Seat Erosion and Wire Drawing
Wire drawing is a cutting operation carried out by the process itself. It happens whenever a valve is used partly open on a high-differential service.
Seat Erosion and Wire Drawing
Wire drawing is a cutting operation carried out by the process itself. It happens whenever a valve is used partly open on a high-differential service.
What it looks like on site
A valve that begins to pass after being used for throttling, often with a visible groove across the seat or disc.
The causes, in order of likelihood
- Throttling with the wrong valve type. A gate or ball valve part-open on a high differential produces a high-velocity jet across the seat.
- Solids in the stream. Particles carried in the jet act as an abrasive and accelerate the cutting.
- Flash steam or cavitation. The velocity at the seat is far higher than the nominal line velocity.
- Undersized valve. A small valve working at a large pressure drop is the classic wire drawing condition.
- Soft seat on an erosive duty. A polymer seat cut by a high-velocity jet fails faster than a metal seat under the same conditions.
Why wire drawing is a specification problem
The damage is not a manufacturing defect. It is the predictable result of asking a valve designed for isolation to regulate flow at a high differential pressure, and the seat is simply the place where the energy is concentrated.
That is why the answer is usually a change of valve type rather than a change of seat material. A globe or control valve with a characterised trim handles the throttling case without exposing a seat to a jet.
Recognising it
Wire drawing shows as a narrow groove or a bright cut across the seat, aligned with the flow path, and it usually starts at the point where the jet first leaves the seat. It appears on one side of the valve rather than uniformly around the circumference.
By contrast, erosion from solids in a fully open valve tends to be more evenly distributed, and cavitation damage looks rough and pitted rather than cleanly cut.
Specifying against it
Where throttling is unavoidable, specify a valve designed for it: a globe or control valve with an appropriate trim, and hard facing where the medium is abrasive. Where a throttled isolation valve must be used, hard-faced seats and discs last considerably longer than soft ones.
The other lever is the operating regime. Throttling at a smaller differential, or staging the reduction across two valves, reduces the velocity at each seat and spreads the damage.
Damage Patterns
| Item | Detail | Note |
|---|---|---|
| Narrow groove aligned with flow | Wire drawing from throttling | Change the valve type or hard-face the trim |
| Even erosion around the seat | Solids at velocity | Hard facing, and review the medium |
| Rough, pitted surface | Cavitation | Anti-cavitation trim |
| Soft seat cut and displaced | High-velocity jet on a polymer | Metal seat for that duty |
| Damage on one side only | Jet direction | Confirm the preferred flow direction |
The checks, in order
Confirm the operating regime
How often is the valve operated, and is it ever left part-open?
Establish the differential
The velocity at the seat follows from the pressure drop, not from the line velocity.
Inspect the seat pattern
A groove aligned with the flow points to wire drawing.
Check the medium for solids
Abrasion accelerates every erosion mechanism.
Review the valve type against the duty
Throttling duty belongs to a valve designed for it.
From the works and the projects
Questions this page answers
The trim can be replaced, and hard-faced trim lasts far longer under the same conditions. The underlying cause remains until the valve type or the operating regime changes.
A larger valve at the same pressure drop works at a lower velocity, which reduces the rate of damage. It does not change the fact that the valve is being used to throttle.
A globe or control valve with a trim characterised for the duty. Gate and ball valves are isolation valves and their geometry does not support sustained throttling.