Boiler Water Hammer
Water hammer in boiler systems causes noise, pipe damage, and potential rupture. Learn what causes it, how to prevent it, and what to do when it occurs.
What Causes Water Hammer in Boiler Systems?
Water hammer in boiler systems is caused by condensate accumulating in steam lines and being picked up by high-velocity steam flow. The mixture of steam and water slams into pipe bends, valves, and fittings, creating a loud banging noise and potentially damaging forces.
The root cause is almost always inadequate condensate drainage: failed steam traps, missing drip legs, or improper pipe slope.
Common Scenarios
- Cold startup: When steam enters cold piping, it condenses rapidly. If drip legs and traps cannot handle the warm-up condensate load, water accumulates and is driven by steam flow.
- Failed steam trap: A trap that has failed closed (not draining) allows condensate to back up into the steam line. The next steam flow picks up the water column.
- Missing drip legs: Without drip legs at regular intervals, condensate cannot drain from the steam line. It accumulates until steam flow picks it up.
- Incorrect pipe slope: Steam pipes should slope in the direction of flow. Reverse slope causes condensate to pool against the flow.
- Oversized PRV: An oversized pressure reducing valve can cause steam to flow in slugs rather than steadily, picking up condensate.
Prevention
- Install drip legs every 30 to 50 metres along the steam main, at all low points, and at the base of every riser.
- Ensure correctly sized and functioning steam traps on all drip legs. Survey annually.
- Slope steam pipes in the direction of flow at 1:100 minimum.
- Warm up steam lines slowly during startup. Open the bypass valve first to introduce steam gradually.
- Never open a steam valve into a cold, undrained line. Drain condensate first.
- Size PRVs correctly for the load. Oversized valves cause hunting and unstable flow.
Water hammer can rupture pipes
The forces generated by water hammer can be enormous. A slug of water driven by steam at 10 bar can generate impact forces exceeding 10 tonnes on a pipe bend. Repeated water hammer will eventually crack or rupture pipes and fittings. If you hear water hammer, investigate and fix it immediately.
Tip: If water hammer occurs during startup, reduce steam flow immediately. This stops the hammering. Then slowly warm the line using the bypass valve, ensuring drip leg traps are discharging condensate before increasing flow.