Pipe size calculation is necessary for planning and identifying the pipe to be utilized during project execution. The size of the pipe is determined primarily by
- the flow rate required,
- the pressure of the piping system, and
- the end connection equipment.
Identification of pipe size is vital for transmitting the exact idea from the piping design team to the piping execution team for using the correct pipe size for spool production. Pipe size also has a significant impact on cost. Oversizing a pipe result in additional costs, more complex pipe design, more footing, and, in some cases, process difficulties.
In the field of engineering, there are many and varied materials & types of pipes used, each with their own unique credentials, and often with their own sizing. This results in some confusion in the industry around the outside diameter of piping, and the sizing of required pipe supports. That is why understanding to pipe sizing is very important.
Pipe size in referred in various terms like – Nominal Diameter (DN), Nominal Pipe Size (NPS), or Nominal Bore (NB). However, despite having certain similarities and variances in terms of their uses and places of origin, these are all notations for pipe sizes.
Pipe size is specified with two non-dimensional numbers,
- Nominal Pipe Size (NPS)
- Schedule Number (SCH)
And the relationship between these numbers determines the inside diameter of a pipe.
- Nominal pipe size and its importance
The Nominal Pipe Size (NPS) is a North American standard size for pipes that are suitable for high or low pressures and temperatures. It defines the diameter of pipe. Nominal pipe size only refers to the outside diameter (OD) of a pipe. When it is said that pipe size is 2 NPS, it indicates any pipes with an outer diameter of 2.375-inch (or 60.3 mm), regardless of wall thickness and hence internal diameter.
Nominal pipe sizing
Nominal Bore (NB) and Nominal Diameter (DN) are sometimes used alternately with Nominal Pipe Size (NPS). Nominal Bore (NB) is the European equivalent for NPS. The Nominal Diameter (DN) for NPS 5 and larger is equal to the NPS multiplied by 25.
Pipe size illustration
2.1.1 Nominal diameter (DN): The Nominal Diameter (DN), is frequently referred to as “mean or average outer diameter.” The metric unit system uses nominal diameter, sometimes known as Diameter Nominal (DN). The pipe’s Inner Diameter (ID) and Outer Diameter (OD) are not equal to the nominal diameter.
Although not exactly equal, the value of DN is close to the inner diameter of the pipe. The connecting dimensions of pipes and pipe fittings are denoted using this notation. Pipes come in a variety of DN sizes, and using standard tables and pipe schedule charts, the DN is used to determine the pipe’s final dimensions.
NPS vs DN measurement
2.1.2 Nominal Bore (NB): A European standard for indicating pipe size is Nominal Bore, or NB. In the case of pipes, the terms bore, and nominal refer to hollow structures, respectively. The nominal bore is a rough internal measurement across the diameter of the pipe. In other terms, a nominal bore refers to the pipe’s bore’s approximation in size.
A pipe is exactly 10.75 inches long when measured in inches; hence, 10.75′′ x 25.4 = 273.05 mm. This is the reason why the outside diameter is not a straightforward number like 250NB.
Pipe Size Indication
What is schedule Number?
The wall thickness of the pipe is described using steel pipe schedules. This crucial variable as it is directly related to the strength of the pipe and the suitability for specific applications. Given the design pressure and permitted stress, a pipe schedule is a dimensionless number which is derived using the design formula for wall thickness.
Pipe wall thickness is determined by pipe schedule. The mechanical strength of the pipe increases as the wall thickness, allowing it to withstand higher design pressures.
Schedule number = P/S
- P is the service pressure in (psi)
- S is the allowable stress in (psi)
Two pipes of the same diameter with different schedules will have different wall thicknesses. So, when specifying a pipe for a high-pressure application, a larger number signifies a larger schedule (wall thickness).
Pipe schedule illustration
The schedule numbers 40 and 80 are the most popular ones. As the schedule number increases, the wall thickness of the pipe increases. Since there has been advancement of industrial age, pipe sizes and their standard has also been changed. There is wide range of wall thicknesses: SCH 5, 5S, 10, 10S, 20, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160, Standard (STD), Extra Strong (XS) AND Double Extra Strong (XXS).
Steel pipe schedules are a way to specify the pipe’s wall thickness. This is an important metric since it is directly related to the pipe’s strength and applicability for various applications. A pipe schedule is a dimensionless number that is calculated using the design formula for wall thickness and the permissible stress.
Pipe sizing is one of the first important actions a process engineer performs throughout the P&ID preparation process. Pipe size is a key consideration in a well-designed process. It will have an impact on fluid velocity, pressure drop, flow regime, and so on. A poorly sized pipe can disrupt the entire process and, in extreme situations, lead to plant shutdown That is why one should always know about the differences between Nominal pipe sizes and pipe schedule numbers.