Port Placement for Stacks >12" in Diameter

It is very important that sample ports be available on the day of the source test. Most air districts will not accept sampling over the top of a stack, as such measurements may not be representative of pollution emissions or total volumetric flow rate from an emission source. If sample ports already exist, please check on their condition to see if they can be opened easily. A welding torch may be required to loosen ports that have excessive rust.

Sample ports should be installed in accordance to EPA Method 1, which is summarized below:

• Look for the longest straight sections of stack or duct that is also accessible. Ports should be located 8 stack diameters downstream and 2 diameters upstream (8 and 2 criteria) from any flow disturbance, such as bends, expansions, contractions, junctions or dampers.
• If there is not 10 diameters of straight run to place ports, then ports can be placed 80% of the straight run distance downstream or 20% upstream from any flow disturbance, as long as there is a minimum of at least 2 diameters downstream and 0.5 diameters upstream.
• Two ports is required for circular stacks, placed 90° apart in the same plane. For stacks greater than 5-feet in diameter, four ports are recommended (see figure).
• Ports should be installed flush with the inside wall of the stack and a minimum of 3" Inside Diameter (I.D.) for particulate sampling, 5" I.D. (Method 201A) for PM 10 testing, 4" I.D. (Method 501) for particle sizing, and 1-1/2" for Continuous Emissions Monitoring (CEM) and velocity traverses. A simple drill hole is often sufficent for non-particulate testing.
• There should be a minimum of 1.5 stack diameters clearance behind the port for inserting and removing probes.
• For rectangular stacks, determine the port location using the calculated equivalent diameter using the formula De = (2*L*W) / (L+W). e.g. A rectangular stack that measures 30" x 45" has an equivalent diameter of 36".

The table below calculates the placement of ports as a fraction of the rectangular stack width.