Check Valve Slam and Water Hammer
Slam is not a noisy check valve. It is a pressure transient, and the noise is only the part you can hear.
Check Valve Slam and Water Hammer
Slam is not a noisy check valve. It is a pressure transient, and the noise is only the part you can hear.
What it looks like on site
A pressure spike and a bang when a pump stops or a valve closes, often followed by pipe movement and damage to gaskets or supports.
The causes, in order of likelihood
- Closure too slow for the transient. The disc is still travelling when reverse flow arrives, so a large reverse velocity has to be stopped by the seat.
- Undersized or oversized line. Low flow velocity keeps a swing disc hovering, and it has further to travel when the flow reverses.
- Air in the line. Entrained air collapses and amplifies the transient.
- Fast-closing control valve downstream. An unrelated valve closing quickly can generate the transient that the check valve then amplifies.
Why the closure type matters more than the brand
The distance the closure travels before it seats is the main variable. A dual-plate design has a short travel and closes on spring assistance; a large swing disc has a long arc and relies on the flow itself to shut it.
That is why the fix for a slam problem is so often a change of closure type rather than a change of supplier. The valve was doing what its geometry allows.
Confirming it is slam
Slam shows a characteristic signature: a pressure spike immediately after a pump stop, a bang synchronised with the event, and often evidence at the supports and at nearby gaskets rather than at the check valve itself.
A surge analysis is the way to confirm it. The analysis also quantifies the fix, which is useful because it compares the cost of a different closure against the cost of a surge vessel or a different pump control sequence.
What actually works
Faster closure: dual-plate or axial designs, or a non-slam design where the transient is severe. Better pump control: a controlled run-down so the flow decelerates gradually. And removing air, because entrained air makes every transient worse.
Transient Controls
| Item | Detail | Note |
|---|---|---|
| Faster closure | Dual-plate or axial designs | Shortens the travel before seating |
| Purpose-built non-slam design | Severe transients | Designed for the case rather than adapted to it |
| Controlled pump run-down | Longer deceleration | Reduces the reverse velocity that has to be stopped |
| Air removal | Air valves at high points | Entrained air amplifies the transient |
| Surge vessel | Where closure speed cannot be increased further | Higher cost, but it decouples the transient from the valve |
The checks, in order
Confirm the event is a pump stop
Slam is normally synchronised with a trip, a valve closure or a power failure.
Check the check valve type and travel
A long-travel swing disc on a large bore is the common profile.
Look for damage away from the valve
Gasket weeping, support distress and pipe movement indicate a transient rather than a valve fault.
Check for air
High points without air valves, and a line that has been drained and refilled, are candidates.
Commission a surge analysis
It identifies the fix and prices the alternatives.
From the works and the projects
Questions this page answers
It is a pressure transient, so it can damage gaskets, supports and connected equipment. The noise is the least of the problems.
Usually not. A larger valve has a longer travel and often a lower velocity, both of which make closure slower rather than faster.
Sometimes, by controlling how the pump runs down or by removing air from the line. Where the transient is severe, the closure type is usually the item that has to change.