Notation of Pipe size is important for designing and indication of pipe to be used during project execution. The selection of pipe size majorly depends on Flow rate requirement, the pressure of the piping system, and end connection equipment. For transferring the exact idea from the piping design team to the piping execution team for using the correct pipe size for spool fabrication, identification of Pipe size is important.
What is Nominal Pipe Size (NPS) ?
Nominal pipe size or NPS is an American standard of pipe size used for different piping systems. This measures and indicate pipe size in “INCH”. Nominal pipe size refers only to the outside diameter, i.e. OD of the pipe. For Example: When someone says 6 NPS, this refers to a pipe having a 6.625-inch outer diameter of the pipe. Here according to wall thickness can be anything as per application design and hence internal diameter is not fixed.
Specific pipes are identified by two non-dimensional numbers, pipe diameter and wall thickness measured in Schedule (SCH). The pipe schedule sets the dimension for pipe wall thickness. An increase in pipe wall thickness results in a decrease in the Inner Diameter (ID) of the pipe as the outer diameter (OD) of the pipe is always fixed. Increase pipe wall thickness also increases the mechanical strength of the pipe to handle its higher design pressure.
What is Nominal diameter (DN) ?
The nominal diameter is also known as “mean or average outside diameter” and it is represented by DN. Diameter Nominal (DN) or Nominal diameter is used in the metric unit system. The nominal diameter is neither equal to the outer diameter (OD) of the Inner diameter (ID) of the pipe. The word “Nominal”, is the self denotation of non-specificity and approximate. In this case, DN identifies the approximate inner diameter.
The value of DN is close to the inner diameter of the pipe but not equal to it. This notation is adopted in order to indicate the connecting dimensions of pipe and pipe fittings. Pipes are available in many DN sizes and the DN is used to finalize the pipe dimensions with standard tables and pipe schedule charts.
The actual Inner diameter of pipe from different manufacturers can deviate by several millimeters and such type of pipe can be weld together if DN is indicated with reference standards. To convert NPS into DN, We need to multiply this value of NPS by 25, not 25.4.
What is Nominal Bore (NB) ?
Nominal bore or NB is a European standard for Pipe size indication. In the case of pipes, Bore means a hollow structure and nominal is for approximate. The nominal bore is actually the approximate internal measurement of the pipe across its diameter. In other words, a Nominal bore refers to the approximate size of the bore of the pipe.
When a pipe is measured in inches, 250NB pipe turns out to be exactly 10.75 inches i.e. 10.75″ x 25.4 = 273.05mm. This is the reason why outside diameter is not that simple number like 250NB. There is some standard that must be followed by manufacturers to select outside diameter of the pipe.
In the case of a 3-inch pipe, its nominal bore will not be 75mm but it will be denoted with 80NB.
NPS vs NB vs DN: Is there any difference?
Yet many features are similar of NPS, NB, and DN but there is few obvious difference that we should keep in mind during using these notations:
- NPS is American Standard of pipe size notation while NB is European and DN is standard from metric unit system.
- NPS gives information about related outer diameter of pipe, NB provides information related to internal diameter of pipe and In case of DN its niether Outer or Internal diameter but close to internal diameter of pipe.
- NPS indicates dimension of pipe in “Inches“, where NB and DN provide same in “mm“.
- Neither NB, NPS or DN provides exact information about pipe size. Consideration for standard shedule chart for pipes need to have to find out pipe internal and outer diameter.
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