EXPERT TIP #2: PIPES – HOW TO TAKE AN ACCURATE PIPE MEASUREMENT (ID or OD)?
As in any industry, accurate communication is key to minimizing errors in system designs and material supply.
Take pipes. Cathodic protection (CP) systems are designed for many different types and sizes of pipe. In the design, it is common to specify certain types and sizes of pipefittings, such as an insulated pipe union, in order to produce an effective and efficient CP system. To accurately measure and order pipes and accessories, it is important to follow ANSI Pipe Schedule Standards.
Pipe Measurement Miscommunication Example
The following is an example of typical miscommunications when pipe measurements are required:
Conversation 1:
Customer calls the supplier to order insulated pipe unions. Supplier confirms stock and asks for size. Customer ‘measures’ the pipe and orders 2” unions
Conversation 2: One week later
Customer calls the supplier to report that the fittings are too large.
What Happened?
In this case, the customer measured the outside diameter of the pipe and ordered based on a 2” pipe. The actual outside diameter (OD) of 2” pipe is 2.375” while the inside diameter (ID) is more likely 2”. The customer measuring the pipe was thinking actual inch values instead of the actual ANSI nominal pipe size. It is difficult to accurately measure a round pipe by holding a tape measure across it. 2” would be very close to the actual OD of a 1 ½” (nominal) pipe size, not a 2” pipe. Therefore, 2” nominal fittings would be too large for the pipe measured.
This issue occurs in many industries and can result in project problems.
How to Measure
The best way to measure the OD of a pipeline is to wrap a measuring tape around the pipe to measure the pipe’s circumference. Then, divide the circumference by 3.1416 to obtain the actual OD of the pipe.
Once you have that number, you must look up the “nominal” pipe size on the ANSI Pipe Schedule Table (below) to determine the actual ANSI pipe size.
History of Pipe Sizing
Pipe sizes can be confusing because the terminology used today relates to historical dimensions.
Around 1930, the pipe industry standardized sizes for pipes. At the time, all pipes had an ID (inside diameter) that was the same as the designated pipe size, i.e., a 2” pipe actually had an ID of 2”. Yet, the OD (outside diameter) was 2.375” for this pipe. Given the metallurgy technology of the time, this was the dimension required for a standard low-pressure pipe.
As time progressed, there was a need for higher-pressure pipe. Rather than increase the OD, the decision was made to maintain the same OD and increase the wall thickness, which reduced the ID. With this scheme, all the threaded fittings of a given size pipe would be compatible. In time, the metallurgy improved, allowing manufacturers to build pipes with thinner wall thickness and still retain the pressure rating. The consequences of the evolution in pipe technology resulted in pipe sizes that are now a name rather than a descriptive term. In fact, the term "nominal", which is the current way of defining pipe sizes, means "in name only."
Today, the OD is fixed for a given pipe size and the ID will vary depending on the wall thickness of the pipe. Therefore, a higher-pressure rated pipe will have a heaver wall thickness, represented by Schedule Rating. The higher the Schedule, the heavier the wall thickness and pressure rating of the pipe. For example, a 2" Schedule 80 pipe has a thicker wall and therefore, a smaller inside diameter as compared to a 2" Schedule 40 pipe. Interestingly, all of the above confusion is non-existent in pipes that are 14” and above.
ANSI (America National Standards Institute) Pipe Schedule Table
