Surface and Bottom Blowdown Valves: Differences and Automatic Blowdown Systems

As water evaporates in a steam boiler, the substances that come in with the feed water stay behind and accumulate. Some remain dissolved in the water (TDS – Total Dissolved Solids), while others precipitate and form sludge at the bottom of the boiler. Surface and bottom blowdown valves remove these two different types of build-up and can now be operated fully automatically.

This article explains why blowdown is necessary, the difference between surface and bottom blowdown and the components of the automatic blowdown systems supplied by ORFA.

What Is Blowdown and Why Is It Needed?

Blowdown is the controlled discharge of part of the boiler water so that it is replaced by cleaner feed water. Without blowdown:

• Dissolved salt concentration rises; foaming and carryover begin, and the steam becomes wet and contaminated.
• Salts carried into the condensate raise its conductivity and damage valves, steam traps and heat exchangers.
• Settled sludge impairs heat transfer, increases the risk of local overheating and corrosion, and shortens boiler life.

Surface vs Bottom Blowdown

For bottom blowdown it is important that the valve opens quickly and briefly: the sudden flow scours the sludge from the bottom of the boiler. Keeping the valve open longer does not remove more sludge; it only wastes water and energy.

Automatic Surface Blowdown System Components

• Conductivity probe (detector)
• Transmitter
• Blowdown control valve
• PID conductivity control panel

The system measures boiler water conductivity continuously and opens the control valve when the set point is exceeded, blowing down only as much as necessary. Optionally, a plate heat exchanger and circulation pump can be added to recover heat from the blowdown water.

Automatic Bottom Blowdown System Components

• Timer (digital time relay)
• Full stainless steel 3-piece ball valve
• Pneumatic actuator
• Solenoid valve

At set intervals the timer signals the solenoid valve, which supplies air to the pneumatic actuator so that the ball valve opens and closes quickly for a short time.

Recovering Heat from Blowdown

Water removed by surface blowdown is at boiler pressure and saturation temperature. Its heat can be used to preheat boiler feed water or make-up water through a plate heat exchanger. The achievable saving depends on the blowdown rate, boiler pressure and feed water temperature and should be calculated for each plant.

Benefits

• Protects steam quality and prevents foaming and carryover.
• Reduces sludge build-up, protecting the boiler and extending its life.
• Avoids unnecessary blowdown, saving water, chemicals and fuel.
• Reduces the need for manual blowdown and saves labour.
• Can be adapted to different types of steam boilers.

Frequently Asked Questions

Conclusion

Surface and bottom blowdown valves automatically remove dissolved salts and sludge from steam boilers, protecting steam quality, extending boiler life and preventing unnecessary water and energy losses. For automatic blowdown systems, see our Boiler Surface and Bottom Blowdown Systems page.