What Is Boiler Blowdown?
Boiler blowdown is the controlled removal of water from a steam boiler to manage water quality. As steam leaves the boiler, dissolved and suspended solids remain, concentrating in the boiler water over time. Without blowdown, high TDS (Total Dissolved Solids) causes scale, foaming, carry-over into steam, and boiler corrosion. Two distinct blowdown methods are used — intermittent (bottom) blowdown and continuous (surface) blowdown — each serving a different purpose.
PureSys India supplies GESTRA blowdown valves and systems for both types. Visit our boiler equipment page or GESTRA brand page for the full range.
Intermittent Blowdown (Bottom Blowdown)
Purpose
Intermittent blowdown removes suspended solids, sludge, and sediment that settle at the lowest point of the boiler shell. These non-dissolved materials cannot be controlled by surface blowdown — they must be physically expelled from the bottom of the boiler.
How It Works
A large-bore blowdown valve at the bottom of the boiler is opened fully for a short duration — typically 10–30 seconds — once or twice per shift. The high internal pressure of the boiler expels the sludge-laden water rapidly into a blowdown vessel or pit.
Valve Type Used
- Quick-opening blow-off valves: Designed for rapid full-bore opening to create the high-velocity purge needed to dislodge and remove sludge
- Slow-opening safety valves (in series): A slow-opening valve is often installed in series upstream of the quick-opening valve to prevent water hammer and pressure shock during blowdown
- GESTRA bottom blowdown valves — forged steel, hardened stainless trim, rated PN40 to PN160
Characteristics
- Periodic/manual operation — not continuous
- High-volume, short-duration discharge
- Results in fluctuating (“sawtooth”) TDS levels between blowdowns
- Less suitable for heat recovery due to intermittent nature
- Essential for removing physical debris — cannot be replaced by surface blowdown
Continuous Blowdown (Surface Blowdown)
Purpose
Continuous blowdown removes dissolved solids (TDS) from the boiler water surface. As steam evaporates, dissolved impurities remain and concentrate — continuous blowdown removes a steady, small stream of this concentrated water to maintain TDS within prescribed limits.
How It Works
A small-bore connection at the boiler waterline draws off a continuous, metered flow of boiler water. This is typically automated via a conductivity sensor and TDS controller — the blowdown rate adjusts automatically to maintain the target TDS setpoint.
Valve Type Used
- Needle/vernier valves: Fine-threaded valves that allow precise, adjustable flow control
- Motorised TDS control valves: Automated valves that modulate in response to a conductivity controller signal
- GESTRA continuous blowdown systems include conductivity sensors, TDS controllers, and motorised control valves working together
Characteristics
- Continuous, low-volume, steady flow
- Maintains stable, consistent TDS level
- Ideal for integration with blowdown heat recovery (flash vessel + heat exchanger)
- Can be fully automated — reduces operator attention required
- Cannot remove sludge or sediment — must be used alongside bottom blowdown
Side-by-Side Comparison
| Feature | Intermittent (Bottom) | Continuous (Surface) |
|---|---|---|
| Primary purpose | Remove sludge & sediment | Control TDS level |
| Extraction point | Bottom of boiler shell | Boiler waterline / steam drum |
| Operation | Periodic — manual or timer | Constant — automated |
| Volume | High — short burst | Low — steady trickle |
| TDS control | Poor — sawtooth fluctuation | Excellent — stable |
| Heat recovery | Difficult | Efficient — flash vessel |
| Valve type | Quick-opening blow-off valve | Needle/modulating control valve |
| Automation | Manual or timer | Conductivity-controlled |
Do You Need Both?
Yes — most industrial boiler installations require both systems:
- Continuous blowdown handles TDS management automatically and efficiently
- Intermittent blowdown provides periodic sludge removal that continuous blowdown cannot achieve
Running both systems together provides the most complete boiler water management, protects equipment, maintains steam quality, and complies with boiler safety regulations.
Blowdown Heat Recovery
Continuous blowdown water leaves the boiler at boiler pressure and temperature — it carries significant energy. A flash vessel recovers low-pressure flash steam for feedwater preheating, while the cooled blowdown passes through a heat exchanger to pre-heat make-up water. This system can recover up to 80% of the blowdown heat energy.
PureSys India Blowdown Solutions
We supply GESTRA bottom blowdown valves, continuous blowdown control systems, TDS controllers, conductivity sensors, and blowdown heat recovery vessels. Contact our engineers for a complete blowdown system design or visit our boiler equipment range.