Bottom Blowdown is used to remove suspended solids that build up in the boiler water such as those formed by calcium and magnesium salts, iron, copper, phosphorus and silica. Although preventing these contaminants from entering a boiler through proper makeup water and feedwater pretreatment is always preferred, it is inevitable some contaminants manage to enter the boiler. Failure of pretreatment equipment or corrosion of condensate systems are common causes of boiler water suspended solids. High levels of these contaminants will produce sludge (suspended solids) in the boiler water and can result in energy-robbing deposits on heat transfer surfaces. Bottom blowdown, the removal of boiler water and sludge from the lower portions of a boiler, is a common practice used to prevent deposition on heat transfer surfaces.
The amount and frequency of bottom blowdown varies depending on the type of boiler, quality of feedwater and operating pressure. Superior feedwater quality can reduce the frequency of bottom blowdown to as little as once per week while fair to poor feedwater quality may necessitate daily or multiple bottom blows per day.
Boiler bottom blowdown systems should consist of piping sized according to ASME code
. The pipe size is usually the same size as the blowdown tapping on the boiler. This piping should be equipped with either two slow opening valves or one quick opening valve. If it is equipped with two slow opening valves, one slow opening valve should have the quick opening valve closest to the boiler. If there is more than one bottom blowdown port on a boiler, they can each have a quick opening or slow opening valve close to the boiler which is then piped to a common header with a common slow opening valve. Slow opening valves typically require 5 complete 360o turns to go from a completely closed to a completely open position. Quick opening valves go from completely closed to completely open in one continuous motion. It’s always a good idea to pipe the bottom blowdown system on the side of the boiler with the sight glass so the boiler water level can be monitored during the blowdown process.
Our next blog will go through the steps of bottom blowing a boiler.