The thresholds are not arbitrary; they align with shipbuilding practice and classification society requirements. Small-bore valves (≤ DN 40) predominantly serve instrumentation lines, sampling points, tank level indicators, and auxiliary drain circuits, where compact dimensions and fine flow regulation are essential. Medium-bore valves (DN 50–300) form the backbone of shipboard systems such as ballast, bilge, fire‑fighting, fresh water, and fuel oil service lines, delivering a balance between flow capacity and installation flexibility. Large-bore valves (DN 350–1200) are found in main seawater cooling overboard lines, cargo pumping systems, and ballast mains on medium‑ to large‑sized vessels, demanding robust construction and reliable actuation under significant static and dynamic loads. Extra-large-bore valves (≥ DN 1400) are reserved for highly specialised applications—most notably on liquefied natural gas (LNG) carriers, ultra‑large container ships, floating production units, and offshore platforms—where extreme volumetric flow rates must be handled safely.

Notably, the minor gap between DN 1200 and DN 1400 reflects the fact that DN 1300 is seldom specified in marine piping systems, as it lies outside standard manufacturing series and offers little practical advantage. Adopting this clear-cut, size‑based classification streamlines valve selection, ensures compliance with class rules, and facilitates the integration of piping systems from initial design through outfitting and commissioning.