The term nominal diameter is used with different meanings in different parts of pipeline code. Be careful you understand the meaning for the code section involved. For example, in the PIR calculation in Part 192.903 nominal diameter refers to the nominal pipe size where outside diameter is used for NPS 14 inch and above and inside diameter is used from NPS 12 inch through 1/8 inch. See attached diagram. From a discussion with the creator of the PIR formula please note:
- One intent of the PIR formula was easy calculation knowing only the MAOP and the outside diameter. The result is a slight over specification of the effective hole diameter for 14-inch diameter and above pipe. This is conservatism and is simple.
- For pipes 12-inch and less, the conservatism goes away unless you explicitly use the external diameter but is acceptable because effective hole diameter is roughly equivalent to the inside diameter
- For plastic pipe, where wall thickness can be quite thick, if you want to avoid a gross over-estimation, you can use the internal diameter. This is still in the spirit of the referenced report developing the PIR.
Another example is 195.11 below. Note in the interpretation that line pipe in incorporated by reference (IBR) standard API 5L, nominal diameter is defined as outside diameter.
- 195.11 What is a regulated rural gathering line and what requirements apply?
Each operator of a regulated rural gathering line, as defined in paragraph (a) of this section, must comply with the safety requirements described in paragraph (b) of this section.
(a) Definition. As used in this section, a regulated rural gathering line means an onshore gathering line in a rural area that meets all of the following criteria—
(1) Has a nominal diameter from 65⁄8 inches (168 mm) to 85⁄8 inches (219.1 mm);
Interpretation Response #PI-21-0008
Below is the interpretation response detail and a list of regulations sections applicable to this response.
Interpretation Response Details
Response Publish Date: 09/01/2021
Company Name: Audubon Field Services
Individual Name: Mr. Bryan Louque
Location State: OK Country: US
View the Interpretation Document
Response text:
Mr. Bryan Louque
Vice President Pipeline Integrity
Audubon Field Services
9920 E. 42nd Street
Tulsa, OK 74146
Dear Mr. Louque:
In a letter to the Pipeline and Hazardous Materials Safety Administration (PHMSA), dated March 10, 2021, you requested an interpretation of the 49 Code of Federal Regulation (CFR) Part 195. Specifically, you requested an interpretation on the term “nominal diameter” as it relates to the requirements of § 195.11(a).
You asked for clarification on the size of pipelines intended for inclusion in § 195.11 and that PHMSA consider adding a definition to § 195.3.
The nominal pipe size (NPS) is related to the inside diameter in inches, and NPS 12 and smaller pipe has outside diameter greater than the designated size. Therefore, a 6 inches nominal diameter pipe is a 6 ⅝-inches nominal outside diameter pipe and an 8-inches nominal diameter pipe is an 8 ⅝-inches nominal outside diameter pipe. However, for NPS 14 and larger, the NPS is the same as the nominal outside diameter. That is, a 14-inch nominal diameter pipe is the same as a 14-inch nominal outside diameter pipe. API Specification 5L, Specification for Line Pipe, 45th Edition incorporated by reference in 49 CFR § 195.3(b)(13) (API 5L) uses “specified outside diameter” for the nominal outside diameter of pipe. Sections 195.2 (gathering line) and 195.11(a) reference terms “nominal diameter” or “nominal outside diameter” for pipe, with both references being the same nominal outside diameter for pipe that is referenced in API 5L. In addition, the Research and Special Programs Administration (RSPA) (the predecessor to PHMSA) clarified and addressed this issue in a 1994 final rule.1 Also, it should be clear that outside diameters are furnished by pipe mills in nominal outside diameters that are not in whole numbers for less than or equal to 12-inches pipe diameters. For example, pipe with an outside diameter of 2 ⅜-inches is used by pipe mills for a nominal outside diameter of 2-inches. Other examples include 8 ⅝-inches instead of 8 inches, and 10 ¾-inches instead of 10-inches.
Therefore, PHMSA has addressed this issue by regulation as explained by the June 28, 1994, final rule which added using both 6⅝-inches and 8⅝-inches pipe nominal diameters under the § 195.11(a) requirements as pipe outside diameters.
If we can be of further assistance, please contact Tewabe Asebe at 202-366-5523.
Sincerely,
John A. Gale
Director, Office of Standards
and Rulemaking
1 RSPA published the final rule in the Federal Register on Tuesday, June 28, 1994 [59 FR 33388].
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