Seven Steps for Boiler Blowdown

While most people think boiler water is pure, it’s actually chemically treated in a manner similar to tap water. If you were to take a pot, fill it with water, and boil it, you may find a black or brown residue at the bottom. These are impurities that couldn’t be boiled away with the water.

The purpose of blowdown is to control the number of solids and sludge in the boiler water. The blowdown process involves partially draining the boiler to remove sludge and to maintain predetermined concentration levels of solids.

As the water is turned into steam, the solids remain behind. Unless there is 100% condensate return, the solid content tends to build up when the boiler takes on make-up water. On hot water systems, there is generally no make-up water. Therefore, the solid concentration remains constant and no blowdown is needed.

The frequency of blowdown differs from each boiler application and should be determined by your water management consultant. Blowdown is affected by the type of boiler, operating pressure, water treatment and the amount and quality of make-up water.

Blowdown piping should be at least the same size as the blowdown tapping on the boiler. Blowdown valves should be sized in accordance with the ASME Code and piped to a safe point of discharge.

There should be either two slow-opening valves or one quick-opening valve and one slow-opening valve piped to a series. A slow opening valve is defined as needing five complete 360° turns to go from fully closed to fully open. A quick-opening valve goes from fully closed to fully open in one complete motion and should be located closest to the boiler. If possible, both blowdown valves should be piped on the same side of the boiler as the water column gauge glass.

Steps to blowdown the boiler:

Steps to blowdown the boiler:

1. Open the quick-opening valve (closest to the boiler) first
2. Open the slow-opening valve last
3. Blowdown the boiler for the required amount of time, per your water management consultant, by opening and then closing the slow opening valve. Important: pay close attention to the water level in the gauge glass. Certain loads may require several blowdown cycles of shot duration to maintain proper water level in the boiler. 
4. Close the slow-opening valve first
5. Close the quick opening valve last (the valve closest to the boiler – front and rear of boiler) * Repeat steps 1- 5 for both the front quick opening valve and then the rear quick opening valve 
6. Open the slow opening valve, again, to drain the line between the quick and slow opening valve
7. Close the slow opening valve again, and double-check for tight shutoff after the valve has cooled-off

NEVER pump the quick opening valve to blowdown the boiler! This may cause water hammer, which could damage piping and valves and may cause personal injury. Also, NEVER leave an open blowdown valve unattended!

Remember: The quick opening valve (the valve closest to the boiler) is opened first and closed last, which ensures its protection from wear associated with blowdown. This will make this valve more reliable, so maintenance and repair can be performed on the slow-opening valve furthest from the boiler, without draining the boiler.

If you have questions regarding the bottom boiler blowdown procedure, contact Thermal Tech Inc.