Valve types
| Type | Motion | Typical use |
|---|---|---|
| Globe | Linear | The classic control valve: precise throttling, high ΔP, many trim options |
| Segmented / V-ball | Rotary | Control of slurries, fibrous media, wide rangeability |
| Butterfly (high-performance) | Rotary | Large lines, low ΔP, cost-effective control and isolation |
| Ball (full bore) | Rotary | Tight on/off isolation, ESD valves |
| Plug | Rotary | On/off, some throttling, dirty services |
| Gate | Linear | Manual isolation only; never throttle |
| Check | Self-acting | Prevent reverse flow |
| Needle | Linear | Instrument root valves, fine manual adjustment |
Flow characteristics
The inherent characteristic is the relationship between travel and flow at constant ΔP:
- Linear: flow proportional to travel. Used where the valve takes most of the system pressure drop, and often for level control.
- Equal percentage: each increment of travel changes the flow by the same percentage of the current flow. It is the most common choice, because it compensates for the falling ΔP across the valve as flow increases (the installed characteristic becomes closer to linear).
- Quick opening: most of the flow in the first part of the travel. Used for on/off and relief duties.
Aim for the valve to operate between roughly 20 % and 80 % open across the normal operating range. A valve that runs at 5 % open is oversized and will control badly.
Actuators and fail-safe action
Pneumatic spring-and-diaphragm actuators are the most common. Piston, electric and electro-hydraulic actuators are used where force, speed or air availability demand. The spring decides what happens on loss of air or signal:
- Fail Closed (FC) / air-to-open: e.g. fuel gas, feed to a reactor.
- Fail Open (FO) / air-to-close: e.g. cooling water, compressor anti-surge, pressure relief to flare.
- Fail Last / Fail In Place (FL): double-acting actuators with lock-up valves.
Positioners
A positioner compares the controller signal (4–20 mA) with the actual stem position and adjusts the air to the actuator until they match. It overcomes friction, packing load and process forces. Digital (smart) positioners add auto-calibration, valve signatures, friction trends and partial-stroke testing, which are very valuable for predictive maintenance.
Cv sizing basics
The flow coefficient Cv is the US gpm of water at 60 °F that flows through the valve with a 1 psi pressure drop. For a non-flashing liquid:
Cv = Q · √(SG / ΔP) Q in US gpm, ΔP in psi
Kv = 0.865 · Cv Kv in m³/h at 1 bar
Gas and steam sizing must check for choked (critical) flow, roughly when ΔP reaches half of P1 (absolute). Get a first-pass Cv with the Control Valve Cv calculator; final sizing should follow IEC 60534-2-1 or the vendor's sizing software.
Cavitation and flashing
As liquid accelerates through the valve's vena contracta, its pressure dips. If it falls below the vapour pressure, bubbles form:
- Cavitation: the pressure recovers downstream and the bubbles collapse violently. You get noise like gravel in the pipe, vibration, and pitting of the trim and body.
- Flashing: the downstream pressure stays below the vapour pressure, so the liquid remains partly vapour. This causes erosion with a smooth, polished look and high outlet velocity.
Remedies include anti-cavitation (multi-stage) trims, hardened trim materials, splitting the pressure drop across valves or restriction orifices, and angle-body valves discharging into a larger outlet for flashing.
Seat leakage classes (ANSI/FCI 70-2)
| Class | Allowed leakage | Typical |
|---|---|---|
| II | 0.5 % of rated capacity | Double-seated globe |
| III | 0.1 % of rated capacity | Balanced trim, metal seat |
| IV | 0.01 % of rated capacity | Common standard for metal-seated control valves |
| V | Specified ml/min per inch of port diameter per psi | Tight shut-off, metal seat |
| VI | Bubbles per minute (by port size) | Soft seats: "bubble tight" |
On/off and safety valves
- ESD / on-off valves: usually ball or butterfly valves with solenoid valves, limit switches and fast-acting actuators. Stroke time, fail action and partial-stroke testing are key for SIS duty.
- Pressure relief / safety valves (PSV): self-acting last line of defence. They are sized to API 520 and specified to API 526, with set pressure, blowdown and backpressure checks. They are tested and re-certified at regular intervals.