Valve Basics - What Is a Valve? Five Functional Classes and Six Nameplate Parameters Explained
From the shutoff valve on a household water line to metre-tall valves on petrochemical piping, every line that carries fluid needs valves — they decide whether the fluid passes, how much flows, and in which direction. This article skips the product talk and covers the fundamentals: what a valve actually does, how valves are classified, and what the letters and numbers on the nameplate mean.
What a valve does
In one sentence: a valve is the switch and regulator of a pipeline. Open it and media flows through; close it and the flow stops; change the travel and the flow rate follows. Beyond that, valves can prevent backflow, divert flow paths, and relieve overpressure automatically. Whether a process runs safely and steadily depends, to a large degree, on these unassuming components doing their job.
2. Classification by function: five classes
| Class | Core function | Typical valves |
|---|---|---|
| Isolation | Fully open or fully closed to isolate the media | Gate, ball, butterfly, plug |
| Regulating | Regulate flow and pressure | Globe, control valve, needle |
| Non-return | Prevent reverse flow | Check valve, foot valve |
| Diverting | Change or split the flow path | 3-way ball valve, 4-way changeover |
| Safety | Automatic pressure relief | Safety valve, relief valve |
One valve can serve more than one role: ball valves are primarily isolating, while V-port ball valves add throttling capability.
3. Reading the nameplate: six key parameters
The first step in choosing a valve is reading its nameplate. Six parameters decide whether a valve can serve on a given line:
| Parameter | Meaning | Typical notation |
|---|---|---|
| Nominal size DN | Nominal pipe size, decides interface matching | DN50, DN300 |
| Pressure class | Pressure rating the valve is built for | PN16 (EN), Class 150 (ASME) |
| Body material | Sets temperature, corrosion and strength limits | WCB carbon steel, CF8 (304), CF8M (316), ductile iron |
| End connection | How it joins the pipeline | Flanged, wafer, threaded, butt-weld, grooved |
| Actuation | How it is operated | Handwheel/lever, gear, electric, pneumatic |
| Temperature rating | Allowed media temperature range | Set by body material and soft seals combined |
Note: PN and Class belong to two different standard systems — the numbers cannot be converted directly; always check the temperature-pressure ratings in the applicable standard. For any given class, the maximum allowable working pressure also falls as temperature rises.
4. The "personality" of common valve types
Gate valve: a wedge that rises perpendicular to the seat; for open/shut duty. Low pressure drop, but long travel — not for throttling or frequent cycling.
Globe valve: the disc moves along the seat centreline; reliable sealing and coarse flow adjustment, at a higher pressure drop.
Ball valve: a bored ball turns 90° to open or close — fast, low pressure drop, and full-bore versions barely obstruct the flow.
Butterfly valve: a disc rotates 90° on a shaft; compact and light, especially economical on large-diameter lines.
Check valve: opens and closes by the flow itself, allowing one direction only and protecting pumps and equipment from backflow surge.
Diaphragm valve: a flexible diaphragm isolates the media from the body interior — suited to corrosive and hygienic services.
Safety valve: opens automatically above its set pressure — the last line of defence for the system.
5. The first question of valve selection
Many selection mistakes trace back to questions never asked. Before picking any valve, answer three things: what flows through it (media, temperature), how big is the line (size, pressure class), and who operates it (manual or automatic; on/off or modulating). With those three answers, the range of candidate types narrows quickly.
A valve is a small component carrying heavy responsibility. Get the basics clear, and reading drawings, nameplates, and supplier conversations all become easier. Treat this article as the entry index of our Knowledge column — we will expand each valve family in detail in future posts.