How to Blow Down a Boiler
Boiler blowdown controls dissolved solids and prevents scaling and carryover. Learn the difference between bottom and surface blowdown, and the correct procedure for each.
Why Blow Down a Boiler?
As water boils, it leaves dissolved solids behind. Over time, these solids (measured as Total Dissolved Solids, or TDS) concentrate in the boiler water. If TDS gets too high, it causes foaming, priming, carryover, and scale formation — all of which reduce boiler efficiency and can damage the boiler.
Blowdown is the controlled discharge of concentrated boiler water and its replacement with fresh feedwater. This dilutes the TDS and keeps it within the acceptable range.
Two Types of Blowdown
| Type | Purpose | Frequency |
|---|---|---|
| Bottom blowdown | Removes sludge and sediment that settles at the bottom of the boiler | Once per shift or daily |
| Surface (continuous) blowdown | Removes dissolved solids concentrated at the water surface | Continuous, regulated by TDS controller |
TDS target
Most fire-tube boilers operate at 3000–3500 ppm TDS. Water-tube boilers may have lower limits. Always follow the boiler manufacturer's specification and your water treatment consultant's recommendations.
Safety Precautions
Blowdown is dangerous
Boiler water is at saturation temperature (100–200°C+). A sudden discharge of blowdown generates flash steam. Never stand in front of a blowdown valve. Ensure the blowdown discharge pipe terminates in a safe location (blowdown tank or cooling pit). Always wear full PPE.
Bottom Blowdown Procedure
Bottom blowdown removes sludge from the lowest point of the boiler. It is done in short, sharp bursts to lift sediment — not in a long, slow drain.
- Check that the boiler is firing at a low rate (if possible) — blowdown is more effective when the boiler is at low load because the water is less agitated and solids settle.
- Verify the blowdown discharge pipe and blowdown tank are clear and not blocked.
- Ensure no one is working near the blowdown discharge point.
- Open the quick-opening valve (the one closest to the boiler) fully.
- Slowly open the slow-opening valve (the one further from the boiler) to start the blowdown.
- After 3–5 seconds (or as per your water treatment program), close the slow-opening valve.
- Close the quick-opening valve.
- Repeat steps 4–7 two or three times — the short bursts are more effective at lifting sludge than one long blowdown.
- Allow the blowdown to cool in the blowdown tank before discharge to drain.
Water level
Monitor the boiler water level gauge during and after blowdown. If the level drops below the low-water mark, stop immediately and do not resume until the level is restored. Never blow down a boiler below its minimum safe water level.
Surface (Continuous) Blowdown
Surface blowdown is done continuously through a skimmer pipe that draws water from just below the normal water level, where dissolved solids concentrate. The flow rate is controlled either manually or automatically by a TDS controller.
- Verify the TDS reading from the boiler water sample or online TDS meter.
- If TDS is above target, open the surface blowdown regulating valve slightly.
- Take a water sample after 15–30 minutes and check TDS again.
- Adjust the valve until TDS stabilises within the target range.
- If fitted with an automatic TDS controller, verify it is functioning and the probe is clean.
- Log the blowdown rate and TDS reading in the boiler log.
Tip: The ideal blowdown rate is the minimum needed to keep TDS within specification. Excessive blowdown wastes heated water and fuel. A 1% reduction in blowdown can save 0.5% in fuel costs.
After Blowdown
- Verify the boiler water level is normal — top up with feedwater if needed.
- Take a boiler water sample and test for TDS, pH, alkalinity, and hardness per your water treatment program.
- Record the blowdown (time, type, duration, and TDS reading) in the boiler logbook.
- Check the blowdown valve for leaks — a leaking blowdown valve wastes water and energy continuously.