Bulletin 1006: Proper Boiler Blowdown Procedure


Feb 09, 2016

Columbus, OH –

Why do I need to blow down my boiler?:

Only pure water leaves the boiler as steam is produced; any dissolved solids are left behind. Even though the condensate is pumped back into the boiler, some is inevitably lost and must be replaced with make-up water. Over time, the concentration of dissolved solids from the make-up water increases in the boiler as pure water is removed as steam. The relationship between the level of dissolved solids in the boiler and the dissolved solids in the feedwater is termed cycles of concentration. Water is drained, or blown down, from the boiler periodically to maintain a consistent number of cycles of concentration. This process is essential to the prevention of scale and corrosion in your boiler.

What is the difference between bottom and skimmer blow down?:

Water can be removed from the boiler from two main points: the bottom drain valve and a skimmer drain valve which enters the boiler somewhere just slightly below the water surface. The concentration of sludge is greatest at the bottom of the boiler and bottom blowdown is performed to remove sludge which precipitates during boiling. The concentration of dissolved solids is greatest at a point 6-8 inches below the water surface. Water removed from this depth, termed skimmer blowdown, removes the greatest amount of solids in the least amount of boiler water. Skimmer blowdown is the most efficient means of controlling cycles of concentration, but bottom blowdown is essential to prevent the build up of sludge.

How often will my boiler need to be blown down?:

Blow down frequency and volume depends primarily on the amount of condensate returned to the boiler and the quality of your make-up water. Develop a routine which ensures your boiler receives the proper amount of blowdown at regular intervals. Always consult your Chardon Master Service Technician before changing your blow down routine.

How should I do the blowdown?:

The bottom drain line on your boiler usually has two valves: a gate valve and a knife valve. The proper way to bottom blowdown a boiler is to start with both valves fully closed. Start by opening the knife valve, then completely open and then close the gate valve. Do this to both front and back drain valves if your boiler is so equipped. Repeat this process three times to rock the water in the boiler and work the sludge toward the drain line. Skimmer blowdown should also be on a regular schedule. Use a properly calibrated conductivity meter to determine the amount of skimmer blowdown needed. Maintain the conductivity of your boiler water in the range determined by your Chardon Master Service Technician. Finally, it is important to blowdown the safety valves, level switch and sight glass once every week to ensure that they will work when needed.
If you have questions about this or any other technical matter, call one of our Technical Support Team Members at 800-848-9526.