and Ninth Edition of the Massachusetts State Building Codes CMR 310 and CMR 780. At the time of writing this, Massachusetts has not officially adopted the tenth edition, nor has it adopted IMC 2021 code.

Within these codes there are specific requirements for the discharge stack location and height above adjacent structure based on the engine KW output rating, indoor vs outdoor locations and fuel type. State code 320 CMR 7 references engine power output, which is different than the generator KW output.

With all these requirements is there any wonder why there is confusion and misunderstandings?

The distance requirements spelled out in the codes could be blindly followed however they there the overarching language found when applying the codes for hazardous exhausts. ASHRAE delves further into re-entrainment issues. The goal is to keep people safe from harmful fumes.

Breaking down the code references:

Massachusetts State Building Code, CMR 310 7.26 (42-3) Emergency Engines and Emergency Turbines. Section 3 states the following:

3. Stack Height and Emission Dispersion.

a. All engines or turbines shall utilize an exhaust stack that discharges so as to not cause a condition of air pollution (310 CMR 7.01(1)).

i. Exhaust stacks shall be configured to discharge the combustion gases vertically and shall not be equipped with any part or device that impedes the vertical exhaust flow of the emitted combustion gases.

ii. Any emission impacts of exhaust stacks upon sensitive receptors including, but not limited to, people, windows and doors that open, and building fresh air intakes shall be minimized by employing good air pollution control engineering practices. Such practices include without limitation:

(i) Avoiding locations that may be subject to downwash of the exhaust; and

(ii) Installing a stack of sufficient height in locations that will prevent and minimize flue gas impacts upon sensitive receptors.

b. An engine or turbine with a rated power output equal to or greater than 300 kW, shall have an exhaust stack with a minimum stack height of ten feet above the facility rooftop or the emergency engine or turbine enclosure, whichever is lower.

c. An engine with a rated power output equal to or greater than one MW shall be equipped with a exhaust stack with a minimum stack height of 1.5 times the height of the building on which the stack is located. If the stack is lower than 1.5 times the building height or lower than the height of a structure that is within 5L of the stack (5L being five times the lesser of the height or maximum projected width of the structure), the owner/operator shall submit documentation that the operation of the engine or turbine will not cause an exceedance of any National Ambient Air Quality Standard.

5. Visible Emissions. Engines and turbines shall comply with all the requirements of 310 CMR 7.06(1)(a) and (b)

International Mechanical Code IMC 2015

This section 915.1 of IMC provides specific minimum clearance requirements.

At least 10 feet from property lines,
3 feet from exterior walls and roofs,
10 feet from operable openings into buildings
10 feet above adjoining grade.

IMC references 2015 Edition of NFPA, Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines for the installation of liquid fueled stationary internal combustion engines and gas turbines. The requirements in NFPA 37 requires generators located on roofs or outdoors to be at least 5 feet from openings in walls or from structures having combustible exterior walls unless a 1-hour rated enclosure is provided (NFPA 37, 4.1.3.1 & 4.1.4)

In addition to the specific generator exhaust requirements, exhaust locations has to follow the same rules as all hazardous exhaust (ASHREA CLASS 4 AIR) as outlined in 2015 International Mechanical Code (IMC) Section 501.3, as referenced by 780 CMR 28.00.

The code requires that any air removed by a mechanical exhaust system be discharged outdoors at a point where it will not cause a public nuisance and will not be readily drawn in by a ventilating system (re-entrainment) in the subject building or any other building. Additionally, the IMC provides requirements for minimum exhaust outlet clearances including at least 10 feet from property lines, 3 feet from exterior walls and roofs, 10 feet from operable openings into buildings, and 10 feet above adjoining grade. The design engineers should also look at a wind rose to determine which way the prevailing winds are from. This will impact the location and height of the stack.

This section of IMC is widely thought of being dedicated to fan driven equipment and not engine driven exhaust, however the intent of the code is to avoid re-entrainment of the air, regardless of the source.

If the generator is serving a hospital, compliance with ASHRAE 170 is recommended, which changes the distance from the source to outside intakes. For this application the generator exhaust should be 25 feet away from any intake.

For engines or turbines with more than 1MW of power output, the code states “exhaust stack with a minimum stack height of 1.5 times the height of the building on which the stack is located”. This can result in stacks that could exceed 100 feet or more. Installing a free-standing stack in downtown Boston is unrealistic. To address this issue there have been considerations and on-going discussions within the Board of Building Regulation and Standards (BBRS) and with the AHJ about changing the 1MW exhaust requirements to the same as the smaller generators. The code has not been changed but there is a different pathway to reduce the stack heights.

The code provides an option to perform a wind dispersion study to determine the height of the stack. A dispersion study will generate a 3 D model of the surrounding area topographic and through computational fluid dynamics (CFD) analysis determine how the effluent will react. In areas where there are numerous tall buildings it is difficult to determine how the wind will react. The modeler can raise to lower the stack height and provide guidance on the final height.

Case Study

Strictly following the distances outlined in the codes can lead to problems if the overarching fundamental intent of avoiding re-entrainment is not met.

On a recent project, the owner wanted to install a diesel fired generator on grade between three tall buildings. The engine output was for rated for more than 1 MW of power, so the governing code stated the exhaust stack had to be 1.5 times the height of the tallest building within the 1.5L distance. Applying the code would require the stack to be 30 stories tall. The building served by the generator was the lowest building of the three adjacent buildings, however following the distance requirements and the building had operable windows. The prevailing winds were blowing towards the building with operable windows which became more of a concern as the prevailing winds were creating a down force on the effluent that would create issues. We determined that a dispersion study was required to inform the design team of the best location on the building roof and the height requirements. Based on the study's results, the design team relocated the roof stack towards the front of the building and extended it to 18 feet tall.

Considerations were given to reducing the generator KW rating to be less than 1MW, however the lower stack height was still creating re-entrainment issues and a study was required.