ENGINEERING EXPLAINED / GATE
A gate is for isolation.
Move the wedge, compare valve characteristics, and see why a smaller opening is not the same as controlled throttling.
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01 / MOVE THE WEDGE
The opening changes.
The jet changes with it.
At half lift, an ideal straight gate exposes a semicircle: half the bore area. The thin crescent appears nearer closed. Real wedge, seat and body geometry changes the opening. The reel compresses these positions into one explanation; this drawing separates them.
A partly open gate produces a narrow jet and can expose the seat and disc to erosion and vibration. It is generally intended for isolation. Local speed is Q/A for a stated flow, but actual flow also changes with pressure drop, piping and valve geometry. The velocity readout holds Q constant only to show the area effect; it is not a valve sizing result.
02 / FLOW VERSUS TRAVEL
Compare the characteristics.
These are illustrative inherent characteristics at constant pressure drop, not measured curves for the pictured gate. A 90%-flow-at-50%-travel shape is one quick-opening example. Spirax Sarco illustrates other quick-opening proportions; the exact curve depends on trim. A globe valve can use different trim characteristics.
Equal percentage: Q/Qmax = 50x−1
Here x is normalised travel over a rated range and 50:1 is the chosen rangeability example. Equal increments multiply flow by the same factor at constant pressure drop. The plotted low end is 2% flow; shutoff and leakage are separate from the rated characteristic. Installed pressure drop changes the installed response.
03 / CHOOSE FOR THE DUTY
The valve type is only the start.
| Valve | Typical duty | What to check |
|---|---|---|
| Gate | Isolation | Full travel; erosion and vibration if throttled |
| Globe | Modulating control | Trim, characteristic, pressure drop, cavitation and noise |
| Ball | Isolation; characterised designs also control | Port geometry, seats and control-rated construction |
| Butterfly | Isolation or control, with suitable design | Operating range, torque, leakage and cavitation limits |
A P&ID symbol identifies the valve function and type but does not replace the datasheet. A simulator's coefficient and opening can predict a flow without establishing acceptable wear, materials, vibration or service life.
- Emerson: Control Valve Handbook, valve types and application considerations.
- Spirax Sarco: inherent and installed valve characteristics. The area calculation uses a straight edge crossing a circular bore; the quick-opening curve is an explicit teaching model.