To distinguish which pipe it is, one must consider several factors such as material, size, shape, and application. Pipes are categorized based on their composition, which can be metal, plastic, or composite materials. Size and shape are determined by the pipe's internal diameter and wall thickness, respectively. Additionally, the application of the pipe, whether it's for water supply, gas, or drainage, can also help in identification.

In the United States, the most common types of pipes are:

1. Metal Pipes: These include steel, copper, and cast iron pipes. Steel pipes are often used for water supply and gas lines due to their durability and strength. Copper pipes are commonly used for plumbing and are known for their corrosion resistance. Cast iron pipes are typically used for drainage systems due to their longevity and resistance to corrosion.

2. Plastic Pipes: Plastic pipes are widely used due to their lightweight, corrosion resistance, and ease of installation. Polyvinyl Chloride (PVC) pipes are common for water supply and drainage systems. Polyethylene (PE) pipes are used for gas lines and are known for their flexibility and resistance to chemicals.

3. Composite Pipes: Composite pipes combine materials like steel and plastic to offer the benefits of both. These pipes are used in various applications, including water supply and gas transmission.

To identify a pipe, follow these steps:

1. Examine the Material: Look for any visible markings or labels on the pipe. These can indicate the material type. For instance, "Steel" or "PVC" may be written on the pipe.

2. Measure the Diameter: Use a caliper or ruler to measure the internal diameter of the pipe. This will help determine the size of the pipe, which is crucial for understanding its intended use.

3. Check the Shape: Pipes can have different shapes, such as round, oval, or square. The shape can provide clues about the pipe's application.

4. Consider the Application: The intended use of the pipe can also help in identification. For example, a pipe used for water supply will likely have different specifications than one used for gas transmission.

According to the American Society of Mechanical Engineers (ASME), the correct identification of pipes is essential for ensuring safety and compliance with regulations. Incorrect pipe identification can lead to leaks, failures, and even accidents. For instance, a study by the U.S. Department of Transportation found that improper pipe identification was a contributing factor in 15% of pipeline accidents between 2000 and 2010.

In conclusion, distinguishing which pipe it is involves examining the material, measuring the diameter, checking the shape, and considering the application. Proper identification is crucial for safety and regulatory compliance.