DN identifies the nominal valve size, while PN identifies the nominal pressure-rating system. The two markings describe different properties of a valve. DN relates to component size and connection compatibility; PN relates to pressure rating.
These markings are useful, but neither is a complete specification. DN is not necessarily the valve’s exact inside or outside diameter. PN does not guarantee the same allowable working pressure for every material and operating temperature.
What DN Means on a Valve
DN means nominal diameter or, more strictly, a nominal size designation. It is used to match valves, pipes, flanges, fittings, strainers and other piping components within the same size system.
Under ISO 6708, DN is a reference designation rather than a directly measured dimension. The number following DN is related to component size, but it does not define every physical dimension of the valve.
DN is not the same as the actual bore
In this drawing, DN250 identifies the nominal size. The same row separately lists L, D, D1, D2, flange thickness and bolt-hole dimensions. This shows that DN250 does not mean every valve dimension is 250 mm. The actual face-to-face and connection dimensions must be read from the drawing and applicable dimensional standard.
The actual flow opening depends on the valve type and construction. Full-bore and reduced-bore ball valves can carry the same DN designation while having different bore diameters. A butterfly valve, globe valve or control valve of the same nominal size can also have a different internal flow area.
Seat design, disc position, trim construction, pipe wall thickness and end-connection standard can all affect the opening available to the fluid. DN therefore identifies the size family, not the exact flow passage.
A ball-valve general arrangement drawing illustrates this difference clearly. DN appears as the nominal size entry, while separate columns specify face-to-face length, flange diameter, bolt circle, bore-related dimensions, flange thickness and bolt-hole details. Those dimensions must be read from the drawing and the applicable dimensional standard.
What DN helps you check
DN helps establish the basic connection size when comparing a valve with its pipe, flange or fitting. It must still be checked together with the connection type and standard, such as flanged, threaded, socket-weld, butt-weld, wafer or lug.
Matching DN markings do not automatically confirm flange drilling, face-to-face length, bore design or connection compatibility. Two valves can have the same DN designation but belong to different dimensional standards or product series.
DN is also not a flow-sizing result. This matters particularly for control valves, where the required Cv or Kv, pressure drop, flow characteristic, trim design, cavitation risk and noise limits may lead to a valve body size that differs from the line size.
When an inch-based specification is involved, DN may be compared with NPS as an initial size reference. The final connection should still be checked against pipe outside diameter, flange standard and project drawings. A metric and imperial conversion table can assist with the first comparison, but it does not replace dimensional verification.
What PN Means and Why It Is Not a Fixed Working Pressure
PN means nominal pressure. It is a numerical reference designation used with many metric valves, flanges, fittings and other piping components. The number following PN identifies the pressure-rating series; it does not describe the valve’s size.
ISO 7268 and its 1984 amendment address PN designations for piping components. PN provides a common rating reference, but it does not state the allowable working pressure for every valve and service condition.
How to read PN16, PN25 and PN40
PN16, PN25 and PN40 belong to different nominal pressure series. In common valve specifications, the numbers are associated with pressure ratings expressed in bar under defined reference conditions. They help match valves with compatible flanges and piping components within the applicable standard system.
The PN number should not be treated as an unconditional continuous working-pressure limit. A valve that meets a rating at a reference temperature may have a lower allowable pressure at an elevated temperature.
Body material also changes the pressure-temperature rating. Carbon steel, stainless steel and alloy steel do not necessarily retain the same allowable pressure as temperature rises. Seat, seal and packing materials impose additional limits. PTFE, EPDM, graphite and metal seats behave differently in water, steam, hot oil, gas and corrosive media.
How the drawings help explain PN
A useful comparison is to hold the nominal size constant while examining two drawings with different PN designations. In the globe-valve drawings, the nominal size remains separate from the pressure rating, while pressure-test data and pressure-boundary dimensions change with the valve design.
This comparison shows that PN identifies the pressure-rating system rather than valve size. It also shows why a PN designation cannot be converted into a complete set of valve dimensions or operating limits. The values shown on each drawing apply to that particular design and should not be treated as universal values for every valve carrying the same PN mark.
PN is different from test pressure and allowable working pressure
A valve drawing or datasheet may list shell-test and seat-test pressures above the nominal rating. These values apply to defined factory tests. They are not continuous operating-pressure recommendations.
The actual allowable working pressure must be checked against the governing valve standard, body material and operating temperature. Seat, seal and packing limits may impose a lower service limit than the pressure-retaining body.
PN also does not convert directly to ANSI Class by comparing the numbers. The two systems use different pressure-temperature tables, material groups and dimensional standards. The ANSI Class vs PN guide can provide an initial comparison, but the applicable valve and flange standards must determine final compatibility.